The European gypsy moth, introduced to North America in 1869 near Boston, Mass., has steadily spread from there, devastating forests from eastern Canada to Wisconsin to North Carolina and thwarting all attempts at control. In a paper just published in the journal Ecology Letters, 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.

It has long been known that for some species, populations will not persist unless a certain critical density exists—a minimum number below which a population cannot thrive. Near that threshold, there is positive density dependence: the population’s growth rate increases along with its density. Scientists have predicted there also is a minimum area required for a small population to survive, but had not been able to confirm it in nature.

Now, 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.

“Critical points have been known in physics for decades,” said study co-author John M. Drake, an associate professor in the Odum School. “Now, with new methods and more sophisticated theories, we are discovering critical phenomena in many areas of science. What’s especially interesting about this one is the role played by space. It’s a deeply geometric aspect of ecology, one that until now had been only an idea. This paper shows it to be true in nature as well.”

“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.”