Understanding Food Webs

What exactly is a food web?

Nearly a century ago, the ecologist Charles Elton introduced us to the concept of food chains –linear series of links that shows how food, which provides the environmental currency of nutrients and energy, moves through an ecosystem. No doubt you’ve seen illustrations of food chains.  These often begin with some kind of plant, like a grass; that gets eaten by a plant-eater (a herbivore), like a grasshopper. The grasshopper gets eaten by a mouse.   The mouse gets eaten by a snake.  The snake gets eaten by an eagle… Not many things eat eagles in the prime of its life, so the eagle is shown at the top of the food chain, making it an apex predator.

Image compares and contrasts food chains and food webs.

Food chains illustrate the dependence of consumers on producers and energy losses across trophic levels.

In a food chain, species are identified as producers (ex. plants) and consumers (either herbivores, ominvores, or carnivores).  Some food chains will also describe the decomposers- organisms that eat dead stuff.  However, in general, a food chain is a linear representation that illustrates why consumers, including people, are dependent on producers like plants, even when people eat meat.

A notable message that a food chain conveys well is that at each level of the food chain, energy is lost.  Only about 10% of the energy consumed by the grasshopper is transferred to the mouse, so in order to survive, a mouse must eat many grasshoppers over its lifetime.

The energy availability at lower levels of the food chain explains why some who are concerned about feeding the planet argue that people should eat less meat.   As omnivores, humans can choose between plant and animal-based diets, but an acre of quality farmland can produce more tons of corn or apples than, say, beef. This thinking is as linear and incomplete as the illustration of a food chain.

Food webs illustrate interdependence between species and recycling of energy and nutrients

While today we talk about food chains in popular conversations, most ecologists recognize that the true process of energy movement across trophic levels is nonlinear.  Food, energy, and nutrients move through the ecosystem through complex, interconnected webs. For example, while some mice get eaten by snakes, others get eaten by the eagle.  Still others die accidental deaths, and are partially eaten by ants or other insects before their little mouse bodies become food for decomposers.

The realm of decomposition may include some animals (ex: vultures, dung beetles, and earthworms), but decomposition is also catalyzed by billions of living fungi, bacteria, archaea, and other microorganisms.  Most of these are impossible for the unaided human eye to behold, but these organisms are essential to food webs because they recycle the nutrients that producers (plants) use as food.

Diverse food webs support cleaner environments and more productive food systems.

Healthy food webs are rich in diverse microorganisms and other decomposers that transform waste materials-including toxic substances and environmental contaminants-into nutrients that support one or more sectors of the food web. This decomposition and recycling is crucial for maintaining a clean environment.

Each organism present in the food web presents unique mechanisms through which energy and nutrients can be captured and recycled within the ecosystem.  This means biodiversity is essential to ensure a clean environment and robust food production.   For example, if you can imagine a food web in which the only primary producers are deciduous trees, it becomes clear that these trees will only capture the solar energy that lands directly on the leaves.  Sunlight that lands on the branches or hits bare ground will not be captured.  In the winter, trees will lose their leaves, and no energy will be captured.

By contrast, a food web that contains the same deciduous trees, but also contains evergreens, low-lying ground covers, grasses, wildflowers, and vines will be more efficient because the energy that misses the tree leaves can be captured somewhere else.  Energy that might have bounced off the tree trunk gets captured by vines. Energy that hit the ground in the tree-only system would now land on grasses or other ground covers.

Energy and nutrients are the currency that drives the food web economy.

Energy from the sun cannot be converted into food in the absence of essential nutrients.  These nutrients provide the raw materials, the building blocks, from which living cells are made.  Since nutrients are distributed throughout the soil and delivered to plants via root-associated fungi and other microorganisms, it stands to reason that biodiversity underground is as crucial as biodiversity on the surface. When many plant species are present, the bacterial and fungal communities can disperse themselves more uniformly through the soil, acquire more nutrients, and trade these more efficiently with plants.

Similar relationships between species diversity and nutrient availability occur among herbivores and other consumers. Like having many businesses helps support a productive economy, having many species (many food sources) helps ensure a productive food web.  Biodiversity then, is an important key to the food-web economy.  By protecting biodiversity, we protect complex food webs that feed society and ensure a clean environment.

Create habitat to support biodiversity in food webs

Creating habitat is key to supporting biodiversity in food webs is a job anyone can undertake at some scale.  A few are listed below:

  1. Plant a garden. Gardens that contain many plant species offer numerous ways to create habitat above and below ground.
  2. Rethink your lawn. Expansive stretches of smooth green grass are unnatural systems that often require pesticides, herbicides, and other chemicals to maintain.  These chemicals disrupt microbial food webs, making it more difficult to maintain a healthy lawn.  Try blending the grass with other species to give a prairie wildflower effect, or add mixed flower beds and border plantings.
  3. Focus on soil development. Our Building Better Soils class shows growers how to increase biodiversity in the soil. Restricting use of synthetic chemicals that kill microorganisms.
  4. Compost your organic wastes. Plant clippings, food scraps, animal wastes and more can easily be turned into bioactive composts that house billions of beneficial microbes.  These can be sprinkled on lawns and gardens or on farmlands to increase biodiversity in the soil. Learn how here.
  5. Add diversity to your farm with bioactive compost, multi-species cover crops, pollinator hedgerows along field borders, etc.

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