Rethinking extinctions that arise from habitat loss
Does the loss of species through habitat decline follow the same pattern whether the area lost is part of a large or a small habitat? An analysis sheds light on this long-running debate, with its implications for conservation strategies.
Understanding how habitat size affects the abundance of all the species living in a community provides ecological insights and is valuable for developing strategies to boost biodiversity. Writing in Nature, Chase et al.1 report results that might help to settle a long-running debate about the relationship between the area of a habitat and the diversity of species it can host.
Land transformation by human activity is a major component of global change. The loss of natural habitats reduces the local diversity and abundance of species2, and has been implicated in more than one-third of animal extinctions worldwide between 1600 and 19923. A report from the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services estimates that currently more than half a million species — about 9% of all terrestrial species — might lack the amount of habitat needed for their long-term survival4. Moreover, their disappearance would compromise many key ecosystem services, such as pollination or the control of pests or disease-causing agents.
The effect of habitat loss on biodiversity has been conventionally estimated on the basis of the relationship between area and species richness, which was first described more than 150 years ago5. This seemingly universal relationship is simple: the larger a given habitat’s area, the more species it holds, although the number of species increases with area in a nonlinear way6. There is a limit to the number of individuals of ecologically similar species that can persist in an area, owing to the limited resources that it harbours7. When a habitat loses part of its area, therefore, for many species, it also loses its capacity to support populations that are large enough to be viable. These species become extinct as habitat area diminishes with land-use intensification8.
Chase and colleagues propose an elegant and simple approach to account for the dynamics of communities occupying habitat patches of different size. Rather than considering only the overall number of species in each habitat fragment, the authors focused on the number and relative abundance of different species in samples obtained from such fragments. This allows the structure of ecological communities to be compared directly2, while avoiding problems that can arise when taking into account the differences in the effort needed to sample large and small areas9. The authors’ approach also allows a comparison of variations in the relative abundance of individuals of all species, a measure of community structure that is associated with ecosystem dynamics10.
Thanks to this method, Chase et al. could distinguish between three patterns of change that might occur as an outcome of habitat loss (Fig. 1). In the pattern described by the ‘passive sampling’ model, the structure of the community remains the same in large and small fragments. Therefore, each sample provides similar species richness (the number of species), abundance (the number of individuals) and evenness (the allocation of individuals to the different species), regardless of the total habitat size. In this case, species decline will mirror the loss of habitat area under the classical species—area theory5, and the total number of species in the entire fragment would depend solely on its size.
The other two patterns are described as types of ecosystem decay — a hypothesis proposing that a habitat that shrinks undergoes a disproportionately high loss of organisms compared with the loss of habitat area. One type of ecosystem decay is proposed to occur owing to excessive loss of individuals. Smaller habitat fragments will contain fewer individuals per sample than will larger ones, and all species are equally affected. This generates communities with fewer species in smaller fragments, but no changes in the relative abundance of species per sample between small and large fragments.
The other type of ecosystem decay occurs owing to uneven changes in relative species abundances coupled to species loss. In this scenario, the species present have different responses to habitat loss, and therefore species become relatively more or less abundant in smaller fragments than in larger fragments. Their relative abundance becomes more uneven in samples from smaller fragments as some species increase their numerical dominance, impoverishing the community and causing it to become species poor.
Using data from around 120 human-transformed landscapes worldwide, Chase et al. show that, in general, samples from small fragments of natural habitat have fewer individuals, fewer species and a more uneven abundance of species than samples taken from larger fragments do. This outcome is consistent with a generalized pattern of ecosystem decay, mainly as a result of a decline in evenness (see Fig. 1), and this result holds, regardless of the type of habitat or organism studied. This implies that the alteration of natural habitats causes major functional changes in ecosystem dynamics that go beyond simply losing populations and species. Therefore, current estimates of extinctions associated with habitat loss made on the basis of the passive-sampling model might be underestimating not only the number of species that are threatened or already gone, but also the consequences of their loss for ecological functioning and the provision of ecosystem services.
Changes in biodiversity after habitat loss alter many ecological processes11, eventually causing catastrophic effects that accelerate the extinction process12. But local extinctions are often not immediate. Some species persist with reduced abundances and declining population dynamics — known as ‘extinction debt’ — that lasts until the final individuals perish13. This causes an uneven distribution of species abundance that is vividly demonstrated by Chase and colleagues’ method. Their analysis reveals a few ‘winning’ species that dominate the community in small habitats, and a very large number of rare species, many of which are probably heading towards extinction.
Declining species can be replaced by others coming from the neighbouring human-altered landscape, particularly in habitat edges14, producing what are described as ‘edge effects’ that are comparatively more important in smaller fragments. Indeed, in the early stages of land transformation, communities in small fragments are more different from pristine communities than are those in large fragments, with communities in small fragments becoming more similar to those in large fragments over time, as they recover from the effect of land transformation2. According to Chase and colleagues, the degree of decay in diversity and species abundance found between large and small fragments is smaller in the older or ‘softly’ transformed European landscapes than in the more recently and dramatically transformed North American ones. This indicates that, over time, species moving in from the edges of the human-altered habitats might compensate, at least in part, for the ecological functions carried out by native species in larger habitats, causing small fragments to reach a new — yet different — ecological balance.
Although this work underscores the key role of habitat area in maintaining ecosystem processes, there is little exploration of how these processes are altered by habitat loss. Species from higher trophic levels (the upper levels of the food chain), such as predators, require larger areas to maintain their populations compared with species from lower trophic levels, so the number of individuals supported by smaller habitat fragments might not suffice to maintain populations of top predators or consumers, and hence would produce shorter food chains and alter the ecosystem structure15. Differences in extinction rates between trophic levels can cause striking changes in ecosystem functioning at habitat edges16, jeopardizing the functioning and ecosystem-service provision as natural habitats diminish in size11.
Chase and colleagues’ results call for a reconsideration of the debate over whether a single large area devoted to conservation would preserve more species than would several small ones that combine to make up the same total size17. Some current evidence suggests that one continuous habitat might host fewer species than do many small patches that total the same area18. However, the large ecological changes that these small fragments might undergo could end up resulting in massive reductions in ecosystem function and, ultimately, increased extinction rates of native species over the long term compared with the case for a single, large protected area.
Chase and colleagues’ approach is good for providing a general overview of the extent of these effects, but to understand exactly how ecological processes are changing locally, a higher level of detail will be needed. This will require going beyond the studies of trophic chains14,16 to assess more-complex food webs15, and to gather information on changes in species’ functional responses and trait diversity in increasingly smaller habitats. Ultimately, this information will reveal which ecological processes are decaying, and what the consequences of such ecosystem decay are for the maintenance of fully functional biodiversity.