The missing link to a 163-year-old ecological puzzle has been discovered by a group of UH researchers, and scientists are now able to put the pieces together and make sense of the mystery that is Bergmann’s rule, which dates back to 1840s.
Chuan-Kai Ho, who holds a UH doctorate in ecology and evolution, and UH professor of biology and biochemistry Steven Pennings came upon the discovery. Ho studied latitudinal variation in plant-herbivore interactions in Pennings’ lab, which Ho focused on in his six-year dissertation.
While writing his dissertation, Ho stumbled upon Bergmann’s rule, which states that animal and insect body mass correlates with the temperature of the area that they inhabit. Ho determined this was partially correct, but was also determined by nutrition and the taste of the plants insects eat.
“We don’t disagree with the idea that temperature can affect body size,” Pennings said via e-mail. “But our results suggest that other factors can also be important. We found that a diet of high-latitude plants produced a larger body size than a diet of low-latitude plants. The reason was that the high-latitude plants were more nutritious.
“This explanation might not apply to all possible species, but it could apply to many species, and so would suggest a new avenue of research into Bergmann’s rule,” he said.
The team of researchers on the study, including Ho and Pennings, worked in salt marshes along the Atlantic Coast of the U.S., examining interactions between different species of insects from Florida to Maine.
Ho said in an e-mail that while he studied plant hoppers and grasshoppers along the U.S. coast, some of the studies’ co-authors examined sea hares in Japan.
Pennings says they believe that similar communities of plants and animals had similar interactions. But after examining taste tests of ten species of plants eaten by hundreds of insects and crabs, they found differences between plant-herbivore interactions.
“Herbivores are more abundant and do more damage to plants at low-versus-high latitudes. Perhaps this intense herbivore damage selects for plants to be resistant (less tasty),” Pennings said. “Whatever the reason, plants definitely taste better at high-versus-low latitudes.”
Ho said he believed that explanations of Bergmann’s rule have failed in the past because they were based on examinations of abiotic factors. Ho’s study focused on biotic factors, such as diet quality.
Ho published his team’s findings in the February issue of American Naturalist.
“Since its publication, this study has been reported in the U.S. and other countries. It is a good advertisement for UH,” Ho said.
Ho was able to continue research through a doctoral dissertation improvement grant given to him for the years of 2007-2008 by the National Science Foundation.
“It provides around $10,000 to help improve dissertation projects that are already strong. Chuan-Kai was able to use his funding to perform chemical analyses of the plants that he worked with in order to determine their nitrogen content,” Pennings said. “Nitrogen is a limiting nutrient for herbivores, because most plants are low in proteins.”
