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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