plant world, where reduced individual growth can benefit the broader community. The findings from the University of Michigan's Paul Glaum and André Kessler of Cornell University help explain the persistence (持续存在) of some plant communities when theory predicts they should go extinct(绝种的).

"We looked at how chemical defense cues (信号) from plants, meant to prevent herbivores (食草动物), can also prevent pollinators," said Glaum, a doctoral student in the U-M Department of Ecology and Evolutionary Biology. "The surprising model result is that while this can lead to fitness losses for individuals, the population effects can be positive for pollinators and plants under some circumstances."

　　Many plants, including the wild tomato species used in this study, produce chemical compounds to repel (排斥) insect pests and other hungry herbivores. But those same chemical defenses can reduce the number of visits to the plant by pollinators such as bees, resulting in less pollination of individual plants and decreased growth.

　　"Biologists have puzzled over how such a costly defense mechanism (机制) can be maintained in these plant populations," Glaum said. "How would a plant population with such a strategy persist?" Glaum and Kessler developed a computer model showing that decreased growth of individual plants can benefit overall populations and community resilience (恢复力) by indirectly controlling herbivore population growth. The results introduce mechanisms of persistence into communities previously found to be prone (易于遭受的) to extinction in theoretical models.

　　Tomatoes and other plants produce repellent chemicals called volatile (易挥发的) organic compounds in response to herbivore attacks. The presence of these so-called herbivore-induced volatile organic compounds can make the plant less attractive to pollinators, which can reduce pollen deposition (沉淀物) and negatively affect individual plants, an effect known as herbivore-induced pollinator limitation.

Previous modeling studies have looked at the direct effects of herbivory on a three-species community: flowering plant, pollinator and herbivore. Some of those studies predicted extinctions because growing herbivore populations would reduce the number of plants, limiting resources available to pollinators. In response, the pollinator population would decline, lowering plant reproduction.