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

1. Management of the effects of stress on populations - for instance in eco toxicology - requires understanding of the effects of stressors on populati ons and communities. Attention to date has too rarely been directed to rele vant 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 popula tion effects that may occur through interactions between individuals. Here we introduce an approach that allows direct measurement of the effects of s tressors on carrying capacity and density dependence. 2. Using the marine copepod Tisbe battagliai Volkmann-Rocco, we report repl icated experiments establishing the effects of 100 mu g L-1 pentachlorophen ol (PCP) in combination with varying diet and food concentrations. Populati on density was measured as population biomass in 10 mL volumes. Diet was ei ther the alga Isochrysis galbana Parke (here designated 'poor diet') or a m ix of two algal species (I. galbana and Rhodomonas reticulata Novarino: 'go od diet'). Each was given at three food concentrations (520, 1300 and 3250 mu gC L-1), selected on the basis that at low population density these cove r the range between limited and maximal population growth. 3. Carrying capacity increased linearly with food concentration. On the poo r diet the increase was 1.2 mu g L-1 for each mu gC L-1 increase in food co ncentration. On the good diet the increase was 2.3 mu g L-1/mu gC L-1 in th e absence of PCP, and 1.9 mu g L-1/mu gC L-1 with PCP. Maximum carrying cap acity was in the region of 60-80 mu g per 10 mL volume. Population growth r ate (pgr) decreased linearly with population biomass when the latter was pl otted on a logarithmic scale. Increasing biomass reduced pgr by 1.70 week(- 1) for each unit increase in log(10) 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 e xcept that at some higher food concentrations non-PCP populations initially increased faster than PCP populations, and at high concentration on the go od diet the effect of density-dependence was decreased in PCP populations. 4. The results show that a stressor's effects at high population density ma y differ from its effects at low density, and emphasizes the importance of finding new protocols, such as those introduced here, with which to study t he joint effects of a stressor and population density. Managers and researc hers 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.