Species distributions are often limited by environmental factors and according to the abundant-centre hypothesis, abundance should be highest where the environment is most favourable for the species. So, do the same environmental factors determine occurrence and abundance patterns inside the range? I examined this question using Arthroleptella lightfooti, a species of frog from the family of Pyxicephalidae, endemic to the mountains of the Cape peninsula, South Africa.
I used density estimates obtained from acoustic Spatially Explicit Capture Recapture (aSCR) methods and data from an acoustic survey using an array of 6 microphones to construct the first peninsula wide population-density surface for this visually cryptic but acoustically active species. The analysis consisted of three stages. The first involved creating two sets of data from the original: one shows whether the species is present or not and the other indicates the density when the species is present. The second stage consisted of fitting a Hurdle Model to the data where the presence data is modelled using logistic regression and the density data is separately modelled using ordinary linear regression. The third stage involved combining the two models to estimate the expected density of the species. Confidence intervals were built using non-parametric bootstrapping.
It was found that covariates explaining variation in occurrence were not the same as those explaining variation in density, suggesting that processes determining occurrence were not always those determining density. Of the environmental conditions examined, although predictive of occurrence, were generally poor predictors of A. lightfooti density. Presence of the Lightfoot’s moss frog was largely explained by topographic features and availability of water. In contrast, predictions of density were only weakly related to these same environmental factors and in some cases contradicting one another.
The second part of this study produces the first Peninsula wide population density surface of A. lightfooti. At the same time, it assesses the ability of using opportunistically collected presence-only records in combination with the higher quality density data to improve the estimation of expected population-density surface of A. lightfooti. The presence-only records were constructed into a habitat suitability map using an ensemble of species distribution models. The habitat suitability map was then integrated in the modelling framework as a covariate in order to improve the estimation of expected population-density surface of A. lightfooti. However, the habitat suitability covariate resulted as being uninformative.
Reference: Poongavanan, J. (2019) Modelling the range-wide density patterns of the Arthroleptella lightfooti using acoustic monitoring data. Master’s Thesis, University of Cape Town.