Abstract
While the prevalence of density-dependence is well-established in population ecology, few field studies have investigated its underlying mechanisms and their relative population-level importance. Here, we address these issues, and more specifically, how differences in body-size influence population regulation. For this purpose, two experiments were performed in a small coastal stream on the Swedish west coast, using juvenile brown trout (Salmo trutta) as a study species. We manipulated densities of large and small individuals, and observed effects on survival, migration, condition and individual growth rate in a target group of intermediate-sized individuals. The generality of the response was investigated by reducing population densities below and increasing above the natural levels (removing and adding large and small individuals). Reducing the density (relaxing the intensity of competition) had no influence on the response variables, suggesting that stream productivity was not a limiting factor at natural population density. Addition of large individuals resulted in a negative density-dependent response, while no effect was detected when adding small individuals or when maintaining the natural population structure. We found that the density-dependent response was revealed as reduced growth rate rather than increased mortality and movement, an effect that may arise from exclusion to suboptimal habitats or increased stress levels among inferior individuals. Our findings confirm the notion of interference competition as the primary mode of competition in juvenile salmonids, and also show that the feedback-mechanisms of density-dependence are primarily acting when increasing densities above their natural levels.
Highlights
Negative density-dependence, through reduced survival and/or fecundity, is a well-established process for hampering population growth as resources become limited, the underlying mechanisms and their relative importance are less well known [1]
While growth rate was different between blocks, no effects of initial body-size, removed biomass, or treatment were detected (Fig. 2; Table 3)
We tested for differential effects of adding and removing large and small individuals on a group of intermediate-sized individuals
Summary
Negative density-dependence, through reduced survival and/or fecundity, is a well-established process for hampering population growth as resources become limited, the underlying mechanisms and their relative importance are less well known [1]. Among juvenile stream-living salmonids, interference through territoriality and dominance hierarchies is generally assumed to be the most prevalent mode of competition with strong densitydependent effects on mortality and migration, especially noticeable at the early stages after emergence [9]. If exploitation and interference competition co-operate as density-dependent mechanisms in populations of stream-living salmonids, small individuals may have a competitive advantage under certain conditions and alter the resource acquisition of the remaining members ( larger individuals) of the population [14]. While individuals with a large body-size (in relation to the rest of the population) are likely to successfully defend a resource against smaller individuals at low density (i.e. through interference competition), such a pattern may shift to indirect exploitation competition, to the benefit of smallsized individuals, as the competitor density increases and/or as the size-difference between competitors diminishes
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