Macroinvertebrate Diversity and Food Web Dynamics in a Guano Subsidized Cave Ecosystem: Bracken Bat Cave
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Caves are generally oligotrophic ecosystems and highly limited by energy sources due to the lack of primary productivity within the system. Therefore cave ecosystems rely almost entirely upon allochthonous energy supplies originating from surface ecosystems. Bracken Bat Cave is unique in terms of the magnitude of allochthonous energy resource input in the form of guano, which is provided by the largest bat community in the world (a colony of more than 20 million Mexican free-tail bats (Tadarida brasiliensis)). For this reason it is important to gain an understanding of food web dynamics within Bracken Bat Cave. The objectives of this study are: 1) provide baseline biological information of the macroinvertebrate taxa richness and abundance of the cave, 2) quantifying the importance of other environmental factors such as distance from entrances, depth within the guano substrate of the cave floor, or seasonality on macroinvertebrate abundance and richness, 3) investigate how anthropogenic disturbances affect macroinvertebrate abundance and richness, 4) examine the key nutrient relationships in this ecosystem, the nutrient properties of the guano subsidy and its effect on the macroinvertebrate populations, and 5) investigate the food web interactions of the cave macroinvertebrates through stable isotope analysis. Our results indicate that seasonality does not have a major effect on macroinvertebrate abundance and richness, despite the fact that guano deposition significantly varied between months. An estimated dry weight total of 50,521.72 kg of guano were deposited in the cave (3078 m2) August 2009-August 2010, with the highest guano deposition rate being recorded in September and the lowest deposition rate in January-March. On the other hand, macroinvertebrate abundance and richness were significantly correlated with guano depth and distance from the entrance. In addition, disturbances such as large amounts of guano removal led to an increase in macroinvertebrate abundance and richness. Nutrient analyses indicated that guano nutrient properties remained constant with respect to seasonality and guano depth, with the exception of carbon content, which decreased with guano depth. Lastly, the stable isotope analyses suggested that guano has a high range of variability in carbon and nitrogen stable isotope ratios. More specifically, δ13C only varied with respect to sample type (guano, hair, macroinvertebrates) and δ15N varied with respect to sample type and guano depth. Our findings thus describe Bracken Bat Cave as a relatively stable ecosystem that experiences few fluctuations with seasonality, and can withstand the effects of disturbances due to the large amount of guano subsidy found in the cave. The information gained from our study increases our knowledge of such cave ecosystems and has ecological implications on the conservation of such a distinctive ecosystem and bat community.