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The question "Why are there so many species?" has puzzled ecologist
for a long time. Initially, an academic question, it has gained
practical interest by the recent awareness of global biodiversity
loss. Species diversity in local ecosystems has always been
discussed in relation to the problem of competi tive exclusion and
the apparent contradiction between the competitive exclu sion
principle and the overwhelming richness of species found in nature.
Competition as a mechanism structuring ecological communities has
never been uncontroversial. Not only its importance but even its
existence have been debated. On the one extreme, some ecologists
have taken competi tion for granted and have used it as an
explanation by default if the distribu tion of a species was more
restricted than could be explained by physiology and dispersal
history. For decades, competition has been a core mechanism behind
popular concepts like ecological niche, succession, limiting
similarity, and character displacement, among others. For some,
competition has almost become synonymous with the Darwinian
"struggle for existence", although simple plausibility should tell
us that organisms have to struggle against much more than
competitors, e.g. predators, parasites, pathogens, and envi
ronmental harshness.
The number of species found at a given point on the planet varies
by orders of magnitude, yet large-scale gradients in biodiversity
appear to follow some very general patterns. Little mechanistic
theory has been formulated to explain the emergence of observed
gradients of biodiversity both on land and in the oceans. Based on
a comprehensive empirical synthesis of global patterns of species
diversity and their drivers, A Theory of Global Biodiversity
develops and applies a new theory that can predict such patterns
from few underlying processes. The authors show that global
patterns of biodiversity fall into four consistent categories,
according to where species live: on land or in coastal, pelagic,
and deep ocean habitats. The fact that most species groups, from
bacteria to whales, appear to follow similar biogeographic patterns
of richness within these habitats points toward some underlying
structuring principles. Based on empirical analyses of
environmental correlates across these habitats, the authors combine
aspects of neutral, metabolic, and niche theory into one unifying
framework. Applying it to model terrestrial and marine realms, the
authors demonstrate that a relatively simple theory that
incorporates temperature and community size as driving variables is
able to explain divergent patterns of species richness at a global
scale. Integrating ecological and evolutionary perspectives, A
Theory of Global Biodiversity yields surprising insights into the
fundamental mechanisms that shape the distribution of life on our
planet.
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