General Concepts in Ecology

Abstract

This Chapter details the hierarchy of ecological assembly, from organismic, to population, to community, to ecosystem and to landscape levels. The processes of evolution by natural selection, genetic differentiation and speciation are described. Measures of species richness, evenness, diversity, endemism and categories of threat are important to conservation needs assessment and are illustrated with examples from Africa. The use of models of life history patterns (such as r- and K-strategies), and of the concepts of the ecological niche and guild are explained. Interactions between consumers and their food source provides a basis for their classification into herbivores, carnivores, omnivores and decomposers. To these are added interactions between two species with reciprocal effects—predation, parasitism, mutualism and commensalism. At landscape scales, interactions within and between populations and communities of existing and immigrant species, and the rates of immigration and extinction, are explored using the theories of island biogeography and of metapopulations. The impact of invasive species on rural livelihoods is used to demonstrate the fragility of natural communities to anthropogenic disturbance.

FormalPara Key Concepts and Questions: This Chapter Explains

• How ecosystems are structured and function at successive levels within a hierarchy of interactions.

• Why evolution by natural selection is a founding principle of biology.

• How populations are structured spatially and in terms of age, movement, and life history patterns.

• Why species richness, evenness and dominance are key elements of community structure.

• How the interrelationships of species within a community create food chains and food webs.

• Why a landscape view of community structure is needed to identify patterns and relationships in and between habitats and biota.

• How the theories of island biogeography and metapopulations explain the diversity and dynamics of isolated populations.

• Why the competitive advantages of certain non-native (invasive) species can exert strongly negative impacts on rural livelihoods.

Context: Ecological Assembly: Key Levels and Related Terms

Textbooks on the fundamentals of ecology usually follow a logical sequence from the organismic to population to community levels. Thereafter, processes are examined at higher scales of integration such as ecosystem energetics, patterns of distribution and abundance, and the emergent properties of ecosystems. Here we will introduce some of the foundational concepts and principles at these successive levels, and provide selected examples of their characteristics in the chapters that follow. Key terms mentioned in bold print will be discussed in more detail in succeeding chapters. The structure of this outline follows that presented by Smith and Smith (2015) in their excellent volume Elements of Ecology.

Organismic-Level Concepts

As emphasised in the definition of ecology, the science starts at the study of organisms, and therefore has to consider both the genotype (where genes are the units of inheritance in the process of adaptation by natural selection) and the phenotype (which is the physical expression of the genotype as molded by the environment). Adaptation by natural selection (or other processes such as genetic drift) occurs through the mechanism of genetic variation within populations and can result in genetic differentiation, and in certain environments the formation of geographic isolates and subspecies. Over longer periods, differentiation might lead to the creation of distinct species.

Population-Level Concepts

A population is a group of individuals of the same species that inhabit a given area. Populations vary in terms of the area encompassing all the individuals of the species, representing their geographic range. A species might comprise many, often isolated, populations. These individual populations form part of what is referred to as a metapopulation (Sect. 9.7). Endemic species and subspecies occupy a defined and limited area, usually referring to a single country or biome, such as Giant Sable of central Angola and Welwitschia mirabilis of the Namib Desert. Ubiquitous species occur over a wide geographic area, often across many countries and habitats, such as the dominant tree of the miombo, Brachystegia spiciformis. The term dominant is use both for a species that might, due to its size, abundance or other features, be visibly prominent, or where it is most abundant in numbers of individuals, or dominant in terms of biomass. A population’s abundance and distribution within its range provides a measure of density, and the spatial pattern within the range might be uniform, random or clumped. While plant populations are fixed where they germinate, root and grow to maturity, most animal species are mobile and might be migratory or nomadic (as in many birds and some antelope) or the populations might be sedentary within a seasonally or permanently defined home range and, in many cases, a defended territory.

The structure of a population usually varies over time, both in sex and in age. An understanding of the demography of populations, in terms of rates of birth and death, and the timing of such events within the life history of individuals, is a requirement for the study of the dynamics of populations and communities. The life history characteristics of species can be used to understand how they relate to environmental forces, through what are known as r- and K-strategies.

Within populations of a single species, intraspecific interactions include competition for resources, where environmental factors (such as water and light availability, food and shelter resources) might limit growth according to the habitat’s carrying capacity. Intraspecific competition might result in self-thinning within plant populations, as occurs in monospecific stands of trees in miombo woodlands. Self-thinning is the progressive decline in density and increase in biomass (growth) of remaining individuals caused by the combined effects of density-dependent mortality and growth within a population. Within antelope species, territorial behaviour might function as a regulator of population growth.

Community-Level Concepts

The structure of communities is one of the central topics of ecological research. Key measures of community structure are species richness and species evenness. Related to richness and evenness is the concept of dominance, which can refer to the number or size of individuals of a species. Species which predominate (in number or size) in a community are called dominants. In arid savannas, individuals of a single grass species might vastly out-number the scattered trees. Trees such as baobabs, through their size, the shade they cast and the alteration of soil water and nutrients around their base, might dominate both the functioning of the savanna community and the visual appearance of the landscape. Conversely, grasses might overshadow baobab seedlings, or through the accumulation of fuel and a resulting fire, might prevent the growth of baobab seedlings and thus have a dominant role in the life history and demographics of the physically much larger trees. Size or abundance are not the only measures of importance or dominance. Keystone species are those that influence community structure and function disproportionally to their size or numbers. For example, porcupines have been shown to influence savanna tree mortality at a level similar to the great ‘ecosystem engineers’—elephants.

The concept of structure relates not only to physiognomic form (grassland, savanna, forest) or species composition (floral and faunal) but also to the physiological, morphological and behavioural adaptations that species have evolved to succeed in a particular location along an environmental gradient. The particular set of conditions, resources and adaptations needed to succeed in a particular ecosystem determine the fundamental niche of a species. Other factors, such as competition, determine the realised niche of a species within the range of conditions within which it can physiologically succeed.

Within communities, interspecific competition for resources is defined as a relationship that affects the populations of two or more species adversely. Two main forms of competition are recognised by ecologists. Interference or contest competition occurs where some individuals—using direct threats such as fighting or poisons—access resources while excluding other competitors (some winners, some losers). Exploitation or scramble competition occurs indirectly where the increasing intensity of competition depletes each other's resources. This process reduces growth and reproduction in all individuals in a population (all individuals losing equally) and described in examples in Sect. 9.5.

The concept of interspecific competition is one of the cornerstones of evolutionary ecology, and the subject of many classic studies. The fundamentals were established independently by two mathematicians, the American Alfred Lotka and the Italian Vittora Volterra, in the 1920s. Lotka and Volterra both described four possible outcomes of interspecific competition. The key outcomes of interspecific competition, according to the Lotka-Volterra equations, and as summarised by Smith and Smith (2015), are:

• Species 1 may outcompete species 2;

• Species 2 may outcompete species 1.

Both of these outcomes lead to the competitive exclusion principle, which states that two species with exactly the same ecological requirements cannot coexist. The other two outcomes involve coexistence.

• An unstable equilibrium, in which the species that was most abundant at the outset usually outcompetes the other.

• A stable equilibrium, in which two species coexist but at a lower population level than if each existed without the other.

A further outcome of the Lotka-Volterra models of interspecific competition is that which suggests mutual control between predator and prey populations that oscillate through time. Here another classic study demonstrates the concept—that of the relationship between Snowshoe Hare and Canadian Lynx, described in Sect. 9.6.

One of the important evolutionary consequences of competition is the coevolution of often complex structural features such as those that contribute to the successful pollination of plants by insects. Competition for similar resources, such as for soil moisture by trees and grasses, might result in partitioning of the use of such resources in space and time (Box 10.1). When a species’ niche expands in response to the removal of a competitor, the result is known as competitive release. An example of rapid and major changes in herbivore populations, following removal of competitors (other herbivores), and of predators and poachers. This has occurred in the past several decades in Gorongosa National Park, Mozambique, as illustrated in Fig. 9.1. Box 9.1 provides a synopsis of the problem of invasive species in Angola, demonstrating the result of competitive interactions between invasive species and natural communities, mediated by human influences.

Fig. 9.1

Changes in the structure of herbivore biomass in Gorongosa National Park, Mozambique, following civil war and the reduction of carnivores and mega-herbivores. Mega-herbivores (greater than 600 kg body mass), meso-herbivores (less than 600 kg). From Stalmans et al. (2019) PLoS ONE 14 (3): e0212864

FormalPara Box 9.1 Human–Environment Interactions: Invasive Species Threaten Rural Livelihoods

Invasive species are among the top threats to biodiversity globally and are reported to be affecting livelihoods in 70% of African countries.

Munyaradzi Makoni (2020).

One of the cornerstones of evolution and ecology is the concept of competition. The equilibrium of natural ecosystems is provided by a dynamic playing field—the demand for resources between competing species reaching a balance until some factor alters the availability of specific resources to the advantage of one of two competing species. Nowhere is the imbalance better illustrated than in the interactions between indigenous (native) species and alien (exotic, non-native) species, occasionally leading to the surge in dominance of the ‘invasive’ species. Many factors can account for a sudden proliferation of the population of a non-native species. First, the newly arrived species might be free of the factors (predators, competitors, parasites, diseases) that control its abundance in its natural habitat. Second, invasive species usually have high reproductive rates, effective dispersal mechanisms, rapid growth, and broad or generalist niche requirements. Third, human interventions frequently provide ideal conditions for newly arrived species.

During the past few centuries, the role of humans, as agents of long range dispersal (ships, cars, planes) and as transformers of landscapes through agriculture, have accelerated the distribution of species across the globe. The majority of immigrant species die unless supported by human intervention, or survive because they can adapt to their new environment. On average 10% of new arrivals do not survive and reproduce, less than 10% become naturalised. Of these, less than 10% become invasive. But the few species that have become invasive are today threatening sustainable food production, human wellbeing and biodiversity.

Despite the global impacts of invasive species, surprisingly little attention has been given to the problem in Angola. This is in contrast to the situation in Europe, where annual damage worth 12 billion Euros results from invasive species impacts on human health, damaged infrastructure and agricultural losses. In South Africa, with over a century of investment in invasive species research (Moran et al., 2013), such species currently cost the country over US$450 million a year and are responsible for about a quarter of its biodiversity loss.

Several explanations might been offered for the absence of a concerted programme of invasive species management in Angola. These include the lack of information, limited scientific capacity and inadequate technological and financial resources. But a key factor is that despite the five centuries of inter-continental contact and trade between Angolan ports such as Luanda and Benguela with Europe, the Americas and Asia, surprisingly few invasive plants and animals have established themselves in Angola. The country has relatively few aggressive invasive plant species, perhaps less than a dozen, compared with South Africa where 775 invasive species of plant have been identified (Van Wilgen et al., 2020). Yet several invasive plant species are currently spreading rapidly across valuable agricultural land, reducing the production potential of rural communities and threatening rare plant and animal species and biodiversity hotspots. Very limited information is available on invasive animal species, but a rapid survey of invasive plants was undertaken in 2014 (Rejmánek et al., 2016), providing insights into the seriousness of Angola’s invasive species problem.

Here the focus is on invasive plants. Three categories of non-native plants with invasion potential are recognised (Pysek et al., 2004):

• Casual alien plants: Non-native plants that may flourish and even reproduce occasionally outside cultivation in an area, but that eventually die out because they do not form self-replacing populations, and rely on repeated introductions for their persistence.

• Naturalized plants: Non-native plants that sustain self-replacing populations without direct intervention by people (or in spite of human intervention) by recruitment from seed or ramets (tillers, tubers, bulbs, fragments, etc.).

• Invasive plants: A subset of naturalized plants that produce reproductive offspring, often in very large numbers, even at considerable distances from the parent plants, and thus have the potential to spread over a large area.

In a 4000 km roadside survey across nine provinces and 13 vegetation types during August 2014, Rejmánek et al. (2016) identified 44 naturalised plant species (excluding crop or horticultural species), of which 19 species were conclusively invasive. Three of these are of immediate concern.

The spiny cactus Opuntia stricta, which is rapidly invading the southern escarpment and the lowlands from Dombe Grande to north of Luanda, is listed among the world’s 100 worst plant and animal invaders (Lowe et al., 2004). It is an extremely aggressive invader of savannas, and has been the subject of intense control projects in Kruger National Park for over 50 years (Foxcroft et al., 2004). Fortunately, it can be suppressed through inexpensive biological control methods, using the herbivorous insect Dactylopus opuntiae. Opuntia stricta is invading much of Quiçama National Park, and an urgent response is needed to prevent loss of habitat for the park’s wildlife population.

The second most invasive plant species in Angola is another of the world’s 100 worst invaders—the triffid weed, Chromolaena odorata, which is infesting agricultural lands in the moist forests of Cuanza Norte, Uíge, Zaire and Malange. This plant spreads through the wind dispersal of its seeds. It is unpalatable to livestock and has no traditional use. It is difficult to control and manual methods need to be supplemented by chemical poisoning. Biological control methods have been used with variable success.

The third invasive species of concern—the fast growing South America tree Inga vera—has been invading escarpment forests, notably in Cuanza-Sul, where it was introduced to provide shade in coffee plantations. It rapidly out-competes indigenous forest trees, and thus threatens the biodiversity of the critically threatened escarpment forest remnants. It is resilient to control by mechanical means as it coppices if not treated with chemicals.

The control of these three species is of the highest priority. But prevention is better than cure, and several species of great concern elsewhere in Africa, but not yet recorded in Angola, should be detected at an early stage and eradicated before they become established. These include Azolla filiculoides, Broussonetia papyrifera, Clidemia hirta, Parthenium hysterophorus, Rubus rosaefolius, and Salvinia molesta. The planting of highly invasive species, such as the Australian Acacia mearnsii—recently planted as a shade tree at Tundavala, Huíla, should be prohibited.

Ecosystem-Level Concepts

The ecosystem concept, like that of genes, species, populations and communities, is one of the founding elements of ecology. First defined by Tansley (1935) and elaborated by Odum (1983), the concept encompasses both structural and functional attributes. These attributes include all the organisms in a given area interacting with the physical environment, so that the flow of energy, and cycles of water and nutrients, create a defined trophic structure and biotic diversity. These include the exchange of materials between living (biotic) and non-living (abiotic) components of the system.

The trophic structure of an ecosystem comprises primary producers (the autotrophic component, which is dependent on solar energy, water and mineral nutrients, i.e. plants), primary consumers (the heterotrophic component, the consumers of plant tissue, i.e. herbivores); and secondary consumers (consumers of primary consumers, comprising carnivores and omnivores). Finally, decomposers, which mainly include invertebrates and microorganisms (bacteria, protozoa and fungi), break down the dead organic matter (detritus) produced by the higher trophic levels, releasing nutrients in a form that can be used by primary producers (Sect. 10.6).

The structure of the interconnections between the different trophic levels is termed a food web. Of special interest to conservation biologists are the consequences of any disruption to a food web. This often takes the form of a trophic cascade, where a predator reduces the abundance of its prey (herbivores) such that the impact cascades down to the next trophic level (grasses and browse—primary producers) with a rapid increase in the abundance and biomass of the latter. A different trophic dynamic has been recorded in the rapid explosion of the Waterbuck population in Gorongosa National Park, Mozambique, following the extinction of its main predator (lion) and the dramatic reduction of its herbivore competitors (buffalo, elephant, hippo) during the civil war. The Waterbuck (a mesoherbivore) population increased by an order of magnitude to over 55,000 from 1970 to 2020. Lion and other carnivore populations have been supplemented by re-introductions, but in the absence or low biomass of competitors, the waterbuck population continues to increase (Stalmans et al., 2019). The dynamics of herbivore populations affected by war and by the removal of carnivores is well illustrated by the Gorongosa data (Fig. 9.1).

The study of the processes of production, consumption, decomposition and the transfers of energy and materials between trophic levels has become a major branch of ecology, with specialised branches known as systems ecology or ecological energetics, as will be described in Chap. 10.

Landscape-Level Concepts

At the scale of a country with as much environmental diversity as Angola, details of community structure are often lost in the breadth of seemingly uniform landscapes that stretch from horizon to horizon. A landscape is defined by geographers as a part of the Earth’s surface that can be viewed from one place. A landscape is usually made up of a collection of landforms such plains, valleys, hills, mountains and plateaus. Ecologists study the spatial patterns in landscapes and the habitats they offer for plants and animals, especially in terms of diversity and heterogeneity. The pioneers of Angolan ecology, from Welwitsch (1859) to Diniz (1973, 2006) took a landscape view of the country’s ecosystems. Today, the patterns of Angola’s landscape diversity are most easily detected with the aid of satellite imagery and drones, but much can be still learned from ground-based observations (Box 14.1).

Throughout this book, pattern is a central theme. It is defined by the spatial arrangement and connectivity of patches, large and small, across landscapes. As will be described, these patterns range from the scale of termitaria, to catenal sequences, to the extensive patterns of biomes, pyromes and herbivomes. Connectivity, through corridors, and disjunction (physical separation) are key features of pattern, and are responsible for the maintenance and creation of biodiversity through speciation and extinction processes. Boundaries between the units that create pattern can be diffuse or sharp, usually reflecting underlying environmental gradients, but also resulting from ecological processes. These processes might be immediate, by fire or herbivory, or very slow, as the result of the oscillations of climates and geological and evolutionary processes that cover millions of years. Gradual transitions between landscape units and plant communities are often called ecotones. Some ecotonal zones support edge species that have very narrow habitat preferences, such as Swierstra’s Francolin, that occupies the scrubby margins of Afromontane forests of Mount Moco and Mount Namba.

Pattern and connectivity have become important concepts of conservation science, and have led to classic theories such as those of island biogeography and of metapopulations, which have direct importance for the understanding of Angola’s Afromontane forest remnant patches and of fragmented populations of iconic species such as Giant Sable, Forest Buffalo and Forest Elephant (Sect. 9.7).

1 The Evolutionary Basis of Ecology

Overview

The foundations of ecology, and its underpinning by the concept of evolution by natural selection, go back to the writings of Malthus, von Humboldt, Wallace and Darwin. All these scholars were pioneers in the future science of ecology. Foremost of their publications was Darwin’s (1859) seminal work On the Origin of Species which provided a revolutionary framework for understanding the diversity of life on Earth. A general introduction to the evolutionary basis of ecology is thus an important step in the learning process.

Throughout this book, several recurrent themes will have become familiar to the reader:

• Life on land imposes diverse constraints on organisms. Each environment presents a different set of constraints on processes relating to the survival, growth, and reproduction of organisms.

• Species richness varies widely across the face of the Earth, yet the set of characteristics that enables an organism to succeed in one environment typically precludes it from doing equally well under a different set of environmental conditions.

• The theory of natural selection is arguably the most unifying principle in biology. It provides a basis for understanding the distribution, diversity, and abundance of species, and is an essential point of departure for an understanding of ecology.

It is not intended that a detailed introduction to evolutionary biology and genomics be presented in this introduction to terrestrial ecology. An excellent and concise introduction to the subject of natural selection, adaptation, and evolution is given by Smith and Smith (2015) and is summarised here. It is relevant, however, to note that the modern tools of genomics, especially phylogenetics, have provided new insights into the evolution and age of many features of African ecosystems. These advances are described in relation to spinescent trees and shrubs in savannas (Sect. 8.1), hypsodont teeth in ungulates (Box 8.1), the evolution of C₃ and C₄ grasses (Sect. 10.2), the speciation of Afromontane birds (Box 18.1) and of the underground geoxyles of mesic savannas (Box 15.3).

Natural Selection and Differentiation

Adaptations are those heritable characteristics that enable an organism to survive and reproduce in a given environment. An adaptation is the product of natural selection—a heritable behavioural, morphological or physiological trait that an organism evolves over an extended period of time. Natural selection results from the differential fitness of individuals within a population to interactions with their environment. Fitness is a measure of the proportional contribution an individual makes towards future generations. In a given environment, some individuals will survive better, reproduce more, and leave more descendants—they will be fitter—than other individuals. The process of change in heritable characteristics carried from one generation to the next is called evolution. Natural selection and evolution occur within species.

Darwin laid down the key ideas defining the process of natural selection, summarised by Townsend et al. (2008) as follows:

• Individuals that form a population of a species are not identical.

• Some of the variation between individuals is heritable—that is, it has a genetic basis and is therefore capable of being passed down to descendants.

• All populations could grow at a rate that would overwhelm the environment; but in fact, most individuals die before reproduction and most (usually all) reproduce at less than their maximal rate. Hence, for each generation, the individuals in a population are only a subset of those that ‘might’ have arrived there from the previous generation.

• Different ancestors leave different numbers of descendants (descendants, not just offspring): they do not all contribute equally to subsequent generations. Hence, those that contribute most have the greatest influence on the heritable characteristics of subsequent generations.

The units of heredity, passed from parent to offspring, are called genes. Genes are arranged on chromosomes, with the genetic information being carried by molecules of DNA (deoxyribonucleic acid). A gene is a stretch of DNA coding for a sequence of amino acids. The different forms of a gene are called alleles. The pair of alleles present at a given locus defines the genotype. The sum of heritable information carried by an individual organism is the genome.

A phenotype is the physical expression of the genotype. The genotype can give rise to a range of phenotypes under different environmental conditions, a property known as phenotypic plasticity. The adaptive characteristics of phenotypes are referred to as traits. Genetic variation occurs within subpopulations, among subpopulations of the same species, and among different species. The gene pool is the sum of genetic variation within all members of a population. Where genetic variation occurs between subpopulations of a given species, it is referred to as genetic differentiation. All individuals of a species are part of the species’ metapopulation, which might comprise many subpopulations, often geographically isolated from one another, as discussed in Sect. 9.7.

The process of natural selection acts on the phenotype, and involves altering a heritable characteristic of the genotype within the population and requires that this variation results in differences among individuals. Natural selection eliminates most deleterious genes from the gene pool, leaving behind only genes that enhance an organism’s ability to survive and reproduce—which is their ‘fitness’ as described earlier.

Adaptations can only result from natural selection. Genetic variation can result from mutations (heritable changes in a gene or chromosome) or non-random mating. Non-random mating is the selection of a mate based on some phenotypic characteristic such as size or colouration. In small populations, non-random (or selective) mating can result in in-breeding between closely related individuals, which in turn can result in the inheritance of deleterious genes. These genes can result in decreased fertility, even death, a consequence known as in-breeding depression.

Genetic differentiation provides individuals and populations with the ability to survive and reproduce within a given environment. It also leads to variation in morphological, behavioural and physiological characteristics as a consequence of different selective pressures through natural selection. Species with a wide geographic distribution are exposed to significant environmental gradients. The subtle changes in genotype and phenotype across such gradients is called a cline. In Angola, the Forest Buffalo ranges widely across the country, from Quiçama on the coast to Luando in central Angola, to Lunda-Norte in the northeast and to Cabinda in the northwest. Across this geographic range a clear gradient (cline) in horn size and shape, and body mass and colouration is found in the different populations of Forest Buffalo.

Where abrupt changes in the habitat or environment occur, the population might be recognised as an ecotype. An ecotype is a population of a species that survives as a distinct group through environmental selection and isolation and that is comparable with a taxonomic subspecies. Clines and ecotypes are common in Angola, given the very sharp environmental gradients that occur, for example, along the Angolan Escarpment. The fragmented Afromontane forests of the Highlands have many examples of geographic isolates, where barriers to the free flow of genes within the populations have caused the evolution of subspecies, as illustrated in many bird species (Box 18.1).

Speciation

Speciation is the evolutionary process where populations evolve to become distinct species that are reproductively isolated from one another. Species can therefore most simply be defined as populations that can potentially breed together and produce fertile offspring. The concept is illustrated by the classic study of hybridization in the Cangandala population of Giant Sable. The isolated Cangandala population had been severely reduced, with a small group of females, but no males, surviving the impact of poaching. However, the Giant Sable cows mated with a lone Roan Antelope bull, producing a generation of hybrid, but infertile calves. When a Giant Sable bull was translocated from Luando to Cangandala, the group of cows successfully raised a new generation of fertile Giant Sable calves, saving the population from ultimate extinction through hybridization (Vaz Pinto, 2019).

Speciation may be allopatric, where all divergence occurs in subpopulations in different places, which is especially likely for island species. Sympatric speciation occurs where divergence takes place in subpopulations in the same place. It should be noted that the origin of a species, whether allopatric or sympatric, is a process, not an event. Speciation might occur as an isolated process, with few new species being produced. However, under special conditions, best illustrated by Darwin’s classic studies of the Galapagos finches, a single species might give rise to multiple species, each exploiting a particular environmental resource or opportunity. This process is called adaptive radiation. In Africa, several rapidly radiating taxa in genera such as Hyperolius (Reed frogs), Brachypodion (Dwarf Chameleons), Pachydactylus (Geckos) and Aizoaceae (Mesem succulents) have displayed niche differentiation in response to fine-scale environmental conditions.

It is not only island archipelagos that are characterised by high levels of speciation. The biota of mountain ecosystems can become ecologically isolated from one another during periods of climate change, as described in Box 18.1. The bird faunas of the mountains of the Angolan Escarpment are characterised by high levels of species of very restricted range, with many endemic species, which occur nowhere else on Earth.

1.1 Species Richness, Evenness and Diversity

One of the central questions in ecology is seemingly quite simple: why do some communities contain more species than others? Before one can answer this question, one must have an objective measure of the number of species in a community—a statistic referred to as its species richness. Quantifying species richness is one of the first steps in describing community structure, and is a basic ecological tool. The relationship between species richness and habitat is a consistent feature of ecological patterns, varying according to temporal, edaphic, climatic, biotic and disturbance factors. Equally important is the relationship between species richness and area.

Species Richness

The species-area relationship is used in many studies of biodiversity. Counts of the species of a taxonomic group or trophic level present in samples of a habitat generally increase as sample sizes, or replicates, of the sample increase. The most common species usually appear in the first samples, with rarer species appearing as the sample size increases. Empirically, the increase in numbers with increasing sample size follows a mathematical relationship which decelerates as the area increases on arithmetic axes and looks like a straight line on log–log axes (Fig. 9.2). Plant ecologists often use the species-area curve to determine the size of a sample needed to adequately characterise a plant community. Adequacy can be judged as the sample size where the addition of more samples ceases to add significantly more species per work effort. This is where the species-area curve approaches a plateau, where increasing effort is not rewarded by an equivalent increase in information collected. Experience working in different habitats provides guidance on how many samples of what size are needed to characterise the community.

Fig. 9.2

The species-area relationship for a contiguous habitat. Note the different shape of the species-area curves of arithmetic and log–log axes. Creative Commons Attribution 4.0

Challenges in quantifying richness come with the identification of species. This is especially difficult in Angola where the fauna and flora are poorly known, particularly within certain taxonomic groups. However, many regional guides to major taxonomic groups are available to assist ecologists to identify organisms to at least the genus level, while several taxa (amphibians, reptiles, birds, butterflies) have detailed accounts of Angolan species.

Species richness and diversity indices vary widely between communities, and thus using the same sample design for grassland, savanna and rain forest habitats would give results that are not logically or statistically comparable. Botanists often use a standardised cluster of samples (quadrats) of increasing area to measure species numbers, growth form, abundance and other characters within the series of subsamples, and repeat the sampling across the landscape. Abundance is defined as the total number of individuals in a population. Population density is defined as the number of individuals per unit area. This information can thereafter be analysed using various data analysis techniques such as association analysis, principal components analysis and other approaches relevant to the purpose of the study. In practice, one size does not fit all.

Species Richness and Evenness

On its own a measure of species richness provides a good indication of the relative importance of an area for biodiversity—but it is only a list of species and does not provide any insight into how important the area is for different species and in terms of conservation value. Ecologists need some measure to make comparisons between similar habitats and communities, based on more than a simple list of species present. A more meaningful measure is the diversity index which combines both species richness and the evenness or equitability of the distribution of numbers of individuals among those species (Fig. 9.3). More complex measures of the distribution of species abundance in a community have been developed. The most commonly used index is the Shannon-Weiner diversity index which determines, for each species, the proportion of individuals which that species contributes to the total number of individuals in the sample. It gives a measure of the relative dominance, commonness or rarity of one species in relation to others in a community. Such information is important in experimental studies where the impacts of different treatments (fire, fertilization, grazing) are being tracked. Diversity indices also provide an indication of a community’s ‘health’, where high evenness indicates a healthier state than a low diversity index that might be due to dominance of pioneer or invasive species.

Fig. 9.3

A simplified representation of the difference between species richness and evenness. Each icon represents an individual of one species

Another measure of populations and diversity is that of density—the number of individuals of a species or a taxonomic group per unit area. Estimates of African bird densities indicate that the actual number of birds per km² increases following a rainfall gradient and habitat. Brown et al. (1982) gave estimates of 25 birds per km² in desert, 125 birds per km² in sub-desert shrubland, 1500 birds per km² in arid savanna, 3500 birds per km² in mesic savanna, 6000 birds per km² in forest/savanna mosaics and 8000 birds per km² in rain forest.

Alpha, Beta and Gamma Diversity

Species diversity measures differ according to their spatial scale. A count of species (or a taxonomic group) at the scale of communities is called alpha diversity. Defining a community is a complex task, as the structure and scale of communities vary considerably from one ecosystem to another. In Quiçama National Park, the Setaria welwitschii grasslands, comprising less than a dozen plant species, cover many thousands of hectares in unbroken blocks. In contrast, the species-rich gallery forests of the Muengueje River are confined to a narrow belt of a few metres width and a few km length. Alpha diversity of forests also differ from one continent to another, influenced by tree size and density. A single hectare of rain forest in Peru average as many as 618 trees of greater than 10 cm dbh (diameter at breast height); compared with 377 trees per ha in Gabon. Peru has many small tree species, Gabon forests have fewer species of trees, but larger trees per ha. On average, about 200 tree species are found within one ha of Peruvian forest, compared with only 50–75 species in equatorial Africa (Terborgh et al., 2016).

Over larger areas, of mixed habitats and communities, the variation in species composition is called its beta diversity. Beta diversity describes the rate at which species composition changes (the species turnover) across a region, or along a gradient or transect. It increases with habitat and environmental heterogeneity. Beta diversity is higher in the patchy mosaic of ecosystems on young landscapes of broken relief such as the Angolan Escarpment, and lower on older landscapes such as the African and Post-African peneplains of the interior plateau. Quiçama, which has some 28 plant communities within its 9960 km² has a much higher beta diversity than the less heterogenous landscapes of Bicuar National Park of 7900 km², as illustrated in the maps presented in Figs. 14.4 and 15.8.

Within tropical rain forests such as the Maiombe of Cabinda, the secondary forests are significantly poorer in species than primary, climax forests. At a landscape scale, however, the mosaic of mature, stable climax forest and disturbed secondary forest has a higher beta diversity than either of these two forest types. The mix of disturbance regimes in communities has led to the intermediate disturbance hypothesis (Connell, 1978) which holds that in such communities, the richest diversity is found not in climax forest, nor in secondary forest, but in communities where there is a mix of disturbance levels. In reality, the scale of measurement determines species richness, and in rain forest this is particularly important, as the size of disturbance area, period of recovery, and maturity of climax forest are the key determinants of species richness, as discussed in Sect. 12.4.

At a broader geographic scale, the total number of species across all communities is called gamma diversity. The gamma diversity of the Guineo-Congolian rain forests, at ca. 8000 plant species, although much poorer than similar forests of the Neotropics and Indo-Pacific, is nevertheless extremely rich compared with other African biomes (Chap. 12). The richness of rain forests is due to a combination of factors. First, long periods of climatic stability, and the perennial hot, moist climate of tropical evergreen forests, without widespread and lengthy episodes of extinction (such as happened due to glaciation of the northern temperate regions during the Pleistocene), allowed continued accumulation of species. Second, the structure and microclimates below the forest canopy and the temporal niches provided by forest-gap succession, supports a great diversity of life. Third, the interactions of animals and plants strengthens and diversifies the webs of life that characterise tropical rain forests. The gamma diversity of Angola, including representatives of six biomes, is much higher than that of Botswana, which has only three biomes. Namibe province, for example, with only three ecoregions, has a much lower gamma diversity than Huíla province, with seven ecoregions.

2 Endemism, Threatened Species and Hotspots

Endemism

Species richness is not the only measure used by biologists and conservationists to compare the importance of biomes, ecoregions and communities. Endemism is a much used metric. An endemic species is one that is only found within a defined geographic area, usually an ecosystem, biome or country. Narrow endemics are species with a very limited range, usually within a specific habitat or geographic region. Near-endemics refers to species that have 70% of their range within one country, such as some species that mostly occur in the Angolan Namib, but also across the border in Namibia. Examples include Black-faced Impala, Angolan Giraffe, Hartlaub’s Spurfowl, Cinderella Waxbill and Anchieta’s Python. Paleo-endemics are species that might have had a wide distribution in much earlier times, but are now restricted to isolated populations, such as certain bird species of the Afromontane forests of Mount Moco. Neo-endemics are species that have recently evolved through divergence and reproductive isolation. Ubiquitous species are those that occur across a wide range of habitats, landscapes or countries.

Categories of Threat

The science of ecology is the cornerstone of conservation biology, the study of the world’s biodiversity, the threats to its sustainability, and the prevention of extinction of species, ecosystems and their services to society. Threats to species survival are a key concern of conservation biologists. Categories of threat are measures used in conservation assessments of the threat to the survival of individual species. Categories of threat have been standardised by the International Union for the Conservation of Nature (IUCN) and are widely applied. In descending order of threat, the IUCN Red List threat categories are as follows (Rodrigues et al., 2006):

• Extinct: known only from museum, herbarium or other historical records.

• Extinct in the Wild: with the last remaining individuals or populations alive in zoos, aquaria or botanical gardens.

• Critically Endangered: if there is considered to be more than a 50% probability of extinction in 10 years or three generations, whichever is longer.

• Endangered: if there is more than a 20% chance of extinction in 20 years or five generations.

• Vulnerable: if there is a greater than 10% chance of extinction in 100 years—species threatened with global extinction.

• Near Threatened: species close to the threatened thresholds, or that would be threatened without ongoing conservation measures.

• Least Concern: species evaluated with a lower risk of extinction.

• Data Deficient: no assessment is possible because of insufficient data.

The term ‘endangered’ is applied to the following categories: extinct in the wild, critically endangered, endangered, and vulnerable. Species falling within these groups are recorded in global or national ‘Red Lists’ such as those published by many national conservation agencies. Based on these criteria, the IUCN global Red List includes 36% of tree and shrub, 14% of bird, 27% of mammal, 34% of reptile and 41% of amphibian species currently assessed as being threatened with extinction.

Gap Analysis and Hotspots

Evaluating the level of protection afforded species or habitats at national or global scales (gap analysis) has been used for conservation planning and priority setting purposes since the 1970s. In Angola, Huntley (1973, 1974a, 1974b) assessed the conservation status of 80 mammal species and measured the proportion of vegetation types falling within conservation areas, in order to draw attention to priorities for the establishment of new conservation areas. Based on the percentage of Angola’s 32 vegetation types falling within conservation areas, Huntley found that only 11 vegetation types were represented within the existing protected area system. Of these vegetation types, the species-poor desert and sub-desert systems, which occupied only 1.1% of the country, had 81% of their area protected. At the opposite extreme, the moist Guineo-Congolian forests and grasslands, occupying 10.2% of the country, and with possibly more than 70% of the country’s biological diversity, had 0% of their area within national parks or reserves. A similar situation prevailed for the tiny patches of Afromontane forests. Nearly 50 years after the assessment, the recommendations are currently being implemented (GoA 2018). Since the early measures of gap analysis were tested in Angola, a vibrant science of systematic conservation planning has evolved (Margules & Pressey, 2000) and is being implemented in many countries with considerable impact.

At global scale, British conservationist Norman Myers (1988) introduced the concept of biodiversity hotspots, using a combination of high species richness, high levels of endemism and high levels of threat. The baseline for a hotspot was set at 1500 endemic plant species, with at least 70% of the region having lost its original habitat. The original global hotspot assessment included 25 sites, increased to 35 in 2004 (Mittermeier et al., 2004). The most significant finding of the assessment was that these 35 sites, covering 15.7% of the Earth’s land surface, had already lost 86% of their intact habitat, which by 2004 totalled only 2.3% of the Earth’s land surface, while being home to over 50% of the world’s endemic plant species.

Triage Approaches

Decisions on what habitats or species to conserve with the limited resources available to governments is a question frequently placed before ecologists. In response, Myers (1979), introduced the triage concept to conservation. The term triage was used during the First World War (1914–1918) by the French medical corps at the Battle of the Somme and other battle-fields where they were overwhelmed by the numbers of wounded and dying. By assigning degrees of urgency to casualties, they were able to more efficiently and effectively prioritise treatment. Casualties would be divided into three categories:

• Those who are likely to live, regardless of what care they receive

• Those who are likely to die, regardless of what care they receive

• Those for whom immediate care might make a positive difference in outcome.

The concept has been used in recommendations to prioritise the most urgent conservation needs in Angola (Huntley, 2017), but has yet to be tested or implemented. More recently, triage approaches have been used by some medical facilities confronted by the surge in numbers of Covid-19 patients.

3 Life History Patterns: r- and K-Selection Strategies

Great thinkers often think alike. In considering two different species responses to environmental extremes (long-lived and constant; short-lived and unpredictable), two of the great ecological thinkers of the twentieth century, Robert MacArthur of Princeton University and Edward Wilson of Harvard University, USA, developed the concept of r- and K-selection (MacArthur & Wilson, 1967). While there has been much debate on the general application of the system, for our purpose of illustrating patterns in the Namib Desert and other arid zone ecosystems of Angola, the concept of r- and K- selection in life history strategies is useful, despite its detractors.

The r-/K- strategies are based on familiar observations. Reproductive patterns of organisms can be grouped into a few life history categories. These include factors such as the organism’s size, rate of growth and of development, the number of progeny it produces, the investment it makes in its progeny, and the stage in its lifetime when it reproduces. The terms r is derived from the instantaneous or intrinsic per capita rate of growth (r). Carrying capacity (K) is the maximum sustainable population size for the prevailing environment. Carrying capacity is a function of the supply of resources such as food, water and space.

The concept argues that organisms lie somewhere on a continuum or spectrum delimited by two extreme life-history patterns.

• r-selected species are typically small, have relatively fast growth rates, reproduce at an early age, produce large progeny, and are short-lived. They typically live in unpredictable habitats. In such habitats, resources, often in limited amounts, periodically become available (flood or famine) and the potential for pulses of rapid population growth is enormous. Under these conditions there is little competition, but adult mortalities are high because the resources are short-lived.

• K-selected organisms are typically larger, develop more slowly, are longer lived, reproduce at an older age, and produce fewer progeny. They live in constant or seasonally predictable habitats. K-selected species live at or near the carrying capacity of their habitat. Conditions being constant, the population growth levels off at carrying capacity in response to competition between organisms for limited resources. Because of stiff competition, the young suffer highest mortalities.

Angolan animals and plants include a mix of r- and K-strategies. Elephant and rhino are typical K-strategists, producing few calves, providing high levels of parental care, maintaining their populations (when in natural systems) at carrying capacity, and being long-lived. The opposite strategy is displayed by annual grasses and herbs. Annual plants often produce vast numbers of seeds with varying germination cues, some germinating after the first rains, but others having germination-inhibiting substances that delay germination for one or two seasons, by which time the inhibiting substances are leached out. The recruitment of new generations is thus spread over several years. This staggering of recruitment allows plants to hedge the bets for success over unpredictable seasons, one of which might be ideal for germination, growth, reproduction and further seed dispersal. This is particularly the case in Namib grasses (typical r-strategists), which need at least a 10 mm rainfall event before germinating. In good years, the mass of grass seeds stored in the soil germinate, turning the desert green and then gold as the grasses mature. As the annual grasses die the litter decays and much of this is blown into the dune sea, to form the detritus that is the foundation of dune food webs.

In considering r- and K-strategies, scale must be considered. As a simple generalisation, r- and K-selection models are best suited to comparisons of organisms that are functionally or taxonomically similar. An example of a K-selected life history is that of the dune-sea dwelling tenebrionid Onymacris plana, which lives for more than three years, unusually long for a small beetle. It feeds on the reliable food source of detritus blown into the dunes, and water from the regular morning fog. It breeds throughout the year, producing small batches of eggs. By way of contrast, the much larger, Brown Locust Locustana pardalina of Africa’s arid zones dies after just two months. In the swarming ‘gregaria’ phase, Brown Locusts have massive outbreaks numbering many millions, even billions, of locusts. They are the continuing expression of one of the ten biblical plagues—the epitome of r-selection.

4 The Concepts of the Ecological Niche and the Guild

The Ecological Niche

The ecological niche is a much-used concept in ecology, developed by Hutchinson (1957). An organism’s niche describes how and where an organism can live, grow and reproduce, influenced by multiple environmental conditions, resource needs and tolerances. Conditions such as temperature, humidity, pH and wind velocity, together with resources such as space, nutrients, water and shelter determine an organism’s habitat and its way of life.

Most species are capable of using much more of an environment’s resources than they actually do. Hutchinson described the environmental space where a species could survive and reproduce in the absence of competition from other species, as its fundamental niche. Competition for resources with other species might restrict the environmental space of a species to what Hutchinson called its realised niche (Fig. 9.4). However, not all interspecific interactions are negative. A species’ fundamental niche can be expanded where interactions are beneficial for one species but neutral for another (commensalism) or where both species benefit (mutualism), as discussed in Sect. 9.6.

Fig. 9.4

The relationship between a fundamental niche and a realised niche. A species can survive and reproduce within the environmental conditions of its fundamental niche, but competition from other species will reduce the niche to a more limited space of its realised niche

A species’ fundamental niche can also be expanded where there is a shift in the availability of certain resources (food, breeding sites). However, it is clear that within the realised niche, reproductive success is maximised, whereas outside of the realised niche the environmental limiting factors will depress breeding success.

Competitive Exclusion

The niche concept led to the formulation of the competitive exclusion principle, which states that two species cannot occupy exactly the same ecological niche, as discussed previously. However, under certain circumstances, two species competing for similar resources can co-exist in a stable environment through the partitioning of their needs in time or space—the differentiation of their realised niches. If there is no differentiation, one species will exclude or eliminate the other, or result in an evolutionary or behavioural shift in the weaker competitor towards a different ecological niche. The complexity of natural communities and interactions and the effects of stochastic (random or unpredictable) events create disruptions to competition ‘running its course’ and competitive exclusion is seldom observed in natural ecosystems. An example of partitioning is provided by the interactions between trees and grasses in savannas. Partitioning of resources can be spatial or temporal of both, as described in Box 10.2. Another example of temporal partitioning is that of diurnal raptors, such as the Black-shouldered Kite and the nocturnal Eagle Owl, both using the same food resource (rodents) but at different times of the day.

Guilds

A concept closely related to the niche is the guild. As exemplified in many bird communities, groups of species that use similar resources, or different resources in a similar fashion, are called guilds. Guilds may occupy the same habitat but use different components of the habitat, or different nesting choices. Examples include forest canopy leaf-eaters and forest floor insect scavengers; cavity nesters and nest weavers. In mammals, the term would apply to browsers or grazers in savannas. Some ecologists distinguishing between mega-, meso- and micro-grazers and browsers according to body sizes. Botanists sometimes use the term for different life-forms, such as graminoids, forbs, trees, shrubs, vines, parasites, epiphytes or saprophytes. Increasingly, the concept of functional traits has gained greater traction than guilds for such ecological groups. A functional trait is a morphological, biochemical, physiological, structural and phenological adaption to survive, grow and reproduce.

5 Herbivory, Predation, Parasitism and Mutualism

The structure and dynamics of communities is largely determined by the relationships between consumers and their food source. The manner in which animals acquire their food (and therefore energy) helps classify them into those that feed exclusively on plants (herbivores), exclusively on other animals (carnivores), both plants and animals (omnivores) and those that feed on the remains of dead animals and plants (detritivores/decomposers).

A further system of classifying the relationships between two species is that of the reciprocal effects—neutral, positive or negative—between them. Predation, parasitism, and parasitoidism have positive results for one, and negative consequences for the other organism. Mutualism has positive effects for both; competition has negative results for both. Amensalism has a negative effect on one, but no effect on the other. Commensalism has positive results for one and neutral outcomes for the other where a pair of species are closely associated. The interactions are summarised in Table 9.1.

Table 9.1 Positive (+), negative (−) and neutral (0) interactions between individuals of two species (A and B)

Herbivores

Herbivory is the consumption of plant materials by animals (herbivores). Some ecologists regard herbivory as a form of predation (the plant being the prey, the animal—from grasshoppers to elephants—being the predator). Here we restrict the term predation to those actions in which animals hunt and consume their prey.

The physical and chemical composition of plants plays a dominant role in determining the acceptability of plants to herbivore consumers. Plants are made up of carbon-based compounds (carbohydrates), mostly in the form of the cellulose and lignin of their cell walls. Most vertebrate herbivores lack the cellulolytic enzymes needed to break down the walls of plant cells to release their metabolic contents, including nitrogen, which is essential in the production of protein. As described in Box 8.1, the role of bacteria is critical in the specialised digestive systems of ruminants. This form of mutualism is known as a commensal relationship.

Animal tissues are high in fats and protein, used as structural building blocks. Plants, as described above, are built of carbohydrates. The ratio of carbon to nitrogen is about 50:1 in plants, and 10:1 in animals. The concentration of proteins in the diet of herbivores is highest in the young leaves of grasses and growing tips of tree shoots. The level of protein in plant material declines as the plant parts age and senesce. In savannas, if the protein content of grasses drops below 6%, the condition of the mammalian herbivores feeding on them will decline and survival, growth and reproductive success might be compromised.

The removal by grazing or browsing of photosynthetic tissue (shoots and leaves) from a plant does not necessarily lead to the plant’s death. In many cases, especially tropical grasses, defoliation of mature leaves can stimulate new growth and avoid the grass tussock becoming moribund due to shading by old leaves. Plants can also respond to the removal of roots, shoots or fruit by compensatory growth. Repeated defoliation can also trigger a response of increased production of defensive chemicals. Predation-prey relationships have evolved adaptations in the form of defence mechanisms in both animals (cryptic colouration, behaviour) and plants (poisons, spines, architecture) and a response from animal predators in terms of hunting techniques, as described in Sect. 11.4.

Predators and Carnivores

Predation is the consumption of all or part of one living organism by another. Predators kill their prey, a behaviour that is best characterised by carnivores. Prey choice might vary during the course of a year. Carnivores, unlike herbivores, consume high energy food, and are not faced with the problem of digesting cellulose. Predators such as Lion, Leopard and Cheetah often attack the most vulnerable (weak or old) prey individuals. The impact of predation on prey populations is therefore not necessarily greater than mortality from other causes in the absence of predators. Predators have inherited specific foraging strategies that result in the highest levels of hunting efficiency—behavioural patterns called optimal foraging strategies. These provide the best energy and nutrient return on hunting investment (energy gained per unit of energy expended). These strategies include the selection of habitats in which to hide, ambush or search for prey; or decisions on how long to sit and wait, depending on how many competing predators, or alternative prey, are present. A further decision for the predator is what prey is acceptable. Most predators have a fairly wide prey menu, but with preferred species. In productive environments populated with herding antelope, such as nutrient-rich arid savannas, a narrow and specialised diet is feasible for large carnivores, but in low-nutrient mesic savannas, a wide and opportunistic diet is more appropriate. However, studies of large predators such as Lion have revealed unexpected prey items, such as Porcupine, or cases where Honey Badgers attack and eat highly poisonous snakes. In such cases, both prey species can inflict serious injury on the predator.

Generalist predators spend little time searching, but consume low-value prey, while specialist predators spend more time and energy targeting high value but less abundant prey. In Angola, despite the decimation of most mammal species over the past 50 years, a very low abundance but widely distributed population of large carnivores (Lion, Leopard, African Wild Dog, Spotted Hyaena) survives. This situation can only exist where these predators have become opportunistic feeders, surviving as metapopulations by active dispersal over extensive hunting areas.

The availability of prey changes with season and patterns of climate from wet to arid periods, with consequent predator–prey population cycles or oscillations, often with large fluctuations in predator and prey abundance. Shorrocks (2007) provides a review of predator–prey dynamics in African ecosystems, where the complex interactions between multiple species (of both predators and prey) and rainfall and habitat, make simple generalisations impossible. Such cycles are more typical of strongly seasonal temperate climates. The best known of these are the Snowshoe Hare Lepus americanus and Canadian Lynx Lynx canadensis. Both hare and lynx populations across the boreal forests of North America follow an approximately 10-year cycle of abundance (Fig. 9.5), suggesting a predator–prey causal relationship. The hare population responds to the availability of food plants (twigs of trees and shrubs) and the predatory pressure of lynx, which in turn responds to hare abundance. The two populations function as a density-dependent regulator on the other. Predators regulate the growth of the prey population by functioning as the driver of density-dependent mortality. The prey population functions as the driver of density-dependent regulation on the birthrate of the predator population. High levels of prey mortality are attributed to the impacts of stress on hare fecundity, exacerbating the direct impacts of predation. However, the highly complex food webs of both predator and prey could provide alternative hypotheses for the synchronicity of the two cycles.

Fig. 9.5

The apparently synchronous oscillations in the abundance of prey (Snowshoe Hare) and predator (Canadian Lynx) as reflected in pelts sold to the Hudson’s Bay Company. Redrawn from Townsend, Begon and Harper (2008) Essentials of Ecology. Blackwell, Oxford

Parasites

Parasites draw on resources directly from their host. One partner draws benefit from the association while the other is harmed, but not necessarily lethally. Parasites include microparasites such as bacteria, protozoa and viruses. Macroparasites include worms, lice and ticks. Transmission of parasites between hosts is usually direct, but some parasites have complex life cycles which include other hosts, called vectors. While most parasites are non-lethal, with death usually occurring as a result of co-morbidities (such as in the case of the Covid-19 virus), some parasites are highly lethal.

In Africa, the most important microparasitic impact on wildlife populations in recorded history was that of the rinderpest (cattle plague) pandemic of the 1890s. Rinderpest is a zoonotic disease transmitted by the Morbillivirus virus. Introduced to Africa from Asia via Eritrea in 1889, within a decade it had swept across Africa, decimating the populations of nearly all even-toed ungulates (cattle, buffaloes and wildebeest). Rinderpest depopulated not only livestock and wildlife, but also the human populations of pastoral communities. Since the eradication of recurrent outbreaks of rinderpest from the Serengeti in the 1960s, the wildebeest population has increased ten-fold. The interactive impacts of rinderpest on herbivore population size and structure, grazing pressure, grass availability, fire frequency, survival and growth to maturity of trees, and the balance between grazers, browsers and carnivores provides the most complex but revealing demonstration of parasite-mediated trophic dynamics in African ecosystems (Sinclair, 2012).

Mutualisms

Mutualisms are associations that are beneficial to both parties. These include the critically important symbiotic relationship (termed commensalism) between microorganisms and their hosts in the transfer of nutrients in the specialised digestive systems of ruminants described above and in Box 8.1. In the fog belt of the Namib Desert, mutualisms between algae and fungi produce the diverse lichen flora (Sect. 11.1), while in the rain forests of the Congo Basin and in the arid savannas of the southwest, ant/plant mutualisms provide food and defence for the partners. One of the most important mutualisms in African ecosystems is that of between trees and nitrogen-fixing bacteria that form nodules on the infected root hair cells of the host plant. Bacteria receive carbon from the tree in exchange for the nitrogen that they fix and make available to the host plant. A similar association is that of fungal mycorrhizae which mobilise nutrients and water in exchange for carbohydrates provided by their hosts, as in the dominant trees of the Angolan miombo (Sect. 10.6; Box 14.2).

6 Theories of Island Biogeography and of Metapopulations

The evolutionary dynamics of Guineo-Congolian and Afromontane forests will be described in Chaps. 12 and 13, where the expansion and contraction of these forests over tens of thousands and even millions of years is described. Today, a much more rapid dynamic is at play—the dramatic reduction in the size and stability of Angola’s moist forest ecosystems. The ongoing destruction of the isolated forests of the Angolan Highlands and Escarpment is the country’s most urgent biodiversity conservation crisis. Furthermore, the fragmentation of once widespread and interconnected populations of large mammals such as Lion, Cheetah, Forest and Cape Buffalo, Giant Sable and Forest and Savanna Elephant has led to their survival in small, isolated and critically threatened populations that are below the viable population size for their survival. These processes can be used to illustrate two of the most influential concepts in ecology—the theories of island biogeography and of metapopulations.

The Theory of Island Biogeography

The vulnerability of Angola’s remnant forests is directly related to the size and shape of individual patches and their connectivity with other patches. The endemic bird fauna of Mount Soqui has been eliminated since it was first surveyed in the 1930s. The original forest patches have disappeared. The avifauna of Mount Moco and Cumbira have been seriously reduced since 2000 as their forest margins and interiors are increasingly damaged by fire, wood cutting and agricultural activities (Mills & Dean, 2021). Along the Chela Escarpment, tiny forest remnants are being reduced by fires and charcoal production. Only the forests of the deep ravines of Mount Namba have thus far been free of major damage, due to their larger size and inaccessible location. In addition to their current vulnerability to human activities, Angola’s Afromontane forests lie over 1900 km from similar forests on the Cape Peninsula, Mount Cameroon, Ruwenzori and the Eastern Arc Mountains of Tanzania—the ancient links of their faunistic and botanical diversity. The fragmented chain of Guineo-Congolian forests of the Angolan Escarpment are losing their connectivity with similar forests through land transformation around and between them.

In 1967, the American ecologists (Robert MacArthur of Princeton University and Edward O. Wilson of Harvard University) proposed a theory of island biogeography in a classic paper that has strongly influenced thinking and research on the relationships between size, connectivity and patterns of species richness on islands. The theory was built on the observations of early explorer-naturalists who had noted the increase in animal and plant species richness on islands of increasing size. A general pattern emerged of a tenfold increase in island size leading to a doubling of the number of species inhabiting the island. What MacArthur and Wilson (1967) described for islands was a dynamic equilibrium between survival of new colonising species and the extinction of previously established species on islands dependent on mainland sources of potential immigrant species. As species numbers increase on islands through immigration, competition for resources increases the extinction rate until immigration rate equals extinction rate. The composition of the biota might change through different rates of extinction of immigrants and original populations. Over time, there will be a continual turnover of species. For any given island a stable number of species (the equilibrium species richness) will ultimately be reached. Larger islands will support more species than smaller islands, and those close to mainland sources will accumulate more species than remote islands (Fig. 9.6).

Fig. 9.6

a Immigration rates are distance related. Islands near a mainland have a higher immigration rate and associated equilibrium species richness (S) than do islands distant from a mainland. b Extinction rates relate to area and are higher on small islands than on large ones. The equilibrium number of species varies according to island size, and larger islands have greater equilibrium species richness than do smaller islands. From Smith and Smith (2015) Elements of Ecology. (9th Edition). Pearson, Boston

MacArthur and Wilson’s theory can be summarised in the following predictions:

• The number of species on an island should eventually become roughly constant through time.

• This should be a result of a continual turnover of species, with some becoming extinct and others immigrating.

• Large islands should support more species than small islands.

• Species number should decline with the increasing remoteness of an island.

The theory does not only apply to oceanic islands. As already discussed, the forests of the Afromontane ‘archipelago-like centre of endemism’ represent islands in a sea of grasslands and savannas. The fragmented Guineo-Congolian forests of the Escarpment, and Afromontane forests of the highlands, are islands in rapidly transforming agricultural landscapes. Applying the theory of island biogeography to Angola’s remote, isolated Afromontane and Escarpment forests offers special opportunities to explore the evolution of their endemic avifauna, as initiated by Hall (1960) and taken further by Vaz da Silva (2015), as described in Box 18.1. The theory is also of relevance to conservation measures for small populations of critically endangered species, such as Giant Sable. Conservation areas are increasingly isolated from one another due to land transformation through agriculture and urbanisation. Viable breeding populations, within networks of conservation areas linked by corridors, are fundamental to the maintenance of metapopulations, as discussed below.

Metapopulation Theory

A metapopulation is a group of spatially separated local populations of the same species which interact at some level. Each of these local subpopulations has a chance of going extinct or being established again through recolonisation. Metapopulation theory was developed by Richard Levins of Harvard University (Levins, 1969) in studies of agricultural pest insects and is now widely applied in the conservation biology of fragmented habitats and populations. In simple terms, it is ‘the study of populations of populations and their interactions and dynamics.

The theory differs from that of island biogeography, in that the distinct local populations of a single species are separated, not by uninhabitable oceans of water (or savannas or deserts), but by areas of suitable habitat that are not currently occupied by populations of the species. Further, island biogeography looks at processes of colonization and extinction of many species of a given island or habitat from a large and distinct species pool. Metapopulation studies typically focus on populations of a single species.

Metapopulation theory is applied at the landscape scale, where small local populations within the larger habitat matrix can go extinct through random events (demographic stochasticity), including inbreeding depression. The smaller the local population, and the greater the distance between local populations, the greater the chance of extinction. Under natural conditions, members of expanding local populations might immigrate to a suitable but unoccupied habitat and initiate a new local population. Interchanges between local populations through dispersal might occur. Where one population emigrates to a population in decline, it might prevent extinction in the smaller population, a phenomenon known as the ‘rescue effect’. Across the whole metapopulation, some stability is achieved by the balance between extinction and recolonization of local populations. In common with island biogeography theory, the achievement of equilibrium and long-term maintenance of the metapopulation is a function of the influence of patch size and isolation on the processes of colonization and extinction.

In southern Africa, local populations under threat of extinction due to landscape transformation or over-exploitation have been artificially rescued by re-introductions from genetically similar populations within the metapopulation. The application of the concept in the study of fragmented populations of several iconic Angolan species, such as Giant Sable, Forest Buffalo (pacassa) and Forest Elephant is particularly appropriate. A classic example of a subpopulation approaching extinction was that of the Cangandala local population of Giant Sable, which had been reduced to a small group of females, hybridizing with a single male Roan Antelope. In an elegant model of conservation biology in action, the Cangandala population was rescued through the removal of the Roan male, and the introduction of Giant Sable males from the closest local population in Luando Strict Nature Reserve (Vaz Pinto, 2019) as described previously.