Department of Geography, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America.
PLoS One. 2012;7(3):e32748. doi: 10.1371/journal.pone.0032748. Epub 2012 Mar 30.
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.
两个物种（同域）共存的区域通常被认为是在生物条件有利于两个物种的地区出现的，因此处于单独存在物种的区域（异域）之间的中间位置。然而，取决于历史因素或物种之间的相互作用，同域可能与异域没有区别，或者相反，同域的生物条件可能比异域更加极端。在这里，我们评估了同域与异域生物条件之间存在的三种假设模式。我们使用两种同种的锄足蟾属蟾蜍，即 Spea multiplicata 和 S. bombifrons，作为我们的研究系统。为了检验这些假设，我们为这两个物种创建了生态位模型（特别是使用 Maxent），以创建两个物种共同出现的联合概率图。利用这些模型的结果，我们确定了三种类型的位置：两种预测只有一种物种出现的位置（即 S. multiplicata 的异域和 S. bombifrons 的异域），以及一种预测两种物种共同出现的位置（即同域）。然后，我们比较了这三种位置类型的非生物环境，发现同域的温度和干燥度明显高于异域。因此，同域不是替代异域地点之间的中间位置。相反，同域发生在两个物种所占据的条件的一个极端。我们假设这些极端环境中的生物相互作用促进了共存。具体来说，S. bombifrons 雌性和 S. multiplicata 雄性之间的杂交可能通过减少蝌蚪的发育时间来促进共存。此外，在更极端的条件下存在替代食物资源可能会排除一种物种对另一种物种的竞争排斥。这项工作对预测相互作用的物种如何应对气候变化具有重要意义，因为物种相互作用可能有助于在极端栖息地中生存。
