Influence of vegetation clutter on the capacity of ground foraging bats to capture prey.
The management of the habitat of insectivorous species often targets increasing the availability of arthropod prey. However, this may be hindered by lack of knowledge of the mechanisms that determine food availability for insectivores. Prey abundance is often used as a surrogate for availability, but this may be incorrect wherever habitat factors limit access to prey. Ground vegetation clutter is likely to be such a limiting factor for ground foraging insectivorous bats, and we investigated this possibility using Myotis myotis as a model. We performed captivity experiments to determine how ground vegetation density affects foraging. Bats were provided with crickets hidden in sparse, medium and dense grass cover. In addition, we used bat radio tracking data, prey abundance measurements, and geographic information system (GIS) modelling to determine if clutter influences how bats select foraging areas in a Mediterranean region. The experiments demonstrated that ground vegetation clutter greatly reduces access to prey, affecting both capture success and time to capture. Bats detected prey in the dense vegetation, but did not attempt to capture them or did so only after a prolonged delay. Their attempts often failed, because the bats landed over prey with open wings, presumably to increase the catching surface, and the dense vegetation prevented them from reaching the ground. In the study area, cover types with the densest ground vegetation harboured the most prey, but clutter made access to prey by M. myotis difficult. Corroborating this, the GIS models showed that bats avoided foraging in habitats with high prey abundance but in which availability was decreased by dense ground vegetation. Ungrazed grasslands reach vegetation densities that limit access to prey by ground foraging bats, as observed in the study area. However, grazing by cattle reduced clutter to levels equivalent to the sparse treatment in our captive experiments, in which bats captured prey easily. Synthesis and applications. Conservation of ground foraging bats may require the management of their feeding grounds, to increase or maintain prey availability, particularly near important colonies. While dense ground vegetation may promote prey density, it dramatically reduces access to prey and usage by bats; hence, this should be considered by managers. Moderate grazing can be used to reduce ground vegetation cover to levels that permit foraging by bats.