Context dependent effects of plant species and functional group loss on vegetation invasibility across an island area gradient

1 There has been much debate about how losses of species and functional groups may affect the invasibility of vegetation, but little is understood about how invasibility differs across ecosystems or is driven by environmental context. 2 We studied the invasibility of field plots in two ongoing removal experiments set up across thirty lake islands in northern Sweden. These islands differ in size, and therefore soil fertility and productivity. One experiment involves full factorial removal of three functional groups (dwarf shrubs, mosses and tree roots), and the other involves full factorial removal of three species of dwarf shrub (Vaccinium myrtillus, V. vitis-idaea and Empetrum hermaphroditum). 3 We investigated the effects of removal treatments in both experiments on the invasibility of each of three species (Betula pubescens, Pinus sylvestris and Picea abies). This included a seed sowing study, and a seedling planting study, for each of the three species. 4 For the functional group experiment, removal of shrubs promoted invasibility by all species, and removal of mosses also had positive effects. For the species removal experiment, the two Vaccinium species exerted the strongest effects against invasibility. The floristic components that had the greatest effects represented only a small proportion of total plant biomass. 5 The effects of the removal of shrubs (or of either Vaccinium species) on invasibility often varied across island size classes. In these cases, removals usually had the greatest positive effects on the largest and most productive islands. In contrast, the effects of moss removals on P. sylvestris seedling survival were greatest on small islands. 6 These results show clearly that the effects of loss of components of the resident flora (at either the functional group or species level) on invasibility at the plot scale are context dependent, and can vary greatly across ecosystems. 7 Synthesis. Our results contribute to the ongoing debate about how loss of species and functional groups influences community-level processes, by showing that the effects of loss of resident biota on invasion of new species depends on the attributes of the biota that are lost and the ecosystems that they are lost from.