Mary E. Barbercheck, Penn State University
No insect population exists as an isolated entity. Rather, in any location there are many populations of organisms that interact to varying degrees in a community. Different species within an ecological community interact in a number of ways. Interactions between species in a community are usually described according to their beneficial, detrimental, or neutral effect. In agroecosystems, the interactions that we are often most interested in tends to involve feeding interactions—individuals in a population feed on, and in turn are fed upon by individuals in other populations (Tscharntke and Hawkins, 2002).
In all types of ecosystems, energy flow determines productivity. Green plants capture and store energy from the sun. Organisms that feed on plants use and convert the energy embodied in plants to grow and live. Energy flow is often conceptualized in food chains and food webs (Schowalter, 2006).
A food chain is a succession of organisms in which food energy is transferred from one organism to the next as each consumes one type of organism in the food chain and in turn is consumed by another type of organism in the food chain. There are three general categories of organisms in food chains in agroecosystems:
At each step in the food chain, some of the energy is assimilated and used by the organism and the rest is used in respiration and released in waste products.
A food web is comprised of the complex interrelated food chains in a community. The goal of crop production is to maximize ecosystem energy into a harvestable product, and to minimize the use of plant energy by pests in a food web.
In crop monocultures, the diversity in the species, physical structure, and age of plants tends to be reduced compared with polycultures or natural ecosystems. Therefore, food webs in crop monocultures are often comprised of simpler and shorter food chains than in more complex systems. The uniformity of monoculture systems encourages pest outbreaks. Increasing spatial and temporal diversity in a cropping system can help keep pests in check. For more information, see the eOrganic articles on using diversity for pest management in organic farming systems and farmscaping. Several concepts help explain how diversity helps to keep pest populations low, including: The Enemies Hypothesis, the Resource Concentration Hypothesis, and Island Biogeography Theory.
According to this hypothesis, diversity leads to stability because of favorable conditions created for enemies of plant-feeding insects (Colauttiet al., 2004, Price, 1997). The diversity of plant-feeding species available as food for natural enemies provides a stable alternate food supply when individual plant-feeding species fluctuate in abundance. More species of natural enemies provide a greater probability that prey species will be maintained at low densities. There are more generalist predators in diverse communities than in low diversity communities. Predators may increase the biodiversity of communities by preventing a single species from becoming dominant. Predatory and parasitic arthropods often require other food sources, for example, pollen, nectar, or alternate insect hosts. To favor “top-down” control of pests by natural enemies, producers should provide these resources on their farms. For more information, see the related eOrganic article on farmscaping.
Concentration or dispersion of food resources have a direct influence on insect populations (Denno et al. 2002; Price, 1997). Plant-feeding insects, especially those that specialize on specific types of plants, are more likely to find a food resource when it is concentrated, for example, in a monoculture, compared to when a resource is disperse in space, for example, in a polyculture, or in time, in well-planned crop rotations. For more information, see the related eOrganic article on cultural practices. Once the insect pest finds the crop, it is more likely to stay there and reproductive success is likely to be greater where resources are concentrated. Several factors influence this hypothesis:
The theory of island biogeography is used to explain factors that affect the biodiversity of an ecological community (MacArthur and Wilson, 1967). In this context the “island” can be any area of habitat surrounded by areas unsuitable for the species on the island—not just true islands surrounded by ocean, but, for example, fields surrounded by deserts, forest fragments, or urban areas. The theory of island biogeography suggests that the number of species found on an island is determined by the distance from the mainland (or source of species) and the size of the island. Distance and size affect the rate of extinction on the islands and the level of immigration. Islands closer to the mainland are more likely to receive immigrants from the mainland than those farther away from the mainland. The number of species on an island or habitat close to a source of species will be larger than that of one found farther away. Large islands can hold more species than small islands because on smaller islands, the chance of extinction is greater than on larger ones. The interaction between distance and size effects can be used to understand how many species an island, or habitat, can support. Island biogeography theory led to the development of habitat corridors as a conservation tool to increase connectivity and foster movement of organisms between habitat islands. In agroecosystems, habitat corridors, for example beetle banks or hedgerows, can increase the increase the number of species that can be supported (Altieri et al., 2005).
An idea related to island biogeography that helps explain the relationship between habitat and biodiversity is patch dynamics. Patch dynamics emphasizes the dynamic nature of habitats within landscapes (Forman, 1995). A patch is defined as any discrete area that is used by a species for obtaining resources. Diverse types of patches of habitat created by disturbance help maintain diversity. There are three states that a patch can found in: potential, active, and degraded. Patches in the potential state are transformed into active patches through colonization of the patch by dispersing species arriving from other active or degrading patches. Patches are transformed from the active state to the degraded state when the patch is abandoned, and patches change from degraded to potential through a process of recovery. Because agricultural systems are highly disturbed, patches of habitat are constantly transitioning through the the patch states. A diversity of patch states helps to prevent overall extinction of a species in a location, and therefore helps to maintain diversity.
This is an eOrganic article and was reviewed for compliance with National Organic Program regulations by members of the eOrganic community. Always check with your organic certification agency before adopting new practices or using new materials. For more information, refer to eOrganic's articles on organic certification.