By Elizabeth Graham
Food webs are maps of “who eats who” within an ecosystem (Figure 1a). Each node, or point, in the web represents a species or group of organisms; nodes are connected by a link if there is a known feeding relationship between the two groups. Though they are built on simple predator-prey relationships, food webs integrate complex information about biotic communities and key ecosystem processes, such as energy and nutrient flow, and are increasingly being used to study both biodiversity and function of freshwater ecosystems.
Food webs are also commonly represented by biomass pyramids, split into trophic levels. At the base of the food web are autotrophs, organisms which produce their own energy, such as plants. This level of the food web also includes decomposing organic material, known as detritus. Plants and detritus together are often referred to as basal resources. The remaining trophic levels consist of heterotrophs, organisms that obtain energy by consuming other organisms. Each trophic level feeds on lower levels, passing energy and nutrients up the food web. Some organisms are omnivores, and feed on multiple trophic levels. Aquatic food webs are typically upright pyramids because there are many small organisms at the base of a food web and only a few large predators at the top.
Both environmental and biological changes affect food web shape and structure. Insertion (i.e. invasion) or removal (extinction) of a species or trophic level will affect the height of food web (Figure 1b), while loss of a basal resource would narrow the width of the food web at the bottom. A change in one trophic level also impacts adjacent trophic levels. This effect is known as a “trophic cascade,” because it ripples down through the food web. For example, when trout were introduced as top predators in NZ streams, invertebrate abundance and biomass declined, and as a result algal biomass increased (Flecker and Townsend 1994; Figure 1b). Trophic cascades can be either top-down, in which a change near the top of the food web affects lower levels, or bottom-up, in which there is a change in resource availability at a lower trophic level.
Some food webs appear to be more resistant to change than others. This resistance, known as food web stability, has been hypothesized to be linked to the particular combinations of links within the food web. Biodiversity is predicted to enhance food web stability because even if a species is lost, there is a greater probability that another species will be present which can fill the same role. However, not all organisms or links within a food web have equal importance. Some organisms have a disproportionately large effect on other organisms and the environment relative to their abundance. Such organisms are referred to as keystone species. For example, a top predator which creates a trophic cascade would be considered a keystone species.
The strength of the link between species, also known as interaction strength, varies within a food web as well. The distribution of strong and weak interactions has been found to have a large influence on food web stability; food webs with more weak links tend to be more stable than those with predominately strong links. Food webs with a greater number of total links are also predicted to be more stable than those with few links, as there are more potential pathways for energy transfer to higher trophic levels. It is important to understand the factors which contribute to food web stability because more stable communities are more likely to persist under environmental stress, such as floods, droughts, or anthropogenic impacts.
There is still much more to learn about the role and function of food webs in freshwater ecosystems, and expanding our understanding of food web structure and dynamics will help us develop more effective and holistic freshwater management and restoration tools.
Flecker and Townsend (1994). Ecological Applications. Community-wide consequences of trout introduction in New Zealand streams. 4(4) 798-807.
Dr Elizabeth Graham is a freshwater ecologist at NIWA.