Newswise — When it comes to biodiversity, the ecological roles filled by species matter more than the number of species in the system.
That's the conclusion reached by Washington State University entomologists Deborah Finke and William Snyder, whose recent study of parasitic wasps and their aphid prey showed that the key to biological diversity in an ecosystem is having species with non-overlapping roles.
"Diversity is beneficial because at some point as you're adding more species, you're adding species that do different things," said Snyder. "It's not biodiversity [the number of species per se] that we need to preserve, so much as it is species that do different things."
In addition to answering an important ecological question, Snyder said, their study provided empirical support for sustainable agriculture practices and offered guidance on how best to do biological control of agricultural pests. Their paper appears in the September 12 issue of Science magazine.
Snyder and Finke, who is now at the University of Missouri, found that increasing the number of wasp species on a test plot increased resource use—consumption of aphids—only if each wasp species targeted a different kind of aphid.
Snyder said the findings show that for efficient biological control of pests, the best recipe is to use multiple biocontrol agents that don't have overlapping prey preferences. He said the study also suggests why sustainable agricultural systems have relatively few pest problems despite their minimal use of pesticides. "A basic mantra in organic agriculture is that you need more diversity. But it's been hard to really pin down, what specifically does that mean?" he said. "Maybe one reason that people are able to grow things under organic agriculture is because natural diversity is being restored to the system."
The experiment provided the first unambiguous demonstration of "niche partitioning," the notion that multiple species can live in the same area only if they don't compete for the same resources. Snyder said the idea that niche partitioning is essential for biodiversity has been amply supported by mathematical models, but has not been definitively shown in experiments before now.
"It's been thought to be impossible to test," said Snyder, because there has been no way to examine the use of a resource versus the number of species while holding everything else constant. What researchers needed was a way to have different members of one species do different things (such as eat different prey), and members of different species do the same thing (such as eat the same prey).
Finke and Snyder realized they had prime candidates for such an experiment in the parasitic wasps, which are used in many agricultural areas as biocontrol agents against aphids. What's special about the wasps, and what made them ideal for this study, is that individuals of one species can be raised to prefer one kind of aphid prey over all others. The wasps reproduce by laying their eggs in the body of an aphid. (These wasps were the inspiration for the monster in the movie Alien.) When the larvae hatch out of the eggs, they eat the aphid from the inside, eventually emerging into the air as adults. An adult wasp will strongly prefer to lay her eggs in the same kind of aphid in which she herself grew up. The preference is so strong that a wasp may forego the chance to reproduce if the favored target prey is not available.
Finke and Snyder took advantage of the wasps' trainability to produce colonies of wasps that honed in on a single kind of aphid. They used three species of wasp, raising some individuals of each species to parasitize one of three kinds of aphid. Then they placed aphids and wasps into cages enclosing radish plants. The aphids fed on the radish leaves, and the wasps laid their eggs in and fed on the aphids.
All the cages held three kinds of aphids. By changing the number of wasp species (one versus three) and whether each wasp species was a specialist (all of the individuals parasitizing just one kind of aphid) or a generalist (different individuals parasitizing different kinds of aphids), Finke and Snyder were able to separately test the effects of species number and resource use.
In one test, all three species of wasp were put into the cage, with some individuals of each species trained to attack each of the three kinds of aphid. In that case, there was species diversity but not niche diversity, because all the wasp species were attacking all the aphids—they overlapped in their use of the resource. The wasps didn't parasitize any more aphids than when a single species of wasp was used to attack the same kinds of aphids.
Another test had all three species of wasp, with each species trained to attack a different kind of aphid. In that case the wasps had niche diversity as well as species diversity. The parasitism rate was nearly double that seen when the wasp species had overlapping niches.
"Species diversity in and of itself doesn't seem to do anything," said Snyder. "It's only when you have species diversity and they're partitioning the resource that you see this improvement."
Photos of experimental cages in the field are available.
The paper: "Niche partitioning increases resource exploitation by diverse communities," by Deborah Finke and William Snyder. Science, Sept. 12, 2008.