Herbivore suppression may be strongest when predator communities are species rich (high richness) with similar abundances among those species (high evenness). These patterns have been justified theoretically and demonstrated experimentally, however, environmental drivers of community evenness are not well known. Therefore, it remains unclear how predator evenness emerges in nature, and if even predator communities are indeed linked with increased herbivore suppression in the field. Working on over 50 mixed-vegetable farms across three US states, we used structural equation models to examine relationships between evenness of ground-active generalist predators and two potential ecological predictors. First, we hypothesized that invasive predators would reduce the evenness of remaining predators due to common traits of invaders that enable them to persist and dominate native communities. Second,we predicted that structurally-complex refuge habitat provided by non-crop plants may preserve predator evenness by reducing predator-predator interference and intraguild predation.
Results/Conclusions
We found that densities of an invasive predatory ground beetle, Pterostichus melanarius, correlated with reduced evenness. Non-crop habitat encouraged higher densities of P. melanarius, neutralizing any positive effects that structural complexity might have contributed to community evenness. High evenness among native predators associated with higher predator densities and lower herbivore densities, however, P. melanarius had positive or neutral effects on herbivores, potentially due to intraguild predation. Altogether, these results suggest that while predator community evenness indeed strengthens top-down suppression across numerous diverse field sites, the presence of a dominant invader may complicate any effort to manage habitat to conserve evenness among native ground-foraging natural enemies.