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

Abstract

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.

Landscape structure influences the use of social information in an insectivorous bat

“…Another, not mutually exclusive explanation for the comparable low feeding buzz rate may be that even distribution of prey items across the forest dominated landscape led to lower prey density at small scales, when compared to insect rich patches above the cropland dominated landscape. Both explanations suggest that common noctules flying above the forested landscape may have mainly competed for prey (Cvikel et al , Roeleke et al ), whereas bats flying above cropland might have been limited by the search for scarce food patches, but not by the competition for single food items within such insect rich patches.…”

“…Indeed, past studies showed that foraging efficiency of bats can be impaired at high densities (Amichai et al ). Bats might thus avoid aggregations of conspecifics (Roeleke et al ) or even engage in agonistic behaviour when competing for prey (Voigt‐Heucke et al , Corcoran and Conner ). Interestingly, an individual's recent feeding activity had no influence on the probability to switch from DM to ARM.…”

“…Thus, a common strategy to detect insect aggregations is the use of inadvertently provided social information (Danchin et al ) via eavesdropping on hunting calls of other bats. These may originate from conspecifics (Gillam , Dechmann et al , Übernickel et al ) or heterospecifics with similar prey preferences (Dorado‐Correa et al , Roeleke et al ).…”

“…These observations suggest the occurrence of facilitation through social hunting (Bruno et al ). However, such a social foraging strategy may only increase foraging effectiveness if resources are patchily distributed (Ryer and Olla , Egert‐Berg et al ) because the aggregation of hunting bats at distinct prey patches may also result in increased competition, either for a limited number of prey items or for undisturbed hunting space (Voigt‐Heucke et al , Corcoran and Conner , Roeleke et al ). Thus, bats face a tradeoff between finding prey and avoiding competition when foraging on patchily distributed prey.…”

Interspecific competition in bats and diet shifts in response to white‐nose syndrome

“…Insectivorous bats Nyctalus noctule (noctule) and Nathusius’ pipistrelle ( Pipistrellus nathusii ; Roeleke et al. ) are among many other species which have been observed to exhibit exploitative competition, including mudsnails (Hybroiidae; Fenchel and Kofoed ); wild dog ( Lycaon pictus ), spotted hyena ( Crocuta crocuta ), and lion ( Panthera leo ); Caro and Stoner ). Exploitative competition is difficult to measure without controlled removal experiments, which are exceedingly difficult with bats (Clare et al.…”