Why Do Species Co-Occur? A Test of Alternative Hypotheses Describing Abiotic Differences in Sympatry versus Allopatry Using Spadefoot Toads

Areas of co-occurrence between two species (sympatry) are often thought to arise in regions where abiotic conditions are conducive to both species and are therefore intermediate between regions where either species occurs alone (allopatry). Depending on historical factors or interactions between species, however, sympatry might not differ from allopatry, or, alternatively, sympatry might actually be more extreme in abiotic conditions relative to allopatry. Here, we evaluate these three hypothesized patterns for how sympatry compares to allopatry in abiotic conditions. We use two species of congeneric spadefoot toads, Spea multiplicata and S. bombifrons, as our study system. To test these hypotheses, we created ecological niche models (specifically using Maxent) for both species to create a map of the joint probability of occurrence of both species. Using the results of these models, we identified three types of locations: two where either species was predicted to occur alone (i.e., allopatry for S. multiplicata and allopatry for S. bombifrons) and one where both species were predicted to co-occur (i.e., sympatry). We then compared the abiotic environment between these three location types and found that sympatry was significantly hotter and drier than the allopatric regions. Thus, sympatry was not intermediate between the alternative allopatric sites. Instead, sympatry occurred at one extreme of the conditions occupied by both species. We hypothesize that biotic interactions in these extreme environments facilitate co-occurrence. Specifically, hybridization between S. bombifrons females and S. multiplicata males may facilitate co-occurrence by decreasing development time of tadpoles. Additionally, the presence of alternative food resources in more extreme conditions may preclude competitive exclusion of one species by the other. This work has implications for predicting how interacting species will respond to climate change, because species interactions may facilitate survival in extreme habitats.

两种生物的同域分布区（sympatry）通常被认为形成于非生物条件适宜二者生存的区域，其环境条件介于仅存在其中任一物种的异域分布（allopatry）区域的环境条件之间。然而受历史因素或物种间相互作用的影响，同域分布区的环境条件可能与异域分布区并无差异，抑或反而相较于异域分布区更为极端。本研究针对非生物条件下同域分布与异域分布的三种假说模式展开评估。我们以锄足蟾属（Spea）的两个物种——多突变锄足蟾（Spea multiplicata）与邦氏锄足蟾（S. bombifrons）——作为研究对象。为验证上述假说，我们针对两个物种分别构建了生态位模型（ecological niche models，具体采用最大熵模型Maxent），进而生成二者联合出现概率的空间分布图。基于模型结果，我们划定了三类区域：两类为单一物种单独出现的区域（即多突变锄足蟾的异域分布区与邦氏锄足蟾的异域分布区），另一类为两个物种共同出现的区域（即同域分布区）。随后我们对这三类区域的非生物环境进行比较分析，结果显示同域分布区的环境显著更为炎热干燥，区别于异域分布区域。由此可见，同域分布区的环境条件并非介于两种异域分布区域之间，而是恰好处于两个物种所占据的环境条件的其中一个极端。我们据此提出假说：这类极端环境中的生物相互作用可促进物种共存。具体而言，邦氏锄足蟾雌性与多突变锄足蟾雄性之间的杂交事件，可通过缩短蝌蚪的发育时长来促进物种共存。此外，极端环境中存在的替代性食物资源，可避免二者之一被另一物种竞争排斥。本研究成果对于预测物种互作如何响应气候变化具有重要启示，因为物种间的相互作用可助力生物在极端生境中存活。