Modes of Competition
Interference vs. exploitation:
– Direct aggressive interaction between individuals
– Using up resource
Intraspecific:
– Competition with members of their own species
Interspecific:
– Competition between individuals of two species – reduces fitness of both

Resource Competition
Intraspecific Competition - Herbaceous Plants
– Tilman + Cowan (1989):
– Indiangrass grown at two densities
7 per pot; 100 per pot
Different Nitrogen
– Plant growth rates and weights increase in low density populations

Resource competition
Competition for resources is more intense at higher population densities
– Usually leads to mortality among competing plants

Intraspecific Competition Among
Planthoppers
Denno and Roderick (1992)
Experiment: 3,11,40 leafhoppers per cage with salt marsh grass

– Attributed prevalence of competition among leafhoppers to habit of aggregating, rapid growth, and mobile nature of food supply
Fig 13.6

Niches
Niche:
– summarizes environmental factors that influence growth, survival, and reproduction of a species

Gause :
Principle of Competitive
Exclusion
– Two species with identical niches cannot coexist indefinitely
One will be a better competitor and thus have higher fitness and eventually exclude the other

Niches
Hutchinson:
– defined niche as:
– n-dimensional hyper-volume n is number of environmental factors important to survival and reproduction of a species

Niche
– Fundamental niche – hypervolume
All of these environmental factors
– Realized niche: includes interactions such as competition that may restrict environments where a species may live

Feeding Niches of Galapagos
Finches
– Grant (1986): differences in beak size among ground finches - translated directly into diet
Fig 13.8

Size of seeds eaten estimated by measuring beak depths
– Individuals with deepest beaks fed on hardest seeds
Fig 13.9

After 1977 drought, remaining seeds were very hard
– thus mortality was most heavy in birds with smaller beaks
Fig 13.10

Fig 13.11

Mathematical and Laboratory
Models
Models:
– Abstractions and simplifications, not facsimiles of nature
– Man-made construct; partly empirical and partly deductive
– Used to provide insights into natural phenomena

Lotka Volterra
Effect of interspecific competition on pop. growth of each species:
– dN
1
/ dt = r m1
N
1
((K
1
-N
1
-

12
N
2
) / K
1
)
– dN
2
/ dt = r m2
N
2
((K
2
-N
2
-

21
N
1
) / K
2
)

12: effect of individual of species 2 on rate of pop. growth of species 1

21: effect of individual of species 1 on rate of pop. growth of species 2

Paramecia Lab Experiments
Gause tested Lotka-Volterra predictions
He demonstrated resource limitation with
Paramecium caudatum and Paramecium aurelia in presence of two different concentrations of Bacillus pyocyaneus

– When grown alone, carrying capacity determined by intraspecific competition
– When grown together, P. caudatum quickly declined
Fig 13.15

Competition and Niches
Competition can restrict species to fewer environmental conditions
– But if competitive interactions are strong and pervasive, may produce evolutionary response in competitor population
Changes fundamental niche

Competition Examples
Tansley (1971):
2 species of bedstraw that grow in different soils.
Mutual competitive exclusion?
Experiments

Tansley’s results suggested interspecific competition restricts realized niche of each of two species of bedstraw ( Galium spp.) to a narrower range of soil types
Fig 13.18

Connell (1961)
Found interspecific competition in barnacles
Balanus play role in determining lower limit of
Chthamalus within intertidal zone
– Exposure to air did not account for all observed patterns

Fig 13.20

Competition Examples
Brown and Munger (1985) studied competition among rodents in Chihuahuan
Desert
Largescale experiments: 20 ha site
– 24 plots that were 50 x 50 m

Definitions of mammal groups:
Granivores: seed eaters
– Large = Dipodomys - 100 g kangaroo rats
– Small = Perognathus - 15 g pocket mice

Insectivores: eat insects
– Onychomys - 30 g grasshopper mice

Experiments in enclosures:
Wire mesh buried, and too small for rodents
Holes in fence initially to allow all in
Then exclude large kangaroo rats
– Little guys should increase if they were competing for food

Fig 13.23

Character Displacement
Because degree of competition is assumed to depend upon degree of niche overlap,
– interspecific competition is predicted to lead to directional selection for reduced niche overlap

Character displacement:
Fig 13.25
sympatry allopatry

Character Displacement
Taper and Case: Necessary criteria:
– Morphological differences between sympatric species greater than differences between allopatric populations

– Differences between sympatric and allopatric populations have genetic basis
– Differences between sympatric and allopatric pops. evolved in place: not derived from different founder pops already differing in the character

Taper and Case: Characteristics
– Variation in character must have known effect on use of resources
– Must be demonstrated competition for the resource and competition must be directly correlated with character similarity
– Differences in character cannot be explained by differences in resources available to each of the populations

Summary
Studies of intraspecific competition provide evidence for resource limitation
A niche reflects the environmental requirements of a species
Mathematical and laboratory models provide theoretical foundation for studying competitive interactions in nature
Competition can have significant ecological and evolutionary influences on the niches of species