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Given the unprecedented modern loss of biodiversity in response to anthropogenic developments, understanding the ecology of threatened species is paramount to their conservation. Mahony’s toadlet (Uperoleia mahonyi) is a poorly understood threatened frog species, endemic to coastal New South Wales, Australia. Known initially from fewer than ten localities, it was listed as Endangered once described. Still, its habitat, distribution, population structure, reproductive behaviour, and threats, remain unknown. Considering its threatened status, broad knowledge gaps; and range overlap with areas of intensifying urban and industrial development, I investigated the ecology of U. mahonyi for purposes of its conservation management.
I conducted extensive surveying of potential habitat, including, but not limited to, 955 standardised surveys from 204 reference sites alone. From this, and other sources, I developed a database of U. mahonyi localities and breeding sites, culminating in 317 verified records by 2023, nearly a ten-fold increase from 38 records in 2019. Thus, a critical step towards firmer knowledge of its ecology, distribution and habitat.
Uperoleia mahonyi occurs in apparently isolated populations, separated by river systems forming dispersal barriers. My first objective was to establish genetic substructuring of U. mahonyi, to determine number of populations, degree of isolation between them, and where conservation resources should be most directed. To do this I collected tissues from 186 U. mahonyi individuals for genomic analyses and sequencing for Single Nucleotide Polymorphisms (SNPs). I identified six genetically distinct units within the species, comprising four isolated populations, with one of grave conservation concern. All six genetic units corresponded to independent Quaternary sand-bed systems, suggesting a strong association with this habitat type.
Secondly, as U. mahonyi breeds in groundwater-dependent wetlands, of varying ephemerality, in a region prone to extreme climatic variation, I documented its reproductive phenological plasticity. Utilising call data collected with automated recorders over four breeding seasons, I found that calling effort varied with water availability and rainfall. The most variation occurred at more temporary sites. Calling was recorded throughout most of the year, associated with warmer minimum temperatures.
Due to imperfect detection, establishing a species true absence from suitable habitat requires in-depth understanding of environmental covariates that influence its detection. I conducted 157 standardised surveys across five sites occupied by U. mahonyi, to quantify its detection probability. I found detectability of U. mahonyi was influenced by abundance, pond fullness in the breeding season, and relative humidity. Dependent on these factors, one to 15 surveys were required to establish 95% confidence of a true absence at a site of unknown occupancy.
Finally, in order to test the robustness of globally practiced “like-for-like” offsetting, where species occupancy is assumed by presence of specific vegetation communities. Incorporating detection probability, I repeatedly surveyed 199 sites predicted as “occupied” or “unoccupied” by U. mahonyi according to Plant Community Types (PCTs), until presence was confirmed or 95% confidence of an absence achieved. From 798 surveys, U. mahonyi was present in roughly 30% of these occupied sites demonstrating serious flaws in offsetting schemes using vegetation type as a proxy for species occurrence, given 70% of sites expected to be occupied were not. Conversely, I found geology predicted site occupancy with nearly 50% accuracy, and that U. mahonyi occurred only on Quaternary sediments, specifically12 geologic units, largely Pleistocene sands.
My findings have important implications for dictating appropriate surveys, offsetting, and management of U. mahonyi – but also for threatened biota generally. Further, as U. mahonyi is representative of threatened, small-range endemics, offsetting schemes globally must adopt species-specific metrics to successfully achieve biodiversity conservation goals. |
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