Grassland plant species pattern and evenness affect arthropod communities

Background/Question/Methods

Plant species richness positively affects arthropod diversity. However, little research has been done to assess the effects of plant species evenness and the spatial patterns of plant species upon arthropods, which could directly impact habitat restoration projects. We hypothesized that more even plant communities would support a greater number and diversity of arthropods due to increased local resources, an effect which should be reduced by aggregating conspecific plants. To test these hypotheses, in May 2012 we planted 1 x 1 m plots (Species Pattern and Community Ecology experiment) with four grassland species selected from a pool of 16 species (constant functional diversity) at three evenness levels (0.64, 0.8, 1.0) in random or clumped (four conspecific individuals per patch) arrays. Arthropod herbivores (vacuum sampling; two minutes plot^-1) and pollinators (pollinator traps; 2-day interval) were sampled in June and July. Invertebrates were sorted into morphospecies and identified to order and family where possible.

Results/Conclusions

Plot species evenness and pattern affected arthropod numbers, but not diversity. Large herbivores (≥ 5 mm) were marginally positively affected by plant evenness (2-way ANOVA, F_2,20 = 3.19, p = 0.0626) and were more numerous in randomly planted plots than in clumped plots (2-way ANOVA, F_1,20 = 7.03, p = 0.0153). Plot evenness also affected the number of large (≥ 5 mm) pollinators (2-way ANOVA, F_4,20 = 8.87, p = 0.0017) with intermediate evenness plots containing the most large (≥ 5 mm) pollinators. Discrepancies between effects of evenness on pollinators and herbivores likely reflect differences in the sampling scales associated with each technique. Our results indicate that the relative scarcity of plant species within a community (evenness) may affect the pollinator services plant communities provide and that plant species evenness and pattern may affect plant above-ground biomass through effects on herbivory. This research suggests that restored grasslands with greater interspersion (the combined effect of evenness and spatial pattern) among plant species would be able to support a larger number of arthropods.