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Abstract The Comal Springs riffle beetle ( Heterelmis comalensis ; CSRB; Coleoptera: Elmidae) is a flightless species endemic to two springs flowing from the Edwards Aquifer in central Texas. It was listed as federally endangered primarily due to its limited distribution and the threat of water over‐extraction from the aquifer. The listing led to the initiation of a research programme that has improved our understanding of the species' biology and the establishment of captive populations. The primary CSRB population in Comal Springs and a co‐occurring cosmopolitan species, Microcylloepus pusillus (Coleoptera: Elmidae), have been monitored semiannually since 2004. Our overall goal was to investigate responses to environmental variation using an intensive 1‐year population study and long‐term data. The population assessment was more spatially and temporally intensive than biomonitoring and was initiated to assess the distribution of the CSRB, determine which environmental covariates relate to relative abundance and develop relative abundance estimates to inform biomonitoring. The population study indicated that spring flow was the primary driver of adult and larval CSRB populations, suggesting that lower spring flows lead to lower populations. CSRB abundance was also positively associated with biofilm coverage, although its effect size was smaller than that of spring flow. Microcylloepus pusillus was not affected by spring flow or biofilm, emphasizing the unique ecological niche of the CSRB and the importance of maintaining spring flow through droughts to support populations. Model results were adapted into a framework for an annual assessment of populations using biomonitoring data. Long‐term monitoring data suggest that the abundances of both species have declined over the past two decades, but M. pusillus has declined beyond the variation explained by included environmental factors.
Pintar et al. (Tue,) studied this question.