Ecology of Agriculture more stable than monocultures, both agronomically and economically Several mechanisms may lead to greater yield stability in diverse systems When one crop performs poorly because of drought or pest epidemic, for example, the other crop(s) can compensate, using the space and resources made available Such compensation is obviously not possible if the crops are grown separately If the yield advantages of polycultures are greater under stress conditions, then yield stability is higher This polyculture advantage has been demonstrated for crops under nutrient stress and drought stress Moreover, where polycultures lead to reduced pest attack, as they often (Letourneau et al., 2011), then greater yield stability may result from the dampening of pest outbreaks and disease epidemics These processes may operate simultaneously in diverse systems and are consistent with the idea that species richness buffers productivity under conditions of environmental variability and that diversity imparts resistance to perturbation The ability of polycultures to compensate for losses might also be considered to represent resilience While systems with greater resistance will be less impacted by a perturbation, systems with greater resilience return rapidly and reliably to original conditions Ecological research on the role of diversity in natural grassland systems around the world (Hooper et al., 2005; Tilman et al., 2006) indicates that diverse natural communities may be more productive than simple systems, at least partially as a result of increased nitrogen use efficiency in more diverse systems These studies also suggest that more diverse plant communities are more resistant to disturbance and more resilient in the face of environmental perturbations such as drought For example, the productivity of diverse grassland communities appears to decline less during a drought and to return more quickly to predrought levels than is the case for species-poor communities Research in grasslands thus tends to support the conclusions from agricultural research that diverse plant communities have higher productivity and stability Planned Diversity and Pest Regulation The planned diversity of the agroecosystem has important effects on herbivorous insects and the microbial community attacking crops Traditional agricultural systems often include substantial planned genetic and species diversity, and genetically diverse grain crops are used in many parts of the world to control pathogens In contrast, the low planned diversity of most commercial monocultural systems often results in large crop losses from a pest complex that is less diverse but more abundant than that in more diverse systems The trend for higher pest densities in monocultures compared to diverse cropping systems is especially strong for specialist insect herbivores with a narrow host range As planned diversity increases, population densities of these specialist herbivores decrease (Andow, 1991) In a recent meta-analysis of experiments on plant diversification in agricultural systems, Letourneau et al (2011) found that herbivores and crop damage were strongly suppressed and natural enemy 11 populations were enhanced in diversified systems compared to monocultures In a classic paper, Root (1973) offered two hypotheses to explain higher densities of herbivorous insects in simple, monocultural systems: the resource concentration hypothesis and the enemies hypothesis The first hypothesis predicts that herbivores, particularly specialists, in pure, dense host plant stands will be more likely to find their hosts and more likely to survive and reproduce In contrast, herbivores in less dense or more host plant-diverse stands should be less likely to find their hosts and more likely to lose them Although the details of herbivore–host interactions vary considerably, subsequent experimental tests of this hypothesis have generally supported it with respect to the effects of diversity (Letourneau et al., 2011), especially for specialist herbivores Host-finding behavior and insect movement, both colonization and emigration, appear to play important roles in the response of herbivorous insects to agroecosystems Densities of specialists may be lower in diverse systems because (1) they have difficulty locating hosts due to interference with olfactory or visual cues or (2) they leave hosts more often due to lower plant quality and then have difficulty relocating them These behaviors are significantly affected by the chemical, nutritional, and structural diversity that accompanies planned plant species diversity The enemies’ hypothesis predicts that diverse systems should have higher densities of herbivore natural enemies (predators and parasites) because they provide more resources for these natural enemies, such as alternate prey or hosts, nectar, pollen, and refugia This hypothesis has also largely been supported in experimental studies (Letourneau et al., 2011) Compared to monocultures, diverse systems are likely to have higher predation rates, higher parasitism rates, and higher ratios of natural enemies to herbivores, all of which may contribute to lower pest densities Natural enemies comprise one component of the unplanned diversity that increases as planned diversity increases in agroecosystems Microbial pathogens also respond to the planned diversity of agroecosystems, but their response is more variable than that of herbivorous insects The majority of plant viruses are transmitted by insects, and these tend to be found at lower incidence in diverse systems due to the effects of plant species diversity on their insect vectors The impacts of host-specific pathogens are likely to be lower in diverse systems due to density-dependent population growth However, crop diversification can modify the microclimatic conditions that play an important role in the development and severity of many fungal and bacterial diseases Pathogen growth and reproduction may be either encouraged or inhibited in more diverse cropping systems, depending on the particular requirements of the organism The effects of diversity depend on a variety of dispersal processes, infection efficiency, and the rate of disease progress Interestingly, some experiments in grasslands have shown that low productivity in low-diversity communities can largely be attributed to specialized soilborne pathogens (Schnitzer et al., 2011) The genetic diversity of crops can dramatically reduce pathogen impacts on crop productivity Mixtures of genotypes of a single species, such as multiline cultivars and varietal mixtures, have been used effectively to retard the spread and