We have noted that population growth can be limited by resource availability. How an organism makes use of the resources that are available can also affect population growth. As we saw earlier in this chapter, each individual must devote some of its available food and energy intake to growth, some to the maintenance of cells and tissues, and some to reproduction. This allocation is very much like a household budget, in which incoming resources are divided among the day-to-day expenses of acquiring food and paying for housing and transportation, meeting present and future medical and educational expenses, and savings held in reserve for emergencies. When one category goes up in cost—it needs more money, or more resources—another must come down.

In nature, individual organisms strike a balance in how they allocate resources to the basic needs of growth, maintenance, and reproduction, a balance that has evolved to minimize waste while retaining the flexibility to use resources in different ways depending on environmental circumstances. These kinds of trade-off evolve through natural selection and are the rule in ecology, explaining in part why species tend to produce either many small offspring or a few larger ones.

The typical pattern of resource investment in each stage of a given species’ lifetime is called its life history. The amount of energy available for reproduction varies over an individual’s lifetime, and the degree of control an individual has over resource allocation also varies from species to species. Lizards, for example, show considerable flexibility in allocating resources to reproduction. Many lizards show a physiological trade-off from year to year between the number of eggs produced and the average size of each egg. There is a finite amount of resources to put into offspring, and lizards adjust the number and size of eggs depending on conditions in the environment. Larger eggs require more resources to build, leaving less for additional eggs, and larger eggs hatch into larger offspring that are better able to fend for themselves. But if, for example, the environment carries a high predation risk, then laying more eggs scattered in different places may be a better strategy than laying a few larger eggs in one place. Does this flexibility make these lizards K-strategists or r-strategists? The answer is neither. Which strategy is optimal depends on the nature of the risks faced by young.

Plants also show physiological trade-offs in the allocation of resources to seeds. For example, flowers may abort some of their developing seeds to favor the growth of others. Plants also make trade-offs between defense and reproduction. When herbivores are around, plants that invest resources in defenses like poisons or thorns have an advantage over plants that do not invest in defenses. These well-defended plants make a greater contribution to the next generation than poorly defended plants. However, when herbivores are absent, defensive plants are commonly at a competitive disadvantage. Individuals that do not invest in defenses can allocate more resources to reproduction. Such trade-offs in plant defense are especially important on nutrient-poor soils, where the cost of replacing valuable tissues lost to herbivores is higher than in nutrient-rich soils (Chapter 32).

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The key point is that the various ways in which animals and plants allocate resources to growth and reproduction is rooted in their physiology, and their physiological traits are, themselves, evolved traits. Thus, ecology both reflects and determines how organisms function in nature.

Quick Check 4 Why must a plant or animal make trade-offs among growth, reproduction, defenses, and other adaptive traits?