The word "indispensible" in the passage is closest in meaning to
[#paragraph1]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 [#highlight1]indispensable[/highlight1] 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.
[#paragraph2]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 [#highlight3]partake[/highlight3]. 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.
[#paragraph3]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.
[#paragraph4]It requires a great deal of energy to make so many seeds, so much energy that it is unlikely that a pinyon population could produce such a bumper seed crop every year. More important, however, it would be to the severe disadvantage of the trees to produce large seed crops annually, even if they could. Doing so would make the resource not only abundant but also predictable. Seed predators could, over the years, steadily build their populations, eventually increasing so much that they could, indeed, consume virtually all of the seeds. It is much more adaptive for the plants to produce seed cornucopias intermittently. [#insert1]Doing so has several major advantages. First, energy can be stored for some years and then devoted to cone and seed production, ensuring adequate energy to produce a large seed crop. [#insert2]Second, seed-predator populations will decline in years of low seed production, either through starvation, reduced reproduction, or emigration. [#insert3]Pinyons in most areas have a roughly six-year interval between heavy seed crops.[#insert4]
[#paragraph5]Pinyon jays have so completely adapted to the cycle of the pinyons that their reproduction is tied to it. Most species of birds mature sexually in response to changes in day length. In pinyon jays, however, day length is only one stimulus for reproduction. The other cue is availability of pinyon seeds. When seeds are abundant, jays can breed very early in spring, continue breeding through summer, and reenter breeding condition as early as the winter solstice. So the jay population temporarily enlarges in response to one of the intermittent large crops of pinyon seeds. But the increase in the population of jays is less than the increase in the size of the seed crop. Thus many of the seeds of a bumper crop are buried and subsequently germinate, many more than would germinate without planting by the jays.