Testing Fox's assembly rule: does plant invasion depend on recipient community structure?

Fox's assembly rule, that relative dearth of certain functional groups in a community will facilitate invasion of that particular functional group, serves as the basis for investigation into the functional group effects of invasion resistance. We explored resistance to plant invaders by eliminating or decreasing the number of understory plant species in particular functional groups from plots at a riparian site in southwestern Virginia, USA. Our functional groups comprise combinations of aboveground biomass and rooting structure type. Manipulated plots were planted with 10 randomly chosen species from widespread native and introduced plants commonly found throughout the floodplains of Big Stony Creek. We assessed success of an invasion by plant survivorship and growth. We analyzed survivorship of functional groups with loglinear models for the analysis of categorical data in a 4-way table. There was a significant interaction between functional groups removed in a plot and survivorship in the functional groups added to that plot. However, survivorship of species in functional groups introduced into plots with their respective functional group removed did not differ from survivorship when any other functional group was removed. Additionally, growth of each of the most abundant species did not differ significantly among plots with different functional groups manipulated. Specifically, species did not fare better in those plots that had representatives of their own functional group removed. Fox's assembly rule does not hold for these functional groups in this plant community; however, composition of the recipient community is a significant factor in community assembly.