Check out some articles about this work!

Inside Science, Science Daily, The Smithsonian Tropical Research Institute

Forests worldwide showcase diverse species. One of the most notable patterns of diversity is the way in which natural communities typically become more diverse closer to the equator. Yet, scientists still do not fully understand what generates this fascinating pattern. By analyzing data from global forests in collaboration with scientists like Joseph Wright, I explore how natural seasonal climate variation and species interactions shape this biodiversity pattern. Our research uncovers why certain regions sustain more diverse ecosystems, unraveling the secrets behind the distribution of biodiversity across the globe.

Models of tree community dynamics (i.e. for many competing populations of tree species) show that asynchrony between species in population growth, created when reproduction favors different climate conditions, promotes biodiversity. By parameterizing mathematical models with long-term time series of tree reproduction, we can explore how this operates in real forests. Furthermore, forests across the globe seem to support different levels of biodiversity because their seasonality is different.

Shorter growing seasons at higher latitudes force species to reproduce at the same time, increasing competition and limiting diversity. To test this, Joseph Wright, staff scientist at the Smithsonian Tropical Research Institute, and I convened a new global network of scientists to share data and create an international database with tens-of-thousands of species and rare, high-quality demographic data spanning several decades from eight countries worldwide. We found that while coexistence happens in this way in all forests, it was significantly more important in forests closer to the equator. Our results are the first evidence that natural geographic-scale variation in seasonal climate actually changes the way that species interact to such a large extent that it can explain latitudinal diversity patterns.

Publications:

1. 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.

2. Usinowicz, J., S. J. Wright, and A. R. Ives. 2012. Coexistence in tropical forests through asynchronous variation in annual seed production. Ecology 93:2073–2084.