LB 10-100 - Estimating temporal variance in effective population size of an endangered species after an adverse climatic event

Adverse climatic events, such as severe drought, can have devastating effects on populations. For narrowly endemic taxa, prolonged droughts may lead to a dramatic decrease in population size and result in genetic bottlenecks. This loss of genetic diversity can cause inbreeding depression, reducing fitness in populations, and the effects of bottlenecks can be incredibly long-lasting, observable dozens to hundreds of generations after the event. In this study, I investigate the impacts of a major, prolonged drought on populations of the endangered Comal Springs Riffle Beetle, Heterelmis comalensis, known from only two central Texas spring complexes. Central Texas experienced its most extreme drought in over sixty years during the period of 2010-2015, in which aquifer discharge at the Comal Springs complex dropped to < 100 cfs, and multiple springs known to host H. comalensis within the complex ceased flowing entirely. The San Marcos Springs complex also experienced lower rates of discharge during the 2010-2015 drought, but no springs went completely dry. H. comalensis populations at Comal Springs have since re-colonized the springs that ran dry during the drought, but the effects on genetic diversity and population size are unknown. The objectives of this study were to characterize and compare genetic diversity in populations of H. comalensis at both spring complexes before and after the 2010-2015 drought by estimating effective population size (Ne). I implemented temporal sampling, collecting beetles from six spring sites over three sampling periods: 1) pre-drought (2006), 2) post-drought (2016), and 3) post-drought (2020). Using a genotyping-by-sequencing approach, I genotyped 576 beetles collected from six spring sites during the three sampling periods and estimated the variance in Ne from temporal shifts in allele frequencies, allowing me to quantify the demographic changes in Ne. Effective population sizes were estimated as mean values over time periods, allowing me to compare mean Ne from 2006-2016 (pre-drought) and 2016-2020 (post-drought). Using these two estimates, I calculated the proportional change in Ne size across sites. Post-drought mean Ne was smaller for all spring sites compared to pre-drought mean Ne, even for sites that did not go completely dry during the drought period. The proportional decline was dramatic, with post-drought mean Ne ranging from 39.1-57.6% of pre-drought mean Ne across sites. This methodology of estimating Ne through temporal shifts in allele frequency is robust and widely applicable to understanding variance in Ne in other taxa of conservation concern.