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Modeling Loss

Susan Harrison

The Shrinking World: Ecological Consequences of Habitat Loss. Ilkka Hanski. xxx + 307 pp. International Ecology Institute, 2005. 47 Euros.

Conservation biology got its start as an academic field in the mid-1970s, with the proposed application of island biogeography theory to the design of nature reserves. Yet a decade later, when I became a graduate student, it was still considered a post-tenure eccentricity to focus one's research on the application of ecological ideas to conservation problems. Ilkka Hanski was at that time studying the natural history of dung beetles and developing theoretical models to explain the coexistence of multiple species on transient resource patches.

In the two decades since, our field has changed to an almost staggering extent. It is hard to remember the last time I read an application for graduate school that did not touch on the desire to understand and alleviate conservation problems through ecological research. As one indicator of this trend, the number of journal articles listed in Biosis as having "habitat fragmentation" in the title or keywords exploded, from 24 in the five-year period from 1985 to 1989, to 139 in 1990-1994, to 483 in 1995-1999, to 928 in 2000-2004. And during that time Hanski rose to prominence as one of the leaders in understanding the dynamics of species in fragmented habitats. He differs from many others in bringing to this task a solid foundation in mathematical ecology as well as in natural history.

Fragmentation is generally defined by researchers as a change in the spatial configuration, as opposed to the total amount, of habitat. Among the clearest manifestations of the process are so-called edge effects, the generally negative consequences of having a patch of natural habitat juxtaposed with the weedy species and adverse physical conditions of neighboring human-altered areas. More subtly, fragmentation may disrupt the movement of individual organisms, and the resulting genetic and demographic isolation of populations may be a stage on the way to their regional extinction. This is a harder problem to deal with than edge effects, because it involves long-term processes that need to be modeled mathematically.

Hanski has been a leader in applying theory to understand the population consequences of habitat fragmentation. At the heart of his new book, The Shrinking World: Ecological Consequences of Habitat Loss, is an exposition of his theoretical and empirical work on metapopulations—spatially distributed networks of populations that survive only if the connectivity (not just the quantity and quality) of habitat is adequate. One gauge of the success of this body of theory is that the number of research articles reported in Biosis as having "metapopulation" in the title or keywords has risen from between 20 and 30 per year in the early 1990s to a steady level of around 150 to 160 per year for the past five years. A key finding from this theory is the existence of an extinction threshold—a critical level of habitat area and connectivity below which species cannot persist. Hanski holds out the hope that the mathematical models he has developed, which can be parameterized with relatively simple data on species presences and absences, may be useful predictive tools for designing networks of successful reserves.

This book differs from his previous ones, however, in ranging far beyond a consideration of metapopulations. Hanski examines the topic of habitat loss broadly (moving from the definitions of niche and habitat to the estimation of regional and global rates of species loss), using it as an organizing theme with which to weave together multiple threads, some highly academic and others much more personal. The book contains fresher and more concise discussions of many basic concepts in population, community and landscape ecology than I have seen in standard textbooks. That these principles are often illustrated with less-familiar northern European examples makes the book even more interesting to a U.S. reader. Often quite seamlessly, these discussions lead into commentary on current conservation issues and policies, again from a largely European perspective. Apparent everywhere is the author's deep familiarity with natural history, combined with an acute sense of its importance to the human psyche and of the tremendous losses that have occurred even in "civilized" landscapes such as Finland's and even during our own lifetimes.

Hanski received the International Ecology Institute's 1999 Prize in Terrestrial Ecology, which afforded him the opportunity to write this book, one that combines provocative research ideas and quantitative arguments with an informal, readable and sometimes overtly opinionated style.

An unresolved issue to which Hanski alludes repeatedly is whether the spatial configuration of habitat is often likely to have a major effect on species survival, in comparison with the overall quantity and quality of habitat and the deterministic trajectory of population growth. This question underlies the more urgent one of whether the application of spatial theory is likely to help in designing conservation strategies. Although the book is written in the firm belief that the answer is yes, even a skeptic could read it as a rich source of hypotheses that deserve more critical tests. Thus I hope to read more about these ideas in future graduate-school applications.

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