Achieving no net loss in habitat offset of a threatened frog required high offset ratio and intensive monitoring

Abstract

The use of habitat offset to mitigate the impact of development on threatened species is becoming increasingly popular. Despite a plethora of theoretical work on the requirements of habitat offset to achieve no net loss, there are very few examples of successful habitat offset programs and monitoring regimes to detect success. We present a case study of a population of the threatened green and golden bell frog (Litoria aurea) which was impacted by urban development through the removal of nine ponds. Development was concurrent with habitat offset and construction of a large number of ponds which resulted in a 19-fold increase in available pond area. Through the use of mark recapture surveys, the population size was determined pre- and post-development. Despite the creation of ponds in the immediate vicinity of the development there was a decrease in the pond area and a measured decline in the population located within the area where the development occurred. However, the overall pond construction program also involved the addition of considerable habitat away from the immediate vicinity of the development which resulted in a 19-fold increase in pond area and an approximate 1.2–3.5-fold increase in population size. No net loss in population size to 95% confidence was achieved only when including all pond construction. This study demonstrated that to achieve no net loss for a habitat offset program can require extensive levels of habitat creation with intensive monitoring to detect it.