Landscape Influences on Ecological Processes in Agriculture The spatial structure of the landscape can shape many ecological processes in agroecosystems, including water and nutrient uptake by crops, weed dispersal, pest control, and pollination Globally, the intensification of agriculture has caused a simplification of landscape structure through the expansion of agricultural land, increase in field size, loss of field margin vegetation, and elimination of natural habitat This simplified landscape structure can affect the movement of organisms across the agricultural landscape, often limiting the movement of wildlife and organisms beneficial to agriculture if natural habitat corridors are not available (Tscharntke et al., 2005b; Kremen et al., 2007) In contrast, wildlife, pests, natural enemies, and pollinators are more likely to move among agricultural and natural habitats in heterogeneous landscapes with smaller, intermixed patches In some cases, natural habitats provide beneficial organisms with refugia and resources that are unavailable in agroecosystems, thus contributing valuable ecosystem services to agriculture Meta-analyses of published studies on the effects of landscape structure on natural enemies and pests in agriculture indicate that landscape complexity often enhances natural enemy populations and biological control of insect pests Relatively few studies have quantified the direct effects on pest Low (Power and Mitchell, 2004) Despite human efforts to breed crops resistant to pests and pathogens, many crops are excellent hosts for a variety of parasites that can spill over to wild hosts Herbivorous insect pests build up on high quality, nutrient-dense crop hosts and then move to wild host plants, where increased herbivory may have significant effects on plant demography (Tscharntke et al., 2005a) The reproductive success of a number of rare or endangered plant species has been significantly reduced due to herbivores moving from crops (Blitzer et al., 2012) Movement out of crop fields is particularly likely at mowing, harvest, and other times when the crop resource is disturbed, degraded, or diminished Similar movements have been seen for natural enemies of crop pests, which seek alternative prey in natural habitats when crop pests are no longer available As a result, enemies that increase in agricultural systems due to high pest densities can suppress the densities of herbivorous insects in natural ecosystems, thereby changing the dynamics of plant–insect interactions Finally, there is some evidence for spillover of pollinators from agroecosystems to natural habitats (Blitzer et al., 2012) Many pollinators forage in crop fields, but move to natural habitats to complete their lifecycle Studies suggest that spillover of bees from agroecosystems may sometimes benefit native plant species in natural habitats by compensating for the decline in native pollinators due to habitat fragmentation and other changes If pollinators are limiting, however, then crops may compete with native plants for pollination services In general, organisms that build up in agroecosystems clearly have the potential to modify species interactions and the structure of natural plant communities, but the consequences of movement of organisms from managed to natural ecosystems have not been extensively studied High Ecology of Agriculture Increase in biodiversity with farm-level diversiication 14 Cleared Simple Landscape type Complex Figure Predicted relationship between biodiversity and local, farm-level diversification in landscapes of varying diversity Reproduced from Tscharntke T, Klein AM, Kruess A, Steffan-Dewenter I, and Thies C (2005b) Landscape perspectives on agricultural intensification and biodiversity – ecosystem service management Ecology Letters 8: 857–874 populations, but there is some evidence that landscape simplification leads to higher population growth rates of pests and greater plant damage (Chaplin-Kramer et al., 2011) One unresolved issue is whether pests and natural enemies respond to landscape complexity at different spatial scales Some studies have suggested that enemies may respond at smaller scales than pests, and that parasitoids respond at smaller scales than other enemies Meta-analyses not strongly support the overall difference between pests and enemies, but they indicate that specialist natural enemies such as parasitoids may respond to landscape structure at smaller spatial scales than generalist natural enemies such as predators (Chaplin-Kramer et al., 2011) As a rule, natural enemies that are capable of dispersing long distances are more likely than weak dispersers to survive in simplified agricultural landscapes with few or distant patches of natural habitat As described above, crop diversification and management of seminatural habitat, such as hedgerows or field borders, within a single farm can also lead to higher natural enemy populations However, farmlevel diversification is more likely to influence pests and natural enemies if the wider landscape is structurally simple, than if it is already heterogeneous with many patches of natural habitat (Figure 3) Within complex landscapes, pest control services may not be significantly affected by additional farmlevel diversity (Tscharntke et al., 2005b; O’Rourke et al., 2011) Landscapes that completely lack natural habitat, however, may have such a limited species pool of natural enemies that farmlevel diversification may not result in notable increases in pest control services Like natural enemies, pollinators tend to be more abundant in complex landscapes with patches of natural habitat Pollination by bees or other animals significantly improves productivity for more than 70% of globally important crops As honeybee populations have declined due to disease, pollination by wild bees has become an essential ecosystem service provided by natural habitats Agricultural intensification threatens wild bee communities and may destabilize