Ecological effects of the invasive giant madagascar day gecko on endemic mauritian geckos: applications of binomial-mixture and species distribution models

Abstract

The invasion of the giant Madagascar day gecko Phelsuma grandis has increased the threats to the four endemic Mauritian day geckos (Phelsuma spp.) that have survived on mainland Mauritius. We had two main aims: (i) to predict the spatial distribution and overlap of P. grandis and the endemic geckos at a landscape level; and (ii) to investigate the effects of P. grandis on the abundance and risks of extinction of the endemic geckos at a local scale. An ensemble forecasting approach was used to predict the spatial distribution and overlap of P. grandis and the endemic geckos. We used hierarchical binomial mixture models and repeated visual estimate surveys to calculate the abundance of the endemic geckos in sites with and without P. grandis. The predicted range of each species varied from 85 km2 to 376 km2. Sixty percent of the predicted range of P. grandis overlapped with the combined predicted ranges of the four endemic geckos; 15% of the combined predicted ranges of the four endemic geckos overlapped with P. grandis. Levin's niche breadth varied from 0.140 to 0.652 between P. grandis and the four endemic geckos. The abundance of endemic geckos was 89% lower in sites with P. grandis compared to sites without P. grandis, and the endemic geckos had been extirpated at four of ten sites we surveyed with P. grandis. Species Distribution Modelling, together with the breadth metrics, predicted that P. grandis can partly share the equivalent niche with endemic species and survive in a range of environmental conditions. We provide strong evidence that smaller endemic geckos are unlikely to survive in sympatry with P. grandis. This is a cause of concern in both Mauritius and other countries with endemic species of Phelsuma.

Figure 1. Locations of the ten sites with Phelsuma grandis (red) and 11 sites without Phelsuma grandis (green) used to estimate the abundance of endemic species of Phelsuma.

The black dot indicates Baie du Tombeau, the location where Phelsuma grandis was first introduced.

Figure 2. The binary projection and overlap between Phelsuma grandis and four endemic species of Phelsuma.

Each map shows the overlap of P. grandis and one of the endemic species. On each map, grey indicates that no species of Phelsuma were predicted to be present, yellow shows the predicted range of Phelsuma grandis, blue the predicted range of that species of endemic Phelsuma, and red areas of predicted overlap between the two species. The first number in each heading is the % overlap of the predicted range of Phelsuma grandis with that species of endemic gecko and the second number is the % overlap of the endemic species' predicted range with that of Phelsuma grandis. The different maps suggest that Phelsuma cepediana (a) and Phelsuma ornata (c) will overlap more with Phelsuma grandis and thus could be at greater risk.

Figure 3. The binary projection and overlap between Phelsuma grandis and the combined predicted ranges of the four endemic species of Phelsuma on mainland Mauritius.

Grey indicates that no species of Phelsuma were predicted to be present, yellow shows the predicted range of Phelsuma grandis, blue the combined predicted range of all the species of endemic Phelsuma, and red areas of predicted overlap. The first number in the heading is the % overlap of the predicted range of Phelsuma grandis with the combined predicted ranges of the four species of endemic Phelsuma, and the second number is the overlap of the endemic species' combined predicted ranges with the predicted range of Phelsuma grandis.

Figure 4. The mean endemic gecko abundance (with 95% confidence intervals) in sites with and without Phelsuma grandis.

Sites without Phelsuma grandis had a high abundance of endemic geckos, those with Phelsuma grandis a low abundance of endemic geckos. N = 10 sites with Phelsuma grandis, N = 11 sites without Phelsuma grandis.

Figure 5. Effects of habitat type and cloud cover in each habitat type on detection probability.

Figure 5a shows that the detection probability (with 95% confidence intervals) was similar in the three habitat types, with detection probability slightly higher in building sites than palm followed by non palm sites. Figure 5b shows a general decrease in detection probability (with 95% confidence intervals indicated by broken lines) with an increase in cloud cover in the three habitat types. Blue indicates building sites, black palm sites and red non-palm sites.