UI Ecología Terrestre, Centro Nacional Patagónico, Consejo Nacional de Investigaciones Científicas y Técnicas, Puerto Madryn, Argentina.
UI Ecología Terrestre, Centro Nacional Patagónico, Consejo Nacional de Investigaciones Científicas y Técnicas, Puerto Madryn, Argentina ; Patagonian and Andean Steppe Program, Wildlife Conservation Society, CABA, Argentina.
PLoS One. 2014 Feb 19;9(2):e89060. doi: 10.1371/journal.pone.0089060. eCollection 2014.
For large herbivores, predation-risk, habitat structure and population density are often reported as major determinants of group size variation within and between species. However, whether the underlying causes of these relationships imply an ecological adaptation or are the result of a purely mechanistic process in which fusion and fragmentation events only depend on the rate of group meeting, is still under debate. The aim of this study was to model guanaco family and bachelor group sizes in contrasting ecological settings in order to test hypotheses regarding the adaptive significance of group-size variation. We surveyed guanaco group sizes within three wildlife reserves located in eastern Patagonia where guanacos occupy a mosaic of grasslands and shrublands. Two of these reserves have been free from predators for decades while in the third, pumas often prey on guanacos. All locations have experienced important changes in guanaco abundance throughout the study offering the opportunity to test for density effects. We found that bachelor group size increased with increasing density, as expected by the mechanistic approach, but was independent of habitat structure or predation risk. In contrast, the smaller and territorial family groups were larger in the predator-exposed than in the predator-free locations, and were larger in open grasslands than in shrublands. However, the influence of population density on these social units was very weak. Therefore, family group data supported the adaptive significance of group-size variation but did not support the mechanistic idea. Yet, the magnitude of the effects was small and between-population variation in family group size after controlling for habitat and predation was negligible, suggesting that plasticity of these social units is considerably low. Our results showed that different social units might respond differentially to local ecological conditions, supporting two contrasting hypotheses in a single species, and highlight the importance of taking into account the proximate interests and constraints to which group members may be exposed to when deriving predictions about group-size variation.
对于大型食草动物而言，捕食风险、栖息地结构和种群密度通常被认为是物种内和物种间群体大小变化的主要决定因素。然而，这些关系的潜在原因是否意味着生态适应，或者仅仅是一个机械过程的结果，其中融合和分裂事件仅取决于群体相遇的速度，仍存在争议。本研究的目的是在对比生态环境下对羊驼家庭和单身群体的大小进行建模，以检验有关群体大小变化的适应意义的假设。我们在巴塔哥尼亚东部的三个野生动物保护区内对羊驼群体的大小进行了调查，这些保护区内的羊驼栖息在草地和灌木丛的镶嵌栖息地中。其中两个保护区已经有几十年没有捕食者了，而在第三个保护区中，美洲狮经常捕食羊驼。在整个研究过程中，所有地点的羊驼数量都发生了重要变化，为检验密度效应提供了机会。我们发现，单身群体的大小随着密度的增加而增加，这与机械方法的预期一致，但与栖息地结构或捕食风险无关。相比之下，在有捕食者的地方，较小的和有领地的家庭群体比在没有捕食者的地方更大，而且在开阔的草原上比在灌木丛中更大。然而，这些社会单位对种群密度的影响非常微弱。因此，家庭群体的数据支持了群体大小变化的适应意义，但不支持机械理论。然而，这些效应的幅度很小，在控制了栖息地和捕食因素后，不同种群之间的家庭群体大小的差异可以忽略不计，这表明这些社会单位的可塑性相当低。我们的研究结果表明，不同的社会单位可能对当地生态条件有不同的反应，这支持了单一物种中两种相互矛盾的假设，并强调了在得出关于群体大小变化的预测时，考虑群体成员可能面临的直接利益和约束的重要性。
