UGA team studying gypsy moths

Ecologists at the University of Georgia Odum School of Ecology and other researchers studying invasive insects report that the success of new gypsy moth populations is partly dependent upon the size of the patch they occupy—information that could eventually help control the spread of the moths and other invasive pests.

 

Using data from the gypsy moth invasion, the researchers have shown there is indeed a critical area necessary for species that are subject to positive density dependence.

 

“Populations that cover too small an area are likely to fail even if they exceed a previously recognized critical density for establishment,” said Andrew M. Kramer, a postdoctoral associate in the Odum School and another of the study’s co-authors. “This is, to our knowledge, the first empirical demonstration that the persistence of small populations can be determined by the size of the area they occupy.”

 

Using Forest Service data, Kramer and his colleagues determined the density and area of each patch occupied by small populations of gypsy moths over a 12-year period. That allowed them to calculate the critical area needed to sustain a small population.

 

“We found that small populations can be more successful if they’re spread over a larger area,” Kramer said.

 

Although previously predicted by theory, the findings are not intuitive. “It’s a complicated interaction,” Kramer said. “The population has to be dense enough to persist, but not too dense. If a population is compressed into a very small area, it will fail because of the way the male moths disperse. They won’t encounter potential mates as they spread out away from their point of origin—they’ll move out into unoccupied areas. But when a population with the same number of individuals occupies a larger area, the males are much more likely to encounter females as they disperse.”

 

The study’s results have implications for the control of invasive species, potentially helping managers more accurately target their efforts, and for conservation of threatened or endangered species.

 

“Knowing that there is a critical area threshold for certain species could inform decisions about habitat protection or re-introduction,” Kramer said. “For restoration programs, considering not just the minimum number of individuals needed for the population to thrive, but also the optimal area they need, could increase the chances of success.”