Abundance, distribution and spread of the invasive Asian toad Duttaphrynus melanostictus in eastern Madagascar

Abstract

The Asian toad, Duttaphrynus melanostictus, was accidentally introduced to Toamasina (eastern Madagascar) around 2010, and since then has spread at a substantial rate across a larger area. This study documents the expansion of the invasive range of this species, calculates the invasion spread rate, and provides estimates of toad abundance and habitat preferences. Updates of the distribution range revealed a fivefold increase of the invaded area over 3 years, and a doubling of the rate of spread, showing a shift of the invasion towards the North-West, most probably because of the absence of ecological barriers. We used N-mixture models to estimate toad abundance on the basis of repeated count data of six areas in Toamasina and its surrounding countryside. Toad distribution shows heterogeneous density across the distribution range, with an average abundance of 184 toads ha−1 (95% CI 132–263). The toad’s abundance was highest in sites with the presence of organic waste, and was negatively related to the density of road networks in the proximity of study sites. The rapid expansion of the Asian toad in the Toamasina region suggests that this toad is an increasing threat for Madagascar. We propose immediate management actions that could limit the spread of alien toads in this megadiverse country.