ABSTRACT
The lives of biological organisms are a constant struggle to acquire resources in a competitive environment and turn those resources into a genetic contribution to future generations. In Chapter 1 we suggested that increased resource availability would generally lead to higher fertility rates as well as higher juvenile survivorship. There is good theoretical reason to expect this. First, the only reason living organisms should care about acquiring resources at all is if energy and other limiting components of life can be turned into greater genetic contribution. Second, natural selection should have operated to produce efficient physiological and behavioral mechanisms for converting resource advantage into reproductive advantage, since organisms without such mechanisms would have been replaced quickly on the scale of evolutionary time. There is abundant evidence that organisms are indeed designed to convert resources into surviving offspring effectively and that those with greater resource access generally have more surviving offspring (Chapter 1, Tables 1.1 and 1.2). However, a demonstration of significant association between resources and fitness is only marginally instructive because at some point increased resources will likely produce ever-diminishing fitness returns. Thus, an important goal for life history studies should be the characterization of the shape of the relationship between resources and life history traits, rather than a simple demonstration of significant associations. Mathematical functions describing the relationship between estimates of resource availability and vital rates are critical to developing quantitative, testable models about life history traits. The shape of these relationships ultimately determines the fitness implications of alternative life history responses, and thus provides the basis for explaining rather than describing life history patterns. In this chapter we provide some estimates of resource impact on fertility and mortality rates, and then in the next chapter we examine the shape of those relationships and use those estimates to test a mathematical model of one life history trait.
