UNIVERSITY PARK, PA. - The relationship between changes in the environment and evolution in wild animals is not a one-way street, according to a new study that includes a Penn State biologist. The study shows that evolutionary adaptations in wild animals can in turn drive changes in the plants and animals around them, generating a feedback loop.
"For decades, evolutionary biology has focused on how environmental variation, for example in climate or presence of predators, can drive adaptive evolution in the wild," said Sean Giery, assistant research professor of biology in the Eberly College of Science at Penn State and a member of the research team. "Because of this, evidence for an environment-to-evolution pathway is strong and abundant. Our recent paper shows that adaptive evolution is part of a larger system of reciprocal interactions between ecology and evolution - and being able to demonstrate these dynamics in nature with a well-established system of lizard populations really sets our project apart."
The new study, published in the Proceedings of the National Academy of Sciences, showed that an evolutionary change in the length of lizards' legs can have a significant impact on vegetation growth and spider populations on small islands in the Bahamas. This is one of the first times, the researchers said, that such dramatic evolution-to-environment effects have been documented in a natural setting.
"Lizard populations on these islands have traits that differ slightly due to local adaptation to vegetation," Giery said. "We wanted to know whether these fine-scale differences among lizard populations could feed back into the local ecosystem by altering the interactions between these lizards and their arthropod prey."
For the last 20 years, the research team has been observing the evolutionary dynamics of brown anole lizard populations on a chain of about 40 tiny islands in the Bahamas. They previously determined that the length of the lizards' legs is influenced by the characteristics of surrounding vegetation. In habitats where the diameter of brush and tree limbs is smaller, natural selection favors lizards with shorter legs, which enable individuals to move more quickly when escaping predators or chasing prey. In contrast, where the tree and plant limbs are thicker, lizards with longer legs tend to fare better.
"We know from previous work that the brown anoles can adapt rapidly, with limb length changing in just a few generations," Giery said. "In this study, we placed lizards with relatively short legs on one set of islands and lizards with long legs on a different island set. Then, we carefully monitored these scrubby islands - which had both previously been uninhabited by lizards - to see how the differentially adapted lizards might affect the ecology of their new homes."
The islands are small enough to be easily monitored and far enough apart that lizards cannot easily move between them. Because the experimental islands were mostly covered by smaller diameter vegetation, the researchers expected that the short-legged lizards would be better adapted to that environment, where they would be better able to move and catch prey in the trees and brush.
After eight months, the researchers found substantial differences between the islands that supported their expectations. On islands with shorter-legged lizards, populations of web spiders - a key prey item for brown anoles - were reduced by 41% compared to islands with long-limbed lizards. There were significant differences in plant growth as well. Plants flourished on islands with short-legged lizards because the lizards were better at preying on insect herbivores. For example, buttonwood trees - the dominant vegetation on these islands - had twice as much shoot growth on islands with short-legged lizards compared to trees on islands with long-legged lizards,
"These findings help us to close that feedback loop," said Jason Kolbe, professor of biological sciences at the University of Rhode Island and an author of the study. "We knew from previous research that ecological factors shape limb length, and now we show the reciprocal relationship of that evolutionary change on the environment."
Understanding the full scope of interactions between evolution and ecology will be helpful in predicting environmental outcomes, the researchers said - particularly as human activities accelerate the pace of both evolutionary and ecological change worldwide.
In addition to Giery and Kolbe, the research team includes Oriol Lapiedra at the Centre for Research in Ecology and Applied Forestry in Spain; Kelsey Lyberger, David Spiller and Thomas Schoener, at the University of California, Davis; Jessica Pita-Aquino at the University of Rhode Island; Haley Moniz at the University of Nevada; Manuel Leal at the University of Missouri; Jonathan Losos at Washington University in St. Louis; and Jonah Piovia-Scott at Washington State University. The National Science Foundation funded the research.
