Pinyon Pines and Pinyon Jays
Organisms of different species frequently evolve adaptations that make them utterly and specifically dependent on each other for resources. For example, many trees must have mycorrhizal fungi living in their root systems, fungi that are indispensable to the tree because they facilitate mineral uptake from the soil. In turn, the fungi rely on the trees to photosynthesize: they use some of the tree’s chemical food as their only energy source. Such intimate relationships are examples of coevolution through which two species become so interdependent that they can thrive only in each other’s presence.
At first glance, the relationship between pinyon pine trees and the pinyon jay does not appear to be one of coevolution. Pinyon pines produce cones and seeds that attract seed predators, especially the pinyon jay. A seed, of course, contains an embryonic plant for the next generation. Why sacrifice it to an animal? Some plants have poisonous seeds, an obvious adaptation to reduce loss to animals. Pinyons, however, have an array of characteristics that combine to encourage jays to visit the pines and help themselves to the seeds. Cones are positioned upward and outward on the tree, so the seeds inside are in plain sight of the jays, essentially inviting them to partake Pinyon seeds are unusually large, and each seed is high in energy. The seed coat is thin, meaning that birds such as pinyon jays can not only ingest the seeds but also digest them. In many plants, an indigestible seed coat permits the seed to pass unharmed through the bird’s alimentary system. Pinyon seed coats differ in color between edible and nonviable seeds, signaling the jays as to which they should select.
Attracting seed predators would not seem to be a successful survival strategy for pinyon pines. However, pinyon jays are behaviorally adapted to bury any seeds in excess of their immediate survival needs. This is a useful behavior for the jays, providing they can retrieve some of the buried seeds during winter, and good for the trees, as the unretrieved seeds are ready to germinate. Still, for the jays to bury any seeds, there must be an abundance of seeds far beyond the jays’ immediate needs. One tree could never produce so many seeds, but if all the pinyons in a region produced heavy seed crops at once, they would indeed “flood the market” with vastly more pinyon seeds than the local population of jays could consume In fact, that is exactly what the pinyon pines do.
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