We have recently published a paper on Biological Invasions which is my first PhD chapter! In this study we tried to understand the influence of landscape and local characteristics on biological invasions by exploring the level of plant invasion and alien species traits in forest edges in the highly urbanized landscape of the Vallès county (Catalonia, Northeast Spain). Overall we found landscape characteristics the most important determinant of the level of invasion in the forest edge and interior and the type of exotic species we can encounter. This finding suggests that invasion at these sites is a dynamic process primarily related to the propagule pressure from the surrounding landscape. High human-alteration of the landscape contributes to further expansion of alien plant species from forest edges into the interior. The global trend towards forest fragmentation, which increases the length of the interfaces between urban and forest areas, will exacerbate plant invasions by gardening species (Martin et al. 2009).
A bit of methods:
The CREAF team (Nuria Gassó and Joan Pino) identified all plant species in 73 paired plots in the edge and 50 m towards the interior of the forest. For each plot they also collected information about the local chracteristics: habitat and edge type and disturbance regime. We maybe missed information about light availability using the density of trees or LAI but edge type was valid proxy. Once in the office we calculated the landscape variables (% land-cover type and road distance) using GIS. Then I explored the association between alien species richness and similarity in species composition between edge and interior plots with landscape and local variables, using generalized linear models and variance partitioning techniques. I found this last technique very useful because it can separate the importance of several group of variables, landscape and local variables in our case. I took the idea from a paper of Martina Carrete and customized the R function varpart from the VEGAN package. Definitely messing up R codes is the way to learn this powerful tool!
A priori it is very useful to identify which exotic species could become successful invaders. In fact, the analysis of plant traits related to invasiveness has been one of the main focuses in invasion biology (Hayes and Barry 2007) and the basis for invasion risk analyses. However, identifying general patterns is complicated as many plant traits associated with invasion are dependent on the habitat type and the stage of invasion. Our results indicate that these risk analyses should also include characteristics of the landscape matrix of the recipient region because successful or more abundant alien plant traits are likely to be different depending on the surrounding landscape (Vilà and Ibáñez 2011). For example species of Mediterranean origin and introduced for agriculture were associated with higher agriculture use in the landscape (very obvious but not tested yet).
Once on the field, management of plant invasion must also consider the landscape adjacent to natural areas in order to reduce the propagule pressure from human-derived land-uses. Possible landscape management measures could include changes in gardening practices or constant monitoring of plant invasion in urban-forest interfaces.