Competition and Resource Scarcity

And NUH is the letter I use to spell Nutches, 
Who live in small caves, known as Niches, for hutches. 
These Nutches have troubles, the biggest of which is 
The fact there are many more Nutches than Niches. 
Each Nutch in a Nich knows that some other Nutch 
Would like to move into his Nich very much. 
So each Nutch in a Nich has to watch that small Nich 
Or Nutches who haven't got Niches will snitch. 

Dr. Seuss - On Beyond Zebra (1955)
10/26/2008 Format for printing

 We wish to learn:

  • What happens when two individuals, populations or species are sustained by the same resource?
  • Do populations compete for resources, or are their needs sufficiently different that they can be said to occupy non-overlapping niches?
  • What are the consequences of competition for the distribution and abundance of species with similar needs?
  • If there is any relationship between competition for a shared resource and evolution by natural selection?

Basic Concepts and Definitions

Biologists and naturalists have long been fascinated by differences among species. The Galapagos finches are a famous example of the many small but we think important differences among a number of species that otherwise so resemble one another they prompted Darwin to speculate that all were modifications (descendants) of a single type. These finches do differ in important aspects of body size, bill size and shape, whether they feed on insects or seeds, on large or on small seeds, and so on. Each seems to have its own niche, a concept we will define more carefully in a short while. But suppose we look at this group of finches in another way, note how similar they are, and ask whether any two species can be too much alike in their resource needs? How different must two species be to coexist in nature? What really is a niche, and can niches overlap? Do populations in nature compete?

We now have abundant evidence that competition is an important factor influencing present-day plant and animal distributions, and that competition plays a substantial role in evolution by natural selection. Today we will examine that evidence and the development of our understanding of the subject.

We will begin with a definition of competition, because this provides a frame of reference against which to examine evidence.

Competition occurs when two species each require a resource that is in short supply, so that the availability of the resource to one species is negatively influenced by the presence of the other species. It is a "-/-" interaction.

Competition does not occur if the resource is too plentiful to limit the growth, distribution or abundance of at least one of the populations. We expect only one or a few resources to be limiting and therefore to be competed for.

Competition can occur between individuals that are members of the same species. This is called intraspecific competition. Intuitively, intraspecific competition seems likely to be intense because two members of the same species are likely to have very similar resource needs. This sharing of resources among individuals of a single population ultimately limits how large a population can be in a given area (which really means, for a given amount of resources).  Today we focus on interspecific competition, which occurs between members of different species. The intensity of interspecific competition should be related to the similarity between the two species. We expect this to affect the amount of overlap in resource utilization (= niche overlap).

Three Classic Studies of Competition

Paramecium: a laboratory "bottle" experiment- Paramecium is a single-celled, ciliated protozoan that feeds on bacteria. It can be raised in a test-tube, in a broth of bacteria, which 
is its food supply. In fact, there are a number of species of paramecium, which can be raised in this fashion. What will happen if two species are raised separately, and then together? As Figure 1 shows, both species thrive when raised alone, reaching some limit set by the volume of the test-tube and the supply of bacteria. When raised together, the growth rate is slowed for both, until one species declines, and the other then grows to the capacity of the test-tube. Evidently these two ciliates are competing for a limiting resource, share the same niche, and competition determines that only one persists. This study, and others like it, were influential in establishing the fact that competition could occur, and could eliminate one species from a shared environment.

Results of Paramecium bottle experiment

The paramecium study raised a number of interesting questions. Does the same species always win out? Can two species ever coexist? How well can we extrapolate from these laboratory systems to nature? Many such "bottle" studies have now been performed, with flour beetles, Drosophila, yeast, various plants. In general, one species wins, and one species is eliminated. These experiments gave rise to the competitive exclusion principle:

No two species of similar requirements can long occupy the same niche (coexist).

In the case of paramecium, the same species always won. Studies with flour beetles, in contrast, found that changes in micro-environment (temperature, moisture) could shift the outcome in favor of one or the other species. The simplicity of such experiments remains as a nagging concern: do real populations, living in a complex, variable world, also experience competitive exclusion?
Warblers in a spruce forest: a field observational study - Warblers are migratory, spending the winter in the tropics, and flying to northern forests in early spring to set up territories, mate, and rear their young. An important study was made of five species of warblers in the genus Dendroica. These warblers are similar in body size, beak size, and all feed on insects. Moreover, they were studied in a spruce forest in northern Maine, almost as uniform a natural habitat as one could find. Surely if competition occurred in nature, this was the place to show it.
Cape May Warbler

A careful study of the foraging behavior of these five warblers found many differences in the way they made use of the seemingly homogeneous habitat. These species differed in what part of a spruce tree they frequented, whether they captured insects on the wing, from needles, or under bark, and so on. The conclusion was that each species occupied a distinct niche. Rather than showing that competition was important in nature, this study seemed to establish the opposite. Even superficially similar species really are different, upon close inspection. This study helped to establish the idea that each species occupies its own distinct niche.

The niche of a species includes all aspects of its habitat, how it makes a living, and the physical environment in which it is found.

Competition can be thought of in terms of niche overlap. A further consideration, raised but not answered by this study, is the degree of "permissible" niche overlap. Observational studies such as this one don't give a very clear answer to the question, how much did the warblers' niches overlap since all five species coexist, the answer seems to be, "not excessively".

Barnacles in the inter-tidal zone: a field experimental study - Barnacles occur on near-vertical rock walls in the inter-tidal zone, which means that they are submerged at some times and exposed to the air at others. Tidal fluxes of several meters occur twice daily.

tidal zone
 A study in Scotland examined the vertical distribution of two species: Balanus, which occurs lower in this zone, and Chthamalus, which occurs higher, even to the point where some individuals are just submerged during wave surges at the highest tide. Barnacles filter-feed small plankton from the water, so can only feed when submerged. They can close their oral region with calcareous plates to prevent desiccation during their exposure to air.
Observations of the two species could suggest that competition occurs, but not necessarily. The physical environment differs between upper and lower inter-tidal locations. Perhaps the niche of Chthamalus is high in this zone; the niche of Balanus is low in this zone. Barnacles have a larval stage, and larvae settle out of the water column onto hard substrates, metamorphose, and begin to grow. Larvae of each species settle in the zone of the other, but no adults are found (Figure 3). Does this mean neither can live in the zone occupied by the other?
Figure 3.

This study helped to pioneer the approach of field experiments. Using cages to exclude one or the other species, and by transferring slabs of rock from one region to the other, it was established that Chthamalus could thrive in the lower zone if protected from Balanus. Otherwise, Balanus, which is a larger and more robust species, crushed neighboring Chthamalus as it grew in size. Chthamalus was competitively excluded from the lower inter-tidal. At the upper limit, however, both species were limited by their ability to withstand desiccation. Chthamalus, the inferior competitor, was better adapted to desiccation and so occupied the higher portion of the inter-tidal zone.

Mechanisms of Competition

Competition often takes place via the using up or depleting of resources by members of one species, making those resources unavailable to members of the second species. Resources would include nest sites, food items, water or minerals in the soil. This is called indirect competition. It is likely to be most severe between individuals with similar needs for example, two species occupying the same trophic level, and with similar ecological roles or niches.

Direct competition occurs when members of one species interfere with or directly harm members of a second species. Examples include fights over space among animals, release of chemicals by plants (e.g., sage, creosote bush) that inhibit growth of another plant species, and the stinging tentacle of corals.

Competition and Evolutionary Specialization

Competition is likely to result in evolutionary divergence and specialization among closely related species. Consider the process of speciation, which is thought often to be gradual and include a period of selection for reproductive isolation once geographic isolation no longer separates the two diverging populations. We should expect those two species to occupy very similar ecological niches, hence to be likely to compete. Quite possibly, one species will eliminate the other. It also is possible, however, that the two species will diverge ecologically, perhaps via divergence in traits such as bill size that affects the size of food items consumed. This is known as character divergence.
divergence table of Galapagos finches
 The Galapagos Finches

The Galapagos finches appear to be good examples of character divergence. Three species of ground finches make a particularly strong case. If we assume there are seeds of different sizes -- small to large -- it makes sense that beaks of different sizes -- also small to large -- permit efficient handling and cracking of seeds in a specific size range. If only one species of finch exists, perhaps it will occupy the "middle ground" of beak and seed size. However, once speciation occurs, should one species be slightly smaller and one slightly larger, natural selection should favor individuals that are smaller than average within one species, larger than average within the other. Over time, beak size (the character that determines which portion of the resource spectrum is consumed) will diverge.

There is a complication, however. Suppose our large and small finches arose independently somewhere else, and happened to colonize various islands in one's, two's and three's. In the absence of competition, both a small and a large species might evolve towards an intermediate size. This is known as ecological release. Whether one is observing character displacement or ecological release then depends on the order and independence of speciation events. This is hard to know, but a good cladogram would resolve the issue.

Summary - The Consequences of Competition

Competition can be a powerful force affecting the growth, distribution, and abundance of populations in nature. This is supported by evidence from laboratory "bottle' experiments, field observations, and field experiments. Laboratory studies lead to the formation of the competitive exclusion principle (no two species can long occupy the same niche), while field observations suggest that niche differences, while sometimes subtle, are invariably found between co-existing species. Field experiments provide strong evidence of competition in nature. Often, one species is the superior competitor, the other is better able to withstand certain environmental extremes.

Competition is a powerful force of natural selection. There are many examples of closely related species that differ "just enough" to reduce niche overlap and permit coexistence. Genetic variation in a character that influences competition, coupled with selection to reduce competitive overlap, make a strong argument for the evolutionary divergence of characters.

Competition likely affects species diversity. In the short run, competition should cause a reduction in the number of species living within an area, preventing very similar species from co-occurring. In the long run, however, competition is likely to increase species diversity, by acting as a force for specialization and divergence.

Suggested Readings:

    Purves, W.K., G.H. Orians and H.C. Heller. Life: The Science of Biology. Sinauer, Sunderland MA.