Components of life histories

What are the most important components of any organism's life history? Individual size is perhaps the most apparent aspect. As we have seen, it is particularly variable in organisms with a modular construction. Large size may increase an organism's competitive ability, or increase its success as a predator or decrease its vulnerability to predation, and hence increase the survival of larger organisms. Stored energy and/or resources will also be of benefit to those organisms that pass through periods of reduced or irregular nutrient supply (probably true of most species at some time). Finally, of course, larger individuals within a species usually produce more offspring. Size, however, can increase some risks: a larger tree is more likely to be felled in a gale, many predators exhibit a preference for larger prey, and larger individuals typically require more resources and may therefore be more prone to a shortage of them. Hence it is easy to see why detailed studies are increasingly confirming an intermediate, not a maximum, size to be optimal (Figure 4.16).

Development is the progressive differentiation of parts, enabling an organism to do different things at different stages in its life history. Hence rapid development can increase fitness because it leads to the rapid initiation of reproduction. As we have seen, reproduction itself may occur in one terminal burst three types of question optimization and other approaches to understanding life history evolution

25 30 35 40 45 Adult male weight (mg)

Figure 4.16 For adult male damselflies, Coenagrion puella, the predicted optimum size (weight) is intermediate (upper graph), and corresponds closely to the modal size class in the population (histogram below). The upper graph takes this form because mating rate decreases with weight, whereas lifespan increases with weight (mating rate = 1.15 — 0.018 weight, P <0.05; lifespan = 0.21 — 0.44 weight, P < 0.05; n = 186). (After Thompson, 1989.)

25 30 35 40 45 Adult male weight (mg)

Figure 4.16 For adult male damselflies, Coenagrion puella, the predicted optimum size (weight) is intermediate (upper graph), and corresponds closely to the modal size class in the population (histogram below). The upper graph takes this form because mating rate decreases with weight, whereas lifespan increases with weight (mating rate = 1.15 — 0.018 weight, P <0.05; lifespan = 0.21 — 0.44 weight, P < 0.05; n = 186). (After Thompson, 1989.)

(semelparity) or as a series of repeated events (iteroparity). Amongst iteroparous organisms, variation is possible in the number of separate clutches of offspring, and all organisms can vary in the number of offspring in a clutch.

The individual offspring can themselves vary in size. Large newly emerged or newly germinated offspring are often better competitors, better at obtaining nutrients and better at surviving in extreme environments. Hence, they often have a better chance of surviving to reproduce themselves.

Combining all of this detail, life histories are often described in terms of a composite measure of reproductive activity known as 'reproductive allocation' (also often called 'reproductive effort'). This is best defined as the proportion of the available resource input that is allocated to reproduction over a defined period of time; but it is far easier to define than it is to measure. Figure 4.17 shows an example involving the allocation of nitrogen, a crucial resource in this case. In practice, even the better studies usually monitor only the allocation of energy or just dry weight to various structures at a number of stages in the organism's life cycle.

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