Understanding mechanisms that generate range limits is central to knowing why species are found where they are and how they will respond to environmental change. There is growing awareness that biotic interactions play an important role in generating range limits. However, current theory and data overwhelmingly focus on abiotic drivers and antagonistic interactions. Here we explore the effect that mutualists have on their partner's range limits: the geographic “footprint” of mutualism. This footprint arises from two general processes: modification of a partner's niche through environment-dependent fitness effects and, for a subset of mutualisms, dispersal opportunities that lead suitable habitats to be filled. We developed a conceptual framework that organizes different footprints of mutualism and the underlying mechanisms that shape them, and evaluated supporting empirical evidence from the primary literature. In the available literature, we found that the fitness benefits and dispersal opportunities provided by mutualism can extend species' ranges; conversely, the absence of mutualism can constrain species from otherwise suitable regions of their range. Most studies found that the footprint of mutualism is driven by changes in the frequency of mutualist partners from range core to range edge, whereas fewer found changes in interaction outcomes, the diversity of partners, or varying sensitivities of fitness to the effects of mutualists. We discuss these findings with respect to specialization, dependence, and intimacy of mutualism. Much remains unknown about the geographic footprint of mutualisms, leaving fruitful areas for future work. A particularly important future direction is to explore the role of mutualism during range shifts under global change, including the promotion of shifts at leading edges and persistence at trailing edges.

中文翻译：

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共生的地理足迹：共生者如何影响物种的范围限制

了解产生范围限制的机制对于了解为什么在它们所在的位置发现物种以及它们将如何应对环境变化至关重要。人们越来越意识到生物相互作用在产生范围限制方面起着重要作用。然而，目前的理论和数据绝大多数都集中在非生物驱动因素和对抗性相互作用上。在这里，我们探讨了互利共生对其伙伴范围限制的影响：互利共生的地理“足迹”。这种足迹来自两个一般过程：通过依赖于环境的适应性效应改变合作伙伴的生态位，以及对于互利共生的一个子集，分散机会导致合适的栖息地被填充。我们开发了一个概念框架来组织互利共生的不同足迹和塑造它们的潜在机制，并评估了来自主要文献的支持性经验证据。在现有文献中，我们发现互利共生提供的适应性优势和分散机会可以扩大物种的分布范围；相反，互利共生的缺失会限制物种远离其范围内其他合适的区域。大多数研究发现，互利共生的足迹是由互利合作伙伴从范围核心到范围边缘的频率变化驱动的，而很少有人发现互动结果、合作伙伴的多样性或适应性对互利共生影响的不同敏感性发生变化。我们讨论了关于互利共生的专业化、依赖性和亲密性的这些发现。关于互利共生的地理足迹还有很多未知，为未来的工作留下了富有成果的领域。