Ecological Understanding of Insects in Organic Farming Systems: Diversity, Stability, and Productivity

The Benefits of Biodiversity

Biodiversity is variation of life at all levels of biological organization, from genes to whole organisms to populations. In agroecosystems, biodiversity is generally a measure of the relative numbers of types of organisms present. When considering the effects of biodiversity on a system, two concepts are especially important to consider: stability and productivity (Schowalter, 2006).

Most agroecosystems tend to be highly disturbed. Common practices like tillage, planting, application of fertilizers and pesticides, irrigation, and harvest can cause temporary or longer-lasting changes in average environmental conditions that change the functioning of the ecosystem (Altieri et al.,2005). Disturbances can have profound immediate effects on ecosystems and can greatly alter the biological community for an extended period of time. Often, when a disturbance occurs in a natural ecosystem, it provides conditions that favor the success of different species over pre-disturbance organisms. This can be due to changes in both the physical environment and biological community, resulting in changes for a period longer than the period over which the immediate effects persist. In the absence of further disturbance, many ecosystems will eventually return to pre-disturbance conditions. In natural systems, the sequence of changes that occur as a disturbed system returns to pre-disturbance conditions is called ecological succession.

Stability in ecosystems is a measure of resilience (the ability of the system to recover from a disturbance) and the resistance of the system to change (Schowalter, 2006). If an agroecosystem recovers quickly after a disturbance or resists pest invasion, it is considered more stable than one that doesn't. Productivity is a measure of ecosystem function. In agroecosystems, productivity is usually measured as the yield of a crop.

There is no single overarching effect of biodiversity on either productivity or stability. The effects in any particular system will depend on the environmental context and the time scale over which the effects are observed. However, it is generally considered that biodiversity is important for both managed and natural ecosystems. Some researchers consider that an intermediate level of disturbance will result in the highest levels of biodiversity. In systems with low intensity or frequency of disturbance, competitive species become dominant. In systems with high intensity or frequency of disturbance, only species tolerant of the stress can persist.

There are three main ideas about how biodiversity stabilizes the functioning of an ecosystem: the Redundancy Hypothesis, the Rivet Hypothesis, and the Portfolio Effect.

1. The Redundancy Hypothesis assumes that more than one species plays a given role within an ecosystem (Walker, 1992). Environmental conditions in ecosystems are dynamic, and a community of diverse species may be more likely to contain a particular species that is tolerant of stress conditions (for example, during drought, high or low temperatures, soil disturbed by tillage) and maintain overall stability in the function of the ecosystem. According to the redundancy hypothesis, species that play the same roles in the ecosystem (sometimes referred to as a guild) can compensate for each other if some are lost under particular conditions. In this way, species redundancy enhances the ability of an ecosystem to recover from a disturbance (resilience).
2. The Rivet Hypothesis uses the analogy of rivets in an airplane wing to compare the increasingly critical effect that the loss of each species will have on the function of an ecosystem (Lawton, 1994). The effect of the loss of one species on the functioning of a diverse system is relatively small. However, similar to the devastating effect of the loss of too many rivets from an airplane wing, the loss of several species can lead to the complete collapse of the functioning ecosystem. This hypothesis assumes that communities are comprised of relatively specialized species with limited ability to compensate for each other. Therefore, the loss of any species is critical to the performance of the ecosystem. The rate at which the loss of species affects total ecosystem function is especially important in this hypothesis. Non-specific, density-independent mortality factors, such as the application of a pesticide, that indiscriminately remove many species are more likely to have adverse effects on the functioning of an agroecosystem than are more specific, targeted control tactics, that remove few species.
3. The Portfolio Effect compares biodiversity to stock holdings. Diversification of a stock portfolio minimizes the risks associated with volatility of an investment, or in the case of agroecosystems, the risk of loss of stability and productivity (Tilman et al.,1998). A community of species together will respond differently to an environmental disturbance than when considered individually. The stability created by a diverse community acts to preserve the functioning of the agroecosystem in the event tat single species are lost.

References and Citation

• Altieri, M., C. I. Nichols, and M. A. Fritz. 2005. Manage insects on your farm: A guide to ecological strategies. Sustainable agriculture network handbook series book 7. (Available online at: http://www.sare.org/Learning-Center/Books/Manage-Insects-on-Your-Farm) (verified 25 April 2011).
• Lawton, J. H. 1994. What do species do in ecosystems? Oikos 71: 367-374.
• Schowalter, T. D. 2006. Insect ecology: An ecosystem approach. 2nd Edition. Academic Press. Burlington, MA.
• Tilman, D., C. L. Lehman, and C. E. Bristow. 1998. Diversity-stability relationships: Statistical inevitability or ecological consequence? The American Naturalist 151: 277-282.
• Walker, B. H. 1992. Biodiversity and ecological redundancy. Conservation Biology 6: 18-23.

Additional Resources

• Wikipedia contributors. Biodiversity [Online]. Wikipedia, The Free Encyclopedia. Available at: http://en.wikipedia.org/wiki/Biodiversity (verified 11 March 2010)
• Duffy, J. E., J. Lloyd. 2007. Biodiversity. In: Encyclopedia of Earth. C.J. Cleveland (ed.) Environmental Information Coalition, National Council for Science and the Environment, Washington, DC. (Available online at: http://www.eoearth.org/article/Biodiversity)(verified 11 March 2010)
• Wikipedia contributors. Ecosystem services. Wikipedia, The Free Encyclopedia. (Available online at: http://en.wikipedia.org/wiki/Ecosystem_services) (verified 11 March 2010).