Ecological drivers of group size variation in sika deer: habitat structure, population density, or both?
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ORIGINAL ARTICLE
Ecological drivers of group size variation in sika deer: habitat structure, population density, or both? Hayato Takada1 · Akane Washida1 Received: 1 March 2020 / Accepted: 20 May 2020 © Deutsche Gesellschaft für Säugetierkunde 2020
Abstract Habitat structure and population density have long been considered key factors in determining intraspecific variation in the group size of social ungulates. However, the effect of interaction between habitat structure and population density on group size is poorly understood, and whether the underlying causes of these relationships are biological adaptation, emergent properties, or both is still debated. Therefore, over 10 years, we examined group size of the sika deer (Cervus nippon) at different population densities and habitat types (open habitats vs closed forests). We found that deer group size was larger in open habitats than closed forests and increased with population density, and population density and habitat type had a synergistic effect on group size. There was greater density effect in open habitats, while the density effect was very weak in closed forests. Our findings in open habitat support emergent properties, because both population density and visibility enhanced the frequency of group fusion. Alternatively, our findings in closed forests support biological adaptation; the scarce food supply in closed forests may increase the cost of conspecific feeding competition, which consequently limits group size even under high densities. We suggest that both emergent properties and biological adaptation determine group size variability of sika deer, and emergent properties may be the mechanism that contributes to group size increase, whereas biological adaptation may be the mechanism that contributes to group size restriction. Keywords Group size · Cervus nippon · Habitat structure · Population density · Fusion–fission society · Interactive effect
Introduction Ungulate species exhibit inter- and intraspecific variation in social group size (e.g., Lott 1991; Brashares et al. 2000), and ecologists have long sought to identify ecological drivers and determinant mechanisms of social group size. Habitat structure and population density have long been considered key factors in determining intraspecific variation in group size (Gerard et al. 2002), and many studies on social ungulates have shown that group size tends to increase with habitat openness (e.g., Walther 1972; Geist 1974; Jarman 1974; Geist 1977; Hirth 1977; Leuthold 1977; Barrette 1991) and population density (e.g., Wirtz and Lörscher 1983; Lovari Handling editor: Luca Corlatti. * Hayato Takada [email protected] 1
Mount Fuji Research Institute Yamanashi Prefecture Government, 5597‑1, Kenmarubi, Kamiyoshida, Fujiyoshida, Yamanashi 403‑0005, Japan
and Cosentino 1986; Lawes and Nanni 1993; Gerard et al. 1993, 1995). However, the effect of interaction between population density and habitat structure on group size is still poorly understood, although this information is benef
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