Food and the Future Environment

Natural Ecosystem and Agroecosystem Comparison


Natural Ecosystem and Agroecosystem Comparison

Pest species can be present in agroecosystems, but not cause significant crop yield loss or livestock productivity reductions. Why? What factors prevent pest populations from reducing yield? One explanation may be that the crop or livestock is resistant to the pest. For instance, a crop plant may produce compounds that fend off pathogen infection or deter insect feeding. And if environmental conditions and resources are ideal, the plant may be able to grow and recover from pest infestation. What other ecological processes and factors might contribute to agricultural resilience to pests or other stresses such as climate change?

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Question 1 - Short Answer

Draw a food web pyramid and label the trophic levels as categories of organisms with i. primary producers at the bottom, ii. herbivores next, ii. omnivores and carnivores at the top of the pyramid. Chose a natural ecosystem and list all of the species you can think of that are found at each trophic level in the natural ecosystem. Then draw a second food web pyramid for a type of farm that you are familiar with, and list all of the species you might find at each trophic level. Describe how the natural ecosystem and the agroecosystem compare. How do they differ?

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You should have many more species at each trophic level in the natural ecosystem. Additionally, the genetic diversity within species in the natural ecosystem is typically greater than in the agroecosystem.

Question 2 - Short Answer

Odum (1997), an Ecologist summarized some of the major functional differences between natural and agroecosystems that are shown in the table below. Consider how your natural and agroecosystem food pyramids offer examples of the below ecosystem differences. How many predatory and parasitic species are there in the natural ecosystem and agroecosystem? How might the presence of predatory and parasitic organisms impact agricultural pests? How might genetic diversity contribute to pest management and ecosystem stability?

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Although you may not be familiar with parasitic species such as wasps and nematodes, you likely can think of many predatory species: humans, large and small mammals, predatory birds, rodents, fish, and arthropods (ex. beetles, spiders, ants, etc.)

In natural ecosystems there tend to be more niches and a higher diversity of species compared to most managed agroecosystems that are simpler, have fewer predatory and parasitic species, and less genetic diversity within a species. As the table below indicates with fewer trophic interactions, there are fewer species to reduce pest populations and prevent them from reducing agricultural yield and quality. Further, with low genetic diversity within agricultural species and across the landscape, the agricultural system is more vulnerable to pest outbreaks than natural ecosystems.

Natural Ecosystems and Agroecosystems
Property Natural Ecosystem Agroecosystems
Human Control Low High
Net Productivity Medium High
Species and Genetic Diversity High Low
Trophic Interactions Complex Simple, Linear
Habitat Heterogeneity Complex Simple
Nutrient Cycles Closed Open
Stability (resilience) High Low