Competition Chapter 13

Outline Resource Competition Niches Mathematic and Laboratory Models Modes Niches Mathematic and Laboratory Models Lotka-Volterra Competition and Niches Character Displacement

Competition The effects of competition in nature can be observed through observation & experimentation. Many organisms compete for limited resources and may defend territories to protect their access to these resources.

Modes of Competition Interference Competition: Involves direct aggressive interactions between individuals.

Modes of Competition Intraspecific: Competition with members of the same species.

Modes of Competition Interspecific: Competition between individuals of different species. Reduces the fitness of both species. Reduction in fitness may not be equal.

Intraspecific Competition Among Herbaceous Plants Competition in a forest can be as intense as competition on a coral reef. Takes place mostly underground.

Intraspecific Competition Among Herbaceous Plants Plant growth rates and weights have been found to increase in low density populations. Competition for resources is more intense at higher population densities. Usually leads to mortality among competing plants - Self-Thinning

Intraspecific Competition Among Herbaceous Plants The self-thinning process can be represented by plotting the average weight of individuals vs. density. Slope averages -3/2. -3/2 thinning rule.

Intraspecific Competition Among Planthoppers Denno and Roderick attributed prevalence of competition among Homoptera to their habit of aggregating, rapid growth, and the mobile nature of their food supply. Demonstrated intraspecific competition within populations of planthopper Prokelesisia marginata. Probably result of limited resources.

Interference Competition Among Terrestrial Isopods Survival was lower at higher population densities. Food was not limited. Cannibalism may have been a factor. Interference competition can occur in the absence of obvious resource limitation.

Niches Niche: Summarizes environmental factors that influence growth, survival, and reproduction of a species. An organism’s role within the environment.

Niches 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 equals the number of environmental factors important to survival and reproduction of a species. Fundamental niche – hypervolume or the entire area where a species could survive in the absence of biotic interactions. Realized niche includes interactions such as competition that may restrict environments where a species may live.

Feeding Niches of Galapagos Finches Grant found differences in beak size among ground finches translates directly into diet. Size of seeds eaten can be estimated by measuring beak depths.

Feeding Niches of Galapagos Finches Individuals with deepest beaks fed on hardest seeds.

Feeding Niches of Galapagos Finches After 1977 drought, the remaining seeds were very hard. Thus, mortality was most heavy in birds with smaller beaks.

The Habitat Niche of a Salt Marsh Grass Spartina angelica is a species produced by allopolyploidy – a process of speciation initiated by the hybridization of two species. S. maritima (European) S. alterniflora (North American) The hybrids doubled their chromosome number and were then able to reproduce as a new species.

The Habitat Niche of a Salt Marsh Grass This species has an increased tolerance for water-saturated soil and can inhabit lower zones in a salt marsh.

Mathematical and Laboratory Models Models provide a theoretical foundation for studying interspecific competition in nature.

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

Modeling Competition The effect of competition on the growth of a population can be predicted by mathematical models.

Lotka Volterra Lotka-Volterra competition models are based on the logistic equation of population growth – the s-shaped curve.

Lotka Volterra Four possible outcomes of the Lotka-Volterra competition equations: Species 2 eliminated. Species 1 eliminated. Either species 1 or species 2 eliminated, depending on starting conditions. Both species coexist.

Lotka Volterra In general, LV predicts coexistence of two species when, for both species, interspecific competition is weaker than intraspecific competition.

Paramecia Lab Experiments Gause demonstrated resource limitation with Paramecium caudatum and Paramecium aurelia in presence of two different concentrations of food - Bacillus pyocyaneus.

Paramecia Lab Experiments When grown alone, carrying capacity was determined by intraspecific competition. When grown together, P. caudatum quickly declined. Reduced resource supplies increased competition.

Flour Beetle Experiments Tribolium beetles infest stored grain products. Park studied interspecific competition between T. confusum and T. castaneum under varied environmental conditions.

Flour Beetle Experiments Growing the two species together suggested interspecific competition restricts the realized niches of both species to fewer environmental conditions. Hot-wet conditions favor T. castaneum. Cool-dry conditions favor T. confusum. Unpredictable results with intermediate conditions.

Competition and Niches Competition can restrict species to their realized niches. But if competitive interactions are strong and pervasive enough, they may produce an evolutionary response in the competitor population. Changes fundamental niche.

Niche Overlap and Competition Between Barnacles Connell examined the relationship between two kinds of barnacles – Balanus balanoides and Chthamalus stellatus. Chthamalus is found in the upper intertidal. Limited by competition with Balanus. Balanus is found in the mid & lower intertidal. Limited by exposure to air in upper intertidal.

Niche Overlap and Competition Between Barnacles The fundamental niche of Balanus is equal to its realized niche. The fundamental niche of Chthamalus is much broader than it’s realized niche. Both are limited by predation in the lowest part of the intertidal.

Competition and Niches of Small Rodents Brown studied competition among rodents in Chihuahuan Desert. Three factors make this experiment stand out. Large scale Experimental trials well replicated Long term

Competition and Niches of Small Rodents Brown predicted if competition among rodents is mainly for food, then small granivorous rodent populations would increase in response to removal of larger granivorous rodents. Insectivorous rodents would show little or no response. Results supported hypothesis.

Competition and Niches of Small Rodents

Character Displacement Individuals that compete less should have higher fitness than those that compete more. Because degree of competition is assumed to depend upon degree of niche overlap, interspecific competition has been predicted to lead to directional selection for reduced niche overlap. Evolutionary divergence in niches.

Character Displacement The process of evolution toward niche divergence in the face of competition is called character displacement.

Character Displacement Example: Galapagos finches, G. fortis and G. fuliginosa. Allopatric (living apart) on some islands. Both species have similar beak sizes. Sympatric (living together) on another island. No overlap in beak sizes. Different feeding niches – reduces competition.

Character Displacement

Character Displacement Taper and Case: Necessary criteria: Morphological differences between sympatric species are statistically greater than differences between allopatric populations. Differences between sympatric and allopatric populations have genetic basis. Differences between sympatric and allopatric populations evolved in place, and are not derived from different founder groups already differing in the character.

Taper and Case: Characteristics Variation in the character must have a 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.