TOEFL Academic Passage Practice 018Coevolution of Pinyon Pines and Pinyon Jays (Variation A)

Species in nature often evolve alongside one another in a way that makes them interdependent. One example of this is the relationship between trees and certain fungi. Mycorrhizal fungi live in tree roots, assisting the tree in absorbing nutrients from the soil, while the fungi rely on the tree’s energy-rich sugars for survival. This mutual dependence is a result of coevolution—a process in which two species adapt to each other over time to such an extent that neither can thrive as well without the other. Coevolution does not require that the two species have equal effects on one another; it requires a continuing pattern of reciprocal influence.

At first glance, the interaction between pinyon pines and pinyon jays does not seem to qualify as coevolution. Pinyon pines produce cones with seeds that attract animals, particularly pinyon jays. Since these seeds contain the plant embryos for the next generation of trees, it may seem counterintuitive for the trees to produce seeds that attract predators. Other plants, for example, have developed toxic seeds or tough outer coverings to discourage animals from eating them. Pinyon pines, however, appear to do the opposite. Their cones grow in prominent locations, their seeds are highly nutritious, and they even have thin, easily digestible seed coats. Moreover, the seeds differ in color in ways that indicate which are viable, essentially guiding jays toward seeds that can grow into new trees. These features make the seeds conspicuous and valuable rather than protected and difficult to consume.

This seemingly risky strategy actually benefits the trees. Pinyon jays gather and bury large quantities of seeds to save for winter. A jay can carry seeds away from the parent tree and place them in many separate hiding sites. Although the bird later recovers some of these stores, it cannot relocate every seed, especially when snow, distance, or competing demands interfere. Many of the buried seeds therefore go uneaten, allowing them to germinate and grow into new trees. Burial also places seeds in locations where they may be better protected from some surface-feeding animals. For this system to work, though, the pines must produce seeds in such abundance that jays cannot consume them all. Individual trees cannot manage this on their own, so pinyons across a region coordinate their seed production. Every few years, they produce massive crops of seeds—a phenomenon known as a “mast year.”

Producing large crops of seeds requires significant energy, and annual mast years would be unsustainable. A tree that invested heavily in reproduction every year might have fewer resources for growth, maintenance, and resistance to environmental stress. In addition, regular seed production would make it easy for seed predators to expand their populations, which could result in the loss of all seeds before they germinated. By varying the intervals between mast years, usually about six years, pinyons keep seed-predator populations lower during lean years, ensuring that some seeds survive during heavy production cycles. The irregular schedule also makes it more difficult for predators to adjust their numbers to a predictable annual supply.

Pinyon jays have adapted to this cyclical pattern. Unlike most birds, which breed primarily in response to seasonal changes in daylight, pinyon jays also use seed abundance as a cue for reproduction. When pinyon seeds are plentiful, the jays breed earlier and more frequently. This response allows the birds to take advantage of a temporary food surplus, but the resulting increase in jay populations is temporary and never fully matches the abundance of seeds during a mast year. After the surplus ends, food becomes scarcer and the jay population does not remain at its temporary peak. Consequently, many seeds remain buried and able to germinate.

The two species therefore affect each other through more than simple consumption. The pines’ irregular abundance influences when and how successfully the jays reproduce, while the jays’ collecting and burying behavior helps distribute pinyon seeds and establish future trees. Neither species controls the entire cycle alone: the trees depend on the birds to move and conceal seeds, and the birds depend on the trees for a major seasonal food supply. The relationship between pinyon pines and pinyon jays is, therefore, an intricate example of coevolution, in which reciprocal adaptations create a long-term ecological dependence.