An Examination of Occupancy on a Coastal Refuge and Mercury Concentrations in Texas Bats
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Texas bats are threatened by habitat loss, contaminants such as mercury (Hg), disease, and wind turbines. In east Texas, specifically along the Gulf Coast, much of the native landcover is fragmented from anthropogenic activities. East Texas also has 5 of the top 10 Hg emitters in the United States, which contribute to Texas producing more Hg pollution than any other state. Being consumers of prey connected to aquatic ecosystems, Hg bioaccumulates in bats and has been documented to reach toxic levels in some species. Knowledge gaps exist regarding critical foraging habitat for bats on the upper Texas Gulf Coast and Hg concentrations in bats throughout Texas. I identified habitats in which bats were active using multi-state occupancy models for 5 bat species: tri-colored (Perimyotis subflavus), evening (Nycticeius humeralis), eastern red (Lasiurus borealis), northern yellow (Lasiurus intermedius), and Brazilian free-tailed (Tadarida brasiliensis) surveyed with fixed acoustic detectors on the San Bernard National Wildlife Refuge (SBNWR) and measured total Hg (THg) concentrations in the fur of 7 bat species: cave myotis (Myotis velifer), hoary (Lasiurus cinereus), P. subflavus, N. humeralis, L. borealis, L. intermedius, and T. brasiliensis from eastern and central Texas. All bat species were widely distributed within the SBNWR; thus, I assessed high and low activity areas using multi-state occupancy models. Occupancy rates of T. brasiliensis were lower in areas with greater canopy cover whereas high activity rates of N. humeralis decreased in open habitats. Additionally, I collected 427 fur samples from 32 sites to investigate inter- and intraspecific variability in fur THg concentrations. Two species, P. subflavus and N. humeralis, had greater mean THg concentrations (6.04 and 5.87 µg/g, respectively) than other species with several individuals exceeding the 10 μg/g threshold cited as having deleterious health effects in bats. Nycticeius humeralis was the only species that demonstrated intraspecific variation with adults having greater mean THg concentrations than juveniles (7.45 and 4.29 µg/g, respectively). Nycticeius humeralis fur THg concentrations were greater along the upper Gulf Coast than in central and southern Texas and were positively influenced by the density and distance to coal-fired power plants. My research concluded that Hg may be a greater threat to bat populations on the upper Gulf Coast than other areas in the state. To aid future conservation efforts of bats in Texas, I recommend maintaining a matrix of native habitats for bats to use in areas where bats are widely distributed, like SBNWR, and also facing multiple stressors including habitat loss and the accumulation of harmful contaminants like Hg.