Seeing both the woods and the trees

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A novel approach to classifying biodiversity may aid attempts at conversation.

Terrestrial Ecoregions of North America: A Conservation Assessment

et al. Island Press: 1999. 485 pp. $75, £50 (pbk)

Metaphorically, population ecologists study single species of trees, community ecologists study the woods that are the collections of tree species, and ecosystem ecologists view woods and trees as so many kilograms of carbon. While important questions fall neatly within these divisions, the continuing destruction of biodiversity imposes questions across their boundaries. ‘Biodiversity’ means the full variety of life — between individuals of a species, the different species themselves, and the different assemblages they create. In practice we count and map species. Documenting within-species variability is too daunting a task for all but a few species. And no two ecologists agree on what constitutes an assemblage. It can be an ‘association’, ‘habitat’, ‘ecosystem’, ‘biome’, ‘life-zone’, ‘landscape’ or ‘biotype’. Neither do they agree on where, say, the woodland biome ends and the grassland biome begins. Unfortunately, the hegemony enjoyed by species creates its own problems. Perhaps only 15 per cent of all species have names, and we have range maps for only a tiny fraction of them. How can we conserve biodiversity when its measures are either arbitrary or incomplete?

The solution most often found in the pages of this journal is to test whether small samples of known species are representative. They are not. Peaks of species richness of one species group rarely match the richness peaks of another group. Neither do they match the ‘hotspots’ — the concentrations of endemics, those species with the smallest geographical ranges and therefore greatest vulnerability. Ricketts and his colleagues have chosen the less trodden path, bravely classifying the United States, Canada and Mexico into about 150 inevitably arbitrary “ecoregions”. In doing so, they follow their parent organization, the Worldwide Fund for Nature (WWF), in expressing concerns about purely species-based exercises in setting priorities. Importantly, this book documents the patterns of species within ecoregions and so uniquely integrates the two views of biodiversity.

A comparison of WWF's list of 200 global priorities and the list of hotspots drawn up by Conservation International or BirdLife International uncovers many identical choices. It is the differences that are interesting. Florida's Everglades or Brazil's Pantanal, for example, do not rank as hotspots, but achieve prominence because flooded grasslands are globally scarce and uniformly vulnerable. Other regions attain prominence because of the biological phenomena they house, such as the Arctic tundras and their migratory shorebirds and caribou. Most of this book describes the individual regions, the features that make them biologically distinctive, where the largest blocks remain, and what threatens them. No eco-traveller should leave for an ecoregion without it.

However one may debate the lines drawn between ecoregions, they indisputably provide a more compelling base onto which to map the patterns of species richness and endemism than political boundaries or lines of latitude and longitude. On a larger geographical scale and with more taxa than previous studies, the book documents these patterns. Butterflies, tiger beetles, reptiles, birds and mammals have similar patterns of richness, concentrating in the southwest. Base your conservation priorities on these groups alone, however, and you miss the Appalachian concentrations of amphibians, snails and vascular plants, the southeastern concentrations of trees and the western concentration of conifers.

Idiosyncratic patterns of richness and endemism provide a strong argument for an ecoregional approach: “We cannot wait until we have mapped out many taxa, nor for taxonomists to complete the species catalogue. (The end of this new millennium at the current rate of progress.) We know an ecologically distinct region when we see one and we see its threats. Seeing the woods is enough; don't sweat the trees.” Indeed, the summary list of a dozen key regions provides compelling priorities not found in other exercises. The more pluralistic and more inclusive ecoregional approach may better anticipate hitherto unknown patterns of species richness and endemism than those we know from the few well-studied taxa. Moreover, it is readily exportable to continents where even fewer species are known and mapped.

Ecologists readily erect ad-hoc explanations for these patterns. Unfortunately, none of the explanations predicts which taxa should be similar and which different. I may believe the explanation of why snails have an inordinate fondness for Appalachia, but what in that explanation tells me that it is one that amphibians share, but not reptiles? Surely we must integrate what we know about how trees create a wood with the history of how the trees got there in the first place. Reading this book is a great way to start such integration. Its conservation message is a powerful enjoinder to do so immediately.

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Pimm, S. Seeing both the woods and the trees. Nature 402, 853–854 (1999) doi:10.1038/47183

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