One of the most persistent questions in Ecology is, Why does the number of species increase closer to the equator? For example,the diversity of forests varies massively across latitudes, ranging from over a thousand species in a 0.5 km2 of equatorial rainforest to only several in an equivalent area of boreal forest. Evolutionary studies have shown that diversification rates are higher in the tropics. But can ecological mechanisms also slow or even prevent extinction rates more in the tropics, thereby promoting the latitudinal biodiversity gradient? Using a unique combination of theory and long-term demographic data from hundreds of tree species across a broad latitudinal range, we can identify a gradient in a temporal coexistence mechanism known as the “storage effect” that reduces competitive exclusion; this mechanism can therefore contribute to geographic variation in biodiversity across latitudes. I have found that the strength of the storage effect can vary with latitude in association with climate. More specifically, longer growing seasons in the tropics reduce constraints on the seasonal timing of reproduction, which permits lower recruitment synchrony between species, thus enhancing niche partitioning via the storage effect. The generality of the coexistence mechanism means that it could operate for other taxonomic groups along latitudinal gradients, and it also alludes to a clear mechanistic pathway between variation in climate and biodiversity patterns.
Usinowicz, J., C. Chang-Yang, Y. Chen, J. S. Clark, C. Fletcher, N. C. Garwood, Z. Hao, J. Johnstone, Y. Lin, M. R. Metz, T. Masaki, T. Nakashizuka, I. Sun, R. Valencia, Y. Wang, J. K. Zimmerman, A. R. Ives, S. J. Wright. 2017. Temporal coexistence mechanisms contribute to the latitudinal gradient in forest diversity. Nature 550:105–108. Link to code on figshare
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