Based on Mader, Sylvia S. 1996. Biology - 5th Ed. WCB and Cox, G.W. 1997. Conservation Biology - 2nd ed. WCB
In any biological community, there may be several things in short supply for the organisms. We call these things resources, and the struggle to obtain them competition. On this page, we will examine the causes and effects of competition. If you haven't reviewed basic ecology yet; perhaps you better do so now: Review Basic Ecology
When discussing competition, one of the most important, basic concepts is that of the niche. The niche is the functional relationship of an organisms to its physical and biological environment. Niches are multidimensional in that they include a wide variety of aspects of the environment which must be considered. Normally, when defining a niche we look at 3 sets of parameters:
of physical factors for survival and reproduction: temperature, humidity,
pH, soil, sunlight, etc.
Gardeners of course are familiar with many of these parameters. When you buy plants or seeds you buy them with your garden in mind; some plants cannot live in wet soils, others require shade, still others need acid soils. Animals likewise have similar requirements; a tropical lizard like the iguana cannot survive the cold winter of a temperate forest.
With a definition of niche in hand, we can now turn to competition. Simply put, competition can be defined as follows:
The reason is simple enough. If two organisms have the same requirements - for food, water, nesting sites, whatever - there will not be enough of that thing to go around (technically speaking, the things the organisms are competing for are called resources). Competition can be divided into two main types, intraspecific and interspecific.
Intraspecific competition occurs within the
species. The niche overlap here is near 100%. Individuals of the
same species compete for the exact same things in the environment, therefore
this is the strongest type of competition. Interspecific competition
is waged between species. It can be fierce, if the competing species are
similar, but it is never as strong as intraspecific competition. It is
easy to see why. Take squirrels and chipmunks, for example. They
compete for acorns and a few other resources, but they do not compete with each
other for nesting sites (squirrels nest in trees, chipmunks underground), or for
mates (one hopes). Therefore while a chipmunk might have to work very hard
to get acorns before a rival squirrel does, he will compete more strongly with
another chipmunk who will also be after his nest and any female chipmunks in the
Whatever the type of competition, it will be strongest at high population densities. The more organisms there are, the more strongly they will compete for the remaining resources.
shapes the natural world to a great extent. One principle of ecology is
exclusion, which states that two species competing for the same limiting
resource in an area cannot coexist. This means that it is rare indeed to
find two very closely related species in the same area. If they are
closely related they will compete for almost all of the same things, and this
will mean that inevitably they will compete for some item that is in short
supply (the limiting resource). One species or the other will be better at
the competition and will displace the other. If two similar species are
found in the same place, careful; examination often finds that they differ in
the way they use the resources is some critical, but often not apparent
way. For instance, a series of birds might feed in the same trees for the
same insects. However, each species might forage only on a particular part
of the tree - such as the top, the bottom, the inner branches, etc. In
this way they reduce the competition and are able to co-exist. Another
example from the eastern deciduous forests of North America is provided by the nuthatch
and the brown creeper. These two birds forage on the same trees for
insects, however the creeper works its way UP the trunk, while the nuthatch
forages Down the trunk. In this way they see (and eat) an entirely
different set of insects. Another way of stating the competitive exclusion
principle is two species cannot share the same niche.
One trick that some organisms use to reduce competition within the species is to partition the habitat between the young and the adults. Many insects do this; caterpillars, for instance, feed on plants while butterflies only sip nectar. Likewise, tadpoles live in the water feeding on algae while the adult frogs are carnivorous, and, in many species, live away from the water.
Competition in Detail:
We should start by defining resources more specifically. Resources include food, water, mates, a place to grow (for organisms like plants, mussels, etc.), a hiding place or other shelter, etc. By definition, a resource is something that can be used. It is important to differentiate resources from conditions; the latter include things like temperature or pH. Conditions may affect organisms, but they cannot be used up (on the other hand, basking sites that allow turtles to warm up in the sun are a resource). Resources can be renewable or non-renewable; the former are regenerated while the latter are not. Food and water are usually considered renewable, while space is generally non-renewable. Some renewable resources like rainfall can't be controlled by the organisms; others, like food, are under the control of the organisms. If one particular resource is in short supply and thus limits the size of the population, we call that a limiting resource.
The first experimental demonstration of competition between related species was done in a common garden experiment carried out by A.G. Tansley. A common garden experiment is a classic study designed to control for many of the variables that would otherwise creep into an experiment. Because the plants are in the same garden, day length, sunlight, rainfall, temperature and a host of other variables are the same. The experimenter imports the plants and perhaps their native soils to conduct the experiment. Tansley worked with 2 species of plants in the same genus, one from acidic soils and one from alkaline soils. In nature, neither of the species was found outside its normal soil type. In the common garden, Tansley placed both types of soil. He found that each species outcompeted the other on its "home turf", although if grown without the competitor, both plants could survive on the "foreign" soil. The fact that neither could survive with its competitor on the competitor's soil type would thus explain the fact that the two species are not found growing in the same place in nature. Presumably, such competition could explain many distributions in nature.
We've already mentioned that competition within a species (intraspecific competition) is stronger than interspecific competition. But, each type of competition also has different effects. Intraspecific competition is the mechanism by which some populations control their size - poorer competitors get less food and die. It is also a strong force in evolution; better competitors get more resources, survive better, and have more offspring. Those offspring will carry the genes of the better competitors; the genes of the weaker competitors will be lost. As mentioned above, interspecific competition probably is one of the main factors controlling the distribution of species. Also, when two species compete, the population of both species will be lowered as a result.
The concept of competitive exclusion arose from the work of G.F. Gause, a Russian ecologist who grew different species of Paramecium in cultures. Grown by themselves, both species did well and typically leveled off at a certain population size. Grown together, one of the species dominated and would gradually eliminate the other. Another ecologist, Vito Volterra, was able to produce a mathematical model of competition. Volterra's model uses a competition coefficient to simulate the effect of each population on the other. In effect, Volterra's model works something like this: You derive an equation to simulate the growth of a squirrel population on its own. Next, you determine the effect that another species - say chipmunks - would have on the squirrel population. You might determine that in terms of common resource use, one chipmunk is the equivalent of 1/4 of a squirrel, so you would use the coefficient of 0.25 and multiply that by the number of chipmunks to get the overall effect (squirrel equivalents) that the chipmunks have on the squirrels. You make a similar determination to see what effect the squirrels have on the chipmunks. If both of these coefficients are less than 1, then the species may find a balanced equilibrium. If one or both numbers are greater than one, then usually the species with the larger coefficient will replace the other - i.e. competitive exclusion. Closely related species are more likely to have numbers greater than one. A graph showing the possible outcomes is shown here:
In the graph, the alpha values are the competition coefficients. Note that a stable equilibrium is possible only when both of the alpha values are low; all other cases end in elimination of one population or the other. Competitive exclusion in "nature" has most often been demonstrated with the release - accidental or otherwise - of introduced species, which often outcompete the natives (other introduced species cannot outcompete the natives and die out, but these examples are less obvious).
We have looked at the two types of competition - intra- and interspecific; but there are also two mechanisms of competition: exploitation and interference. In exploitation competition, organisms use up resources directly. Once used, the resource is no longer available for other species to use. In interference competition, one organism prevents other organisms from using the resource. Of the two mechanisms, exploitation competition is the more common. Interference competition can occur, particularly where the resource is "patchy" - only occurring in discrete patches - and thus able to be defended. An elephant might be able to prevent other animals from using a water hole, but would be unlikely to be able to chase them away from a river with its long banks.
Interference competition is not restricted to cases where one mobile answer physically chases off another. Sessile organisms and plants can practice interference competition as well. One common method is allelopathy, in which toxic chemicals are produced by one organism - often a plant - and directed at potential competitors. Walnut trees (right) are a common example; they produce toxic chemicals both at the leaves and from the roots that kill competitors. Some sponges and corals likewise engage in "chemical warfare". Desert plants space themselves - thus protecting an areas from which their roots will absorb infrequent rains - by putting chemicals into the soil as well.
Competition for space is one of the strongest competitions around, particularly if sessile (non-moving) organisms are involved. Once a plant or a barnacle or a mussel attaches itself to a place it is very unlikely to move; thus the space is out of play for other competitors until the organism occupying the space dies.
Here, open space on the rock has been settled by a number of young (small) barnacles. These shrimp relatives encase themselves in moveable rocky shells that are permanently glued to the rock. Some larger barnacles were already there, and some of the small barnacles have settled on them. This isn't good for them, since the bigger barnacles will get all the food (barnacles reach out from their shells to filter food from the water). The little guys have about as much chance of getting the food as Muggsy Bogues has of getting a rebound from Shaquile O'Neal - it may happen once in a while, but nobody's going to get fat from it. For barnacles, the trick is to get ot an open place before anyone else does and settle there.
|Left to themselves, barnacles can reach huge densities:|
But, barnacles are rarely left to themselves. Often, they have to compete with other organisms, such as mussels (above right), for space on a rock. Like the barnacles, the mussels attach themselves to a rock and filter food from the water. Competition for space is so severe that both mussels and barnacles will grow on rocks exposed at low tides, even though this means that they will not be able to feed for part of the day and that they will have to survive the hot sun and the dangers of desiccation. Often, the barnacles or the mussels will outcompete and eliminate the other in certain microhabitats on the rock. Consider the image below:
Over the surface of the rock the barnacles dominate, but in the crevices (inset) the mussels have the edge. Perhaps the barnacles are more able to withstand the rigors of the open rock but are unable to compete effectively in the moister, protected areas of the crevices. You can read more about barnacles, including a different rebound analogy, here.
The ecologist Robert Paine discovered that sometimes the winners in competition are mediated by other species, particularly predators. When he excluded the Purple Sea Star (Pisaster ochraeus) (below) from areas of rock, the resulting community often shifted. Apparently, the sea stars preferentially prey on the most abundant organisms (the best competitors) and these are often mussels. With the most abundant competitors removed, other, weaker competitors have a chance. Similar effects have been seen in some cases with grazing animals which remove the dominant grasses and allow others to flourish.
Above - The Purple Sea Star (Pisaster ochraeus), below, several of these sea stars (which range in color from purple to orange) are feeding. Once again, the mussels are black and the barnacles a lighter color.
Above: In the early 1980's, an invasive red algae, Bangia, was commonly found in the western basin of Lake Erie. This marine algae was able to compete well for space with the native green algae Cladophora, at least for the higher areas of rocks that were not submerged, but swept by waves. In the photo above a band of Bangia is indicated by the red area while the green Cladophora closer to the water is indicated by the green arrow. By partitioning the environment in this way the algae reduce competition and avoid competitive exclusion of either species. Other invaders of the Great Lakes? Click Here.
|Left: Competitive exclusion in action? Florida's native anole, the Green Anole (Anolis carolinensis, far left) is a direct competitor with the introduced Brown Anole (Anolis sagrei, near left). The Brown Anole was introduced into Florida in the late 1800's and has spread over much of the state. I find them much more commonly that the native Green Anoles, at least in habitats where humans are present. The Green Anoles still dominate in more natural settings. Where the two are found together, the Green Anoles are more common on the vegetation and higher up, while the Brown Anoles tend to occupy the ground and the tree trunks. Wheter the Brown Anoles will completely exclude the Green Anoles or whether they will partition the habitat remains to be seen. More here.|
|The spacing of plants in deserts is often the result of allelopathy. In order to take advantage of whatever rains fall, many desert plants have extensive, shallow root systems. To prevent seedlings or adjacent plants from sending their roots into the area staked out by an established plants, those established plants may leach toxic chemicals into the soil to kill their competitors. The final result is a landscape of plants with more or less uniform open spaces between them. The photos above are from the Mojave Desert of California; the photos below are from the Sonoran Desert of Arizona.|
In a forest, plants compete for light. Taller plants get more light and may block most of the light from reaching the forest floor. The Trillium, left, is like many other plants of the deciduous forest floor; it leafs out and blooms in the spring before the big trees get their leaves and shade the forest floor.
Strangler figs (below left) start their lives as seeds dropped high in the canopy. Here they leaf out and grow downward to get their roots to the soil. The roots continue to grow, strangling the tree which originally supported the fig. Using this top down approach to life, the strangler figs "win" the competition for light. Other tropical rainforest plants have to wait for a tree to fall, opening a light gap in the forest. Then, it is a mad scramble to grow towards the sun (below right).
The American Alligator (below left) and the American Crocodile (below right) are similar species but they avoid competition by occupying different habitats. The alligator is found in freshwater while the crocodile prefers saltwater. Because the crocodile's saltwater habitat was smaller to begin with - and has been more intensively impacted by humans - it is on the endangered species list. Alligators were also threatened with extinction, but regulations on hunting have allowed them to recolonize much of their extensive range.
|Like trees, corals depend on light. The coral animals harbor photosynthetic algae which supply the coral with much of its nutrition. To support photosynthesis, the coral needs light, and many corals will grow towards the sun, shading out their competitors as they do so. Other corals grow flat. They run the risk of being shaded out, but they are better able to survive hurricanes. The frequency of hurricanes determines which growth strategy is the better competitive bet.||
|Sometimes other factors affect competition. In the
Everglades, trees will replace the sawgrass that covers much of the
habitat. Sawgrass cannot compete with the trees for light, however,
sawgrass is better able to withstand fires than are the trees. Under
natural conditions, fires swept the Everglades with enough regularity to
ensure the sawgrass' dominance in most habitats. Human intervention,
however, reduced the number of fires and trees began to take hold.
Prescribed burning now seeks to mimic the natural fire cycle. The
trees in the image to the left have been killed by a fire and will be
replaced by sawgrass.
Many communities are so-called fire climaxes, where the dominant species are those adapted to fire. Without fire, other competitors would dominate.