Introduction

The competitive exclusion theory states that two species cannot coexist indefinitely using the same limiting resource. Several species of bats in Illinois, however, appear to feed during the night on the same insects in the same habitats. Theory suggests that this apparent niche overlap may be more limited than it appears and these species likely partition their food resources in some manner. Various strategies may be adopted by species to specialize their foraging patterns and reduce competition with other insectivorous species. The purpose of my study was to examine foraging niches and test whether northern long-eared bats (Myotis septentrionalis), little brown bats (Myotis lucifigus), and eastern pipistrelles (Pipistrellus subflavus) reduce interspecific competition by feeding in subtly different ways.

Methods

Bats were live-trapped at Copperhead Cave, an abandoned coalmine in Vermillion County, Indiana, using both double-frame harp traps and mist nets. Bats were identified, sexed, weighed, and tagged with an individually numbered wing tag. Time of capture was also recorded. Bats were then fixed with a chemiluminescent (light) tag positioned between the scapulas and attached using a non-toxic surgical glue. These tags emitted light for about two hours and allowed for the visual monitoring of bats during foraging flights.

After release, bats were allowed to recuperate for about five minutes or until they began feeding. Observations were then recorded every fifteen seconds if bats were in constant view or recorded with each sighting when not in constant view.

I looked for evidence of niche partitioning on four niche axes: foraging schedule, foraging height, foraging habitat, and diet. Foraging schedule was determined by noting the time that each species exited the cave to begin foraging. Foraging height was measured by noting the height at which bats foraged. Habitat selection was measured by noting the type of habitat in which bats foraged during the night. Differences in the diets of each species were based on previously published dietary studies.

Niche Breadth

A total of forty-two Myotis septentrionalis, thirty-seven Myotis lucifigus and twelve Pipistrellus subflavus were observed and used in this study. Myotis septentrionalis tended to be the most selective of foraging habitat (B_A=0.2623) but fed at a variety of heights and over a wide period of time (B_A=0.4867 & 0.5565). This bat species spent much of its time foraging on wooded slopes (72.5%) just below the canopy (47.1%). Pipistrellus subflavus tended to be more selective of foraging height and schedule (B_A=0.3803 & 0.3151) than Myotis septentrionalis, more general with foraging habitat (B_A=0.5713) and showed a relatively narrow foraging schedule with 66.6% of its captures during a 1-hour period. Also, 40.0% of this species' diet consisted of insects in the Cicadellidae and Carabidae families. Overall, Myotis lucifigus tended to have the widest niche breadths. It showed the widest foraging schedule (B_A=0.5967) of the three species but tended to be more selective with foraging habitat and diet (B_A=0.3903 & 0.3928). Contrasting with Myotis septentrionalis, Myotis lucifigus did much of its foraging in valleys (60.3%) and in the canopy (53.5%). These bats tended to focus their diets on Lepidopteran, Trichopteran, Dipteran and Cicadellidae insects (59.0%).

Niche Overlap

Measurements of percent niche overlap revealed the extent to which each pair of species overlapped on each axis. Calculations on Myotis septentrionalis and Myotis lucifigus indicated a high level of overlap in foraging height use (P_jk=86.0%) but a much smaller level of overlap in foraging habitat (P_jk=50.5%) use. Myotis septentrionalis and Pipistrellus  subflavus tended to have smaller overlaps in foraging times (P_jk=47.5%) and rather high overlaps in foraging height (P_jk=75.5%) use. Myotis lucifigus and Pipistrellus  subflavus demonstrated very high levels of niche overlap in both foraging height and habitat (P_jk=87.5% & 87.1%) use but managed to offset this trend with their differing use of foraging times (P_jk=36.2%). Their use of food showed an overlap of 53.0%.

Conclusion

These three bat species coexist in the same habitat and forage in a similar manner. My results suggest that they reduce interspecific competition through niche partitioning and specialized foraging patterns. Understanding how bats utilize their foraging habitat may assist researchers in providing essential information for the management and conservation of bat biodiversity.

Acknowledgements

Heather Fraser, Stacey Fuhrmann, & Katie Dunn
Dr. John Whitaker, Indiana State University
Eastern Illinois University Honors Program