Wednesday, February 17, 2010

Living on the Edge: Insect Repellent

Most everyone knows habitat loss is one of the major reasons we are losing biodiversity across the globe. Many species have particular habitat requirements, like the gopher frog, which breed in seasonal wetlands within longleaf pine forests, or red-cockaded woodpeckers, which nest in forests containing mature pine trees. When these habitats are gone, the animals can’t persist. That much is fairly straightforward.

But, when half of a forest is destroyed, is the remaining forest as good as ever (albeit a bit smaller)? The answer is no, and it’s due to what ecologists refer to as edge effects. When the border of a habitat is altered, the interior may be affected in many different ways. These edge effects need not be generated from complete conversion to other land uses; a road, a powerline corridor, or even a footpath can influence surrounding habitats. For example, when a road is constructed through a forest, the temperature of the woods increases because there is less canopy cover to block the sunlight. This may also result in a decrease in humidity levels and more wind, again because there are less trees to serve as a buffer. In the northeastern United States, roads are salted frequently after winter storms to melt the snow and ice; these salts are known to seep into surrounding wetlands and influence tadpole development.

Those are all examples of what we’d call direct effects of edges, but there are also more subtle consequences. For example, edges may serve as a barrier to dispersal for many species. Some species, like insects or other invertebrates, may be unable to cross an open area because they dry out or get eaten. Larger but slow moving animals, like turtles, may be run over by cars on roads that cut through their habitat. This functions to isolate groups of animals, which is dangerous for two reasons. Smaller populations are more likely to go extinct simply because of chance events (like a disease outbreak, for example) and smaller populations have reduced genetic diversity, which is never a good thing.

During our stops through Costa Rica, we break up into small groups and conduct short-term research projects to investigate the biology and ecology of tropical systems. While staying in Cerro de le Muerte, my group undertook a small study to determine how a lightly used and unpaved road might influence assemblages of invertebrates. To explore the issue, we placed a one meter square (made with PVC pipe) over the ground at two, 10, and 40 meters (6.5, 33, and 131 feet) from the road (repeated four times for a total of 12 squares. Within these squares, we collected all the leaf litter and small sticks and tied it all up within a garbage bag for storage.

The Study Site

Back at the lab, we sifted the leaf litter through a sieve and collected all the spiders, cockroaches, beetles and everything else alive that fell through. By the time we had sorted through all of the leaf litter, we had collected nearly 800 individual critters. Not bad for just 12 small squares. Although the few large animals that we see get most of our attention most often, if nothing else we should all be aware of the millions of invertebrates that are underfoot all the time, providing an important link in the food webs of every habitat on earth.

We weren’t really sure what to expect from our results. One might think that when a habitat is degraded, such as after road construction, the diversity and abundance of organisms would decrease. That’s the most straightforward expectation. But, when a habitat is altered, a new habitat is created, and this new habitat may suitable for new species that invade the area. If the original species still hung around, then you would actually expect a greater diversity of organisms after the disturbance than were present before.

We documented the first scenario, a clear trend indicating the diversity of invertebrate animals increased sharply as we sampled further away from the road and into the forest interior. This was relatively surprising, as the road we sampled was small, unpaved, and traveled by only a few cars each day. If the number of small, ecologically important animals is affected so drastically from just a glorified footpath, one must wonder at the magnitude of how we our actions are affecting the surrounding landscape at a larger scale.

Photos courtesy of Kim Morrell

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