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Comparative study of the life history tactics of two species of marine intertidal isopods Buskey, Edward Joseph

Abstract

Advocates of the theory of r-K selection claim that high levels of density-independent mortality select for a high intrinsic rate of natural increase (r- strategist), while density-dependent mortality selects for greater competitive ability and a higher carrying capacity (K- strategist). Furthermore, environmental stability is thought to affect the amount of density-independent or density-dependent mortality that a species encounters with unpredictable and highly variable environments leading primarily to density-independent mortality, and more predictable, less variable environments leading primarily to density-dependent mortality. Species can be positioned on an r-K continuum on the basis of several life history characteristics which are correlated to the intrinsic rate of natural increase. Thus it is suggested that selection should favour rapid development, single reproduction, early maturity, smaller body size, and higher birth rate for r- strategists, and slower development, repeated reproduction, later maturity, larger body size and lower birth rates for K-strategists. I compared two species of isopods from the upper and lower intertidal zone for the reproductive parameters correlated with high intrinsic rate of increase. I considered the upper intertidal environment to be more variable and unpredictable because of the increased time of exposure to air, and the resulting changes in temperature and humidity, compared to the more stable conditions which exist when submerged. Thus the species from the upper intertidal, Idotea wosnesenskii, was expected to have life history characteristics which were r- selected with respect to those of Idotea montereyensis, from the lower intertidal. I found the differences in two of the correlates to be in the direction predicted by r- and K- selection, two other correlates to be in the opposite direction, and a fifth not to differ between the two species. By ranking these parameters with respect to their influence on the intrinsic rate of population increase, I found the two most important parameters, age of first reproduction and birth rate, to support the predictions of r-K theory, and I concluded that I. wosnesenskii was an r- strategist relative to I. montereyensis. Reproductive effort, the proportion of food energy resources allocated to reproduction, was also used as a measure of a species' position on the r-K continuum. Three different measures of reproductive effort supported the prediction of r-K theory, and showed that I. wosnesenskii had a higher reproductive effort that did I. montereyensis.

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