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.

中文翻译：

---

梅花鹿种群规模变化的生态驱动因素：栖息地结构、种群密度，或两者兼而有之？

长期以来，栖息地结构和种群密度一直被认为是决定群居有蹄类动物群体规模种内变异的关键因素。然而，栖息地结构和人口密度之间的相互作用对群体规模的影响知之甚少，这些关系的根本原因是生物适应、涌现特性还是两者兼而有之，仍存在争议。因此，10 多年来，我们检查了不同种群密度和栖息地类型（开放栖息地与封闭森林）下梅花鹿 (Cervus nippon) 的群体规模。我们发现开放栖息地的鹿群规模大于封闭森林，并随着种群密度的增加而增加，种群密度和栖息地类型对种群规模有协同作用。开放栖息地的密度效应更大，而密林中的密度效应非常弱。我们在开放栖息地的发现支持紧急属性，因为人口密度和能见度都提高了群体融合的频率。或者，我们在封闭森林中的发现支持生物适应；封闭森林中稀缺的食物供应可能会增加同种饲养竞争的成本，因此即使在高密度下也限制了群体规模。我们认为紧急特性和生物适应共同决定了梅花鹿的种群规模变异性，紧急特性可能是导致种群规模增加的机制，而生物适应可能是导致种群规模受限的机制。因为人口密度和知名度都提高了群体融合的频率。或者，我们在封闭森林中的发现支持生物适应；封闭森林中稀缺的食物供应可能会增加同种饲养竞争的成本，因此即使在高密度下也限制了群体规模。我们认为紧急特性和生物适应共同决定了梅花鹿的种群规模变异性，紧急特性可能是导致种群规模增加的机制，而生物适应可能是导致种群规模受限的机制。因为人口密度和知名度都提高了群体融合的频率。或者，我们在封闭森林中的发现支持生物适应；封闭森林中稀缺的食物供应可能会增加同种饲养竞争的成本，因此即使在高密度下也限制了群体规模。我们认为紧急特性和生物适应共同决定了梅花鹿的种群规模变异性，紧急特性可能是导致种群规模增加的机制，而生物适应可能是导致种群规模受限的机制。因此即使在高密度下也限制了群体规模。我们认为紧急特性和生物适应共同决定了梅花鹿的种群规模变异性，紧急特性可能是导致种群规模增加的机制，而生物适应可能是导致种群规模受限的机制。因此即使在高密度下也限制了群体规模。我们认为紧急特性和生物适应共同决定了梅花鹿的种群规模变异性，紧急特性可能是导致种群规模增加的机制，而生物适应可能是导致种群规模受限的机制。