12 - Life cycles of some Mediterranean invasive plants

Summary

In this chapter we will consider some life history traits, such as longevity and fecundity, dispersal in space and in time, and the type of reproductive system (inbreeding or outbreeding). Some of the factors which may act on these traits, as reviewed by Stearns (1976, 1977) and Charlesworth (1980), can be summarised as:

1. Perennial types (polycarpic or monocarpic) (Hart, 1977) are to be favoured if, because of environmental conditions, juvenile survival is low compared to adult survival (Michod, 1979), or alternatively, if the environmental conditions are less predictable during the juvenile stage than later in the life cycle (Murphy, 1968).

2. Perennial genotypes are also favoured in species in which population sizes are usually stable or decreasing, whilst annual types are favoured in species in which populations are increasing most of the time. As pointed out by Caswell (1982), however, no population is always increasing or stable, so that the result for a given genotype depends on the relative importance of phases of increase and decrease.

3. High dispersal rates are selected against within each population and selected for at the establishment of new populations. Van Valen (1971) and others (e.g. Slatkin & Wade, 1979; Olivieri & Gouyon, 1985) emphasised the importance of the rate of extinction on the evolutionary stable dispersal rate. We shall see subsequently that there is another important environmental factor which should also be considered.