Rise of the supertramp beetles

Natural History, July-August, 1998 by Raphael K. Didham, Nigel E. Stork

Imagine you had spent your life wandering the rainforests of the Amazon Basin. Every day you would see the signs of large rainforest animals: peccary mud wallows, carpets of tapir dung, and tracks of the jaguar. Now imagine you are standing in the middle of a forest fragment surrounded by scrubby cattle pasture baking in the sun. The scene appears pristine, but you are immediately aware that something is wrong: large creatures and their telltale signs are missing. Closer inspection reveals that many of the smaller creatures are missing, too; there are far fewer insects, and far fewer kinds of insects, than you used to encounter in the undisturbed forest.

Early naturalists' accounts of the Amazonian forest give the impression that one can easily observe and collect vast numbers of insects there. Actually, except for the seething masses of ants and termites endemic to the rainforest floor, the casual hiker or collector can see many more insects during summer in a European or North American woodland. But in the rainforest, the richness is not so much in numbers as in diversity; almost every new insect one encounters is a species different from the last. Indeed, a single square mile of pristine tropical rainforest generally contains about 50,000 different insect species--more than all the world's mammal, bird, fish, reptile, and amphibian species combined.

The most important actors in Earth's global ecosystem (leaving aside bacteria), insects perform such vital functions as assisting plant pollination and decomposing larger organisms. But what happens to insects and the services they provide when the forest is cut into small patches? To find out, we first needed to establish how many different species live on the forest floor. We began by scraping up and sieving more than 10,000 square feet of leaf litter--almost two tons of material. Then, using simple contraptions known as Winkler bags, we extracted more than 80,000 large insects and tens of millions of other small arthropods, such as mites and springtails. (Winkler bags allow the leaf litter to be dried for several days without any insects escaping. As the litter dries out, the moisture-seeking creatures crawl downward and end up in a collecting jar at the bottom of the bag.)

Ironically, one of the earliest and most obvious effects of forest fragmentation seems to be a huge increase both in insect abundance and in the kinds of insects in disturbed areas. At first, we found that insects were two to three times more common at forest edges adjacent to pasture and in small forest fragments than they were in undisturbed forest. A decade ago, these "edge effects" were thought to be desirable, and forest managers deliberately created edges to increase the insect population. We have since learned, however, that the increase in diversity at the edge is caused by the invasion of an array of very common, widely distributed insects that we now call supertramp species.

When the Amazonian rainforest had an unbroken canopy stretching from one coast of the continent to the other, the supertramp insects survived mainly in gaps where trees had fallen and in other naturally disturbed areas where light and heat could reach the forest understory. In recent years, with the rising rate of tropical deforestation, there has been a corresponding increase in habitat for these hardy insects. Tolerant of disturbance, they are usually in no danger of extinction. The insects that normally inhabit the deep forest, however, have decreased markedly in both abundance and number of species in forest fragments.

Our studies focused on beetles. More than 1,000 kinds of beetles may live in any one patch of undisturbed Amazonian leaf litter, but we looked most closely at 300 species--ground beetles (Carabidae), rove beetles (Staphylinidae), and dung beetles (Scarabaeidae). Of these 300 species, 70 percent are very rarely found and were sometimes represented by just a single individual in our samples. Just six species of beetle represent 30 percent of the total number of beetles in undisturbed forest. Oddly, these very common species are the most affected by forest fragmentation. When the forest is cut up, the common, deep-forest species disappear from the fragments. But why? Why can't these beetles, which are less than one-tenth of an inch long, survive in half a square mile of forest?

At present, we do not have the answer to this very important question. Maybe the slightly drier environment does not suit them, or perhaps the decaying large-mammal dung and fallen fruits that they feed on have become scarce. Fragmenting of the insect population may also be a factor; when populations become too small, individuals have difficulty finding mates, and inbreeding may result. Small populations may also become less adaptable to small environmental changes; a short-term fluctuation in rainfall, for instance, may cause a species to become locally extinct. Often, new colonists from the continuous forest will not be able to cross the open pasture to repopulate the forest island.