“How extremely stupid not to have thought of that!” (Thomas
H. Huxley, commenting on the obviousness of Darwin’s theory of natural
selection)
Sometimes I read a paper and Huxley’s famous quote seems
exceedingly appropriate. Why I say this is that a new idea or concept
is presented which seems both so simple and at the same time a potentially
powerful explanation of patterns in nature. This was my reaction to a recent paper from Omri Allouche and colleagues published in the Proceedings ofthe National Academy of Science. The paper presents a simple conceptual model,
in the same vein as Connell’s classic intermediate disturbance hypothesis,
which accounts for large-scale diversity patterns based on aspects of species
niche requirements as well as classic stochastic theory. Merging these two
aspects is a critical step forward, as in ecology, there has been a tension in
explaining diversity patterns between niche-based processes requiring that
species exhibit differences in their needs, and stochastic (or neutral)
explanations that ignore these differences, but seem to do well at large
scales.
The classic stochastic model in ecology, the theory of island biogeography, simply predicted that the number of species increases with
the size of an island or habitat, and ultimately is the balance between species
colonizing and going extinct. Allouche et al. also assume this stochastic
colonization and extinction, such that in a uniform environment, the number of
species increases with area. However, they then add the fact that species do
not do equally well in different habitats, that is they have specific
environmental niches associated with a particular environment. Thus as you
increase the amount of heterogeneity in a landscape, you increase the total number
of species, because you’ve captured more niches. However, there is a trade-off here. Namely, as you increase the heterogeneity in a landscape, the amount of
area for the dominant habitat type decreases, thus reducing the number of
species. So if you increase the heterogeneity too much, the individual habitat
types will be too small to support large numbers of species and the numbers of
species will be less than regions with less heterogeneity –paradoxically.
Their heuristic prediction is that diversity is maximized at
intermediate levels of heterogeneity, as long as species have intermediate
niche breadths (i.e., they could perhaps use a couple of different habitats).
However, if their niche breadth is too narrow (i.e., they can only exist in a
single habitat type), then diversity may only decline with increasing
heterogeneity. Conversely, if species have very broad niche breadths (i.e., can
survive in many different habitats) then the tradeoff vanishes and
heterogeneity has little effect on diversity.
They tested this exceedingly simple prediction using
European bird data and found that species richness was maximized at intermediate
heterogeneity (measured by the variation in elevation). Further, when they
classified species into different niche width classes, they found that the
relationship between richness and heterogeneity changed was predicted (i.e.,
strongest for intermediate breadth).
This is a great paper and should have a large impact. It
will be exciting to see what other systems fit this pattern and how specific
studies later the interpretation or mechanisms in this model.
Allouche, O., Kalyuzhny, M., Moreno-Rueda, G., Pizarro, M., & Kadmon, R. (2012). Area-heterogeneity tradeoff and the diversity of ecological communities Proceedings of the National Academy of Sciences, 109 (43), 17495-17500 DOI: 10.1073/pnas.1208652109
Allouche, O., Kalyuzhny, M., Moreno-Rueda, G., Pizarro, M., & Kadmon, R. (2012). Area-heterogeneity tradeoff and the diversity of ecological communities Proceedings of the National Academy of Sciences, 109 (43), 17495-17500 DOI: 10.1073/pnas.1208652109
