How environmental stress affects density dependence and carrying capacity in a marine copepod

Abstract

1. Management of the effects of stress on populations - for instance in ecotoxicology - requires understanding of the effects of stressors on populations and communities. Attention to date has too rarely been directed to relevant ecological endpoints, such as carrying capacity and density dependence. Established procedures are instead based on measuring the Life Tables of individual organisms exposed to differing concentrations of a pollutant at low population density, but this approach does not take into account population effects that may occur through interactions between individuals. Here we introduce an approach that allows direct measurement of the effects of stressors on carrying capacity and density dependence. 2. Using the marine copepod Tisbe battagliai Volkmann-Rocco, we report replicated experiments establishing the effects of 100 μg L-1 pentachlorophenol (PCP) in combination with varying diet and food concentrations. Population density was measured as population biomass in 10 mL volumes. Diet was either the alga Isochrysis galbana Parke (here designated 'poor diet') or a mix of two algal species (I. galbana and Rhodomonas reticulata Novarino: 'good diet'). Each was given at three food concentrations (520, 1300 and 3250 μgC L-1), selected on the basis that at low population density these cover the range between limited and maximal population growth. 3. Carrying capacity increased linearly with food concentration. On the poor diet the increase was 1·2μg L-1 for each μgC L-1 increase in food concentration. On the good diet the increase was 2·3 μg L-1/μgC L-1 in the absence of PCP, and 1·9 μg L-1/μgC L-1 with PCP. Maximum carrying capacity was in the region of 60-80 μg per 10 mL volume. Population growth rate (pgr) decreased linearly with population biomass when the latter was plotted on a logarithmic scale. Increasing biomass reduced pgr by 1.70 week-1 for each unit increase in log10 biomass. Increasing food concentration and improving diet both increased pgr, but did not affect the slope of the density-dependent relationship. Presence or absence of PCP had no effect except that at some higher food concentrations non-PCP populations initially increased faster than PCP populations, and at high concentration on the good diet the effect of density-dependence was decreased in PCP populations. 4. The results show that a stressor's effects at high population density may differ from its effects at low density, and emphasizes the importance of finding new protocols, such as those introduced here, with which to study the joint effects of a stressor and population density. Managers and researchers of threatened species, harvested species and pest species need to know the joint effects of stressors and population density, in order to be able to predict the effects of stressors on carrying capacity and on the course of recovery from environmental perturbations.