Food Quality Effects On Life History Trait Correlations In Daphnia
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Life history theory assumes that correlations among fitness-related life history traits should be negative among individuals, reflecting resource allocation constraints among traits such as growth and reproduction. These traits trade off because they cannot be simultaneously maximized in individuals facing finite resource acquisition. Positive correlations among traits that are expected to be negative (e.g. between energetically costly traits) have been regularly observed, however, and have usually been ascribed to genetic or resource acquisition differences. In the freshwater zooplankton Cladoceran Daphnia, positive correlations have been particularly well documented even when genetic and environmental variation have been controlled. The sign of these correlations represent a problem for life history theory, which is underpinned by the notion of universal and unavoidable costs associated with investing in a given trait. It has been suggested, however, that costs vary with environment and thus can change the sign of a correlation. A change in correlation sign over an environmental gradient may indicate that the assumed universality of costs and constraints are not as universal as expected by life history theory. Few life history experiments have examined traits in multiple environments, and fewer have done so while controlling resource acquisition and genetic variation. Here I ask whether the positive genetic correlations among somatic growth rate, egg production rate, and longevity are present in the face of equal ingestion, clonal individuals, and a finely resolved gradient of food quality, an environmental factor that frequently affects Daphnia. I partition trait covariation into genetic and residual, or environmental, sources. All resulting genetic correlations were non-significant. Residual correlations trended from significantly positive to negative between longevity and growth rate and remained near zero between longevity and egg rate. The residual correlation between growth and reproduction, two expensive traits, displayed significantly positive residual correlations across the food quality gradient remained significantly positive. Given the experimental controls in place, I suggest that differences in individual ontogeny may give rise to differential resource utilization or assimilation efficiency. This may be a mechanism for differential resource acquisition in the absence of ingestion variation. Additionally, the different ways genetic and residual correlations change over an environmental gradient may provide insight on one manner by which genotypes might coexist and provide a candidate explanation for the second paradox of the plankton.