April 15, 2009

Study explores animal survival in inherited habitat

by Tim Crosby

CARBONDALE, Ill. -- Humans have known for centuries that offspring inherit characteristics from their parents. More recently, science explained this phenomenon with genetics; writing formulas that explain how certain traits get passed down generation to generation.

Now, a wildlife researcher at Southern Illinois University Carbondale is helping open the door on a new, poorly understood aspect of heredity that owes more to simple fate and geography than genomes.

Eric M. Schauber, associate professor of zoology and a member of the Cooperative Wildlife Research Laboratory at SIUC, is part of team examining how inheriting favorable or unfavorable habitat affects the overall rise and fall of animal populations. Schauber’s work, which involves studying the relationships among mice and chipmunks, gypsy moths and ticks in several areas around the northeastern United States, promises to mathematically analyze how those variables impact natural selection in those populations.

Schauber theorizes that for some animal species, inheriting a piece of turf has as much impact on an animal’s survival as inheriting intelligence, fertility, camouflage or other genetically transferred characteristics. The ongoing study could eventually provide insight into managing endangered species populations, among other uses.

Schauber is working with researchers from the Cary Institute of Ecosystem Studies, Texas Tech and the University of North Dakota on the eight-year, more than $800,000 study, funded by the National Science Foundation. The study also involves SIUC graduate student Tyler Schartel and undergraduates Jessica Hartshorn of Medway, Ohio, and Karen Heldt of Hoffman Estates. This summer, current and former students Alishia Zyer, Scott Bergeson and Viraj Perera will travel to New York to continue the research.

By examining the relationships between the animals, all of which interact with one another in the forest ecosystem, Schauber can track how specific habitats promote or discourage survival, and whether those habitats can be passed down generation to generation.

For example, one aspect of the study involved Schauber and the team examining population spikes of gypsy moths, which as an invasive species can cause widespread defoliation in American northeastern forests when unchecked. Mice, it turns out, play a key role in preventing such occurrences by decimating the moth population at its pupae stage, which occurs near the ground and makes them easy pickings for the hungry rodents.

“You don’t really think about mice as predators, but for the right kind of prey, they’re pretty ferocious,” Schauber said. “The mice in particular are predators of the gypsy moths.”

The moths play into the spatial inheritance theory in that they typically don’t disperse very far after they emerge from where they are born. The adult female moths are flightless and lay their eggs close to where they emerge from the pupal stage. Because pupae eaten by mice lay no eggs, gypsy moths that survive mainly do so in spots with fewer mice. The offspring caterpillars also don’t disperse very far, and usually enter the pupae stage near where their mother survived, “inheriting” a relatively safe site without even realizing it.

Using measurements of the mouse population density in different areas of the woods, as well as their attack rate on gypsy moths, Schauber initially built computer models examining the moth and mice populations and their interactions.

For some reason, however, the models consistently showed the moth population crashing, which simply did not reflect reality. In the model, gypsy moths only avoided extinction if mouse populations stayed unnaturally low.

Because the moth populations weren’t crashing in real life, Schauber knew his model was missing an important variable.

“What we were missing, it turned out, was the spatial relationship,” Schauber said. “Once we accounted for moths inheriting these hot spots, which is where there are few mice and high chances of survival for moths, the models started getting what we see in the real world, which is the mice control them unless the mouse population drops, and then it’s off to the races for the moths.”

Inheriting the hot spots meant over time the majority of the moth population ended up in places where mice did not live. Hence, the fate of moths being born in a certain geographic area was responsible for their survival, as much as genetically inherited traits might be in other cases.

“It’s a selection process and an inheritance process that works almost the same as natural selection,” he said. “The differences in survival in this case are not based on your characteristics as an animal, but where you happened to be born.”

An animal having a predisposition that encourages it to stay put also plays into its survival chances in this situation.

“It’s spatial inheritance, rather than genetic inheritance,” he said. “The next generations tend to be more or less biased toward these same spots in the environments, too. So if we are seeing spots that are safe year to year, then those offspring basically inherit those spots when they’re born.”

The team’s research has shown that hot spots can be relatively stable year-to-year, as the team reported in their recent paper in the journal “Ecology.” They also are experimentally creating hot spots by doing small-scale mice removal in certain areas and checking for the effects on how many gypsy moth eggs are laid and where.

“We know the players very well and we know how they impact each other so this is a good model system for getting at these questions,” he said.

Living in a hot spot, however, can also lead to other consequences, such as animals more readily passing on diseases. That looks like the case between mice, ticks and the bacterium that causes Lyme disease. The team in this case suspects the hot spot effect could concentrate the disease more readily in ticks, which pick up the bacteria from mice and together pass it on to subsequent generations. They will test this theory in the coming years with the mouse removal experiment.

Overall, the spatial inheritance dynamic, when more fully understood, could be applied to many wildlife management situations, Schauber said.

“It’s not that this is a completely novel idea. Researchers have been thinking about the effect of inheriting a good spot on animals in the ecology literature for a long time,” he said. “In our work, though, we’ve been actually able to apply mathematics and quantify how this works. Because it’s analogous to natural selection, we can use the same formulas geneticists use to understand how this spatial inheritability influences how populations grow and shrink.”