A successful invasive species, such as the gypsy moth, may seem an unlikely choice to inform the study of small populations. At the edge of their current range, however, where they are expanding into new territory, numerous small populations exist in a transition zone. Because of the economic damage they cause, an enormous amount of data about gypsy moths has long been collected through the U.S. Forest Service “Slow-the-Spread” program.

Each spring, gypsy moth eggs hatch and caterpillars emerge to consume the foliage of trees; adult moths appear briefly in late summer to breed. The females are flightless and emit pheromones to attract mates, a strategy that only works well at short distances—making the gypsy moth one of those species that requires a critical density to persist in small populations.

“If there aren’t enough individuals in the area, they are likely to be too far away from potential mates, and the population won’t grow,” said Kramer.

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.”

Besides Kramer and Drake, the research team included lead author Elodie Vercken of the French National Institute for Agronomic Research, whose work focuses on invasive insect pests and who was a visiting scholar at UGA in 2009; and Patrick C. Tobin, an entomologist with the U.S. Forest Service Slow-the-Spread program.

For more information on the UGA Odum School of Ecology, see www.ecology.uga.edu/.