A wildflower, a moth, and a diverse landscape: Shifting relationships offer a rare view of how species interactions evolve

The complicated relationship between a common wildflower and a little gray moth is yielding new insights into how species coevolve, with implications for the conservation of biodiversity. Researchers at the University of California, Santa Cruz, and Washington State University (WSU) described the variable interactions between these two species--a remarkable case study in coevolution--in a paper to be published in the June 13 issue of the journal Nature.

The wildflower, known to botanists as Lithophragma parviflorum, goes by the common names woodland star or prairie starflower. Its life history is entwined with that of an inconspicuous moth called Greya politella. The moth is both a pollinator of L. parviflorum flowers and a consumer of its seeds, a combination of effects that has widely varying outcomes in different habitats. The interaction covers the full range of possibilities, from mutually beneficial to antagonistic, in a complex geographic mosaic.

"This is a particularly interesting interaction for understanding the link between evolution and ecology, and how that plays out over complex landscapes," said John Thompson, a professor of ecology and evolutionary biology at UC Santa Cruz.

"We're finding that much of evolution is about the coevolution of species--how species continually respond to one another, forming complex networks of interaction," Thompson said.

In the Nature paper, Thompson and coauthor Bradley Cunningham of WSU described how these two species--the woodland star and the Greya moth--have coevolved in a variety of habitats throughout western Idaho and adjacent areas of Washington and Oregon.

The female moths of G. politella lay their eggs in the flowers of L. parviflorum, inserting a long ovipositor down the neck of the corolla and cutting into the flower's ovary. In the process, the moths pollinate the flowers, carrying pollen from one flower to another on their abdomens. After the eggs hatch, the moth larvae feed on the plant's seeds.

In extensive field surveys, the researchers found the moths associated with every population of L. parviflorum they sampled. Further studies focused on 12 sites in contrasting habitats, including open grassland, ponderosa pine woodland, and streamside canyons. In all but one of the sites, the moths were completely dependent on L. parviflorum for food (nectar for adults and seeds for larvae) and for mating sites.

At each study site, the researchers assessed the development of Lithophragma seed capsules, comparing flowers that had been visited by moths with those that had not. At four sites, flowers without Greya eggs were far more likely to have aborted seed capsules than flowers with eggs, indicating that the flowers depend on the moths for pollination. In the terminology of coevolution, a mutually beneficial relationship like this is called mutualism.

At four other sites, the moths had no effect on the development of seed capsules, indicating a neutral or "commensal" relationship. Thompson said this was due to the presence of enough other insect pollinators to swamp the beneficial effect of pollination by Greya.

"The moths depend on the plants for food, but the plants are not affected either way, because the copollinators are effective and the moths are not abundant enough for their feeding to have a negative effect on the plants," Thompson said.

The relationship looked antagonistic, however, at the remaining four sites. There, L. parviflorum plants selectively aborted seed capsules containing moth eggs, as if they had decided to rely entirely on other insects for pollination and avoid producing seeds for Greya larvae.

"The negative effects of the moths feeding on seeds seems to select for plants with the ability to rid themselves of the moths," Thompson said.

The mutualistic and antagonistic sites are coevolutionary hotspots, he said, where natural selection is acting reciprocally on both species. "It's a kind of tit-for-tat evolution, where one species responds and the other counterresponds, and the two species end up in a coevolutionary vortex," Thompson said.

The complex mosaic of coevolutionary hotspots and coldspots revealed in this study covers only part of the range of L. parviflorum and G. politella, one of the most widespread plant-insect interactions in western North America. This kind of variation in coevolutionary selection pressures across a broad area may be an important factor shaping the genetic structure of species and the ecological dynamics of natural communities, Thompson said.

"Ecologists have tended to study species interactions at the local level, but these results are telling us that we need to look at larger scales to understand how coevolution shapes biodiversity," he said.

Efforts to conserve biodiversity may also need to encompass large geographic scales.

"If keeping players in the evolutionary game demands a geographic mosaic of coevolutionary hotspots and coldspots, then fragmentation of habitats will fundamentally change the way coevolution maintains biodiversity," Thompson said.