Invertebrate diversity in Bracken Bat Cave: Effects of bat guano subsidy

Background/Question/Methods Cave ecosystems are generally oligotrophic and highly limited by energy sources due to the lack of primary productivity, and therefore almost entirely rely upon allochthonous resources. This has led to the development of special adaptations in cave organisms in order to survive these stable, but extreme conditions. Bracken Bat Cave in central Texas is atypical in terms of the magnitude of allochthonous energy resource input in the form of bat guano. Bracken Bat Cave is recognized as the ecosystem with the largest bat community in the world (a colony of more than 20 million Mexican Free-tail bats). This phenomenon has resulted in an exceptional cave ecosystem. Our study focused on determining the species richness and abundance of cave invertebrates in Bracken Bat Cave with respect to cave depth, guano layer depth and seasonality. We accomplished this by randomly sampling in three preselected cave zones of 45 m length on a monthly basis for 5 months. Each sample contained a core-sample from the surface to 0.91 m in depth. Through the comparison of these two depth factors we aim to establish whether invertebrate species richness and abundance vary at different cave and guano depths, with respect to seasonality.

Results/Conclusions Preliminary data suggests that cave invertebrate population abundances are closely correlated to guano availability. This is supported by the fact that the highest abundance of organisms was found in the top layer of guano. Additionally, the highest abundance of invertebrates significantly occurred in the middle zone, which coincided with the highest quantity of guano deposits or bat abundances in the cave. This data indicates that cave invertebrates at Bracken Bat Cave greatly rely on the subsidy supplies from the cave-inhabiting bat populations. These findings underline the importance of bat conservation at Bracken Bat Cave, since it appears that the bats represent a key stone species in this ecosystem. Our project is crucial in increasing our knowledge of such a unique cave ecosystem and it will have ecological implications on cave conservation for preserving such a distinctive ecosystem and the largest population of bats in the world.