Section: New Results

Modeling of human-mediated dispersal via road network in invasive spreads

In the case of ecosystem invasions, human-mediated dispersal often acts as a vector for many exotic species, both at the introduction and secondary spread stages. The introduction stage is mainly a consequence of human-mediated long distance dispersal and is known to happen at continental or global scales. Secondary spread, however occurs at smaller spatial and time scales (e.g. landscape), and can result from natural or human-mediated dispersal. Despite the importance of local goods and materials transportation (e.g. for landscaping, construction, or road-building) potentially promoting the spreading of invasive species, few studies have investigated short distance human-mediated dispersal. This lack of consideration seems to be the consequence of multiple factors:

• human-mediated dispersal is generally considered as a long distance dispersal process, more important for invasive species introduction than for secondary spread;

• it is difficult to qualify and quantify this mode of dispersal because of the multiplicity of potentially involved human activities;

• for organisms that can disperse naturally, it is complicated to distinguish between natural and human-mediated dispersal, as they may occur at similar scales.

Even though a range of methodologies are available for describing population spread by natural dispersal, only few models have been developed to describe and predict human-mediated dispersal consequences at small scales, and none of them take into account the topology of the transport infrastructure (roads, waterways). In this result, and in order to fill this gap and provide new insights into how invasion dynamics impact ecosystem services, we combined ecological (invasive species occurrence data) and geographical (transportationnetwork topology) data in a computer model to provide estimated frequencies and distances of materials transportations through the landscape. In this study (cf. [7]), we investigated the spreading pattern of Lasius neglectus, an invasive ant species originating from Turkey, which spread into Europe in the last decades. In this species, no mating or dispersal flights are performed, and its spread is therefore solely ensured by the transport of soil materials in which individuals are present. We built a numerical model enabling the estimation of multiple human-mediated dispersal parameters based on ground-truth sampling and a priori minimizing. After having built a model of the landscape-level spreading process that takes explicitly into account the topology of the road network, we localized the most probable sites of introduction, the number of jump events, as well as parameters of jump distances linked to the road network. Our model was also able to compute presence probability map, and can be used to calibrate sampling campaigns, explore invasion scenarios, and more generally perform invasion spread predictions. It could be applied to all the species that can be disseminated at local to regional scales by human activities through transportation networks.