Determining environmental limits of threatened species: the example of the wallum sedgefrog Litoria olongburensis

Abstract

In an environment that is changing due to anthropogenic processes, managers responsible for conservation of threatened species need to know environmental limits beyond which those species are at risk of extinction. We demonstrate estimation of environmental limits for a threatened species using a novel combination of response modeling techniques. Our study species was Litoria olongburensis (wallum sedgefrog), which has a biphasic lifecycle (aquatic larvae and terrestrial adult phases) with larvae developing in naturally acidic wetlands of coastal sandy lowlands (“wallum”) of subtropical eastern Australia. Land development in, and around, areas occupied by the frog is the main cause of the species’ decline, while climate change is emerging as a new threat. The species will continue to decline where these processes destroy, fragment or degrade habitat. We surveyed waterbodies throughout the latitudinal range of the species’ distribution, recording wallum sedgefrog density and environmental variables including abiotic waterbody characteristics, key vegetation types, potential competitors and potential predators. For each environmental variable, we tested the fit of increasingly complex response models to the highest possible quantile of wallum sedgefrog density. The best‐fitting model indicated the most likely response, if any, to the variable. This model was then applied to estimate environmental limits. Our analysis indicated wallum sedgefrogs were less likely to occur in waterbodies with pH outside 3.53–4.61 (± 0.11) and maximum water depth outside 23.4–46.0 (± 3.5) cm, and their density decreased with increasing densities of eastern sedgefrogs, wallum froglets and common froglets. The optimal pH levels and water depths indicated by our analysis provide a necessary baseline for predicting and responding to impacts on wallum sedgefrogs caused by changes in land use or climate. The negative relationship with eastern sedgefrogs supports the hypothesis that competition from eastern sedgefrogs is the mechanism that limits occurrence of wallum sedgefrogs in higher pH wetlands. The modeling framework that we developed for our study can be applied to improve management of other species exposed to anthropogenic threats including climate change.

Assessing acoustic competition between sibling frog species using rhythm analysis

“…That is, while larvae are usually recorded in waters ranging from pH 3.4-4.5 (Hines & Meyer, 2011), the species is known to breed in wallum habitats with an acidity of pH < 6 (Meyer et al, 2006). However, in degraded ponds where this alteration has facilitated the arrival of the eastern sedgefrog, it is not common for the two species to be observed coexisting over long periods of time (Meyer et al, 2006;Shuker et al, 2016). The wallum sedgefrog may experience greater negative effects in areas of cooccurrence due in part to the fitness cost of increasing their call rhythm (Anichini et al, 2018;Kruger & Du Preez, 2016) and the potential influence of call masking on breeding success (Kelley, 2004), but also due to other compounding mechanisms such as competition for food sources (e.g., small arthropods; Curtis, 2012), perch substrate use (Shuker & Hero, 2012), and potential asymmetrical predation of the wallum sedgefrog and the eastern sedgefrog by other introduced species in disturbed areas such as the eastern mosquitofish (Gambusia holbrooki; Meyer et al, 2006;Remon et al, 2016).…”

“…In particular, the eastern sedgefrog, a sibling (closely related, morphologically similar but reproductively isolated) species to the wallum sedgefrog, is a notable example of a species that capitalizes on disturbance to colonize new habitats, including that of the wallum wetlands (Meyer et al, 2006). Once introduced, these species initially occur in sympatry, but as anecdotal observations have noted, the wallum sedgefrog is eventually excluded from what is still considered potentially viable habitat (Shuker et al, 2016). It can therefore be inferred that the eastern sedgefrog may be directly competing with the wallum sedgefrog, excluding them from the area.…”

Distribution mapping of specialized amphibian species in rare, ephemeral habitats: Implications for the conservation of threatened “acid” frogs in south‐east Queensland

“…The modeling approach used in our study was unable to take into account site‐specific factors affecting the occurrence of acid frog species such as water pH, wetland hydroperiod, water quality, the presence and abundance of competitor species and predators (Shuker et al, ; Shuker & Hero, ; Simpkins et al, ), or broader biogeographic factors affecting the distribution of acid frog species (e.g., historical barriers to dispersal). Therefore, the modeled distributions presented here are likely to overestimate the actual distribution of acid frog species, particularly in more heavily populated areas, where historical occurrence records may not reflect recent habitat degradation, as a result of the high level and intensity of human disturbance.…”

“…We developed species occurrence models using 13 predictor variables including environmental, climatic, and anthropogenic factors (see Table S2) that were selected a priori on the basis of an assessment of which of these were biologically most meaningful in the contexts of our investigation (Shuker & Hero, ; Shuker, Simpkins, & Hero, ). Environmental layers included dominant vegetation types (Department of the Environment and Energy, ), soil properties (amount of clay, sand, and soil pH CaCl 2 ; Viscarra Rossel et al, , , ), land zone (from regional ecosystem classification scheme; Department of Environment and Science, ), wetlands (presence and type; Department of Science, Information Technology and Innovation, ), and elevation and slope (Department of Natural Resources and Mines, ).…”

Battling habitat loss: suitability of anthropogenic waterbodies for amphibians associated with naturally acidic, oligotrophic environments

“…Turner 1990; Levin et al 2009), due to human population growth and expansion. Within Australia, the majority of freshwater, coastal waterbodies situated along the eastern seaboard between Fraser Island, Queensland, and Jervis Bay, New South Wales, are unique as they are both naturally oligotrophic (Hines et al 1999) and acidic (Griffith et al 2008;Hines and Meyer 2011;Simpkins et al 2014;Shuker et al 2016). The vegetation community within this area is known as 'wallum', and is defined as the vegetation communities that include Banksia woodland, sedgeland, heathland and Melaleuca swamps (Hines et al 1999;Griffith et al 2003).…”