One of the most venerable questions in all of ecology is: Why are there more species in the tropics? There is no shortage of hypotheses (over 30 in a recent count!) to explain this most famous of biogeographical patterns. Several of these hypotheses fundamentally concern the geometry of the global environment, and essential properties of species ranges. In a 2016 publication in The American Naturalist, former MS student Andrew Snyder-Beattie and I proposed a mathematical model that captures these geometric explanations, and allows us to investigate their implications more deeply. The illustration here shows an Earth-like sphere with a latitudinally varying environment, and three randomly placed species ranges, each of which is determined by the species' environmental niche and a maximum range radius. Surprisingly, our model predicts latitudinal gradients in species diversity that are characterized by nuanced fine structure, such as small Equatorial valleys in species diversity and broad diversity plateaus through the subtropics. We also applied our model to elevational gradients in species diversity, and the model could yet be applied to less traditional species diversity gradients. While a formal confrontation betwee out model's predictions and data is needed, many of our model's more surprising predictions seem to match empirical data. Are you interested in helping to refine this theory, or to confront it with data? If so, please contact me. There are many steps ahead!
The photograph above shows a (small) elkhorn coral, Acropora palmata in Great Lameshur Bay, St. John, USVI. Look closely and you'll also see a sea urchin Diadema antillarum nestled in amongst the coral lobes. Diadema are important herbivores of benthic macroalgae in the Caribbean, and macroalgae in turn compete with corals for open space on the sea floor. It is thought that declines in Diadema population abundance may have contributed to declines in Acropora abundance over the past few decades. Empowered by 25 years of data collected by my collaborator Peter Edmunds, I used statistical models to understand how changes in the biotic and abiotic environment impact coral-reef communities in St John, and to project how the coral-reef communities of the near future may or may not differ from those of the recent past. Our work appeared in Ecology in 2015. Peter, Bob Carpenter, and I are now engaged on a project to understand how ocean acidification will impact coral-reef ecosystems in the South Pacific.