Predator-prey interaction reveals local effects of high-altitude insect migration.

High-altitude nocturnal insect migrations are ubiquitous and represent significant pulses of biomass, which impact large areas and multiple trophic levels, yet are difficult to study and poorly understood. Predation on migratory insects by high-flying bats provides potential for investigating flows of migratory insects across a landscape. Brazilian free-tailed bats, Tadarida brasiliensis, provide valuable ecosystem services by consuming migratory pests, and research suggests migratory insects are an important resource to bats in autumn. We sequenced insect DNA from bat feces collected during the 2010-2012 autumn migrations of insects over southern Texas, and tested the utility of predator-prey interactions for monitoring migratory insect populations by asking: 1) how extensively do bats consume migratory insects during autumn? (2) does the prey community reflect known drivers of insect migrations, e.g. cold fronts? and (3) are migratory insects increasingly important to bats when local food resources decline in autumn? Bats consumed at least 21 species of migratory insects and 44 species of agricultural pests. Prey community richness increased with cold front passage. Bats consumed migratory moths over the entire autumn season, and the proportion of migratory moths in the bat diet increased over the course of the autumn season in all 3 years. This study confirms extensive consumption of migratory insects by bats, links patterns in prey communities to mechanisms driving insect migration, and documents a novel approach to tracking patterns of migratory insect movement. As an important resource for T. brasiliensis in autumn, migratory insects provide stabilizing effects to the local animal community.

Patterns of Bat Distribution and Foraging Activity in a Highly Urbanized Temperate Environment.

Understanding how to manage biodiversity in urban areas will become increasingly important as density of humans residing in urban centers increases and urban areas expand. While considerable research has documented the shifts in biodiversity along urbanization gradients, much less work has focused on how characteristics of dense urban centers, effectively novel environments, influence behavior and biodiversity. Urban bats in San Francisco provide an opportunity to document changes in behavior and biodiversity to very high-density development. We studied (1) the distribution and abundance of bat foraging activity in natural areas; and (2) characteristics of natural areas that influence the observed patterns of distribution and foraging activity. We conducted acoustic surveys of twenty-two parks during 2008-2009. We confirmed the presence of four species of bats (Tadarida brasiliensis, Myotis yumanensis, Lasiurus blossevillii, and M. lucifugus). T. brasiliensis were found in all parks, while M. yumanensis occurred in 36% of parks. Results indicate that proximity to water, park size, and amount of forest edge best explained overall foraging activity. Proximity to water best explained species richness. M. yumanensis activity was best explained by reduced proportion of native vegetation as well as proximity to water. Activity was year round but diminished in December. We show that although bats are present even in very densely populated urban centers, there is a large reduction in species richness compared to that of outlying areas, and that most habitat factors explaining their community composition and activity patterns are similar to those documented in less urbanized environments.

Weather-driven dynamics in a dual-migrant system: moths and bats.

Animal migrations generate large spatial and temporal fluctuations in biomass that provide a resource base for many predator-prey interactions. These interactions are often driven by continent-scale weather patterns and are difficult to study. Few studies have included migratory animals on more than a single trophic level or for periods spanning multiple entire seasons. We tracked migrations of three species of agricultural pest noctuid moths over the 2010-2012 autumn seasons as the moths travelled past a large colony of migratory Brazilian free-tailed bats (Tadarida brasiliensis) in Texas. Increases in moth abundance, mass of bats and duration of bat activity outside of the cave were correlated with passage of cold fronts over the study area and related increases in northerly wind. Moth responses to weather patterns varied among species and seasons, but overall moth abundances were low in late summer and spiked after one or more cold front passages in September and October. Changes in bat mass and behaviour appear to be consequences of bat migration, as cave use transitioned from summer maternity roost to autumn migratory stopover sites. Weather-driven migration is at considerable risk from climate change, and bat and moth responses to that change may have marked impacts on agricultural systems and bat ecosystem services.