Anyone who as ever watched a David Attenborough documentary knows that biodiversity differs in areas with different climates. Only a few species an survive in hot and dry deserts whereas warm and wet tropical forests are teeming with life. But have you every stopped to wonder why this is so?
Why are certain climate conditions able to support many species and others not? More specifically, how does this work mechanistically?
This was the question my co-authors and I set out to answer in our most recent paper just published online at Global Ecology and Biogeography.
In our study, we tested the hypothesis that strong biodiversity-climate correlations are because climate gradients determine individual species ranges, which then sum up to form species richness patterns. This is a fundamental assumption made by macroecologists who, for example, try to model the impacts of climate change on biodiversity by projecting individual species ranges into forecasted climate conditions.
Our reasoning was that if it is indeed the case that diversity is just the sum of processes acting at the population level, then climate-based mechanisms should not only predict the local occurrence of species, but also co-occurrence of species across the entirety of their ranges.
To do this, we studied patterns in the assemblage dispersion field for African mammals. This approach allowed us to examine both local- and range-wide co-occurrence patterns simultaneously. For those of you who might be unfamiliar with it, the assemblage dispersion field can be visualized by superimposing the geographic ranges of all species co-occurring at a certain point in space.
However, as an earlier study of ours showed, we couldn’t just examine correlative patterns between biological and climate variables because these can arise even in the absence of underlying biological mechanisms. Instead, we used a mechanistic modelling approach to first simulate the geographic ranges expected from specific hypotheses that included varying degrees of climate determinism, dispersal limitation and the approximate position of the range in biogeographical regions. We then compared the outputs from these models to real co-occurrence patterns for African mammals.
The map on the left shows how the assemblage field for African mammals changes with latitude (red dot) and the map of the rights shows the same thing for one our simulation models.
Our findings showed that simple mechanisms of range positioning, dispersal limitation and climate determinism could predict local patterns of species richness with remarkable success (we could explain roughly 70% of the variation in mammal species richness in sub-Saharan Africa). Unfortunately, these same mechanisms were terrible at predicting the range-wide occurrence of species in the assemblage dispersion field. Instead, range-wide co-occurrence seemed to be more strongly associated with the boundaries between biogeographical region in Africa.
Species at the cores of biogeographical regions tended to co-occur over large parts of the geographic ranges, whereas species at the interfaces between these regions only co-occurred haphazardly. These patterns where very clear, but none of our mechanistic models were able to predict them.
Interpreting these results was a bit more complicated. We could state with confidence that climate-diversity relationships are unlikely to be the simple consequence of the climate influencing individual species ranges that then sum up to form higher-level correlations. That much was clear. Unfortunately, offering an alternative explanation was a bit more iffy. We assumed that because patterns in the assemblage dispersion field were closely related to biogeographical regions that they were probably the consequences of species interactions at evolutionary time-scales. But, to be honest, this explanation needs much more evidence (which is well on its way!).
We also can’t really explain what causes strong correlations between climate and diversity (other than ruling out the bottom-up effects of individuals species ranges). One possible explanation is that these relationship are not indicative of underlying mechanisms because one of our environmentally neutral models accounted for more than 65% of the variation in mammal species richness (as long as we maintained the approximate points of origin and limited dispersal into adjacent areas). A more likely explanation, however, proposes that historical speciation add species to a regional pool of species which then colonise and establish within local communities. If immigration to, and extinction within, these local communities are dependent on climate, then strong climate-diversity correlation would arise. Again, this explanation needs a bit more data before it can be fully accepted.
We ended up with more questions than answers, but I suppose that is the nature of scientific research. Nevertheless, our study shows that even very simple questions that have been around for decades can produce some complicated answers.
Update – If anyone is interested in the technical details of this study: our analyses were carried out using the R programming language and the command scripts and data are available online at FigShare and GitHub
Buschke, F.T., Brendonck, L. & Vanschoenwinkel, B. (2015) Simple mechanistic models can partially explain local but not range-wide co-occurrence of African mammals. Global Ecology and Biogeography doi: 10.1111/geb.12316
(feel free to contact me if you can’t get access to this paper)