Paper 37 - by Tanner Hawkins

Species-energy theory: an extension of species-area theory by David Wright

Foreword by David Currie

David Currie is a professor at the University of Ottawa. His research areas of interest are: “1) to identify broad-scale patterns in the distribution, abundance and diversity of life; 2) to determine which environmental variables exert the strongest control on those patterns, and 3) to determine how human activities influence them.”

David Wright is a researcher at the US Fish and Wildlife Service. His research interests are in ecology, entomology, and agronomy.

Species-area curves, first proposed by Arrhenius and popularized by Wilson and MacArthur, has inspired other lines of research to find what else determines the number of species in an area. One possible alternative determinant is the energy, or primary productivity of an area controls the number of species. It makes sense intuitively. Every trophic web begins with the autotrophs, in this case land plants, which are ultimately limited by the amount of energy they can capture (with everything else equal). But does that mean that the two relationships--species-area and species-energy--are mathematically similar? In order to find out, Wright modelled primary productivity (in evapotranspirative potential and in accumulated biomass) as a power function similar to Arrhenius’. He hoped the result would be more general than species-area.

While he managed to show a positive species-energy relationship, it’s unclear whether his explanation or Arrhenius’ explanation is definitive. They both make make compelling cases, and it’s hard to just dismiss either. My first instinct when presented with two competing hypotheses in ecology is to ask “why can’t it be both?” An ecosystem is a really complicated thing to study, and it’s not so outlandish to suggest that multiple factors can go into a single phenomenon. The next question to ask then would be how energy and area relate to each other. If one is dependent on the other then you could parameterize (express mathematically) the dependent variable in terms of the independent variable. But if they’re both independent of each other, then there’s no reason why it can’t be both.

But how do you show that, mathematically or mechanistically? That’s a hard question, but at some point we’ll need to know how all this fits together, whether it’s for improving our conservation programs, or crafting policy responses to climate change. The groundwork that Wright and Arrhenius laid is a good first step, now we just need to build on it further.