If aliens sent an exploratory mission to Earth, one of the first things they’d notice — after the fluffy white clouds and blue oceans of our water world — would be the way vegetation grades from exuberance at the equator through moderation at mid-latitudes toward monotony at higher ones. We all learn about this biodiversity gradient in school, but why does it exist?
Even Charles Darwin wondered. Though the pattern is striking, it is difficult to explain. Because it is global in scale, the initial tendency was to suspect long-term or large-scale mechanisms, such as climate stability (no glaciers in the tropics), rates of speciation (higher in the tropics) or rates of extinction (lower in the tropics, according to the fossil record).
In 1970 and 1971, two ecologists independently proposed a radically different mechanism, one that operates at scales not of kilometers but of meters. Daniel Janzen and Joseph Connell suggested that host-specific natural enemies, which kill seeds and seedlings clumped near parent trees, might keep locally common species from dominating a forest and give locally rare species space to flourish.
The Janzen-Connell hypothesis is now nearly 50 years old, but it has been hard to evaluate, especially at the global scale. Few studies have explicitly looked at the connection between self suppression and species diversity, and no study has looked at this relationship across temperate and tropical latitudes.
A year ago, however, Jonathan Myers, an assistant professor of biology in Arts & Sciences at Washington University in St. Louis, and Joe LaManna, a postdoctoral research associate at Washington University’s Tyson Research Center, proposed a test of the Janzen-Connell hypothesis to the principal investigators of an international network of long-term forest dynamics research sites, called the Smithsonian Center for Tropical Forest Science-Forest Global Earth Observatory (CTFS-ForestGEO).
Over the next year, LaManna analyzed the data from 24 research plots, including one at Tyson Research Center. Together, these plots 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 tropical and temperate latitudes. The paper, which has 50 authors from 12 countries, was published in the June 30, 2017, issue of the journal Science.
“This is the first time we’ve had the data to do this kind of in-depth analysis and to look across temperate and tropical latitudes,” said LaManna, who is the corresponding author on the paper.
The analysis also provided a delicious twist on the hypothesis, namely that the plant predators 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 a rare species advantage,” Myers said. 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.