Con-specific neighbours may enhance compensation capacity in an invasive plant.

Facilitation, both by inter- and intra-specific neighbours, is known to be an important process in structuring plant communities. However, only a small number of experiments have been reported on facilitation in plant invasions, especially between invasive con-specific individuals. Here, we focus on how con-specific neighbours of the invasive alien plant alligator weed affect the tolerance of alligator weed to herbivory by the introduced biological control agent, Agasicles hygrophila. We conducted greenhouse and garden experiments in which invasive plant density and herbivory intensity (artificial clipping and real herbivory) were manipulated. In the greenhouse experiment, artificial clipping significantly reduced plant biomass when plants were grown individually, but when con-specific neighbours were present in the same pot, biomass was not significantly different from control plants. Similarly, when compared to control plants, plants that were subjected to herbivory by A. hygrophila produced more biomass when grown with two con-specific neighbours than when grown alone. Real herbivory also resulted in an increased number of vegetative buds, and again when two con-specific neighbours were present this effect was increased (a 55.3% increase in buds when there was no neighbour, but a 111.6% increase in buds when two con-specific neighbours were present). In the garden experiment, in which plants were grown at high density (6 plants per pot), alligator weed fully recovered from defoliation caused by insects at levels from 20-30% to 100%. Our results indicate that the con-specific association may increase the compensatory ability to cope with intense damage in this invasive plant.