Ecological Effects of the Invasive Giant Madagascar Day Gecko on Endemic Mauritian Geckos: Applications of Binomial-Mixture and Species Distribution Models

<div><p>The invasion of the giant Madagascar day gecko <i>Phelsuma grandis</i> has increased the threats to the four endemic Mauritian day geckos (<i>Phelsuma</i> spp.) that have survived on mainland Mauritius. We had two main aims: (i) to predict the spatial distribution and overlap of <i>P. grandis</i> and the endemic geckos at a landscape level; and (ii) to investigate the effects of <i>P. grandis</i> 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 <i>P. grandis</i> 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 <i>P. grandis</i>. The predicted range of each species varied from 85 km² to 376 km². Sixty percent of the predicted range of <i>P. grandis</i> overlapped with the combined predicted ranges of the four endemic geckos; 15% of the combined predicted ranges of the four endemic geckos overlapped with <i>P. grandis</i>. Levin's niche breadth varied from 0.140 to 0.652 between <i>P. grandis</i> and the four endemic geckos. The abundance of endemic geckos was 89% lower in sites with <i>P. grandis</i> compared to sites without <i>P. grandis</i>, and the endemic geckos had been extirpated at four of ten sites we surveyed with <i>P. grandis</i>. Species Distribution Modelling, together with the breadth metrics, predicted that <i>P. grandis</i> 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 <i>P. grandis</i>. This is a cause of concern in both Mauritius and other countries with endemic species of <i>Phelsuma</i>.</p></div

Model selection results. Abundance was modelled with habitat and status as site-level covariates.

<p>K = number of parameters used.</p><p>Delta AICc = difference between lowest AICc model and model AICc.</p><p>AICc weight = model probability among all candidate models.</p><p>Detection probability was modelled with observation-level covariates: cloud = cloud percentage cover; habitat = building, non-palm or palm; status = presence or absence of <i>P. grandis</i>; and temp = temperature. We used the corrected Akaike Information Criterion (AICc) to determine the best supported model.</p

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

<p><a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0088798#pone-0088798-g005" target="_blank">Figure 5a</a> 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. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0088798#pone-0088798-g005" target="_blank">Figure 5b</a> 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.</p

The mean endemic gecko abundance (with 95% confidence intervals) in sites with and without <i>Phelsuma grandis</i>.

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

The binary projection and overlap between <i>Phelsuma grandis</i> and the combined predicted ranges of the four endemic species of <i>Phelsuma</i> on mainland Mauritius.

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

The binary projection and overlap between <i>Phelsuma grandis</i> and four endemic species of <i>Phelsuma</i>.

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

Locations of the ten sites with <i>Phelsuma grandis</i> (red) and 11 sites without <i>Phelsuma grandis</i> (green) used to estimate the abundance of endemic species of <i>Phelsuma</i>.

<p>The black dot indicates Baie du Tombeau, the location where <i>Phelsuma grandis</i> was first introduced.</p