Even though competition is a potentially powerful selective force, the ecological literature is not brimming with studies that have been carried-out in the field. It is a difficult process to study, or even, for that matter, find a good setting in which it can be studied.
Two crayfish that live in ajoining habitats in the upper midwest were the focus of a marvelous study: Bovbjerg,R.V. (1970) Ecological isolation and competitive exclusion in two crayfish (Orconectes virilis and Orconectes immunis) Ecology 51(2): 225-236.
These two species are native to the streams, wetlands and lakes of NW Iowa where the study was done. In this landscape, which even at the turn of the century was mostly unsettled and had extensive marsh lands, O. virilis inhabits the steams and large lakes and O. immunis inhabits the ponds. They rarely encounter one another and are almost never found living in the habitat of the other.
During years when seasonal rainfall is low (about every 10 years or so), the Little Sioux River becomes an intermittent stream; i.e. only the deeper pools retain surface water. These become ponds in the stream bed. During these periods, O.virilis will move downstream where there is flowing water and O.immunis will leave the drying ponds and move into the pools left by the stream.
When the normal rainfall again fills the stream in the spring, O. virilis will begin to move back upstream in the now flowing water. It is during this upstream movement by O. virilis that the two species come into contact. During this year, O. immunis disappears from the stream bed. These are the field observations.
Why don't the two crayfish come into contact in the ponds? Why does one crayfish disappear from the stream during times when the river has flowing water? It is tempting to ring the cause up to competitive exclusion by each species from their prime habitats. But, aren't there other explanations that are equally probable? For instance, isn't it more likely that the physical environments of the stream and pond are the forces that govern this separation of species? How does one sort out the causes? These are the questions.
The Pond Environment
The total respiration of the pond community drives the oxygen levels down during the night and on windless nights in summer the pond becomes anoxic. Can both species tolerate this anoxia?
In late summer, many of the small wetlands dry out. Crayfish can either walk to another pond or burrow to follow the water table. Can both species survive this drying? Is so, how?
The experimental evidence that Bovbjerg shows makes it clear that O. virilis is far less tolerant of low oxygen than O. immunis. But crayfish in nature have a recourse. They can crawl out of the pond during the night and stay along the wet margins of the pond. When a pond dries, however, the problem for O. virilis is much more difficult because it cannot burrow. Behaviorally, these two species are very different in this regard. O. immunis is an efficient burrower and will spend the winter several feet underground during the winter. These two differences lead Bovbjerg to conclude that the physical environment and not competition is the reason O. virilis is not an inhabitant of the ponds.
The Stream Environment
Since O. immunis is found living in the upper reaches of the stream during the early stages of renewed flow, there is no reason to expect that the stream environment per se is hostile to it. The question of the ability to negotiate current, however, needed to be examined. In field trials in a shute in the stream, each species demonstrated its ability to handle current. The results led Bovbjerg to conclude that there was no difference between the two in their abilities to move and maintain themselves in the current of the Little Sioux River.
The posibility then was that competitive exclusion may be a factor in the disappearance of O. immunis from the stream in the presence of O. virilis. But what resource in the stream did they have in common?
The Substrate
A large tank was divided into three sections, each filled with different substrates. Single species groups were placed in the tank and allowed to wander freely. Locations of each animal were plotted three times a day.
Given choices of substates composed of rocks or gravel or pond muck, both species spent most of their time in the rocks. The preferences were clear. In the lighted tank, the ability to hide was crucial. The best places were under rocks.
When both species were introduced and allowed to encounter one another, O. virilis took over the rocks and O. immunis spent most of its time in the muck. Clearly something had happened between these two species. What was the nature of the interaction?
Differential Aggressive Behavior
Intraspecific and interspecific encounters were arranged in large finger bowls and the behavior of the animals recorded. The results were clear. Large animals are dominant over smaller animals, males are more aggressive and dominant over females when they are the same size, and O. virilis is dominant over O. immunis. Given a limited number of rocks, O. virilis will occupy most of the crevices and most of the O. immunis will be left to wander the river bed in search of cover.
Protection given by crevices to crayfish is important during periods of flood, during the day when they vulnerable to predation and during molting, when their carapaces are soft and movements are difficult.
Conclusion: competitive exclusion of O. immunis by O. virilis from the stream is likely.
To assess the probability of competition we can apply a set of criteria to help us decide. They are stated in the form of questions and are not easy to answer. Such is the nature of competition.
The Criteria
Can each species live successfully in the environment of the other? If yes, go to #2.
Do the species have a resource in common? If yes, go to #3.
Is the resource in short supply? If yes, go to #4.
Does the shortage cause a negative effect on the biotic potential of one or both species? If yes, go to #5.
What is the mechanism of the competitive exclusion?
For instance, in the case of the crayfish, the differential aggressiveness of the two species, allowed one to occupy a "preferred" and safer crevice during the day.
Finally, it is important to be very careful when using competitive exclusion to explain some otherwise unexplainable separation of species in nature. There are likely to be many other explanations. Don't use competition without good evidence to support it.