Density Dependence in the Life-Cycles of Animals and its Importance in K- and R-Strategies

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Abstract
A survey of density dependent mortalities was made using ''b'', the slope of the k-value log density plot of Varley and Gradwell. The animals were divided according to whether they lived in more-temporary or more-permanent habitats and this was related to the magnitude and distribution of the b values for their density dependent mortalities. Whereas 86% of the density dependent mortality in the life cycle acted on the young stages for temporary habitat animals (r-stategists), the comparable figure for permanent habitat animals (K-strategists) was only 15%. Instead, parasitism and predation (30%) and reduced fecundity (35%) occurred more frequently in these animals. The relationship between the type of mortality and the size of b was also discussed. Whereas parasitism, predation and reduced fecundity gave b values that were not on the whole over-compensating, intraspecific effects for temporary habitat animals were mainly over-compensating. They tended towards scramble competition. Where permanent habitat animals showed intraspecific effects, b was 1.2 or less. The overall pattern was for permanent habitat animals to have exactly- or under-compensating mortalities administered in the least wasteful manner possible, whereas temporary habitat animals had very small, under-compensating mortalities at low population densities changing fairly sharply to over-compensating mortalities as population density rose. This allowed them to make maximum use of resources for the limited time that they were available. This difference in strategy could also be seen in the differences in reproductive rate between the 2 groups.

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