Food webs are graphical depictions of the interconnections among species based on energy flow . Energy enters this biological web of life at the bottom of the diagram, through the photosynthetic fixation of carbon by green plants. Many food webs also gain energy inputs through the decomposition of organic matter, such as decomposing leaves on the forest floor, aided by microbes. River food webs in forested headwater streams are good examples of this.
Energy moves from lower to higher trophic (feeding) levels by consumption: herbivores consumes plants, predators consume herbivores, and may in turn be eaten by top predators. Some species feed at more than one tropic level, hence are termed omnivores. Figure 1 provides a simplified model of such a food web.
We can look at this food web in two ways. It can be a diagram of the flow of energy (carbon) from plants to herbivores to carnivores, and so on. We will take this approach when we examine energy flow in ecosystems. In addition, members of a food web may interact with one another via any of the four interaction types named above. An interaction between two species in one part of the web can affect species some distance away, depending on the strength and sign of the inter-connections. Often, adding a species (as when an exotic species invades a new area) or removing a species (as in a local extinction) has surprisingly far-reaching effects on many other species. This is due to the complex inter-connections of species in ecological webs.
Ecologists use the following terms to describe various categories of the effects of a change (in abundance, or presence vs absence) of one species on another.
- Direct effects refer to the impact of the presence (or change in abundance) of species A on species B in a two-species interaction.
- Indirect effects refer to the impact of the presence (or change in abundance) of species A on species C via an intermediary species (A --> B --> C).
- Cascading effects are those which extend across three or more trophic levels, and can be top-down (predator --> herbivore --> plant) or bottom-up (plant --> herbivore --> predator).
- Keystone species are those which produce strong indirect effects.
The keystone species concept is one of the best-known ideas in community ecology. Although it
In the rocky inter-tidal zone of Washington state, and in other, similar areas, starfish have been shown to be keystone species
Instances are known where a predator so strongly suppresses its prey (herbivores), that the trophic level below (plants) benefits because it is released from the pressures of herbivory. Such “top-down” trophic cascades, where the community looks more or less ‘green’ depending on the abundance of predators, are well-known in lakes. We also know of examples where fertilizing a system, which increases plant growth, results in more predators, through the increase in abundance of herbivores. This is a “bottom-up” trophic cascade.
Our understanding of these complex species interactions gives substance to the popular phrase, the “balance of nature”. One can also appreciate how a human-induced removal of one species (an extinction event) or the addition on one species (invasion of a community by a non-native species) could result in harm to many additional species, a topic we will consider in the second semester.
We will gain a fuller appreciation of the complex, multi-way interactions among species as we proceed through this series of lectures. However, we can fully appreciate the complexity of these multi-way interactions, it is helpful to first understand the nuances of the various two-way interactions. We will develop our understanding of species interactions in ecological communities based on these building blocks.
Mutualistic Interactions
A mutualism is an interaction where both sides benefit. Pollination is a common mutualistic interaction. The plant gains gamete transfer, the animal gets nectar (and also pollen).Facultative mutualisms are beneficial but not essential to survival and reproduction of either party. Obligate mutualisms are those that are essential to the life of one or both associates. We will examine an example of each.
A fascinating facultative mutualism involves the Boran people of Africa, and a bird known as the honey guide. According to rock paintings, humans have collected honey in Africa for 20,000 years. H
Many other examples of mutualisms may be familiar to you.
- Gut symbionts in herbivores: mammals can't digest cellulose
- endosymbiosis and the origin of eukaryotic cells: mitochondria, flagella, chloroplasts are thought to be derived from free-living bacteria
- pollination systems
- the coral polyp and its endosymbiont "alga" (actually a dinoflagellate)
Commensalism
When one species benefits, and the other species is neither benefited nor harmed, the interaction is "+/0". In the southeastern US and in South America, it is common to see egrets in cattle pastures. They follow the cattle, eating insects that are dislodged or forced to fly as cattle graze in the field. One might suppose that egrets benefit cattle, by consuming insects that might compete with cows for food. The interaction would be a mutualism if this was demonstrated (but it seems a bit far-fetched). Assuming no benefit to the cattle, this is a commensalism. It often is the case, as this example illustrates, that we aren't sure if the interaction is "+/O" or "+/+".The clown fish and anemone also illustrates this point. The clown fish hides from enemies within the stinging tentacles of a sea anemone, to which the clown fish is immune. Some report this interaction as a mutualism, arguing that the clownfish drops scraps of food into the mouth of the anemone. Careful studies have failed to find much support for any benefit to the anemone, so this appears to be a commensalism.
Summary
Species interactions within ecological webs include four main types of two-way interactions: mutualism, commensalism, competition, and predation (which includes herbivory and parasitism). Because of the many linkages among species within a food web, changes to one species can have far-reaching effects. We will next examine competition and predation, and then return to a consideration of more complicated indirect and cascading effects.