Two Washington University researchers are furthering our understanding of plant biodiversity around the globe.
The biodiversity gradient is the increase in species richness that occurs from the poles to the tropics. It is one of the most widely recognized patterns in ecology but no one is sure exactly why it exists. Because the phenomenon is global, the initial tendency was to suspect long-term or large-scale mechanisms, such as climate, speciation or rates of extinction.
But in 1970 and 1971, two ecologists independently proposed a radically different theory: Daniel Janzen and Joseph Connell suggested that host-specific natural enemies in the form of pathogens kill seeds and seedlings near parent trees. This is because pathogens affect trees much in the same way they affect people: they spread more easily if the trees are close together than if they’re isolated. This keeps locally common species from dominating a forest, which in turn allows locally rare species to flourish.
For the past year, Jonathan Myers, an assistant professor of biology in Arts & Sciences, and Joe LaManna, a postdoctoral research associate at WashU’s Tyson Research Center tested the Janzen-Connell hypothesis in conjunction with the Smithsonian Center for Tropical Forest Science-Forest Global Earth Observatory (CTFS-ForestGEO), an international network of long-term forest dynamics research sites.
LaManna analyzed data from 24 research plots, including one at Tyson. Together, they are home to more than 3,000 tree species and roughly 2.4 million trees. The analysis provided the first evidence that the Janzen-Connell effect contributes to the biodiversity gradient across temperate and tropical latitudes.
The analysis provided an interesting twist on the hypothesis: the plant enemies that kill rare species may also keep them from going extinct. “When species get too rare, their enemies also thin out, and they have what is known as rare species advantage,” says Myers. So the specialized predators ultimately stabilize rare species instead of wiping them out.”
We were able to show for the first time that this stabilizing effect may be stronger for rare species in the tropics; this may explain why rainforests harbor so many rare trees,” LaManna said.
The paper has 50 authors from 12 countries and was published in the journal Science. It was carried out during the 2016 CTFS-ForestGEO Workshop in Hainan, China, and was supported by NSF grants.