Academic journal article Michigan Academician

Influence of Structural Complexity on Bat Activity at Palustrine Habitats in the Northern Great Lakes Region

Academic journal article Michigan Academician

Influence of Structural Complexity on Bat Activity at Palustrine Habitats in the Northern Great Lakes Region

Article excerpt

To determine the influence of habitat complexity on bat activity, we investigated search-phase calls and feeding-buzz calls of bats across a diverse array of palustrine habitats in the northern Great Lakes region. We surveyed 40 sites acoustically for bats in summer 2007 using AnaBat II detectors. We also measured vertical (total vegetation volume and Levins diversity index) and horizontal (canopy cover, area of open water) structural complexity. We recorded 5,866 search-phase calls or bat passes and 725 feeding buzzes--representing all six species that occur in the region--at 39 sites. We identified Little brown bats (Myotis lucifugus LeConte) most often (76.4%). Regressions of bat activity measures with habitat structure metrics suggest that either canopy cover or area of open water were influential predictors of relative activity for Little brown and Red bats (Lasiurus borealis Muller) or presence of the remaining four species. In contrast, measures of vertical habitat complexity did not appear to influence community composition of bats. However, the number of feeding-buzz calls relative to the number of search-phase calls was higher in areas with less vegetation volume and greater canopy cover. Although structural complexity was influential in explaining bat activity in palustrine habitats, we suggest that other environmental or biological factors also may be contributing to observed patterns of bat activity.

INTRODUCTION

Understanding use of aquatic habitats by insectivorous bats in forest-dominated landscapes is a step toward responsible habitat management in North America. Although a greater complexity of vegetation poses maneuverability challenges to insectivorous bats, it may provide suitable substrate for invertebrates. Therefore, based on optimal foraging theory, dense vegetation may provide a tradeoff of greater insect densities compensating for greater foraging efforts of bats (Burford et al. 1999).

In addition to providing a prey base, structurally complex vegetation may correspond with insectivorous bat morphology. Multiple studies suggest that bats respond to clutter based on behavioral and morphological characteristics (i.e., echolocation frequency, body size and wing-loading), whether the clutter is artificially constructed (Brigham et al. 1997) or natural (Menzel 2003; Owen et al. 2004; Wund 2006). Smaller-bodied species, such as those in the genus Myotis, often are detected across the entire complexity spectrum, whereas larger-bodied or slower-flying species may avoid highly cluttered spaces and are observed more frequently in open-canopied or less structurally complex environments (Aldridge and Rautenbach 1987; Brooks and Ford 2005; Francl 2008; Wund 2006). Additionally, these larger species (e.g., Big brown bat [Eptesicus fuscus Beauvois], Red bat [Lasiurus borealis Muller], Hoary bat [Lasiurus cinereus Beauvois]) can alter their call structure according to habitat (Obrist 1995), but are possibly limited to simpler foraging areas because of their larger bodies and correlative wing loadings and aspect ratios (Aldridge and Rautenbach 1987; Findley et al. 1972; Korine and Pinshow 2004). One medium-sized species, the Silver-haired bat [Lasionycteris noctivagans LeConte], is capable of utilizing cluttered habitats but often is grouped with larger species because it is slow-moving and relative flight activity is greater in open-canopied habitats (e.g., Brooks and Ford 2005).

Our objective was to examine how structural complexity in palustrine habitats affects bat activity. Previous studies suggest that large, open-canopied aquatic environments support a greater amount of bat activity, as measured by echolocation surveys or captures (e.g., Ellis et al. 2002; Grindal et al. 1999; Seidman and Zabel 2001; Zimmerman and Glanz 2000). In less open-canopied, forested riverine habitats, vegetation composition at the fine scale (measured on-site) may be a stronger predictor of bat activity than stand-level or watershed-level parameters (Ober and Hayes 2008). …

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