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Keywords:

  • associational resistance;
  • biological invasion;
  • biotic resistance;
  • coastal dunes;
  • invasion ecology;
  • invasive exotic grass;
  • mammalian herbivory;
  • plant facilitation

Summary

1. Exotic plant species have become increasingly prominent features of ecological landscapes throughout the world, and their interactions with native and exotic taxa in these novel environments may play critical roles in mediating the dynamics of such invasions.

2. Here, we summarize results from comparative and experimental studies that explore the effects of two factors – herbivory and facilitation – on the performance and distribution of an invasive South African grass, Ehrharta calycina, in a coastal foredune system in northern California, USA.

3. Using a 2-year exclosure experiment, we show that a native herbivore, black-tailed jackrabbits (Lepus californicus), significantly reduced the height, shoot production, fecundity and above-ground biomass of this exotic grass.

4. Data from two comparative studies and a neighbour-removal experiment revealed that Ehrharta frequently escaped herbivores by associating with three neighbouring plant species – an exotic perennial grass, Ammophila arenaria, an exotic perennial succulent, Carpobrotus edulis, and a native perennial shrub, Baccharis pilularis. Ehrharta growing in association with neighbours was taller, had fewer grazed shoots, produced greater numbers of spikelets and had greater above-ground biomass than unassociated individuals. Furthermore, removing neighbours generally eliminated these benefits in 7 months, although effects differed among neighbour species.

5. An additional neighbour-removal experiment conducted in the absence of jackrabbits indicated that neighbour removals did not have significant impacts on Ehrharta height, shoot production, spikelet production or above-ground dry biomass. These results suggest that the primary means by which Ehrharta benefits from neighbouring plants is protection from herbivores – either because they are less apparent to herbivores or less accessible – and that Ehrharta likely incurred minimal costs from associating with neighbours.

6.Ehrharta was more frequently associated with neighbours than expected due to chance, and less frequently found in open dune habitat. These results are consistent with the hypothesis that the effects of herbivory and facilitation have been sufficiently strong to shape the local distribution of this invader in the landscape.

7.Synthesis. Our research has demonstrated that herbivory and facilitation have jointly influenced the dynamics of a biological invasion, and highlights the importance of evaluating the effects of multiple interactions on invasions in a single system.