Data_Sheet_1_How Bats Escape the Competitive Exclusion Principle—Seasonal Shift From Intraspecific to Interspecific Competition Drives Space Use in a Bat Ensemble.DOCX

Predators that depend on patchily distributed prey face the problem of finding food patches where they can successfully compete for prey. While the competitive exclusion principle suggests that species can only coexist if their ecological niches show considerable differences, newer theory proposes that local coexistence can be facilitated by so-called stabilizing and equalizing mechanisms. A prerequisite to identify such mechanisms is the understanding of the strength and the nature of competition (i.e., interference or exploitation). We studied the interaction between two open-space foraging bats by testing if common noctule bats Nyctalus noctula shift their space use in response to simulated aggregations of conspecifics or heterospecific Pipistrellus nathusii. When confronted with playbacks of heterospecifics, N. noctula increased their activity in early summer, but decreased activity in late summer. This pattern was accompanied by a decrease in the proportion of large insects in late summer, suggesting a more intense competition for food in late compared to early summer. When confronted with playbacks of conspecifics, N. noctula did not change their activity, irrespective of season. Our results indicate that in early summer, intraspecific competition is more severe than interspecific competition for insectivorous bats. Likely, conspecifics engage in interference competition for flight space, and may suffer from reduced prey detectability as echolocation calls of conspecifics interfere with each other. During insect rich times, interspecific competition on the other hand may be mediated by fine scale vertical partitioning and the use non-interfering echolocation frequencies. In contrast, when food is scarce in late summer, bats may engage in exploitation competition. Our data suggests that N. noctula avoid aggregations of more agile bats like P. nathusii, probably due to impeded hunting success. Yet, as fast and efficient fliers, N. noctula may be able to escape this disadvantage by exploiting more distant foraging patches.

依赖斑块状分布猎物的捕食者，往往面临着寻觅可成功竞争获取猎物的食物斑块的难题。竞争排斥原理（competitive exclusion principle）指出，物种仅当其生态位（ecological niches）存在显著差异时才能实现共存；而新近的理论则提出，所谓的稳定化机制与均等化机制可促进局域物种共存。明确这类共存机制的前提，是厘清种间竞争的强度与本质——即干扰竞争（interference competition）或是资源利用竞争（exploitation competition）。
我们以两种开阔空间觅食蝙蝠为研究对象，通过实验测试普通夜蝠（Nyctalus noctula）是否会因同种或异种的 Nathusius 伏翼蝠（Pipistrellus nathulii）的模拟聚集行为，调整其空间利用模式。实验结果显示：当暴露于异种蝙蝠的回放声时，普通夜蝠在夏初的活动量有所提升，但在夏末的活动量则出现下降。该活动模式变化伴随夏末大型昆虫占比的降低，这表明相较于夏初，夏末的食物竞争更为激烈。
当暴露于同种蝙蝠的回放声时，无论季节如何，普通夜蝠的活动量均未发生显著改变。我们的研究结果表明，在夏初，食虫蝙蝠面临的种内竞争要比种间竞争更为激烈：同种个体可能通过干扰竞争争夺飞行空间，且同种个体的回声定位声波会互相干扰，进而降低猎物探测效率。
而在食物资源丰富的时期，种间竞争可能通过精细的垂直空间分区以及使用互不干扰的回声定位频率得以缓解。与之相反，当夏末食物匮乏时，蝙蝠可能会转向资源利用竞争。我们的数据表明，普通夜蝠会回避 Nathusius 伏翼蝠这类更为灵活的蝙蝠的聚集区域，这可能是因为其狩猎成功率会受到影响。不过，作为快速高效的飞行者，普通夜蝠或许可以通过探索更远的觅食斑块来规避这一劣势。