Ecoregions are central to global modeling of earth systems & development of conservation plans. There is great variability across taxonomic groups and regions of the world in how strongly ecoregions described community composition.
Two years ago, a team of us published a study entitled ‘A global test of ecoregions’ (Smith et al., 2018). In that paper, we set out with a relatively simple goal, to test whether or not maps of ecoregions, which are popular with both global modelers and conservation practitioners, are actually reflective of the distribution of unique groups of species around the globe? Or are they artifacts of a bygone era when we lacked the data on both the variability in environmental conditions and the distribution of species to properly assess biogeographic patterns?
Above. Our results show that we should expect ecoregions to be much more dissimilar from one another in the tropics than they are in the temperate zone.
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EDITORS’ CHOICE: Smith, JR, Hendershot, JN, Nova, N, Daily, GC. (2020) The biogeography of ecoregions: Descriptive power across regions and taxa. J Biogeogr.; 47: 1413– 1426. https://onlinelibrary.wiley.com/doi/full/10.1111/jbi.13871
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Over the course of that project we believed that we would find that ecoregions did not describe communities of species, but instead that species were distributed more or less individualistically across space. We were continually surprised at how wrong we were. We found compelling evidence that ecoregions described clusters of species across space much better than we would expect by chance alone. As we analyzed and discussed our results, we focused on this high level finding that ecoregions did in fact delineate unique communities of species.
It was not until we got comments back from our first round of revisions that we realized that maybe we missed the more interesting story. Our results showed that there was a large amount of variability, both across taxonomic groups and regions of the world, in how strongly ecoregions described community composition. In the short format of that original paper we were unable to dive into the nuance of these environmental and biological mechanisms, but are now thrilled that the Journal of Biogeography has given us the opportunity to do so in our new paper, “The biogeography of ecoregions: Descriptive power across regions and taxa” (Smith et al., 2020).
However, in this expanded format we were able to revisit some of the most famous biogeographers, and test decades or centuries old hypotheses that would never have been possible before the modern data renaissance. We could test if as Humboldt suggested, “associations of the same species of plants [in the tropics] are less consistently extensive, less numerous, than in temperate climates” (Humboldt & Bonpland, 1807). Or if, as Dan Janzen suggested, that “Mountain passes are higher in the tropics” (Janzen, 1967).
A tree line in the Colombian Andes showing sharp transitions between tree and shrub habitats (photo credit Christopher B. Anderson)
What we quickly realized was that climate played a dominant role in shaping how distinct the communities within neighboring ecoregions were from one another. Those ecoregions with higher mean temperatures and more stable temperatures across the year were more distinct from one another. Similarly, steep slopes led ecoregions to be more distinct from one another. Together, this leads us to believe that it is in fact Janzen, and not Humboldt, who was correct in hypothesizing that communities are more distinct from one another in tropical climates.
We were also able to test out more recent theories on how biological traits, such as body size and functional guild, led animals to be more restricted to specific ecoregions. We were able to add evidence to a growing body of literature suggesting that larger species that feed higher on the food chain are more likely to be found across a large number of ecoregions. Sadly, given the state of trait data available today we were only able to do this for four vertebrate taxa: birds, mammals, reptiles, and amphibians.
Despite the fact that we hoped to use this paper to follow up on questions posed by our initial piece, we find ourselves with just as many questions as we started with. How might we extend our findings to other taxonomic groups that we didn’t consider here, such as plant, fungi, and arthropods? How might climate change and land use change affect the robustness of ecoregion classification schemes moving forward? How can we incorporate data on the distribution of species a priori into ecoregional classification schemes (sensu Kreft & Jetz, 2010). We are incredibly excited to continue diving into these topics, along with all the readers of Journal of Biogeography.
Written by:
Jeffrey R. Smith, J. Nicholas Hendershot, Nicole Nova, & Gretchen C. Daily – Stanford University
Additional information:
@JeffreySmithJRS, @Appalachianary, @NicoleNovaBio, @CCBatStanford, @NatCapProject, @StanfordWoods
A beach in Northwestern Costa Rica showing sharp transitions between different types of ecosystems, with the beach quickly giving way to mangroves, which give way to upland shrublands (photo credit Jeffrey Smith)
References
Humboldt, A. von, & Bonpland, A. (1807). Essay on the Geography of Plants (S. T. Jackson, Ed.; S. Romanowski, Trans.; Reprint edition). University of Chicago Press.
Janzen, D. H. (1967). Why Mountain Passes are Higher in the Tropics. The American Naturalist, 101(919), 233–249. https://doi.org/10.1086/282487
Kreft, H., & Jetz, W. (2010). A framework for delineating biogeographical regions based on species distributions. Journal of Biogeography, 37(11), 2029–2053. https://doi.org/10.1111/j.1365-2699.2010.02375.x
Smith, J. R., Hendershot, J. N., Nova, N., & Daily, G. C. (2020). The biogeography of ecoregions: Descriptive power across regions and taxa. Journal of Biogeography, 47: 1– 14 https://doi.org/10.1111/jbi.13871
Smith, J. R., Letten, A. D., Ke, P.-J., Anderson, C. B., Hendershot, J. N., Dhami, M. K., Dlott, G. A., Grainger, T. N., Howard, M. E., Morrison, B. M. L., Routh, D., Juan, P. A. S., Mooney, H. A., Mordecai, E. A., Crowther, T. W., & Daily, G. C. (2018). A global test of ecoregions. Nature Ecology & Evolution, 2(12), 1889. https://doi.org/10.1038/s41559-018-0709-x