Interactions between flooding and upland disturbance drives species diversity in large river floodplains
Understanding and predicting vegetation patterns in floodplains are essential for conservation and/or restoration of river floodplains subject to hydrological alterations. We propose a conceptual hydroecological model to explain the disturbance mechanisms driving species diversity across large river floodplains. These ecosystems harbor a unique set of flood-tolerant species different from the surrounding upland vegetation. In elevation gradients across pristine floodplains, the greater the flooding, the fewer the number of plant species. As terrain elevation increases, flood depth and duration decrease and it is more likely that species composition is influenced by external natural or human-driven disturbances. The spatial interaction between the natural flood regime and upland factors creates patterns of disturbance gradients that influence how floodplain vegetation establishes. In regions where upland conditions are subject to strong external disturbances, species diversity peaks at intermediate stages along the disturbance gradient. We demonstrate this concept with observations from the Central Amazon and Pantanal in Brazil, the Mekong’s Tonle Sap in Cambodia, and the Okavango Delta in Botswana. We discuss how this model could be further elaborated and validated to inform management of large river basins under the impact of upstream-induced flood pulse alterations.
KeywordsFloodplain ecology Tropical rivers Plant species diversity Flood pulse concept Intermediate disturbance hypothesis Flood hydrology
River floodplains are complex and productive ecosystems that provide essential services to nature and society. They provide rich habitats and food sources for both terrestrial and aquatic organisms, while supplying drought protection, fisheries, and agricultural grounds to riverine human populations. Overall, the value of wetland ecosystems services around the globe is estimated to be higher than any other inland landscape (De Groot et al., 2012), but the appropriate functioning and provision of floodplain ecosystem services to humans and nature depend on the degree and frequency of connectivity with the upstream river system. These two factors, however, have drastically changed in rivers around the world, primarily as a result of water infrastructure developed to regulate water for hydropower, water supply, and irrigation.
Although water infrastructure development has presumably already resulted in drastic alterations to the hydrology and biota of vast areas of floodplains around the world, the future management of those already disturbed ecosystems and other less disturbed ecosystems will bring multiple challenges. On the one hand, water infrastructure development is accelerating in emerging economies, and future plans will likely impact conservation strategies for those large rivers that are still relatively unregulated (Grill et al., 2015). The changing climate is also bringing modifications to the main drivers of floodplains’ productivity, including extreme hydrological events, rising temperatures, and salt intrusion (Hamilton, 2010; Junk et al., 2013). On the other hand, there are an increasing number of opportunities for large scale hydrological and ecological restoration, as demonstrated by proposed and ongoing programs in Europe (European Commission, 2015) and North America, in particular in the Everglades (Sullivan et al., 2014), Mississippi River (Mitsch & Day, 2006), and Colorado River (Glenn et al., 2013). Whether the focus is conservation, climate change adaptation, or restoration, understanding and predicting long-term ecological patterns and links to the hydrology are essential for the sustainable management of large rivers and their floodplain ecosystems.
The objective of this paper is to propose a conceptual model of how inundation patterns in the annual to decadal time scale controls plant species diversity gradients in large river floodplains and how these may be affected by hydrological alterations. The paper first provides an overview of the current status of large floodplains and how they are affected by river regulation. A brief description of two relevant ecological concepts that underlay this paper’s argument is then provided. Following this, the conceptual model proposed is described. Case studies of four large floodplains on three continents—the Mekong, the Amazon, Pantanal, and Okavango—are then reviewed in light of species richness patterns observed in the field and how these observations relate to the conceptual model. The paper then concludes with a statement of opportunities for future research. The concepts reviewed and proposed throughout this paper aim to provide a crucial step in developing a practical tool that can be used to support management and restoration of large river floodplains around the world.
Current status and future perspective of large river floodplains
Hydrological controls in floodplain species: convergence of ecological concepts
In order to conceptualize a link between hydrological controls and diversity patterns in large river floodplains, it is important to provide the theoretical framework underlying this relationship. A general model of spatial biocomplexity across river networks has been previously proposed (the riverine ecosystem synthesis; Thorp et al., 2006), and the intent of our conceptual model is to synthesize and harmonize knowledge from this and previous concepts of critical importance to the understanding and management of large river floodplains. In particular, there are two widely known concepts in freshwater ecology that are highly relevant to the topic presented in this paper: the intermediate disturbance hypothesis (IDH; Connell, 1978) and the flood pulse concept (FPC; Junk et al., 1989). The following paragraphs give a short overview of these concepts and how they relate to the conceptual hydroecological model presented in this paper.
Intermediate disturbance hypothesis
The IDH first proposed that maximum levels of biological diversity occur at intermediate levels of disturbance with respect to time and/or space. Too much disturbance allows only for the establishment of a few pioneering species, whereas low disturbance facilitates the eventual dominance of a selected number of climax species. Hence, intermediate stages of disturbance offer a transitional phase/niche in which pioneering, intermediate, and climax species may co-exist. Connell (1978) first postulated this hypothesis for rainforest trees and coral reefs, but thereafter the IDH has been proven (and disproven) for a wide range of ecosystems (Petraitis et al., 1989; Mackey & Currie, 2001; Molino & Sabatier, 2001; Tanentzap et al., 2013; Jardine et al., 2015). It is important to recognize that the IDH only represents a set of similar phenomena that arise from different co-existing mechanisms (Roxburgh et al., 2004), but it does not explain the underlying mechanisms themselves.
Floodplains are among the list of ecosystems in which the IDH has been studied. Floodplains (which may be described as riparian wetlands) are at interphase zones between terrestrial and aquatic ecosystems (Naiman & Décamps, 1997), thus harboring species from both types of ecosystems. In addition, floodplains may harbor a different set of species altogether that may or may not be greater in number than their terrestrial counterparts (Sabo et al., 2005). Clear unimodal patterns of maximum number of species as predicted by the IDH have been observed as a function of flooding frequency (Pollock et al., 1998), and Tanentzap et al. (2013) stated that diversity patterns in wetlands are a result of differences in species’ niches rather than demographic stochasticity. Contrary to IDH predictions, Zelnik & Carni (2008) found that species richness increased linearly as a terrain elevation along a wetland moisture gradient surrounded by a landscape with low land use intensity.
The flood pulse concept
The FPC states that the regularity and predictability of the seasonal delivery of river water onto floodplains is the main driver of biogeochemical cycles, habitat characteristics, and species distribution in large river floodplains (Junk et al., 1989). The FPC was primarily based on research in the Central Amazon floodplains, where a range of ecosystem components including chemical cycles (Kern & Darwich, 1997; Wassmann & Martius, 1997; Weber, 1997), flora (Junk & Piedade, 1997; Ferreira & Stohlgren, 1999; Wittmann et al., 2011), and fauna (Petry et al., 2003; Lobón-Cerviá et al., 2015) were analyzed over the last few decades. The basic assumption of this concept is that the regular and predictable flood pulse is the main ecological factor influencing all organisms living in that ecosystem. Indeed, recent research has quantified and demonstrated that flood predictability (or rhythmicity) across river systems determines aquatic species richness and plant productivity (Jardine et al., 2015). Flooding regularity and predictability allow the evolution of adaptations and of highly diverse biota. They maintain genetic and species diversity in the floodplain ecosystem. Parolin & Wittmann (2010) suggested that the FPC could also apply to other (sub-)tropical floodplains, and observations in support of this concept have been documented in the Orinoco (Lewis et al., 2000), Mekong (Arias et al., 2013; Holtgrieve et al., 2013), Okavango (Davidson et al., 2012), and Australia’s wet–dry tropics (Warfe et al., 2011; Jardine et al., 2015). Clearly, the simplicity in which the FPC describes floodplain processes has been criticized (Zurbrügg et al., 2012), in particular with respect to the catchment-river-floodplain water exchange, demonstrated to occur in complex ways involving local catchment inputs, secondary channels, and banks (Lesack & Melack, 1995; Zurbrügg et al., 2012; Rudorff et al., 2014). While it is indeed important to underpin hydrological and biological mechanisms in floodplains, the FPC was coined as a conceptual model that summarizes the overall result of all these processes (Bayley, 1995).
A conceptual model of species diversity for river floodplains
We hypothesize that both concepts described above bring important aspects to the spatial distribution of plant species in river floodplains. The FPC suggests that those floodplains areas with an intermediate and predictable flooding regime exhibit the highest biodiversity (Junk & Wantzen, 2004), suggesting that there is an underlying direct connection with the IDH as pointed out by Jardine et al. (2015). Based on these two ecological concepts, we propose a conceptual model in which species diversity varies in floodplain landscapes as a function of flooding patterns and the land use/land cover conditions that dominate the surrounding uplands.
As a result of the flooding gradient, floodplains harbor a unique set of species with different degrees of tolerance to flooding, and which are typically different from species from the surrounding uplands. In an elevation gradient across a pristine floodplain, it is expected that the greater the flooding, the fewer the number of plant species that can tolerate such conditions. As the terrain elevation increases, the more favorable the conditions for rooted vegetation are, and therefore, the number of species is likely to increase to the extent at which upland disturbances become an important factor. If seasonal flooding becomes marginal due to upstream factors (e.g., water resources development), it is more likely that species diversity can be influenced by external (upland) disturbances. These can not only be anthropogenic (e.g., agriculture, deforestation, etc.), but they can also be natural (e.g., drought and fires). The spatial interactions between the natural flood regime and upland factors create different patterns of disturbance gradients that influence how vegetation establishes in a floodplain. In floodplains that are surrounded by uplands with optimal conditions for vegetation growth and little external disturbance—as it is in the case of the Central Amazon—species diversity responds inversely proportional to flooding. However, in regions where upland conditions are subject to strong external disturbances, species diversity peaks at an intermediate stage along the disturbance gradient (Fig. 2).
River floodplain case studies
In order to illustrate the concept presented in this paper, we selected four large floodplain systems across the tropics in South America, Africa, and Asia where species diversity distribution data with respect to flooding have been published (see Fig. 1 for basin locations). These four case study systems experience distinct seasonal flood pulses that have had historically similar low levels of flow regulation. The surrounding uplands, however, represent very different environmental conditions and disturbance types that could partially explain variations in plant species diversity gradients among these floodplains. Species diversity patterns presented below are based on the field plot inventories, but in all four cases the data represent the summary of observations across the entire study sites.
The Tonle Sap: Mekong’s floodplain within the rice paddies
The Mekong is Southeast Asia’s largest river basin, covering an area of approximately 795,000 km2. The floodplains cover over 41,000 km2, distributed primarily between the Mekong Delta in Vietnam and the Tonle Sap in Cambodia. The latter is a lake–floodplain system that exchanges a maximum discharge of 10,000 m3/s with the Mekong via the Tonle Sap River. Overall, water levels in the Tonle Sap floodplain fluctuate by an average of 8 m annually, inundating an area over 13,000 km2 for an average of 4 months per year (Arias, 2013).
The Okavango Delta: an Oasis in the Kalahari Desert
The Okavango river basin straddles three southern African countries and extends 1100 km from upper catchment to the terminal Delta. This basin extends across approximately 120,000 km2 of sub-humid to semi-arid uplands, with a rainfall gradient from 1400 mm in the north to 450 mm in the south. Rainfall occurs between September and April, peaking in December to February, sending annual pulsed flows down the river system, through Namibia into the fault bounded alluvial fan which forms the Okavango Delta in Botswana.
The average annual inflow to the Delta is approximately 10 km3 (Wilson & Dinçer, 1976; Porter & Muzila, 1989). Under current climatological conditions, the maximum extent of inundation varies from 8000 to 12,000 km2, of which about 6000 km2 is perennially inundated (Gumbricht et al., 2004a, b). Each distributary system across the alluvial fan exhibits slightly different flood behaviors; the westerly and central Thaoge and Jao distributaries, which receive primarily overbank flow, are characterized by fluctuations in water level of 1.5–2 m, while the easterly Moanatshira, Kwhai, and Mogogelo systems are fed by base flow, and the amplitude of fluctuation is less than 0.6 m (Porter & Muzila, 1989).
The Pantanal: high biodiversity between inundation and drought
With an area of approximately 160,000 km2, the Pantanal is one of the largest seasonal wetlands on Earth. Located within the neotropical savanna-belt (Cerrado), it is seasonally flooded by the upper Paraguay River and several of its tributaries. The precipitation regime in the region is clearly seasonal, with a dry period lasting from April/May to September/October, while the rainy period contributes approximately 70% of total annual precipitation (1090–1250 mm). This leads to seasonal water-level changes of the upper Paraguay River with a mean amplitude of 3.1 ± 0.9 m (data from 1988 to 2007, Wittmann et al., 2008).
The landscape of the Pantanal consists of a patchwork of seasonally inundated and non-inundated habitats (Zeilhofer & Schessl, 2000). Non-inundated habitats include isolated granitic outcrops and paleo-levees that were mostly formed as river terraces during Pleistocene and Holocene interglacial periods. These elevations are important refugia for savanna species which are characterized by low tolerance against seasonal inundation.
The vegetation of the Pantanal consists of semi-deciduous forests upon non-inundated levees and seasonally inundated riparian forests along the Paraguay River and most of its tributaries. Seasonally inundated savanna vegetation predominates between these formations, varying from open grasslands to shrub and tree savannas, the latter often characterized by mono-dominant formations of shrub or tree species (Prance & Schaller, 1982).
Central Amazon Floodplains: an exception to the model, but for how long?
Opportunities for future research
Once sufficient evidence is gained about how flooding and upland disturbances affect species diversity in floodplains, an important step to follow is the development of a numerical model. Such model would be a useful tool in projecting floodplain changes as a result of hydrological alterations (or restoration), human use, and changes in climate. This model needs to be a practical tool to be used across systems, primarily driven by data that are globally available. What is probably most innovative and challenging is that such a model ought to integrate human land use disturbance as a mechanism that is interrelated to hydrological processes. Moreover, the model should lead to spatially explicit results that are useful to multiple stakeholders, and its inputs should be compatible with hydrological models that are commonly used for basin and floodplain management. A good example of what a baseline for this model could exist for the Everglades (Foti et al., 2012), but much more data collection and model development need to be carried out before a similar tool can be generalized for floodplains worldwide.
This paper argues that the spatial distribution of species diversity along inundation gradients in large river floodplains can be explained by the interaction between two critical environmental drivers: flooding and upland disturbances. We described a conceptual model of how these two drivers interact in unregulated floodplains, and how the frequency and magnitude of flooding can actually shape the extent and type of land use and cover that takes place in a floodplain. In floodplains where there is little external upland disturbance, species diversity responds inversely proportional to flooding. In regions where upland conditions are subject to strong external disturbances, however, species diversity peaks at intermediate stages along the disturbance gradient. Our model links elements from two well-established ecological concepts, the Intermediate Disturbance Hypothesis (Connell, 1978) and the Flood Pulse Concept (Junk et al., 1989), which appear to have an excellent synergy when explaining patterns of species diversity in floodplains.
The concept described in this paper applies to a large extent of the wetland biome, concentrated primarily in large river floodplains within the tropics (Fig. 1). In order to illustrate this concept, we selected four case studies, for which there are published survey data on species diversity. Overall, we found that our conceptual model fits well with the observations in those systems like the Tonle Sap, Okavango, and the Pantanal where agriculture and drought are common in the outer inundation boundaries, creating a distinct species diversity peak in the middle sections of the floodplain and decreasing both toward drier and wetter conditions. Species diversity in the Central Amazon, however, declines proportionally to flooding, primarily driven by the near-optimal conditions for vegetation growth in the Amazon uplands. Since deforestation and drought are serious threats to the Amazon floodplains, species diversity patterns could shift in the future to a state similar to that found in the other case studies. Such shifts could ultimately bring serious consequences to the Amazon floodplains, compromising the functions and services that these ecosystems provide.
It is not only the Amazon, Tonle Sap, Pantanal, and Okavango which face an uncertain future; these are only a selected sample of large floodplains that are increasingly changing as a result of water infrastructure development and climate change. There is a serious need for practical management tools that make use of the little existing information in combination with remote sensing data and mathematical models, and we hope that this paper provides a step forward in scoping such tools that can be used for adaptation and restoration of these remarkably important ecosystems.
This manuscript was completed while M. E. Arias was a Giorgio Ruffolo Fellow in the Sustainability Science Program at Harvard University. Support from Italy’s Ministry for Environment, Land and Sea is gratefully acknowledged. Comments from two reviewers were very helpful in improving the original manuscript.
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