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Preliminary Observations on Tropical Bat Caves as Biogeochemical Nitrogen Sinks

A previously unrecognized aspect of nitrogen biogeochemistry in tropical moist forests (TMF) relates to the spatial biogeochemistry of these systems. In general, high levels of herbivory in TMF’s transfer large amounts of fixed nitrogen and other nutrients into the insect community, which itself is harvested by aerial insectivorous vertebrates (bats and birds), or moves directly into the non-volant vertebrate community through leaf eating mammals and frugivorous mammals and birds. In non-karstic TMF’s bats and birds can be expected to redistribute this fixed nitrogen through defecation and urination throughout the TMF, contributing to the efficient recycling of this potentially limiting nutrient. However, in karst environments, TMF’s may support very large (105 – 106 individuals) populations of bats and in Southeast Asia, birds (cave dwelling swiftlets), that occupy very spatially localized roosts. A single large cave may support a bat or swiftlet population in the hundreds of thousands of individuals that forages for insects over thousands of square kilometers of TMF, but then deposits a substantial fraction of its total excretory nitrogen within that single cave. A large but undetermined fraction of this nitrogen input is retained within the cave, or lost as diatomic nitrogen gas following biological denitrification. Such a cave then functions as a point-source sink for TMF fixed nitrogen. Recent work in Deer Cave, Mulu National Park, Sarawak (Malaysia) has mapped the ammonia plume emanating from a large bat guano accumulation. Relating this plume to airflow, first order estimates of ammonia production are ~ 9 g NH3/m2/day below the main Chaerephon plicata roost. An additional 4 g N/m2, or 45% of the total nitrogen budget, are retained in the guano and eventually lost to the external environment.