Single trait optimization reproductive lifespan

Some organisms display 'big bang' reproduction, after which they die (semelparous), while others reproduce more consistently over a long period of time (iteroparous) (Figure 4.1). The question of whether to be semelparous or iteroparous is a rather simple one because there are only two strategies to consider. We simply have to calculate, for any given organism, which should lead to the highest fitness. Charnov and Schaffer (1973) produced a simple calculation to help, which is derived by asking which strategy produces the highest rate of population growth (their fitness currency). They assumed a plant could display either an annual or a perennial strategy. The fitness of the annual is determined by its own survival to maturity and fecundity once mature. However, that of a perennial is determined by both its juvenile survival, annual fecundity, and adult survival from year-to-year. These are the constraints. If annuals are to be more fit than perennials, annuals must have higher annual fecundities than perennials, because they do not survive for more than one flowering season. In contrast, high adult survival should select for perennials because this allows them to increase population growth rate, the fitness currency.

The data provide confirmation of these predictions. Annual plants generally have higher annual fecundities than their perennial close relatives (generally between 1.5 and 5 times) (Young 1990). Such fecundity differences are most apparent between close relatives inhabiting adjacent environments. On Mount Kenya in Africa, for example, are two species of Lobelia, one semelparous and the other iteroparous (Figure 4.3). The semelparous forms have high fecundity and grow on dry rocky slopes, where adult mortality is very high. Iteroparous forms have lower annual fecundity and grow on moister valley bottoms where adult survival is higher (Young 1990).

The development of simple (classical) models like the above has been repeated for all of the common traits under investigation. But a 'single trait' view of the world is also somewhat limiting, for each organism represents a specific and sometimes characteristic combination of traits. To understand the organism as a whole we would have to apply and test theory for each of the life history traits separately. This is clearly an impractical task on a large scale. Instead, we need an approach that considers, in a single framework, the whole life history of an organism. That has been the recent challenge in life history evolution, and it has led to great things.

Fig. 4.3 Lobelias growing on the slopes of Mount Kenya, Africa. The two tree-like plants are giant groundsels, Senecio keniodendron. The tall feathery spikes are Lobelia telekii, a semelparous species, about 2 m tall. The prickly plant in the right foreground with the shorter flower spike is another species, Lobelia deckenii ssp.keniensis. This is iteroparous and tends to dominate in moist valley bottoms (see distance) where adult mortality is low. Photo courtesy of Truman Young.

Fig. 4.3 Lobelias growing on the slopes of Mount Kenya, Africa. The two tree-like plants are giant groundsels, Senecio keniodendron. The tall feathery spikes are Lobelia telekii, a semelparous species, about 2 m tall. The prickly plant in the right foreground with the shorter flower spike is another species, Lobelia deckenii ssp.keniensis. This is iteroparous and tends to dominate in moist valley bottoms (see distance) where adult mortality is low. Photo courtesy of Truman Young.

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