Do correlated responses to multiple environmental changes exacerbate or mitigate species loss?

Biological communities face multiple global changes simultaneously, and predicting how they will respond remains a key challenge. Co-tolerance theory offers a framework for understanding how species-level responses to multiple stressors affect community properties. Co-tolerance theory predicts that positive correlations in species responses (i.e., species that are susceptible to one stressor are more likely to be highly susceptible to a second) lessen total species loss, essentially because species cannot be eliminated from a community twice. However, it is unclear whether several of the tenets of co-tolerance theory describe real-world communities, and what consequences result from such deviations. Here, we use an empirical dataset of bird community response to land-use change over a climate gradient to examine co-tolerance theory’s tenet that environmental changes only harm species (not benefit them). We show that this tenet is not met, and then use simulations to examine how predictions of total species richness and community intactness vary when multiple environmental changes both harm and benefit particular species in the community. Finally, we conduct a sensitivity analysis, examining how the average species response to environmental change, as well as the variance among species, can further alter predictions. Overall, we find that predictions of co-tolerance theory can break down when communities contain species that benefit from some environmental changes. As a result, the presence of multiple environmental changes can either compound or mitigate species loss when species’ responses are positively correlated, preventing a one-size-fits-all statement regarding the effects of correlated responses. This finding highlights the need to carefully consider the underlying mechanisms of community change when making policy assessments regarding the consequences of correlations of species responses to environmental impacts.

生物群落同时面临多重全球环境变化，预测其响应规律仍是学界亟待攻克的关键难题。协同耐受理论（Co-tolerance theory）为解析物种对多重胁迫因子的响应如何作用于群落属性提供了理论框架。该理论预测，物种响应间的正相关关系（即对某一胁迫因子敏感的物种，更易对第二种胁迫因子同样表现出高敏感性）会降低总物种损失量，其本质在于一个物种无法在群落中被彻底清除两次。但目前仍不清楚协同耐受理论的若干核心假设是否适用于真实群落，以及此类理论与现实的偏离会带来何种生态后果。本研究依托鸟类群落在气候梯度下对土地利用变化响应的实证数据集（empirical dataset），检验了协同耐受理论的一项核心假设：环境变化仅会对物种造成伤害（而非带来益处）。研究发现该假设并不成立，随后通过模拟实验，探究了当多重环境变化同时对群落内特定物种产生伤害与增益效应时，总物种丰富度与群落完整性的预测结果会如何变化。最后，本研究开展了敏感性分析，考察物种对环境变化的平均响应强度以及物种间的响应方差，如何进一步改变相关预测结果。总体而言，研究发现当群落中存在能从某些环境变化中获益的物种时，协同耐受理论的预测可能会失效。因此，当物种响应呈正相关时，多重环境变化的存在可能加剧或缓解物种损失，无法用单一普适性结论概括这类相关响应的生态影响。这一发现凸显了在制定针对物种对环境影响的响应相关性后果的政策评估时，需审慎考量群落变化的潜在机制。