Abstract
Density dependence can have marked effects on social behaviors such as group size. We tested whether changes in population density of a large herbivore (elk, Cervus canadensis) affected sex-specific group size and whether the response was density- or frequency-dependent. We quantified the probability and strength of changes in group sizes and dispersion as population density changed for each sex. We used group size data from a population of elk in Manitoba, Canada, that was experimentally reduced from 1.20 to 0.67 elk/km2 between 2002 and 2009. Our results indicated that functional responses of group size to population density are sex-specific. Females showed a positive density-dependent response in group size at population densities ≥0.70 elk/km2 and we found evidence for a minimum group size at population density ≤0.70 elk/km2. Changes in male group size were also density-dependent; however, the strength of the relationship was lower than for females. Density dependence in male group size was predominantly a result of fusion of solitary males into larger groups, rather than fusion among existing groups. Our study revealed that density affects group size of a large herbivore differently between males and females, which has important implications for the benefits e.g., alleviating predation risk, and costs of social behaviors e.g., competition for resources and mates, and intra-specific pathogen transmission.
Highlights
Density-dependent processes are fundamental to population ecology [1,2], which have important implications for group size dynamics of social species [3,4,5,6,7,8,9]
Changes in group size as a function of population density have been reported for several social species; sex-specific density effects have rarely been considered, sexual segregation is common among social ungulates [13]
Mean group size increased with density as expected (Table 1, prediction 2); for females this relationship relies on observations taken at the highest density (Text S2, Supplementary Material, Tables S1 and S2)
Summary
Density-dependent processes are fundamental to population ecology [1,2], which have important implications for group size dynamics of social species [3,4,5,6,7,8,9]. Changes in group size as a function of population density (i.e., competition) have been reported for several social species (see [12] for a review); sex-specific density effects have rarely been considered, sexual segregation is common among social ungulates [13]. Whether a social behavior, such as group size, varies with density and whether these changes occur in a non-linear (i.e., density and negative density)- vs linear (i.e., frequency)-dependent fashion could reveal how such costs may affect individuals and populations. A non-linear response suggests that thresholds exist below which certain costs (e.g., disease persistence [20]) may no longer be germane
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