Biodiversity

Turning up the heat on hotspots

Different measures are used to define concentrations of biodiversity — so-called ‘hotspots’. More rigorous, global-scale analyses of how they compare will be essential for efficient resource allocation to conservation.

The hottest hotspot of them all? The tropical Andes is a hotspot under three defining criteria5. Credit: T. MORRISON/SOUTH AMERICAN PICTURES

The variety of life on Earth is in rapid decline1, and global spending on nature conservation is inadequate to arrest that decline2. Consequently, resources for conservation must be allocated to secure the ‘biggest bang for our buck’. In recognition of that need, scientists have identified biodiversity hotspots, where extraordinary concentrations of biodiversity exist, as defined by one or more metrics: number of species (species richness); number of species restricted to a particular area (endemic-species richness); and number of rare or threatened species. As well as using these three ‘diversity metrics’, we need to determine the seriousness of the threat to biodiversity hotspots from processes such as land-use changes3.

There is considerable controversy about which metric(s) to use to delineate hotspots, and the consequences of applying different metrics4. Using a newly compiled global database for the breeding distribution of all of the world's birds, Orme et al. (page 1016 of this issue)5 enter the fray. They have discovered an alarming lack of congruence between hotspots defined using different metrics — so raising concern about the notion that species-richness hotspots should overlap with those identified using the other two diversity metrics6. It also calls into question the use of the hotspots principle in setting priorities for conservation.

The science of biodiversity hotspots is part of the emerging interdisciplinary field of conservation biogeography. The seminal definition of this field, by Whittaker et al.7, is “the application of biogeographical principles, theories, and analyses, being those concerned with the distributional dynamics of taxa individually and collectively, to problems concerning the conservation of biodiversity”. The same authors7 also list the factors that reduce the value of such research; some of these illuminate the roots of past controversies.

One factor that can lead to spurious results is a lack of good data on the distribution of groups of species that are not taxonomically well defined. Orme et al.5 largely circumvent this problem by using birds, which have well-known distributions and are relatively stable taxonomically. The spatial scale of the analysis is also critical, both with respect to its geographical extent and the resolution of data. The scales of Orme and colleagues' analysis are respectively the world and equal-area grids of approximately 1° latitude by 1° longitude; such resolution provides consistent and relatively fine-resolution data for a global analysis. Finally, there is a frustrating lack of theory to explain the reasons for the different patterns of biodiversity that have been identified. Orme et al. address this point by proposing mechanisms responsible for the observed geographical patterns of different aspects of biodiversity.

Compelling theories on why hotspots of different kinds occur where they do are beginning to emerge from analyses of data on the global and continental scales5,6. Orme and colleagues' geographical analyses of the three diversity metrics provide further insights into the ecological, evolutionary and human effects that underlie the origin and maintenance of biodiversity as measured in these ways. They argue that these effects are largely associated with large-scale topography. This supports the view that conservation assessment must consider not only contemporary factors, but also historical and ecological factors, such as the distribution of stable environments (which affects the extinction rate of species with small ranges) and the existence of geographical barriers and steep environmental gradients (which generate the diversity). The authors also argue that the distribution of threatened species is determined by an interaction of biological and human factors, and that the human factor helps to explain the lack of congruence between hotspots defined by the number of threatened species and those defined by the other two diversity metrics.

This lack of congruence highlights the potential inefficiencies that arise from using a single metric to delineate hotspots, and taking that to guide conservation efforts. Nevertheless, it is becoming clear that if resources are expended on endemic-species hotspots, they are likely to go a long way in protecting both species-richness and threatened-species hotspots. The endemism hotspots identified by Orme et al.5 (for example, the tropical Andes, pictured) contained a greater proportion of species richness than did the species-richness hotspots, and a greater proportion of threatened species than the threatened-species hotspots. Areas with large numbers of endemic species may also be of special significance in setting conservation priorities, because they may be areas of high past, and potentially future, speciation6.

All is not doom and gloom for the hotspots principle, however, as congruence between different studies using completely different methodologies is invariably high8. All 10 threatened-bird-species hotspots identified by Orme et al. are on the Conservation International list of hotspots9, which is based on plant endemic richness and habitat loss3. And only two of Orme and colleagues' 20 endemic-species hotspots are not on the list — one of these two, the Guyana Highlands of northern South America, is also a species-richness hotspot that may warrant further global attention.

Earlier this year, Hoekstra et al.10 added a new dimension to the debate over conservation priorities. They ignored species, and instead adopted a habitat-based approach11. They show that the temperate grassland and Mediterranean biomes of the world are those most in need of urgent protection, and so counter the prevailing wisdom that conservation resources should be concentrated in tropical habitats. Analyses of taxonomic groups other than birds, and a marriage of species-based and habitat-based approaches, should go a long way to providing a robust vision of conservation priorities for the future.

The amount and quality of global data on biodiversity is increasing rapidly, and there will be a continued refinement of — possibly even consensus about — the location of biodiversity hotspots. However, the cost of conservation action, which varies by several orders of magnitude from place to place2, is an essential factor missing from this research agenda. If hotspots research is primarily an exercise in the study of spatial patterns of biodiversity and threats to biodiversity, costs are irrelevant. But if its real purpose is to guide resource allocation for conservation where time and money are constraints, we must urgently work to include economic and social factors.

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Possingham, H., Wilson, K. Turning up the heat on hotspots. Nature 436, 919–920 (2005). https://doi.org/10.1038/436919a

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