The dynamics and life history of an isolated population
of Ligia oceanica (L. ) inhabiting a sea-wall exposed to
moderate wave action have been studied over a period of
three years. Sex ratio, size at maturity, growth, breeding
phenology, size and age structure, population density and
size, female breeding size, fecundity regression and broodpouch mortality have been determined. From these data
population natality, and schedules of survivorship, fecundity and reproductive value are calculated. Although the supralittoral zone of rocky shores is generally regarded as a severe and stressful habitat, the dynamics of the population exhibit only minor differences between years, and no direct influence of wave exposure can be determined.
This population is compared with another nearby but
isolated population occupying an harbour wall completely
protected from wave action. These populations are presumed
to suffer different levels of density-independent mortality
due to the wave exposure characteristics of the sites. The
sheltered site population shows no significant differences
between the two years of study. Interpopulation differences
in age and size structure, growth rate, population breeding
phenology and breeding sizes are slight. However the sheltered population shows increased adult survival and longevity, decreased weight specific fecundity and reproductive allocation, and increased brood-pouch mortality. Because winter survival is high, when mortality due to wave action is expected to be severe, this factor is rejected as an important determinant of the life history. The reduced survival at the exposed site can be interpreted as a consequence of the increased reproductive allocation by individuals at this site.
The majority of females are semelparous, but within
both populations two distinct waves of breeding activity
occurred each year. Larger, faster growing individuals
breed at the end of their first ýear, LATE in the season,
and have high weight specific fecundities and low absolute
and weight specific egg sizes. Smaller, slower growing
individuals delay reproduction and breed in their second
year, EARLY in the season. Although gaining greatly in size
through growth the effect on fecundity is offset by a decrease in weight specific fecundity and parallel increases in absolute and weight specific egg size.
The following trade-offs are recognised for individual
female Ligia : between growth and reproduction (within
breeding groups), growth and reproduction (between breeding
groups), reproduction and survival (between breeding groups), weight specific egg size and weight specific fecundity (between breeding groups), and between absolute egg size and fecundity (within breeding groups). Two possible life history strategies are recognised. (i) An ability of a female parent to alter egg size seasonally may optimise total offspring fitness in the different environments they experience.(ii) An ability of small, mature females to delay reproduction until the following year may also increase their fitness. By assuming that offspring survival is related to egg size it is possible to model this tactic. The population breeding phenology is seen as a direct consequence of the life history and has the indirect effect of stabilizing the population. The ability of individual females to vary their life history traits depending on individual circumstances (size and season) may represent a flexible approach to the environment which is not envisioned in the more formal models of life history evolution.