Does mating success determine population growth rate at low abundance in marine copepods?

San Francisco State University, San Francisco CA

Investigators

Abstract

Successful mating is a prerequisite for population growth in organisms that reproduce sexually. Unlike other determinants of population growth, mating success is inversely related to population density, resulting in an inverse density-dependent relationship between rate of increase and local population density. The resulting feedback should prevent dilute populations from persisting or growing; yet, they do persist. This project will examine the potential for poor mating success to suppress rate of increase of copepod populations at low density. This study will build on previous studies of mating in copepods that emphasized behavior and physiology; however, the focus of this research will be on consequences of behavior. That is, a series of experiments will support a model of the probability of successful mating as a function of local population density of adults. Copepods from an estuarine environment will be used under the expectation that they may be adapted to high density and therefore most likely to show effects at low density. The mating success probability model describes encounter rate, as in models describing feeding (Gerritsen 1980), together with simplified population dynamics. Although behavioral studies have determined volume search rates for mating, the conditional probability of actually mating upon encounter has not been determined. Model parameters include the product of the volume search rate and this conditional probability; this product, rather than its components, will be determined from experimental results. The experimental work will examine the frequency of mating in unmated copepods confined together under various conditions of density, sex ratio, container size, and food concentration. These experiments will be conducted using copepods of various species with a range of swimming and searching behaviors in an attempt to determine how these behaviors result in different mating frequencies. Preliminary and ancillary experiments will test for experimental artifacts and issues such as time for males to regenerate sperm after mating. Environmental determinants of copepod sex ratio, a key parameter in mating probability, will also be examined with an emphasis on the potential role of dietary lipids. Intellectual merit: Zooplankton occupy a key role in oceanic foodwebs, and their population dynamics has been the subject of considerable research. Among determinants of population growth rate, mating success has received relatively little study, and most of that has been on behavior and anatomical structure. Yet, mating success should be a key determinant of recovery of populations from periods of low abundance, as well as establishment of new populations from initial seeds. This research builds on previous work to include all components of mating (Buskey 1998) rather than only encounter rate. Broader impacts: The results of this research will have impacts in two areas important to society. First, invasive species, a major concern in coastal waters, must establish populations from initially small inocula, which are therefore subject to the positive feedback addressed here. Second, these results may be transferable, at least in principle, to the study of endangered and rare species of other taxonomic groups. The proposed research will also contribute to education by providing an opportunity for a Master's thesis at San Francisco State University, a minority-serving institution.