A species' life history is the pattern of life cycle processes, including growth, development, reproduction, and death. Life history traits, therefore, are traits that relate to the timing and occurrence of each of these stages. Such traits include the number of offspring produced per mating, the seasonal timing of mating, the age at which individuals become sexually mature, at which stage(s) individuals disperse to new ecosystems, and the average life span of an individual, among others. The principle of allocation acts on all of these traits individually, and in complex interactions, producing trade-offs that may even act across different life history stages.
Each stage of the life cycle in Figure 6.1.1 is labeled with some of the major life history traits associated with that stage. While the specifics of each trait will vary between individuals, each species has a particular pattern of traits. For example, in the common blue butterfly, Polyommatus icarus, shown in Figure 6.1.1, females usually lay a single egg per host plant. A caterpillar emerges about a week after the egg is laid and then undergoes five stages of development, called instars, which in total take around 7 weeks. Each instar is distinct in size and coloration, with the final instar approximately 13 mm in length. The caterpillar then forms a chrysalis, and metamorphosis into an adult butterfly takes approximately 2 weeks. The sexually mature adult butterfly has a wingspan of 2.9-3.6 cm, and lives about 3 weeks, for a total life span of about 13 weeks. Like many butterflies, and indeed many insects, adults reproduce only once and then die soon after. Consequently, the only parental care provided to the offspring are resources contained within the egg, which are about 0.6 mm in diameter. Dispersal primarily occurs during the adult stage (which is much more mobile than the caterpillar), though unlike some butterfly species, P. icarus is not migratory.
Semelparity versus Iteroparity
Production of offspring is a critical component of the life cycle of all organisms. Not only does reproduction contribute to the perpetuation of the species, but also the relative offspring production of individuals within a species has serious consequences for evolution and natural selection. It may be surprising, therefore, that some species reproduce only once before death, a trait called semelparity. Other species are iteroparous, meaning they reproduce many times before they die. Note that an individual of an iteroparous species who reproduces only once and then dies by accident would not be considered semelparous; instead, semelparity is a trait by which all or the vast majority of individuals invest all of their resources in a single reproductive event, which results in their death.
One example of a semelparous species is the 17-year periodical cicada, Magicicada septendecim, shown in Figure 6.1.2. This species spends 17 years in underground burrows (Fig. 6.1.2A), before emerging as a sexually immature juvenile stage called a nymph (Fig. 6.1.2B). Within days, the nymphs will molt into sexually mature adult cicadas (Fig. 6.1.2C) and mate (Fig. 6.1.2D). After mating, the female lays her eggs on tree branches; in Fig. 6.1.2E, each slot on the tree branch behind the female contains a single egg. The adult stage of the life cycle lasts only a few weeks; after mating and egg laying, the adults die. The eggs the females have laid will hatch after about 6-10 weeks, and the resulting nymph (Fig. 6.1.2F) will migrate out of the tree and burrow into the ground, where it will grow and develop, re-emerging 17 years later.
Under what conditions would only one reproductive event be beneficial? If high numbers of offspring are important for the survival of the species and for an individual’s evolutionary fitness (both of which are true), then wouldn’t it always be beneficial to reproduce many times? In what cases would it actually increase the individual’s fitness (total number of offspring produced during its life) to invest all its resources in the first reproductive event and save none for its own survival or growth? We can answer these questions by studying species that exhibit semelparity.
Research on semelparous species has shown that individuals produce far more offspring per reproductive event than their iteroparous relatives, presumably due to the investment of all available resources into offspring (rather than saving some resources for survival of the individual). However, the question still remains: how much more offspring must a single reproductive event produce in order to outweigh the additional offspring gained from later reproductive events? The answer lies in the likelihood of later reproductive events. Semelparity becomes beneficial if the probability of an individual surviving long enough to reproduce again is very low. Consequently, in harsh, unstable, or unpredictable environments, semelparity ensures the highest number of offspring out of the only reproductive event the individual is likely to experience. In environments that are more stable, predictable, and conducive to survival, iteroparity is likely to result in the highest number of offspring.