Trophic responses to plant invasion: How a shift from a native to exotic grass community impacts arthropod community structure

Background/Question/Methods

The invasion by non-native plant species causes dramatic changes in ecosystem processes and habitat structure, including nutrient cycling, litter decomposition, hydrologic regimes, and biotic community composition.  Many of these impacts are noticeable when exotic and native plants are of vastly different functional guilds, but fewer studies examine the bottom-up impacts of exotic versus native plant communities of similar diversity and functional guild composition.  Similarly, the impacts of plant species and functional group diversity on arthropods have been documented, but few studies have examined the impact of species replacement by plants of similar habit.  Here we use a manipulative study to examine how the replacement of native perennial grasses by exotic annual grasses affects the structure and composition of the arthropod community in a California grassland.  We crossed plant community manipulations with a cattle grazing treatment, as grazing and plant invasion commonly co-occur in this and other systems.

Results/Conclusions

Arthropod richness, diversity, and abundance did not vary predictably between native perennial and exotic annual grass communities, though there is some evidence that arthropod community composition as a whole was altered by exotic grasses.    Arthropod community was strongly impacted by cattle grazing and was correlated with plant habitat-forming characteristics including species richness and amount of litter.  Grazing, which reduced aboveground plant standing crop and compacted soils, masked the effect of exotic plants as a treatment but not the correlation with plant characteristics.  Thus, it seems that habitat-forming characteristics of plants, rather than their identity or provenance, are important in structuring their bottom-up impacts on arthropods.  However, arthropod diversity and abundance were strongly correlated with geographic coordinates within a plot, indicating that in a patchy landscape, there may be sufficient migration of arthropods to mask bottom-up effects that could occur in a more homogeneous plant community.