In this module, you’ll learn about the concept of energy flow, food webs, and food waste. We’ll also explore how food chains (a hierarchical series of organisms each dependent on the next as a source of food) create complex food webs (a system of interlocking and interdependent food chains) in agroecosystems, and what happens as energy and nutrient flow through each trophic level. By understanding energy flow, food webs and food waste, you can better understand how to design your agroecosystem to minimize energy loss from your system. Let’s explore these concepts:
Food Chains and Food Webs
Food chains show the levels of consumption between organisms. Each level of consumption is considered to be a different trophic level, and the trophic relationships between species in a community become food webs. When predators and parasites (secondary and higher-level consumers) prey on herbivores or other consumers, they continue the biomass conversion process between trophic levels. However, only a small percentage of the biomass at one trophic level is converted into biomass at the next trophic level. This is because a large amount of energy is expended in maintaining the organisms at each level (as much as 90% of the consumed energy). Additionally, a large amount of biomass at each level is never consumed- this biomass is broken down by detritivores and decomposers. The decomposition process releases, in the form of heat, much of the energy that went into creating the biomass. The remaining biomass is returned to the soil as organic matter.
Excerpt 1: Energy Flow in Ecosystems
by Stephen Gliesmann | Excerpt Adapted from Ch 2, Agroecology 2nd ed.
Organisms in ecosystems use energy to carry out physiological processes, and these stores of energy must be replenished. Energy flows into an ecosystem as a result of the capture of solar energy by plants, and through a food chain and food web. Plants capture and store solar energy through photosynthesis, and store chemical energy in the form of biomass, which serves as food for other species. Because of this role, plants are known as producers. The total amount of energy that plants have brought into the system at a point in time is known as the standing crop or biomass of the plants in the system.
The biomass produced by plants become available for use by consumers. Consumers include herbivores, who consume plant biomass and convert it into animal biomass; and predators and parasites, who prey on herbivores and other predators. Whereas plants are classified as autotrophs, because they satisfy their own energy needs; consumers are classified as heterotrophs because their nutritive needs can only be met by consuming other organisms.
In ecology, energy flow is the flow of energy through a food chain. Energy passes from trophic level to trophic level, and each time about 90% of the energy is lost. Therefore, primary consumers get about 10% of the energy produced by autotrophs, while secondary consumers get 1% and tertiary consumers get 0.1%. This means the top consumer of a food chain receives the least energy, as a lot of the food chain’s energy has been lost between trophic levels.
Humans change energy flow in agroecosystems- we change the species in the agroecosystem food web, adding some (like food crops or animals) and removing others (like rodents or large predators). By learning about energy flow in ecosystems, we can better understand how to design agroecosystems to minimize energy loss, making them more efficient and therefore more sustainable.
…it is important to understand these processes in order to address the concepts of ecosystem dynamics, efficiency, productivity and development, especially in agroecosystems where functions can determine the difference between the success and failure of a particular crop or management practice.
Films 1 and 2: Energy Flow and Tropic Levels (choose one)
In natural systems, waste products enter back in to the trophic system- plants or animals die and defecate, and decomposers break down these wastes and they become nutrients for plant growth or organic matter in the soil. In many food systems, food waste is composted and worked back into the agroecosystem. Since so many people now live far away from the farms where their food comes from, food waste often ends up in landfills, and most of the biomass we produce does not return to the soil as organic matter.
Source: Altieri, Miguel, et al., 1995. Chapter 6: Traditional Agriculture. Agroecology: The Science of Sustainable Agriculture. Westview Press.
Explore the following infographic by clicking the button below to learn more about wasted food and how it effects the food system: