According to competition theory, when two species are close competitors (have high competition coefficients), the superior competitor (the one with higher carrying capacity) will often drive the other species to extinction. This is the basis of competitive exclusion.
As discussed chapter 18 of the textbook, periodic disturbance sometimes permits species to coexist that otherwise would not. If the species that is an inferior competitor can either better tolerate disturbances or recover from disturbances more quickly, it may be able to persist despite being in the presence of a superior competitor. For example, even if disturbances reduce the population sizes of both species, and the inferior competitor has a faster intrinsic population growth rate, it should be able to recover from the effects of disturbances more easily than the superior competitor can.
In this simulation, Species 1 (N1) has a high carrying capacity and a low intrinsic rate of increase. The second species (Species 2; N2) has a higher intrinsic rate of increase but a lower carrying capacity. Although Species 1 should outcompete Species 2 in the absence of disturbances, periodic disturbances may allow Species 2 to persist.
In the simulation, each simulation replicate will start with 100 individuals of each species and will run for 500 generations or until one or both species goes extinct (the population size, N , is less than 5). The characteristics of each species are listed below:
Species 1
K1 (Carrying capacity of Species 1) = 1,000
r1 (Intrinsic rate of increase of Species 1) = 0.02
α(Competition coefficient or the effect of Species 2 on the growth of Species 1) = 0.5
Species 2
K2 (Carrying capacity of Species 2) = 400
r2 (Intrinsic rate of increase of Species 2) = 0.05
β (Competition coefficient or the effect of Species 1 on the growth of Species 2) = 0.5
Open the simulation in a new window
Question 1
Check the box that says “No disturbance” and run the simulation in the absence of disturbances. Describe the population dynamics of both species.
Question 2
Repeat the simulation. What happens?
Question 3
Check the “Small disturbance” box and enter “50” for g. This parameter, g, adjusts how often disturbances will occur on average. Given a g of 50, a small disturbance will occur at random, on average every 50 time periods. This disturbance decreases the population sizes of both species by 50%. The randomness with which disturbances occur causes different replicates to vary.
Run 20 replicates of the simulation. Record how often both species persist, just one species persists, and how often neither species persists. Record how often Species 1 and Species 2 persist. In the cases where both Species 1 and Species 2 persist, record which species had the higher population size at the end of the replicate.
Question 4
Describe the ability of each species to recover from disturbances. What factor is responsible for any difference in this ability?
Question 5
Check “Large disturbance” box and enter “50” for g. A large disturbance will occur at random, on average every 50 time periods. This disturbance decreases the population sizes of both species by 75%. Run 20 replicates of the simulation. Record how often both species persist, just one species persists, and how often neither species persists. Record how often Species 1 and Species 2 persist.
Question 6
Explain the difference in the frequency of persistence for Species 1 and Species 2.
Question 7
Were there any commonalities in the replicates in which both species went extinct?
Question 8
Keep the box for “Large disturbance” checked and change g to “25.” A large disturbance will occur at random every 25 time periods, on average. Run 20 replicates of the simulation. Record how often both species persist, just one species persists, and how often neither species persists. Record how often Species 1 and Species 2 persist.
Question 9
Based on this exercise, what level of disturbance best maintains species that have different competitive abilities?
