Human activity can degrade ecosystem function by reducing species number (richness) and by skewing the relative abundance of species (evenness). Conservation efforts often focus on restoring or maintaining species number, reflecting the well-known impacts of richness on many ecological processes. In contrast, the ecological effects of disrupted evenness have received far less attention, and developing strategies for restoring evenness remains a conceptual challenge. In farmlands, agricultural pest-management practices often lead to altered food web structure and communities dominated by a few common species, which together contribute to pest outbreaks. Here we show that organic farming methods mitigate this ecological damage by promoting evenness among natural enemies. In field enclosures, very even communities of predator and pathogen biological control agents, typical of organic farms, exerted the strongest pest control and yielded the largest plants. In contrast, pest densities were high and plant biomass was low when enemy evenness was disrupted, as is typical under conventional management. Our results were independent of the numerically dominant predator or pathogen species, and so resulted from evenness itself. Moreover, evenness effects among natural enemy groups were independent and complementary. Our results strengthen the argument that rejuvenation of ecosystem function requires restoration of species evenness, rather than just richness. Organic farming potentially offers a means of returning functional evenness to ecosystems.

In the southeast United States, a field of peanuts, Arachis hypogaea L., is often closely associated with a field of cotton, Gossypium hirsutum L. The objective of this 4-yr on-farm study was to examine and compare the spatiotemporal patterns and dispersal of the southern green stink bug, Nezara viridula L., and the brown stink bug, Euschistus servus (Say), in six of these peanut-cotton farmscapes. GS(+) Version 9 was used to generate interpolated estimates of stink bug density by inverse distance weighting. Interpolated stink bug population raster maps were constructed using ArcMap Version 9.2. This technique was used to show any change in distribution of stink bugs in the farmscape over time. SADIE (spatial analysis by distance indices) methodology was used to examine spatial aggregation of individual stink bug species and spatial association of the two stink bug species in the individual crops. Altogether, the spatiotemporal analyses for the farmscapes showed that some N. viridula and E. servus nymphs and adults that develop in peanuts disperse into cotton. When these stink bugs disperse from peanuts into cotton, they aggregate in cotton at the interface, or common boundary, of the two crops while feeding on cotton bolls. Therefore, there is a pronounced edge effect observed in the distribution of stink bugs as they colonize the new crop, cotton. The driving force for the spatiotemporal distribution and dispersal of both stink bug species in peanut-cotton farmscapes seems to be availability of food in time and space mitigated by landscape structure. Thus, an understanding of farmscape ecology of stink bugs and their natural enemies is necessary to strategically place, in time and space, biologically based management strategies that control stink bug populations while conserving natural enemies and the environment and reducing off-farm inputs.

The effects on survival of species-species interaction in the face of a changing climate depend on whether the species' interests are conflicting or non-conflicting.

More diverse communities of consumers typically use more resources, which often is attributed to resource partitioning. However, experimentally demonstrating this role of resource partitioning in diverse communities has been difficult. We used an experimental response-surface design, varying intra- and interspecific consumer densities, to compare patterns of resource exploitation between simple and diverse communities of aphid predators. With increasing density, each single consumer species rapidly plateaued in its ability to extract more resources. This suggests intraspecific competition for a subset of the resource pool, a hallmark of resource partitioning. In contrast, more diverse-predator communities achieved greater overall resource depletion. By statistically fitting mechanistic models to the data, we demonstrated that resource partitioning rather than facilitation provides the better explanation for the observed differences in resource use between simple and diverse communities. This model-fitting approach also allowed us to quantify overlap in resource use by different consumer species.