Mechanisms of density dependence in juvenile salmonids: prey depletion, interference competition, or energy expenditure?

Abstract

Density dependence is a strong regulator of animal populations, operating primarily through intraspecific competition for a limiting resource. Because food is generally limited in natural environments, it is typically assumed that increasing animal density leads to reduced individual fitness through food depletion or monopolization. However, recent work demonstrates that density dependence can occur without apparent food depletion, or with virtually unlimited resources, suggesting that other mechanisms must also be important. Alternatively, density-dependent regulation could be achieved through increasing energy costs rather than decreasing food abundance. The relative importance of these two processes (food depletion and energy costs) and their underlying mechanisms remains unclear. Here, we manipulated the density of juvenile stream-dwelling brook trout (Salvelinus fontinalis) in three neighboring populations over two consecutive summers to relate food availability and consumption to density-dependent growth, survival, and their trade-off. Despite strong patterns of density dependence and evidence of food limitation, increasing fish density did not lead to detectable prey depletion in the environment. Instead, behavioral observations suggested that increasing densities resulted in higher energetic costs, primarily via increasing interference competition and the use of less suitable foraging microhabitats. These results highlight that animal populations may be regulated by density dependence without necessarily impacting prey communities.

Recruitment dynamics of juvenile salmonids: Comparisons among populations and with classic case studies

“…One possible explanation is that the maximum abundance in the experimental sections is a function of food availability, as is typical of many populations following BH models (Wootton, 2012). This is plausible because bioenergetic models have shown that food is a limiting factor of somatic growth and survival at higher densities in these populations (Matte et al, 2021). In addition, overcompensation in the Ricker model is often driven by predation, reproductive interference, and/or cannibalism by adults (Wootton, 2012), but the present system does not have any notable predators (Hutchings, 1990), and adults were excluded from experimental sections.…”

“…Recruitment models with biomass were improved by incorporating the effects of temperature, but not models with density. This is not surprising, given that temperature is an important driver of the underlying patterns of density dependence in this system (Matte et al, 2020a;Matte et al, 2021). More specifically, increasing temperatures result in stronger density-dependent mortality per unit of growth in these populations, but at different rates among populations (Matte et al, 2020a).…”

“…To quantify S-R relationships, we built upon the data collected during a field experiment previously described by Matte et al (2020aMatte et al ( , 2021 see Table A1 for a summary of experimental and environmental conditions). The experiment was conducted in three neighboring streams (BC, FW, and WN) separated by $5 km during three consecutive summers (June-August; 2016-2018).…”

“…due to geographic proximity (Wood et al, 2014); and (6) these populations exhibit divergent patterns of density‐dependent growth, survival, and their trade‐off, partially due to environmental differences (Matte et al, 2020a). In these streams, density dependence operates primarily via competition for suitable foraging microhabitats rather than prey depletion (Matte et al, 2021).…”

“…Flow was quantified using transects of 5 snout‐velocity (i.e., depth at which the fish forage) measurements upstream of each section ( FH950 flowmeter). Substrate composition, macrophyte cover, depth, and width were quantified using five transects per experimental sections (for more details, see Matte et al, 2020a; Matte et al, 2021). At the end of the experiments, fish were removed from experimental sections using electrofishing to measure size (length, mass) and survival, which were estimated as a site‐level average, given that fish were too frail to tag individually.…”