Sensitivity of fitness to macroparameter changes: an analysis of survivorship and individual growth in sea urchin life histories.

An analysis for an age-structured demography was conducted to explore the sensitivity of fitness to individual parameters of a growth equation (Brody-Bertalanffy) and a survivorship function (Weibull). Initial parameters were selected to simulate populations of sea urchins and include northern and southern populations of the temperate sea urchin Strongylocentrotus purpuratus and tropical species such as Diadema antillarum. Results of the analysis are intuitively reasonable. Regardless of whether a population is increasing or decreasing, there would always be an increase in fitness associated with increases of: 1) the growth-rate constant, 2) maximum gonad size, 3) maximum lifespan, 4) the shape parameter of the Weibull function; and, decreases of 1) the age at first reproduction and 2) the ratio of number of recruits/spawn weight. If trade-offs are possible then improved survival characteristics and increased lifespan would be favored in declining populations. In general, relative sensitivity is dependent upon which parameter is varied in the simulations. The analysis shows which changes of parameters would bring about the greatest change in fitness but can not show trade-offs or design constraints. An example of trade-off in urchin life history is the relationship between the growth-rate constant, K and maximum lifespan, ω. A doubling of lifespan is associated with a halving of K.