Chapter 13 Competition

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

Studies of Intra-Specific Competition

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

Laboratory Experiments for Inter-specific Competition

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

Field Experiments for Inter- specific Competition Removal Studies

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 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 allopatry sympatry

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