Why tropical island endemics are acutely susceptible to global change
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- Fordham, D.A. & Brook, B.W. Biodivers Conserv (2010) 19: 329. doi:10.1007/s10531-008-9529-7
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Tropical islands are species foundries, formed either as a by-product of volcanism, when previously submerged seabed is thrust upwards by tectonics, or when a peninsula is isolated by rising sea level. After colonisation, the geographical isolation and niche vacancies provide the competitive impetus for an evolutionary radiation of distinct species-island endemics. Yet the very attributes which promote speciation in evolutionary time also leave island endemics highly vulnerable to recent and rapid impacts by modern people. Indeed, the majority of documented human-driven extinctions have been exacted upon island endemics. The causes include over-exploitation, invasive species brought by people and destruction of island’s naturally constrained habitats. Imminent threats include inundation by rising sea levels and other adaptive pressures related to anthropogenic global warming. We review recent work which underscores the susceptibility of island endemics to the drivers of global change, and suggest a methodological framework under which, we argue, the science and mitigation of island extinctions can be most productively advanced.
KeywordsBiodiversityBiogeographyClimate changeDeforestationExtinctionOver-exploitationHabitat lossInvasive speciesSoutheast AsiaSynergistic human impacts
Southeast Asia has the world’s greatest concentration of endemic species, reflecting the unique geographical and geological history of the region (Sodhi and Brook 2006). Tectonic plate movements and glacial-interglacial fluctuations in Quaternary sea levels have, at different times in prehistory, created islands, formed and severed land bridges and exposed or inundated large areas of Southeast Asia (Voris 2000), creating a foundry for speciation due to allopatry and regular environmental change (Mittermeier et al. 1998). Yet the very attributes which promote diversification on evolutionary time-scales also leave island endemics highly vulnerable to recent, rapid human impact (Cronk 1997; Sadler 1999). Indeed, the majority of documented human-driven extinctions, from prehistory through to modern times, have been exacted upon island endemics (Diamond 1989), many of these involving birds (Baillie et al. 2004). Today, over-exploitation, introduced species and habitat destruction continue to threaten island biodiversity—especially birds (Johnson and Stattersfield 1990; Blackburn et al. 2004)—threatening dramatic and permanent shifts in ecosystem function (O’Dowd et al. 2003).
Global extinction rates have soared over the past century, due predominantly to the resource demands of a burgeoning human population (Millennium Ecosystem Assessment, MA, www.millenniumassessment.org, 2005). Shifting land use (e.g. forest clearance for agriculture and logging) and wildlife exploitation (e.g. for market consumption and pet trade) have reduced the range and abundance of many island species in Southeast Asia, directly causing severe biodiversity loss at a local scale (Brook et al. 2003), and indirectly limiting the scope for sufficient ecological and evolutionary adaptation to future climate and land use change across the region (Sodhi et al. 2004a). Mitigation of human-mediated impacts continues to be a clear focus for tropical conservation biology, but with an increasing acknowledgement that these deterministic and stochastic extinction drivers are usually interacting and self-reinforcing (Bradshaw et al. 2009). In particular, the play-offs between these historical threats and an emerging, overarching process—climate change—are becoming increasingly apparent (Lovejoy and Hannah 2005). With this, a greater awareness of the problem of synergistic feedbacks is becoming apparent (Brook et al. 2008).
In the face of mounting evidence of the developing biodiversity crisis in Southeast Asia (Sodhi and Brook 2006), and knowledge that human-induced ecosystem shifts may be rapid, large and sometimes irreversible (i.e. once thresholds are crossed, hysteresis will mean it is impossible to restore former interrelationships; Brook et al. 2008), policy makers and resource managers urgently need new mechanisms for informed decision-making-validated systems which incorporate the complexities and adequately capture the uncertainties underlying the response of biodiversity to human impacts (Zimmer 2007). Behind this backdrop, we argue that islands, as microcosms of continental-scale processes, provide an intriguing opportunity to disentangle environmental and ecological constraints owing to their smaller size, distinctive colonisation and extinction dynamics (MacArthur and Wilson 1967), and simpler food webs (Pimm 1991): enabling a ‘whole ecology’ model approach.
In this overview of the challenges and opportunities offered by the study of tropical island endemic systems, we: (1) describe the historical factors that have combined to influence the high biodiversity and endemism characteristic of archipelagic Southeast Asia (our focal region); (2) underscore the susceptibility of island endemics to human-caused extinction, highlighting the link between biodiversity and ecosystem services; and (3) provide a methodological framework under which, we argue, the science and management of island extinctions can be most productively advanced.
Island endemics in a biodiversity hotspot
Islands are home to many distinct organisms, yet overall species richness is often low on islands compared to continental areas of the same size; diversity builds as isolation time increases (Heaney 1986). This reflects two important prerequisites for high island endemism; long-term environmental stability and geographic isolation (Jansson 2003; Sadler 1999). The former promotes adaptive radiation; the latter reduces inter-specific competition and immigration rates, with less chance of invasion from non-indigenous species and more scope for fine-scale niche partitioning (Cronk 1997). A substantial proportion of the land area of Southeast Asia consists of islands and island groups. The sheer number of islands, both in the oceanic sense and in the sense of one habitat surrounded by a matrix of different habitat forming a barrier that deters movement, is a particular feature of archipelagic Southeast Asia (Sodhi and Brook 2006). Periodic episodes of rise and falls in sea-levels have at various times isolated or permitted connections, allowing freer movement of species. These episodes of isolation, together with thermal buffering during glacial periods, have driven speciation and high endemism in Southeast Asia (Heaney 1986, 1991).
The Pleistocene epoch of the Quaternary period (1.8 million to 11,400 years ago) saw the development of the dramatic alternation between glacial and interglacial periods, triggered by orbital forcing and amplified by greenhouse-gas and ice-albedo feedbacks (Hansen et al. 2007). Southeast Asia experienced warm wet pluvial periods, alternating with colder dry inter-pluvials (correlating with glacial periods in temperate regions). During dry cool periods, seasonal forests and savannah expanded as areas of rainforest contracted and montane vegetation zones shifted to lower elevations (Heaney 1991). Low sea levels during glacial episodes compensated for expanding drier climates, through a concomitant increase in land area of exposed continental shelves, enabling at least some rainforest species to survive in pockets of moist climates (Gathorne-Hardy et al. 2002). The repeated pattern of glaciations frequently connected islands to other land masses (Voris 2000), facilitating biotic interchange and acted as a ‘species pump’ (Haffer 1969). As sea levels rose, the colonisation of newly formed oceanic islands, followed by genetic differentiation and long-term persistence, resulted in exceptional high species diversity in Southeast Asia, with endemism being highest on those oceanic island chains that remained continuously isolated from neighbouring islands during the Pleistocene, such as Sulawesi and the Philippines (Heaney et al. 2005).
In a landmark paper by Myers et al. (2000), 25 terrestrial global ‘biodiversity hotspots’ were identified; regions containing high concentrations of endemic species that were simultaneously suffering substantial habitat degradation and the ongoing threat of future loss. Southeast Asia overlaps with four of these hotspots (Indo-Burma; Sundaland; Wallacea; and the Philippines). During Pleistocene glacial episodes, some temperate species expanded their ranges southwards into Indo-Burma (Jablonski 1993). Episodic sea level changes repeatedly connected the islands of Sundaland to the Asian mainland, facilitating colonisation by mainland biota; rising sea levels caused isolation and promoted speciation (Meijaard 2004). Wallacea derived from land fragments that rifted from Gondwanaland at different geological periods (Audley-Charles 1983). This unique geological history, together with its consistent equatorial-tropical climate and numerous insular refugia, enabled the evolution of highly endemic biotas in Wallacea. For instance Sulawesi has one of the Earth’s most distinctive mammalian faunas, with 98% endemism for non-volant mammals (Whitten et al. 1987). The other geologically unique region of Southeast Asia, the Philippines, consists of some 7,000 islands of varying ages and geological histories, with multiple centres of endemism (Mittermeier et al. 1998).
Insularity and susceptibility to human-mediated impacts
Island biotas are a product of dispersal events and in situ diversification. Natural colonisation and extinction rates are well known to be broadly related to the geographical isolation of the island, its area and the ecological attributes of the dispersing taxa (MacArthur and Wilson 1967). After an initial phase of colonisation or competitive redistribution of niches, the geographical isolation of oceanic islands and their virgin ecology provides the impetus for an evolutionary radiation of unique biological forms (neoendemics); though endemism can also arise through the extinction of the ancestral and continental taxa (palaeoendemics; Whittaker and Fernández-Palacios 2007). Biodiversity on young islands, close to continents, tends to be the result of colonisation, while on old remote islands, speciation occurs faster than colonisation (Heaney 1986); exacerbated by the tendency for species that arrive at islands to undergo a series of evolutionary changes—range contraction, leading to reduced population size, precluding strong competition between species (Ricklefs and Bermingham 2002; Wilson 1961)—which not only allow island populations to diverge from their source population, but also accelerate adaptive change (Gillespie et al. 2008). Environmental stability over geological time encourages the evolution and persistence of island endemics (Jansson 2003). In Southeast Asia, thermal and evapotranspirational buffering of rain forests during glacial periods provided ideal conditions for in situ evolution. However stability and geographical isolation, the very conditions that promoted high species diversity in Southeast Asia, also leave island endemics highly vulnerable to modern human impact.
The human-induced extinction rate is particularly high on oceanic islands (especially for birds; Blackburn et al. 2004) because insular environments typically harbour endemic clades, and island endemics are disproportionately vulnerable to human-driven extinctions (Sadler 1999). Evolutionary processes shaping island communities have provided taxa with behavioural, life-history traits and ecological relationships suited to stable conditions—specialised species with narrow ecological niches which are often naïve towards predators (Cronk 1997). Recent, research has stressed the importance of biological (e.g. life-history traits and ecological relationships) as well as environmental settings (regional threats and stochastic impacts) for predicting human-driven extinction risk (Pimm et al. 2006). For instance some insular species are more susceptible to habitat disturbance due to intrinsic characteristics such as naturally restricted distributions and behavioural or habitat specialisation (Sodhi et al. 2004b) derived from evolution in isolation; yet species most vulnerable to habitat loss can be ecologically and evolutionarily different to species that are threatened by over-exploitation due to the lack, or possession, of fortuitous pre-adaptations (Olden et al. 2007).
Anthropogenic impacts are many and varied, but broadly encompass habitat destruction, over-exploitation and introduced species (Millennium Ecosystem Assessment, MA, www.millenniumassessment.org, 2005). There is growing recognition that these impacts are not mutually exclusive, but rather are interactive and mutually reinforcing (Brook et al. 2008); and that species extinctions or population declines can cause cascading effects, resulting in catastrophic chains of co-extinctions and the disruption of ecosystem services (Koh et al. 2004a). Island biotas are particularly susceptible to invasions by non-native species, because of low species richness and a reduced competitive ability or predator awareness among native species, symptomatic of geographical isolation and evolution under stable conditions (Williamson 1989). In short, low species richness promotes ‘open’ or flexible ecological spaces and in turn facilitates invasability. A low selective pressure for dispersal among indigenous species compared to non-native species, together with poor evolutionary experience in the midst of competing species, enables invasive species to efficiently colonise open spaces on islands following natural or human-induced environmental perturbations (Denslow 2003). For instance, the degree of niche overlap in native plants has been positively correlated with invasion resistance (Mwangi et al. 2007). Accordingly, the history of regular turnover events and establishment of a highly speciose modern biota in Southeast Asia has buffered, to some degree, this region’s island ecosystems from invasive species (Sodhi and Brook 2006). However, the rapid rate at which forests are presently being converted to open and degraded areas is eroding the buffers once provided by rich biodiversity and relatively large land areas (for islands), increasing ‘open’ ecological space for invasive species.
Although islands in Southeast Asia have remained relatively ecologically undisturbed throughout most of the Anthropocene (Flint 1994), today island communities are experiencing a rapid and extensive rate of species decline (Brook et al. 2003). Deforestation is particularly severe in Southeast Asia, where lowland forests are being destroyed at relative rates that exceed other tropical regions (Achard et al. 2002). Consequently, only a few large islands (e.g. Borneo and Sulawesi) retain large tracts of primary forests capable of supporting viable populations of large-bodied vertebrate species (Sodhi and Brook 2006). Furthermore, over-exploitation, concurrent with habitat loss and fragmentation, accelerates species decline (Clayton et al. 1997; Milner-Gulland et al. 2003). Seven vertebrate species are listed as extinct in Southeast Asia; a further species is extant only in captivity (IUCN 2007). Alone, these figures do not appear alarming. However, they fail to convey the level of endangerment presently faced by extant species (Sodhi et al. 2004a) and the high probability that many extinctions in the region have gone unrecorded (Sodhi and Brook 2006). Because large-scale deforestation is a relatively recent phenomenon in Southeast Asia, many of the native species in the region, especially long-lived ones, may be persisting as ‘living dead’, doomed to eventual human-induced extinction, regardless of future conservation actions (Brook et al. 2003).
The Philippines and Indonesia illustrate the gravity of the situation in Southeast Asia, where the exploitation of vital habitats has brought these archipelagos to the brink of ecological ruin (Posa et al. 2008). Deforestation in the Philippines in the twenty-first century continues largely unabated, despite already extensive commercial logging during the twentieth century: forested land cover is now less than a quarter of land area (Bankoff 2007), while primary forest is much less (FAO 2005). The archipelago’s coral reefs are threatened with over-exploitation, harmful fishing practises and siltation (Gomez et al. 1994). The storey is little different in Indonesia, which has the unenviable distinction of being listed among the top few countries with the most threatened bird and mammal species (IUCN 2007) and highest emissions of greenhouse gases from deforestation sources (Santilli et al. 2005). Because most of these species are endemic to Indonesia, their regional loss would represent global extinctions. From a local perspective, species extirpation and population decline will cause co-extinctions and disrupt ecosystem services, further accelerating extinction rates.
Synergistic impacts of global change
The interaction between different threats to the biodiversity of Southeast Asia remain poorly understood (Sodhi et al. 2004a). However, it is becoming increasingly apparent that in general, it is multiple, rather than single factors, that are primarily responsible for recent population or species loss on tropical islands—occurring often in synergy, thereby accelerating extinctions and potentially leading to ecological collapse (Pimm 1996). For example: (1) current methods of timber extraction increase access to forest interiors, amplifying exploitation and invasability (Milner-Gulland et al. 2003); (2) tropical forest clearance and fragmentation causes localised drying and regional rainfall shifts, enhancing susceptibility to forest fires, causing further forest conversion (Laurance and Williamson 2001); and (3) land cover change can transform El Niño Southern Oscillation (ENSO) events into destructive phenomenon that trigger droughts and wildfires with increasing frequency and intensity (Siegert et al. 2001). Failure to recognise that human impacts on island communities are likely to be interactive and amplifying, runs the risk of making overly optimistic projections of the present and future state of biodiversity, and so could lead to inadequate conservation actions.
Ecosystems most severely affected by habitat loss in Southeast Asia—rainforests, savannas, mangroves and coral reefs—are characterised by highly diverse and complex ecological communities, in which many species are inextricably co-dependent. Thus extinction or species decline can also disrupt ecological processes leading to cascading extinctions (Koh et al. 2004a). For example in Southeast Asia: (1) the extirpation of butterfly species has been linked to the loss or decline in abundance of their host plants (Koh et al. 2004b); (2) the loss of frugivorous birds and mammals, which play an important role in seed dispersal in tropical forests, may impact on forest regeneration (Sodhi and Brook 2006). A decline of forest dwelling pollinators may not only impact plant reproduction in forests, but also influence neighbouring agricultural areas, potentially impacting food production (Bradshaw et al. 2009). Thus, the ramifications of rapid and widespread habitat loss and degradation in Southeast Asia are by no means confined to the direct effects of habitat loss. Forest canopies reduce the force of rainfall striking the soil, while tree roots bind soils, reducing erosion and downstream flooding, minimising siltation of natural waterways and marine environments (Sodhi et al. 2007). Siltation from deforestation and agricultural expansion has been identified as a major destructive factor impacting coral reef ecosystems in the Philippines, together with overexploitation and destructive fishing practises such as dynamiting (Gomez et al. 1994).
Over the next few decades climate change is anticipated to cause a major shift in threatening processes, owing to the unprecedented present rate of global climate change and the fact that landscapes are already heavily modified (Lovejoy and Hannah 2005). Evidence of climatic impacts on the biota of Southeast Asia is currently sparse, although regional observations (e.g. Peh 2007) confirm that human-induced climate change is likely to constitute some of the most pressing conservation issues facing tropical species over the coming centuries (Bradshaw et al. 2009). The modification of forest plant cover and soils which sequester carbon, provides a large source of anthropogenically derived greenhouse gases; second only to fossil fuel emissions (Flint 1994). It is estimated that between 1990 and 2000 forest loss in Southeast Asia accounted for 29% of the global carbon released from deforestation over this same period (Phat et al. 2004), contributing to human-forced climate change. Over the last 40 years in Southeast Asia: (1) the number of warm days and nights have increased; (2) the number of cold days and nights have decreased; (3) annually there are fewer rainy days; and (4) extreme flooding events have become more common (Cruz et al. 2007; Manton et al. 2001). The frequency and magnitude of warm-phase ENSO events have increased, although this may reflect a climatic oscillation and not a direct response to human-induced climate change (Malhi and Wright 2004).
Global warming has, and continues to, invoke latitudinal and altitudinal range shifts, altering ecological interactions and threatening the persistence of range-restricted species (Parmesan 2007). Habitat fragmentation limits dispersal, increasing the risk that species which experience a future loss of suitable local climatic conditions will not be able to move to cope (Williams et al. 2007). Tropical species also appear to be physiologically more susceptible than temperate species to small changes in temperature (Deutsch et al. 2008). Climate change-induced sea level rise will lead to greater agricultural intensification on mid elevation habitats, leading to accelerated habitat loss and greater erosion (Millennium Ecosystem Assessment, MA, www.millenniumassessment.org, 2005). Collectively, these changes can be expected to alter ecosystems, increasing the success and spread of non-native species and so accelerating the homogenisations of regional biota (Brook 2008).
Arguably, the impact of climate-induced change will be greatest on oceanic islands because: (1) indigenous biotas tend to have lower genetic diversity and often little evolutionary experience with new predation or competitors, making them prone to rapid shifts in ecosystem processes (Buckley and Jetz 2007); (2) insular taxa tend to be ill-equipped to disperse in response to climate-induced range shifts—a legacy of evolution in isolation (Gillespie et al. 2008); and (3) for those species with the capabilities to migrate, it may be to no avail, because sea level rise will disproportionately affect insular environments (Mimura et al. 2007), cutting off routes of escape and inundating potential refugia (especially on small islands, and low islands with little relief).
Modelling the dynamics of biota in space and time
Models provide conceptual tools to link demographic traits (population growth, dispersal and trends in vital rates) and ecosystem processes (ecological interactions and feedbacks) to how an organism perceives and uses the landscape’s spatial structure, allowing a species’ range and persistence to be projected under different human impact scenarios (Akçakaya et al. 2004). Multi-species approaches enable ‘whole community’ model-based management decisions (Ferrier 2002). Species’ ranges are traditionally modelled by linking presence or abundance data directly to environmental variables, using habitat models (Elith and Burgman 2003). The implicit assumption, that current climatic and topographic constraints act to define a species distribution and reflect its environmental preferences, is reasonable as a broad assumption, but simplistic in detail in that it ignores the potentially strong influence of interactions between distribution and the spatial structure of suitable habitat, and how a species’ life-history traits interact with shifting ranges (Araújo and Rahbek 2006). Susceptibility to human-mediated environmental disturbances will depend on: (1) life history, morphological and physiological traits (e.g. generation length, dispersal ability, body size and heat exchange); (2) ecosystem processes (including species interactions); (3) landscape spatial structure; and (4) changes in allele frequencies through natural selection (Botkin et al. 2007). Thus habitat and bioclimatic niche models provide, at best, only a first approximation of the potential impact of climate-induced landscape change on biodiversity (Pearson and Dawson 2003).
Persistence in a dynamic landscape depends on the interaction between landscape modification (the pattern, scale, rate and direction of landscape change in size structure and quality) and the species’ ecology (its ability to disperse between and grow within habitat patches or utilise the habitat matrix) (Akçakaya and Brook 2008). Most spatial (meta)population models attempt to integrate landscape features with demographic and ecosystem processes, enabling a potentially more realistic forecasting of species’ persistence. However, to date, their utility has been restricted largely to solving iconic and economically important problems, because it is often intractable to gather detailed demographic and dispersal data for a whole community of species, and because, with rare exceptions (e.g. Carroll 2007), they tend to ignore species interactions (Burgman et al. 2005). Alternatively, pattern-oriented approaches, by relating landscape structure to environmental variables that promote species diversity, allow conservation decisions to be made in the absence of demographic information for most species (Ferrier 2002). This technique, although generalised, avoids the pitfalls of using focal species to extrapolate conservation decisions (Lindenmayer et al. 2002). The caveat is that species responses to temporal and spatial variability are complex, making it hard to identify a suite of landscape or climatic properties that will favour most, let alone all species in a given region (Manning et al. 2004).
Future research directions
While appreciating that the fundamental threat to biodiversity in Southeast Asia is primarily socio-economic in origin (Sodhi et al. 2004a), we argue that the development of modelling systems which try to capture some of the complexities and uncertainties underlying biological mechanisms that drive species distribution and abundance, will be essential if conservation efforts in Southeast Asia are to be converted into conservation successes. Validation via field data and ongoing, well designed monitoring programmes (e.g. Milner-Gulland and Clayton 2002; Ancrenaz et al. 2005) will be essential to this process. Policy makers and resource managers must be armed with the most ecologically ‘realistic’ projective artillery if the impending biodiversity crisis is to be averted. The model framework that we suggest, and illustrate in Appendix 1 and 2, should integrate demographic processes (dispersal, population fluctuations and trends in vital rates), landscape spatial structure and ecosystem processes (ecological interactions and non-linear feedback loops), providing tools for exploring the management option trade-offs and sensitivities to assumptions surrounding regional conservation alternatives, enabling a more strategic approach to conservation prioritisation under global change (Zimmer 2007). Island communities tend to have lower functional redundancy and simpler food web structures than continental biota (Denslow 2003; Pimm 1991). Thus they provide ideal candidate systems for developing this ‘whole ecology’ method.
A hard-nosed, strategic approach to adaptation in the face of global change, which aims to ensure that as many species as possible are able to move into new niches or adapt to human pressures, will require ecological triage (Wilson et al. 2006). Forecasting methods which better identify key communities of future conservation importance will: (1) underpin robust reserve design, by revealing ‘pinch point’ habitats of high conservation priority in the face of global climate change and interactions with other human-mediated impacts; and (2) inform ecological restoration, by identifying sites and communities that will benefit most in the future from present rehabilitation efforts, and those which may be too difficult to maintain. Collectively, the goal must be to create the conditions which enable a rich and genetically robust biota to survive in ecologically adequate refugial sites, or to have the capacity to move into new areas to adapt to future environmental and landscape change and altered ecological relationships.
One likely model architecture to achieve these integrative predictions is demographically structured or individual-based, incorporating explicit and dynamic spatial structure (Akçakaya and Brook 2008). Habitat-vegetation modelling approaches are used to quantify the interaction between landscape and species ranges. Statistical techniques which characterise the background environment and differentially weight variables or alternative model structures in ensemble predictions are used to represent species distributions (Elith et al. 2006) and to develop habitat suitability indices (Akçakaya et al. 2004). Ensembles, by generating probability density functions of range shifts, rather than mean-point projections, explicitly capture prediction uncertainty (Araújo and New 2007). Distribution models are validated by assessing their ability to represent real-world data and their sensitivity to environmental gradients (Botkin et al. 2007). Demographic and ecosystem responses to landscape variation are simulated by developing predictive models which link rates and patterns of human impacts (e.g. decreased habitat connectivity, over-exploitation, predation by invasive species and altered fire regimes) to ecological data (e.g. survival and dispersal rates). Future human impact scenarios are modelled under alternative management actions using the latest climate and land use projections, providing policy makers and resource managers with mechanisms for informed decision-making, for both mitigation and adaptation measures. Illustrative examples are given in Appendix 1 and 2.
The tropical island chains of Southeast Asia have an extraordinarily rich biodiversity and endemism, reflecting the exceptionality of the geographical and geological history of the region. Recently human-driven changes, such as rapid and extensive destruction of habitat and over-exploitation, have become serious threats to the native biota, by shifting ecological baselines, eroding resilience and undermining the scope for sufficient ecological and evolutionary adaptation to anthropogenic climate change and impacts by invasive species. We argue that development of integrated modelling systems, which capture the complexities and adequately represent the uncertainties underlying biological mechanisms that drive species distribution and abundance under the drivers of global change, will need to be an essential component of any multi-disciplinary approach which aims to avert, or at least allay, the looming biodiversity crisis in Southeast Asia.