Detection of density dependence in animal populations is a primary goal of population ecology, and the processes causing density dependence play a major role in population regulation. Predation can strongly regulate populations by populational and behavioral responses of predators to their prey. Here I evaluate the existence and strength of density-dependent mortality in local populations of a reef fish, the kelp perch (Brachyistius frenatus), caused by its predator, the kelp bass (Paralabrax clathratus). Specifically, I examine both the functional response and a potential aggregative response by kelp bass as mechanisms underlying predator-induced density-dependent mortality.I calculated the per capita mortality of kelp perch as a result of the functional response of its predator by subjecting a range of densities of kelp perch to a low, constant number of kelp bass and different amounts of giant kelp (Macrocystis pyrifera) as habitat structure and a potential prey refuge. The potential for an aggregative response by kelp bass was determined by exposing densities of kelp perch to natural levels of kelp bass in the field. Per capita mortality of kelp perch in the laboratory was inversely density dependent to density independent with increasing habitat structure. By contrast, per capita mortality in the field was strongly density dependent, with evidence for an aggregative response by kelp bass. Furthermore, emigration and other nonpredatory losses of kelp perch from field plots were negligible, indicating that the product of the functional and aggregative responses by kelp bass induced density-dependent mortality in kelp perch. My results indicated that increasing habitat structural complexity at low densities of kelp perch, coupled with a strong aggregative response by kelp bass, was responsible for the observed pattern of density-dependent mortality. In addition, the distribution and relative abundances of kelp perch and kelp bass at larger spatial scales (entire reefs) were consistent with predation as an important process structuring local populations of kelp perch. The contrasting patterns of short-term per capita mortality found here underscore the need to evaluate both the functional and aggregative responses of predators to their prey. Ultimately, the pattern of mortality expressed will be determined by the relative strengths of these behavioral responses and the degree to which habitat structure provides an effective prey refuge.
Reproduction is closely tied to environmental conditions and the availability of resources, and thus typically varies with season. Consequently, perennial organisms that reproduce continuously are generally restricted to tropical regions with relatively aseasonal climates. The temperate marine alga Macrocystis pyrifera is a rare exception in this regard, as most individuals reproduce throughout the year in a seasonally variable habitat. Here we measure reproductive responses of the giant kelp Macrocystis during a period in which resources and environmental conditions fluctuated greatly and contrast these responses with those of the palm kelp, Pterygophora californica, a sympatric species that exhibits strictly seasonal reproduction.The quantity and quality of spore production tracked resource availability within and among years for Macrocystis, but not for Pterygophora. Reproductive allocation and spore standing stock in Macrocystis were negatively correlated with seawater temperature and positively correlated with the nitrogen content of adult plants. Macrocystis generally displayed two seasonally distinct peaks in spore production per year (winter and spring). The only disruption of this pattern coincided with a warmwater El Nino event. Although seawater temperature and the nitrogen content of adults were inversely related in Pterygophora, neither variable was significantly correlated with the quantity or quality of spore production in this species. Unlike Macrocystis, Pterygophora exhibited a well-defined reproductive season in which plants displayed a single broad peak in spore production that varied little in timing and magnitude among years, even during El Nino conditions. Spore C/N ratios remained relatively constant over time in both species, despite large seasonal fluctuations in C/N ratios of vegetative tissue of adults plants. Nonetheless, spore C/N ratios were positively correlated with seawater temperature in Macrocystis, but not in Pterygophora. Spore viability (swimming and germination) varied considerably, and often unpredictably, over time for both species.Our results support the general idea that environmental conditions and resources exert a much greater influence on the quantity and quality of reproduction in species that reproduce continuously than on the majority of species that are strictly seasonal in onset of reproduction. The differential responses of Macrocystis and Pterygophora may reflect their different morphologies and life-spans. Macrocystis is relatively short lived and may "hedge its bets" by reproducing continuously rather than risk delaying reproduction. Conversely, since Pterygophora lives much longer, plants can afford to release spores only during times when the chances for reproductive success are predictably greatest because these plants are likely to reproduce again in subsequent years.
Unprecedented population declines and extinctions because of human activities, combined with a growing recognition that such losses affect the stability of ecosystems, underscore the need to better understand how populations persist naturally. We provide field experimental evidence that high biodiversity-in particular, the combined effects of predators and competitors-acts in a way that regulates the size of local fish populations within their coral-reef community. These results indicate that complex interactions among multiple species are necessary for the stability of a highly diverse community, and so forewarn that overexploiting such species may have cascading negative consequences for the entire system. community stability ͉ competition ͉ population dynamics ͉ predation ͉ recruitment
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