Population Dynamics and Life History of Two Contrasting Populations of Ligia oceanica (Crustacea: Oniscidea) in the Rocky Supralittoral

Abstract

(1) The population dynamics of Ligia oceanica (L.) at two sites subjected to different degrees of wave exposure were those of stable, equilibrium populations. There were no large-scale variations in any measured population characteristic over 3 years. (2) Individuals were essentially semelparous and minimum age at first reproduction was one year, but there were two distinct breeding groups each summer. It is concluded that ''cohort-splitting'' (whereby two parts of a cohort grow at different rates and breed at different times), observed in other species, may be a consequence of a similar breeding phenology, and that the stability of small populations of semelparous woodlice will be enhanced as a result. (3) Size at first reproduction, weight-specific fecundity, reproductive allocation (an index of reproductive effort) and longevity differed between the two populations. Longevity was higher and fecundity and reproductive allocation lower at the less exposed site. However, these differences were small and the populations were otherwise remarkably similar. These results are discussed in relation to current models of life-history evolution, but although they match the predictions of r-K theory, they do not demonstrate the mechanism of selection. The results also provide more limited support for the alternative life-history theories.