Manipulating wetland hydroperiod to improve occupancy rates by an endangered amphibian: modelling management scenarios.
Environmental managers have the difficult task of ensuring species persistence despite considerable uncertainty about their response to management. Spatially explicit population models provide one solution for simulating the dynamics of species and evaluating alternative management regimes. We used a Bayesian model to investigate wetland occupancy dynamics of the endangered growling grass frog Litoria raniformis at a wastewater treatment plant in southern Victoria, Australia. We coupled prior information from earlier research on this species with our survey data to quantify the effects of patch-scale variables and connectivity on the probabilities of occupancy, population extinction and colonization. Hydroperiods of 13 sites were experimentally enhanced to bolster occupancy rates by L. raniformis. We used simulations to assess the extent to which the enhanced hydroperiod regime improved the viability of the focal metapopulation. Occupancy rate increased by 15% among the enhanced sites in 2013-2014, whereas the rate of occupancy among unenhanced sites fell by 11% in that year. Forward simulation using the dynamic occupancy model suggested that the minimum occupancy rate across the metapopulation would be 18% higher if the enhanced hydroperiod regime was retained over the next 20 years. Mean posterior effects of patch-scale variables and connectivity on the occupancy dynamics of L. raniformis were consistent with the prior effect in all cases, with only small changes to the size of these effects. There was no clear effect of water chemistry on occupancy dynamics. Synthesis and applications. This work suggests that managing the hydroperiod of constructed wetlands can be an effective tool for the conservation of amphibians and demonstrates the utility of spatially explicit models for assessing metapopulation viability. We encourage managers to experimentally test the efficacy of manipulating patch-scale variables to improve occupancy rates within amphibian metapopulations.