Insects and other animals that eat crops have never been considered a farmer's friends. But in the long term perspective of evolution, they may actually have a beneficial relationship with plants.
One indication of this is the long known phenomenon that plants often thrive under moderate grazing. Dr. Melvin I. Dyer of Colorado State University thinks this may at least partly be due to a chemical growth stimulant deposited on the plants as the animals graze.
In his research, as described in the Proceedings of the (US) National Academy of Sciences, he put small amounts of this growth factor (taken from mice) on sorghum seedlings. After three days, treated seedlings showed 15 percent more growth than those not so treated. Growth then slowed. Nevertheless, the treated seedlings still had an edge on the others after a week.
Dyer notes that this growth regulating chemical (a hormone) is likely to be deposited on plants as animals such as mice graze. It also is present in the digestive systems of insects, suggesting that they too could deposit it as they munch on plants. Thus, he concludes, the chemical could well be one of the factors that makes plants thrive under moderate grazing.
This does not mean that pests are actually good for your garden. Crops represent intensive plantings of one or two species. Attacks by grazers easily can reach massive, and hence destructive, proportions. However, among the mixed plant communities in the wild, the grazers' feedings are usually not that heavy.
Dyer thinks that chemicals such as the one he has studied may provide a basis on which the evolution of plants, animals, and insects are interlinked.
Biologists know that the various parts of the biosphere -- the web of organic life that covers Earth -- cannot be understood in isolation. They have evolved as an interactive set of organisms, all of which ultimately are mutually dependent on one another, although the dependency may be obscure and remote.
Thus, considered in the perspective of the planet as a whole, Earth's rich diversity of life can be regarded as a community that, in its largest sense, has evolved as a single living entity. Dyer's discovery illustrates how subtle the interactions of various parts of the community can be.