Competition
Competition Models and in the Lab
Interspecific competition occurs when individuals of a species experience reduced growth as a result of resource exploitation or interference by another species (Begon et al. 1996).
Lotka (1925) and Volterra (1926), independently derived paired equations that describe competive populations.
Species 1 dN1/dt = r1N1 ((K1-N1-aN2)/K1)
Species 2 dN2/dt = r2N2 ((K2-N2-BN1)/K2)
Where r1 and r2 are the rates of increase for species 1 and 2 respectively;
K1 and K2 are carrying capacities for each species independently;
a
is the influence of one N2 on population growth rate of species 1;and
B is the influence of one N1 on population growth rate of species 2.This model makes a number of assumptions: (1) the environment is homogenous,
(2) the environment is stable, (3) migration is negligible, (4) there is no time lag; and
(5) the organisms are in competition for a single resource.
More complex models are required to deal with these and other assumptions.
Interactive Population Dynamics Models
More Interactive Population Models
Gause (1934) conducted competition experiments with two species of Paramecium - P. caudatum and P. aurelia. The two species were fed bacteria. When cultured separately each population exhibited a logistic growth pattern. However, the P. caudatum population decreased while the P. aurelia increased but failed to reach the level attained in monoculture.
Thomas Park studied competition between the flour beetles Tribolium castaneum and T. confusum. In competition, T. confusum usually won over T. castaneum.
The beetles were infested with a parasite that had an important effect on T. castaneum and not on T.confusum.
Park (1954) continued to study competition between these two species of flour beetles.
Park investigated the following combinations of temperature and relative humidity were with the following overall results:
|
|
|
|
Mixed species (% wins) |
Mixed species (% wins) |
|
|
Temperature oC |
% relative humidity |
Climate |
Single species numbers |
T. confusum |
T. castaneum |
|
34 |
70 |
Hot-moist |
T. confusum =T. castaneum |
0 |
100 |
|
34 |
30 |
Hot-dry |
T. confusum>T. castaneum |
90 |
10 |
|
29 |
70 |
Temperate-moist |
T. confusum<T. castaneum |
14 |
86 |
|
29 |
30 |
Temperate-dry |
T. confusum>T. castaneum |
87 |
13 |
|
24 |
70 |
Cold-moist |
T. confusum<T. castaneum |
71 |
29 |
|
24 |
30 |
Cold-dry |
T. confusum>T. castaneum |
100 |
0 |
Based on 20-30 contests for each combination.
Clearly the numbers attained by each species in allopatry was not sufficient to determine the outcome of the competition.
Tribolium are cannibals. T. castaneum is more cannibalistic than T. confusum. The observed population dynamics observed for them involve a combination of predation and competition.
Species may coexist if there are limitless resources, or if they do not have complete overlap of niches.
Competition in Nature
Schoener (1983) divided exploitative and interference competition into 6 categories as a function of mechanism:
The following table adapted from Ricklefs and Miller 2000 and based on data summarized by Schoener 1983 tabulates mechanisms of competion for different groups in several environments.
|
Group |
consumptive |
preemptive |
overgrowth |
chemical |
territorial |
encounter |
unknown |
|
freshwater |
|
|
|
|
|
|
|
|
plants |
0 |
0 |
1 |
1 |
0 |
0 |
0 |
|
animals |
13 |
1 |
0 |
1 |
1 |
5 |
2 |
|
marine |
|
|
|
|
|
|
|
|
plants |
0 |
6 |
4 |
1 |
0 |
0 |
0 |
|
animals |
9 |
10 |
6 |
0 |
7 |
6 |
0 |
|
terrestrial |
|
|
|
|
|
|
|
|
plants |
28 |
3 |
11 |
7 |
0 |
1 |
9 |
|
animals |
21 |
1 |
0 |
1 |
11 |
15 |
6 |
|
Total |
71 |
21 |
22 |
11 |
19 |
27 |
17 |
Some Lovely Experiments:
Hairston (1980) and competition between Plethodon jordani and Plethodon glutinosus
Connell (1961) examined settlement patterns of Chthamalus and Balanus. The Chthamalus zone was located above the Balanus zone in the intertidal. When he protected Chthamalus from Balanus the Chthamalus increased in the deeper zone that had been dominated by Balanus.
Literature Cited:
Connell 1961. Ecology 42:710-723.
Gause 1934. The struggle for existence. Williams and Wilkins, Baltimore.
Hairston 1980. Ecology 61:817-826.
Lotka, A. J. 1925. Elements of physical biology. Baltimore: Williams & Wilkins Co.
Park 1954. Physiological Zoology 27:177-238.
Ricklefs and Miller 2000. Ecology W.H. Freeman and Co. New York
Schoener 1983. American Naturalist 122: 240-285.
Volterra, V. 1926. Variazioni e fluttuazioni del numero d'individui in specie animali conviventi. Mem. R. Accad. Naz. dei Lincei. Ser. VI, vol. 2.