Ontogenetic timing of density dependence: location-specific patterns reflect distribution of a limiting resource

Abstract

Theoretical considerations suggest that the relative abundance of age-specific limiting resources determines the ontogenetic timing of density dependence. Structural shelters may represent one such resource which can become increasingly scarce with increasing body size. Here we use a time series of juvenile Atlantic salmon (Salmo salar) densities and ask whether ontogenetic patterns of density-dependent losses in two separate reaches of a river can be predicted by considering their shelter abundances. The analyses were conducted using sampling site data (n = 30) as well as stream-reach averages. Loss rates from the egg to the young-of-the-year stage were density-dependent in both reaches. For the transition from the young-of-the-year to the yearling stage, when shelters are more likely to become limiting, the results were sensitive to the spatial scale of analysis. On the reach scale, among-year variation in loss rates was positively correlated with density in the reach with the lowest shelter abundance, whereas no such effect was found in the other reach. This demonstrates that the ontogenetic timing of density dependence can vary among areas within populations, and hence among populations, and that this variation can be explained by quantification of age-specific limiting factors. For analyses at the sample site scale this pattern was reversed, with stronger density dependence in the reach with highest shelter abundance. However, this result was clearly driven by immigration into low density sites, which masked the true reach-level effect. Thus, our study also exemplifies how population level regulation inferred from patch- or trap-based data that fails to account for animal movements can be biased.