The Presence and Persistence of Frog Species in the New England Tablelands, Eastern Australia

Abstract

<p><b>Introduction:</b> Frogs are important vertebrate species in freshwater ecosystems. However, they are currently suffering declines worldwide. One way to monitor declines is to compare historical occupancy to present-day occupancy. The aim of the present study was to revisit historical sites on the New England Tablelands and to compare the current occupancy of frog species found to their historical occupancy. As part of this undertaking, some of the processes currently threatening frogs were also explored, including the difference in infection intensity and prevalence of the Amphibian chytrid fungus <i>(Batrachochytrium dendrobatidis)</i> between a still water and running water site, the influence of a known B. dendrobatidis reservoir host, the common eastern froglet <i>(Crinia signifera)</i>, on the other frog species and the influence of an invasive fish species, the eastern mosquito fish <i>(Gambusia holbrooki)</i>, on the occupancy of frog species still persisting in the New England Tablelands.</p> <p><b>Methods:</b> Two hundred of 898 historical sites in the New England Tablelands were revisited during the period extending from 2017 through to 2019. Occupancy estimation models were used to analyse the historical survey data from all sites and the results of the current visits to the selected sites. Initially, single-visit occupancy estimation models were created for each of the species in the historical records to determine historical occupancy. Secondly, single-species, single-season occupancy models with the inclusion of covariate information were generated to model the current occupancy of the species found in the present survey. Finally, two-species occupancy models were used to determine if the presence of <i>Cri. signifera</i> at a site influenced the occupancy probability of each of the other frog species found at that site, and to determine if the presence of <i>G. holbrooki</i> at a site also influenced the occupancy probability of each of the frog species found at the same site.</p> <p>Two hundred and ninety-two individuals from three frog species at two different sites" Thomas Lagoon and Blue Hole, were swabbed for <i>B. dendrobatidis</i>. Of these species, the eastern dwarf sedge frog <i>(Litoria fallax)</i> was a habitat generalist which maintained its occupancy since the historical study and was found at both sites. The spotted marsh frog <i>(Limnodynastes tasmaniensis)</i> also maintained its occupancy since the historical study, but was found only at Thomas Lagoon. The eastern stony creek frog <i>(Litoria wilcoxii)</i> is a habitat specialist, had a lower present average occupancy than its historical average occupancy and was found only at Blue Hole. Swabs were analysed using qPCR at the Australian Museum. qPCR results were modelled for infection intensity and infection prevalence using generalised linear models.</p> <p><b>Results:</b> Eighteen of the original (historical) 39 frog species found in the New England Tablelands were found during the present study. Of these 18 species, only six had increased or maintained their historical average occupancy. Analysis of the results of the current surveys found that latitude, longitude, altitude and emergent vegetation were important covariates to consider when modelling occupancy for the frog species found in the New England Tablelands, while temperature and humidity were important covariates when modelling detection. From the occupancy models with <i>Cri. signifera</i> included as a covariate, the occupancy for two frog species was influenced by the presence of <i>Cri. signifera</i>. Of these two, only one was negatively influenced. From the occupancy models <i>G. holbrooki</i>, none of the frog species were negatively influenced by the presence of G. holbrooki.</p> <p>Infection intensity was higher in <i>Lit. wilcoxii</i> and <i>Lim. tasmaninensis</i> than it was in <i>Lit. fallax</i>. Similarly, infection prevalence was higher in <i>Lit. wilcoxii</i> and <i>Lim. tasmaniensis</i> than <i>Lit. fallax</i>. </p> <p><b>Discussion:</b> In the New England Tablelands, fewer species were found in the present study than were found in the historical study. Frog species which were habitat generalists generally maintained or in some instances increased their occupancy in the current study when compared to the historical study. Species which were less likely to occupy sites with <i>Cri. signifera</i> and <i>G. holbrooki</i>, showed higher occupancy historically than they did currently. Infection intensity and infection prevalence of <i>B. dendrobatidis</i> was higher in species whose occupancy has decreased since the historical study.</p>