Tuesday, October 30, 2012

The contrasting effects of habitat area and heterogeneity on diversity


ResearchBlogging.org“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

9 comments:

  1. I agree this does seem like a really exciting paper, thanks drawing attention to it!

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  2. Their paper also makes me want to go re-read this old classic - http://www.jstor.org/stable/2462417

    Also I'm trying to figure out if one of the hidden assumptions of their approach is that species niche optima are uniformly disturbed along the gradient. I can think of empirical systems where this is not the case and therefore could violate their predictions...

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    1. Daniel, I suspect that allowing for species to have different niche widths would simply increase the variation around the curve. Their way around this was to classify the bird species into niche width classes and run analyses separately on each group. Of course, this assumes you have the information to do this.

      And yeah, in some ways the ideas about landscape heterogeneity are not new, but I think the really novel part is the tradeoff with area, which ties into species-area relationships and thus invoking niche and neutral mechanisms.

      Thanks!

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    2. Hey Marc, thanks for your comments! My point wasn't what would happen if the niche widths were variable, but rather that their approach seems to assume that species optima are evenly distributed along the environmental gradient. I can think of some empirical systems in which this is not the case because the species all share an environmental optima (e.g., a highly productive region in a landscape). I did like the empirical niche width analysis in the Alloche et al. paper that was a nice confirmatory test of their hypotheses.

      The fractal coexistence paper by Palmer also mixes niche and neutral processes and in many ways is similar to the Alloche paper. The biggest difference is that Palmer added the element of spatial dependence (the fractal part) to how the variation was structured. Interestingly Palmer did not find evidence for the area tradeoff (see his Fig 4 and 7) and instead found both microsite and landscape richness increased fairly monotonically with environmental heterogeneity. Makes me wonder how truly universal the unimodal richness-heterogeneity result is.

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    3. Daniel,

      The model is structured as a local landscape receiving immigrants from a regional species rich pool. In the pool, the niche optimum of each species is randomly placed on the entire gradient (available in the pool). In that way, niche optima of species in the local community are an emergent property of the basic demographic processes and competition. Hope my explanation is clear.

      Nice blog!

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  3. Very interesting. Thank you Marc and Daniel.

    jaume

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  5. Hi Marc,

    This is an interesting paper, but I must agree with Hortal et al (2013) (http://www.pnas.org/content/110/24/E2149) that as they use altitude as a surrogate for environmental heterogeneity, their results would be highly associated with species' responses to altitudinal gradients.

    (Great blog!)

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