"Instead, most insects turn out to be specialized not to plant species, but rather to a genus [grouping of species] or family [grouping of genera] of plants," Weiblen said. "Fewer effective plant 'hosts' means fewer herbivores. Where people had assumed that different insects' food sources overlapped very little, we found that many insects share their food plants with other insect species. There are actually few extreme specialists among tropical insect herbivores."
New Guinea's species diversity made the island an ideal laboratory for the study. Its tropical forests are about the same area as Texas but contain five percent of the world's biodiversity, Weiblen said. The island has 12,000 species of plants; in comparison, Weiblen's home state of Minnesota has only about 1200 species. The work began in 1994 with studies of the insects eating 15 species of figs--Weiblen's area of specialty--that co-occur in the rainforest. The study then expanded to include the mulberry family, which is related to figs, then members of the coffee family and more distantly related plants. The team used DNA sequences to sort out the evolutionary relationships between plants and looked at how insect species were distributed on them.
They found that the tropical forest plant community was dominated by clusters of closely related plant species and that insects tend to feed on multiple close relatives in a given plant genus or family. Because the number of insect species was tied to broader--and therefore less numerous--categories of plants, estimates of their numbers had to be diminished, Weiblen said. Once the estimate of herbivorous insects had been made, the researchers plugged that number into equations to estimate the total
Contact: Deane Morrison
University of Minnesota