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Ecological Communities: Networks of Interacting
Species
We wish to learn:
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What is an ecological community and what kinds of interactions take place within it?
How important are the various categories of species interactions, including mutualisms,
commensalisms, competition and predation?
What kinds of interactions among species become important when many species affect one
another?
What consequences do these interactions have for biodiversity
Format for printing
Species Interactions, Food Webs, and Ecological Communities
An ecological community is defined as a group of actually or potentially interacting species living
in the same place. A community is bound together by the network of influences that species have
on one another. Inherent in this view is the notion that whatever affects one species also affects
many others -- the "balance of nature". We build an understanding of communities by examining
the two-way, and then the multi-way, interactions involving pairs of species or many species.
type of interaction
sign
effects
mutualism
+/+ both species benefit from interaction
commensalism
+/0
one species benefits, one unaffected
competition
-/-
each species affected negatively
predation, parasitism, herbivory +/-
one species benefits, one is disadvantaged
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.
Generalized food web. A food web is an
assemblage of organisms, including
producers, consumers and
decomposers, through which energy
and materials may move in a
community
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 interconnections. 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.
1. Direct effects refer to the impact of the presence (or change in abundance) of species A on
species B in a two-species interaction.
2. 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).
3. Cascading effects are those which extend across three or more trophic levels, and can be
4.
affects
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 is true that many species
potentially interact with one another in a food web such as
depicted in Figure 1, in nature there are big players and little
players. The biggest players of all are referred to as keystone
species. This is a species whose presence or absence, or
substantial increase or decrease in abundance, profoundly
other species in the community. Evidence usually comes from
experiments in which one species is added to or removed from a
community. The name derives from the center stone in an arch
supporting its weight by inward-leaning stones. Removal of the
keystone causes the arch to collapse.
In the rocky inter-tidal zone of Washington state, and in other,
similar areas, starfish have been shown to be keystone
species The entire community lives on relatively vertical rock
faces in the wave-swept inter-tidal zone. The community of
marine invertebrates and algae are adapted to cling or adhere
to the rock face, where most fed upon the small animal life
suspended in the water (plankton). A bivalve, the mussel
Mytilus, is superior at attaching to rock faces, making it the
competitive dominant. A starfish (Pisaster) is an effective predator of the mussels, making space
available for other species, and consequently is critical to maintaining a diverse biological
community.
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. Human hunting parties are often joined by the greater honeyguide (Indicator
indicator), which leads them to bee colonies. In unfamiliar areas, the average search time was 8.9
hr when unguided, but only 3.2 hrs when guided by the bird. Borans use fire and smoke to drive
off the bees, break open the nest and remove the honey, but leave larvae and wax behind. The
bird gains access to larvae and wax. The use of fire and smoke reduces the bird's risk of being
stung, and humans increase accessibility of nests. According to the Borans, the honeyguide
informs them of: direction, from the compass bearing of bird flight; distance, from the duration of
the bird's disappearance and height of perch; and arrival, by the "indicator call". Birds and Borans
can survive without the other, but each benefits from this facultative mutualism.
Pseudomyrmex Ants attack a
Katydid placed on an Acacia Plant
A mutualism between certain ants and a small tree, the acacia,
provides an excellent example of an obligate mutualism. This
particular system has been extensively studied in Costa Rica. The
acacia provides a number of benefits to the ants, including shelter
(hollow thorns), protein (beltian bodies at tip of leaflets), nectar
(secreted near base of leaves). The ant (Pseudomyrmex) provides
several forms of protection. It attacks and removes herbivorous
insects, It also removes vines that might overgrow the acacia, and
kills the growing shoots of nearby plants that might become
competitors. It clears away leaf litter from near the plant, and since
the acacia grows in a seasonally dry environment where it
occasionally is threatened by fire, the ant's activities protect the
tree from fire damage as well.
Many other examples of mutualisms may be familiar to you.
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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.
Suggested Readings
Purves, W.K., G.H. Orians and H.C. Heller. Life: The Science of Biology. Sinauer,
Sunderland MA.
Self-Test
Take the Self-Test for this lecture.