Simultaneous estimation of habitat suitability and colonization kernel for an invasive alien tree Bischofia javanica in a subtropical oceanic island

Potential habitat suitability and speed of range expansion are key attributes of invasive alien species in eradication and control projects. Although many studies attempted to estimate habitat suitability and dispersal process separately, zero-inflated data for colonization kernel estimation caused by unsuitable environments, and also zero-inflated data for habitat suitability model in the places where the alien species has not yet arrived at are often found; and probably cause biases in parameterization. In this research, we considered these processes simultaneously (Habitat-Colonization model, HC model). The probability that an alien species occur at a given site was assumed as the product of two probabilities: the probability that the species can grow in the specific environment of the site (logistic equation), and that propagules arrive at the site in the range expansion process (negative exponential function). A model for an invasive alien tree Bischofia javanica (Euphorbiaceae) in Bonin (Ogasawara) Islands of northwestern Pacific was parameterized using a set of distribution maps in two periods (1977 and 2003) and environments calculated from topography (DEM). We compared goodness of fit of HC model with both of conventional habitat suitability model and colonization kernel, and HC model gained better AIC than others. The HC model successfully analyzed an invader’s distribution pattern. The risk map representing the places where the alien tree will not invade due to unsuitable environments, already dominated, and currently rare but it should be dominated future, was obtained based on this model.