Competition

Two or more consumers may compete for a limited resource. If the consumers are of the same species then intraspecific competition is said to occur. Intraspecific competition can be for nest sites, mates, or food. The results of intraspecific competition leads to density dependent  birth and death rates (as density goes up, the birth rates drop while death rates increase).

If the consumers are different species, then interspecific competition is said to occur. Under these conditions, the birth and death rates of one population affect these rates in the second population. While intraspecific competition results in regulation of the specie's population, interspecific competition can result in one species dominating the other, even to the point where the second species will go extinct.

Figure 1 depicts competition between sage (on the left) and a mixed grassland. A 2-meter wide swath of bare ground surrounds the sage (A) while an even wider area where the growth of wild oat and bromegrass is inhibited is seen at "B". Full-growth mixed grassland is shown at "C". Click on the image for an aerial view.

Figure 2 shows the results of competition between two species of Paramecium in the laboratory. Grown separately, both species do well. When grown together, however, P. aurelia out-competes P. caudatum; eventually leading to P. caudatum's extinction in the culture dish.

In both of these cases it is clear that the successful species are "using up" some resource in the environment that is in short supply. In the first example, the sage is removing nutrients from the soil that are needed by the other grasses (the sage and zone A in figure 1). In the B zone, the nutrients are depressed, but grass species other than wild oat and brome grass are able to grow at low concentrations. For our second example, P. aurelia takes up space faster than P. caudatum and quickly over-runs the culture.

Conceptually we can look at the environment as a limited resource that can be shared by one or more species (Figure 3; wait for it to recycle or press the refresh button on your browser, if necessary). Here, an environmental resource, shown in green, must be shared between the little blue and big red species. It's possible to fill the environments with lots of little blue blocks, a few red blocks, or a mixture of reds and blues. Note that in this example, four blues equal one red individual (the blues need 1/4 of the space, food, or other resource as red individuals). From the environment's point of view, the blues are 0.25 of a red (species A as B; we'll term this as "a"), Red individuals are worth 4 blues (species B as A; we'll call this term "b"), Although in this example the two species are symmetric, they don't necessarily have to be.

Recall our original logistic growth model (Figure 4). Species 1 can fill the environment entirely with their own kind (with K₁ individuals), or with species 1 equivalents (K₁/a); Figure 5 shows the growth of species 1 (blue) plotted against species 2 population size. The environment is filled when there are   K₁ individuals of species 1 (filled to the carrying capacity with no species 2 individuals) or there are K₁/a individuals of species 2 (and no species 1 individuals). The line connecting K₁/a and K₁ is an equilibrium line where dN₁/dt=0 (the change in N₁ over the change in time=0). If the population size is small, species 1 will increase in size (arrow contained within the equilibrium) Any time the population "wanders" to the outside of the equilibrium line it is forced back, as shown by the arrows. It is also possible to fill the environment with a mixture of species 1 and 2 (Figure 6). It can be filled, for example, with a mix of 125 species 1 and 200 species 2 (Fig 6A) or 275 species 1 with 100 of species 2 (Fig 6B).

Species 2 growth can be similarly plotted (Figure 5). The environment can be filled with  K₂ individuals of species 2 or  K₂/b individuals of species 1. As long as the populations are below the equilibrium line, species 2 will continue to grow. If the population grows above the equilibrium line, the population is forced back and decreases (arrows for species 2 growth in Figure 5).

The equilibrium lines for both species can be plotted together on the same graph (Figure 7). The results can be used to predict the outcome of a two-species competitive event. In the first diagram species 1 wins because species 2's equilibrium line (red) is contained completely by species 1. If population numbers are below the green area in figure 7A then both species 1 and two will grow (arrows). In the green area, species 1 continues to grow (arrows point to the right) while species 2 is forced back to it's equilibrium (down arrows). Above the green area, both species 1 and 2 will decrease. Since species 1 can outgrow species 2, it will win the competition and species 2 will go extinct. In 7B species 2 wins (it outgrows species 1 in the yellow area).

In figure 7C and 7D the equilibrium lines cross. A stable equilibrium is shown in 7C. In the green area species 1 can increase while species 2 decreases. In the yellow area species 2 is favored. This forces the population toward a stable equilibrium point. In figure 7D an unstable equilibrium is depicted. In the green area species 2 will grow at the expense of species 1, while in the yellow zone species 1 increases at the expense of 2. The equilibrium is unstable because if one population increases or decreases only slightly, the populations will change so that only one wins. For example, assume that the populations are in equilibrium at the cross point in figure 7D. If species 1 increases by only one individual (say, by immigration), then it would find itself in the yellow area. Since species 1 can now grow, it will continue to do so. At the same time, species 2 populations will go down in the yellow area until species 1 finally wins. So, figure 7D shows a situation where an equilibrium point is possible, but highly unstable.

The equations for population growth are shown in Figure 8. Essentially, we have the same equations as our original population model (Figure 4), with the addition of the species interaction terms (a and b). The ((K₁ - N₁ - aN₂)/ K₁) portion of the equation expresses species 1 as species 1 (the  N₁ part) or as species 2 (the aN₂ part). Similarly the ((K₂ - N₂ - bN₁)/ K₂) portion of the lower equation shapes the results for species two.

Figure 9 shows some of the possible outcomes for our two species. Here, the resource continuum represents the limited resource that is at the center of the competition. As an example, assume that the resource is seeds and the X axis is seed size (small, medium, and large). Let's further assume that species 1 and 2 are finches. Finch 1 takes mostly smaller seeds while finch 2 take the larger seeds. Both, however, take medium-sized seeds.

Scroll down to the embedded program or start your competition program if the applet is not visible (NOTE: You must first install the program for this function to work! Install the stand-alone version here (If you're running Vista and are having problems try this version)). VISTA USERS READ THIS TO RUN THE PROGRAMS Note that for species 1, the starting population, reproductive rate, carrying capacity and species 1 as 2 can be entered .
In addition, you can also apply a random (stochastic) effect. The same options are available for species two. Press the "Show Me" button. Species two (red) quickly overwhelms species 1:

You could have seen the outcome anyway by examining the graph on the right of the simulation (Species 1 decreases while 2 increases over most of the area. Let's adjust the inputs to put species 2 at a disadvantage. For species 2 set K₂ to 100 and b to 0.7. Press the "Show Me" button.

Now species 1 (blue) wins. Again, by interpreting the graph to the right on the simulation, you could have predicted the outcome. Let's set up an equilibrium. For species 2 change b to 0.2 (leave K₂ at 100) and press the button.

This equilibrium is stable. We can make an unstable equilibrium thusly: Set  a to 1.0, K₂ to 350,  and b to 2.8 and press the "show me" button.

Although this looks similar to the previous condition, this is unstable (note that species 2 does not survive. Now let's see the difference between the unstable and stable situations from the population's point of view. Reset the variables thusly:
For Species 1: N₁=40, r₁=0.2, K₁=200, a=0.5
For Species 2:  N₂=10, r₂=0.2, K₂=100, b=0.2  Then press the button.

Both species co-exist. What happens if we drastically increase r₂   to 0.9? Do it and press the button.

Both species still co-exist, although the growth curves have changed. Now what happens with the unstable equilibrium? Set the values as follows:
For Species 1: N₁=40, r₁=0.2, K₁=200, a=1.0
For Species 2:  N₂=10, r₂=0.2, K₂=350, b=2.8  Then press the button.

Species 2 goes extinct. Now let's change r₂  to 0.9. Do it and press the button.

Now species 2 wins! While changing r made little difference in the stable model, it can change the outcome of an unstable model.

Competition Activity
Random environmental effects can be added to the simulation as shown in Figure 10 Below the species 1 properties, the random effect box (blue oval) has a value of 10 while the effect on species 2 is also 10. When one or more species is selected, the grid view is shown (since you'll be running a number of simulations. Leave the Fast! check box selected with 50 runs (that automatically runs 50 runs without you having to press the Show Me! button 50 times). You can view the graph by pressing the "Show Grid" button (twice), then you can toggle back and forth between the grid and graph view using the show grid/graph button (I'll fix that glitch later).

The grid records K for each species, alpha and beta, the random environmental effects for each species, the average number of individuals left at the end of the run and the % success (percent success is the number of runs in which the species did not go extinct over the 100 generations). For this situation species 1 reached 100 generations without going extinct 38% of the time, while species 2 was successful 88% of the time (your results will vary). You can copy the table to the clipboard to paste into Excel with the Clipboard button. NOTE: Questions for you to answer are scattered throughout this web page. To help you out, the questions are highlighted in red.

1. Explore the model. Set up a simulation where species 1 will win the competition without environmental effects. What is the effect of varying the environmental change on one or both species (i.e. a stochastic model) ? How does the magnitude of the environmental change relate to the success rate for the species? What happens if the environmental effects are not symmetric (both set at the same level). How does this relate to the real world?
2. Staying with the situation in 1, what happens if you change K for one or the other species? How does this relate to the real world?
3. What happens when you raise or decrease the reproductive rate(s) for one or both species? How does this relate to the real world?
4. Run through situations 1-3 for a stable competitive situation and answer the questions again.
5. Run through simulations 1-3 with an unstable situation and answer the questions again.
6. Try different levels of environmental effects starting with a stable competitive situation putting an r- versus a K- selected species (just change r, not K). Explain what's going on as related to pollution, habitat destruction, and other human-impact events as related to pest- and non-pest species?
   • Note: try the following to get started.
   • Species 1: N=10 K=200 r=.2 α= .5 (This is the K-selected organism)
   • Species 2: N=100 K=100 r=.9 β= .2 (This is the r-selected beast)
7. Re-run question 6 starting with an unstable equilibrium.
8. How do the above scenarios relate to r- and K- selection?
9. Can these same equations be used to model intraspecific competition? If so, come up with a scenario where these models would make sense.
10. Question 10 is located further down this page.

The Competitive Exclusion Principle

The competitive exclusion principle states that "No two species can occupy the same niche for a limited resource". The concept of niche is covered here. Essentially, the niche is a description of the ecological requirements of a species. If two species overlap in these requirements, then the potential for competition exists (Figure 11). Depending on the degree of overlap, the competition may be more or less intense.

Tests of competition typically compare the results of two putting closely-related species together. In laboratory tests, one species usually wins while in field tests, closely-related species tend to exist together indefinitely. Two explanations may explain this discrepancy: (1) Competition is rare in nature, and therefore unimportant ecologically or evolutionarily, or (2) competition has occurred historically in nature (most of the competitive interactions have been worked out among species and competition is important in explaining the evolution of ecosystems.

To answer this question, researchers observed the feeding behavior of warblers (all of genus Dendroica) in the field.

• All birds are insect-eaters and are about the same size
• Food can be found at different levels in the tree canopy (Figure 12).
• The birds also feed in different manners, move in different directions through the trees, and have different nesting needs.

These data suggest that the competitive arguments among these five bird species have been worked out historically. From an evolutionary perspective, we would expect competitive ability to evolve in the same way as other characteristics: If two or more species were competing for a limited resource, then all would benefit by evolving differences to reduce competition. Such differences can be seen in examples of character displacement (Figure 13). If two closely-related species occupy an overlapping geographical range, they should evolve differences in the area of overlap to avoid competition among these local populations. This change in anatomy or behavior is termed "character displacement".

Read these tomes on Character displacement (This is an assignment)

• Wiki Character Displacement
• Ecological Character Displacement and the Study of Adaptation PAY PARTICULAR ATTENTION TO THE EXPERIMENTAL METHODS AND CONDITIONS OUTLINED FOR CHARACTER DISPLACEMENT!!!

We can go to Darwin's finches to see examples of character displacement on the Galapagos Islands (Figure 14). Members of the genus Geospiza are wide-spread among the islands. Geospiza fortis, for example, is found alone on Daphne Island, while G. fulginosa is found alone on Crossman Island. Both ground-feeding birds are about the same size (Figure 14). On Charles and Chatham Islands, on the other hand, the species co-exist. Although G. fortis is about the same size as their relatives on Daphne, G. fulginosa is considerably smaller than their neighbors on Crossman. The shift in size allows the G. fortis to feed on smaller seeds, thus avoiding competion with the larger G. fortis on Charles and Chatham. This and other documented cases of character displacement suggest that competition is important in shaping ecosystems. Displacement is interpreted as evidence of historical competition. Modern communities show little evidence of competition except when habitats are destroyed and species are forced together.

Keep in mind that the environment is not static, and that the species are often going after a "moving target" (Figure 15). What effects would this have on the evolution of competitive abilities when comparing r- and K-selected orgamisms (why?)?

We don't want to leave the laboratory studies out of this discussion. One of the first experiments on competition in the laboratory involved two species of yeast: Saccromyces cervisiae and Schizosaccromyces kephir. When species are grown alone, they nicely fit the logistic growth curve (Figure 16). The researchers asked "what factors might depress their growth leading to the S-shaped curve?". They determined that it was not a food limitation (sugar is still available when the population levels off. Ethanol accumulation was shown to kill buds just before they separate from the mother cell. By artificially adding ethanol to the culture, they could depress colony growth. In addition, they discovered that pH was of secondary importance. The following attributes were estimated for the populations:

The researchers then placed the species together and watched the outcome. When Saccromyces were reared alone, they did much better than when reared in competition with Schizosaccromyces (Figure 17). Using the logistic equations they estimated the following quantities:

From the 3.15 to 0.439 (alpha to beta) ratio, it can be deduced that about 1 volume of Saccromyces equals 1.58 volume of Schizosaccromyces (Saccromyces K1 spaces are filled according to 1 vol of Schizosaccromyces = 1.58 vol Saccromyces). If ethanol is the limiting factor, the changing the ethanol concentration should affect alpha and beta. If we look at alcohol production by the species Saccromyces produces 0.113 units while Schizosaccromyces produces 0.247 units in the same amount of time. From this we can estimate alpha and beta independently:

a = .247/.113 = 2.18
b = .113/.247 = 0.46

These values are within the ballpark based on other observations (our 1.58:1).

Adaptations to Other Organisms
Adaptations occur among species and these adaptations can affect the composition and structure of biological communities. Six classes of interaction are possible, based on the benefit-costs for each species: All except neutralism imply that an evolutionary interaction can occur between the species.