CHAMPAIGN, Ill. - In a unique experiment that may provide ecological insights needed for animal conservation, tiny prairie voles adjusted to both immigration and emigration among different habitats to maintain a stable overall population.
Voles, also called field mice, are the most abundant rodents in grassland ecosystems, and they are a favorite prey of many predatory birds and mammals.
Reporting Aug. 14 to the annual meeting of the Ecological Society of America in Providence, R.I., scientist George Batzli said that the population diversity of the voles, in the first season of a multiyear project, appeared to be regulated by birth and death rates that compensated for the amount of movement among habitats with different environments.
Batzli, a professor and acting head of ecology, ethology and evolution at the University of Illinois, is learning how voles choose where to live and how their choices influence population densities in each habitat. The study, supported by the National Science Foundation, simultaneously looks at how growth, reproduction, survival and movement are affected by environmental factors such as food supply, predation and competition, which usually are studied one at a time.
During the 1995 growing season, Batzli and U. of I. graduate student Yu-teh K. Lin set up four sets of four 30-by-40 meter pens in a field northeast of Urbana, Ill. Each pen had a habitat with a different combination of food supply and vegetative cover, which shields the prairie voles from predators. Pens within each set of four were interconnected, allowing the voles to choose where to live.
When vegetative cover was low in a pen, some voles emigrated to pens with higher cover, where populations reached higher densities. The addition of new animals in the better habitat was eventually offset by increased deaths, which balanced immigration and births so the populations stabilized. In the pens deemed poor habitats because of their low cover and low food supply, population stabilized at lower densities. Here births exceeded deaths, which compensated for the emigration of voles to better habitats, Batzli reported.
3The voles selected their habitat in such a way that their fitness - reproductive success, as measured by the number of young animals recruited per female - was equal in all the pens,3 Batzli said. 3It1s as if the voles sampled the habitats and settled where they could maximize their reproduction.2
Batzli1s team now is repeating the study with meadow voles, a more active species with less tolerance for low cover and which live in competition with prairie voles. In 1997, the two populations will be combined to see how competition affects densities and their movements among different habitats. An anticipated end result, Batzli said, is knowledge that should help wildlife managers and conservation biologists maintain populations of predators and prey in natural areas. -jb-