Ecology Lecture 9 Ralph Kirby

The struggle for Existence Remember Population Interactions –Neutral 0 0 –Mutualism + + –Competition - - Note interspecies competition –Seek a common resource in short supply Food; Living space; etc See squirrels, mice, deer, various birds competing for acorns Model One. Two forms –Exploitation –Interference Model Two. Six forms –Consumption –preemption\ –Overgrowth –Chemical interaction –Territorial –encounter

Consumption –Utilization of a shared resource by 2 species Preemption –Occupation of a site by 1 st organism stops occupation by 2 nd organism Usually sessile organisms Overgrowth –Where organism covers another preventing access to a resource Light in rain forest Chemical interaction –Release of toxin to kill competing organisms Antibiotics in soil by Streptomyces Territorial –Behavioral exclusion of 1 st organism by 2 nd organism defending territory Encounter –Non-territorial encounters cause a negative effect on one or both species Lion and wild dogs over a antelope kill

Mathematical approach N: population size t: time K: carrying capacity r: rate of increase αN 1 and βN 2 : effect of interspecific competition, namely, where α and β per capita effects of competition –Species 1 dN 1 /dt = r 1 N 1 ((K 1 -N 1 -αN 2 )/K 1 ) –Species 2 dN 2 /dt = r 2 N 2 ((K 2 -N 2 -βN 1 )/K 2 ) No interspecific competition, then α and β are 0 and normal growth to carrying capacity Interspecific competition is density dependent

(a)Species 1 alone or no competition (b) Species 2 alone or no competition (c) Species 1 inhibits growth of species 2 and latter goes extinct (d) Species 2 inhibits growth of species 1 and latter goes extinct (e) Unstable situation, both inhibit in a density dependent manner. Depending on initial density, either can make other extinct (f) Each species inhibits its own population growth more than competitor. Neither can eliminate competitor Diagonal lines are zero growth isoclines

Laboratory experiments to verify Lotka-Volterra Equations –Two Paramecium One with higher rate of growth –Extinction of slower grower With different food supplies –Coexistence –Two Diatoms Require silica in water for growth –Adequate silica Coexistence –Inadequate silica One goes extinct

Competitive exclusion principle Two species possessing exactly the same ecological requirements cannot coexist –One species must go extinct Not normal situation –Immigration –Use of other resources –No selection –No changes in the environment Thus most of the time species can coexist

Thus if environment varies Then competition varies See secondary succession success in graph Also moisture –Grass species southern African savanna –Species change 1971 to 1981 Low rainfall in This is the norm

Systems are not simple one resource situations –Usually competition for more than one resource Territorial defense against wide range of other species Plants –Monoculture –Root competition Skeleton weed reduce by 35% –Shoot competition Skeleton weed reduced by 53% –Root and shoot competition Skeleton weed reduced by 69% –Thus clover superior to skeleton weed for all resources

Effect of interspecific competition across an environmental gradient Note changes in response when in mixture

Similar effect for summer annuals and moisture gradient Also happens in nature with water, anoxia and salt stress in a salt marsh Chipmunks –Alpine Cold tolerant –Lodgepole Most aggressive Needs shade –Yellow Pine aggressive –Least Heat tolerant

For competing species to all survive –Partitioning of resources –Cat species in Israel Cats with larger canines can kill bigger prey Note regular spacing of sizes Therefore distinct prey groups

Evolution is thus affected by competition Selection causes changes in resource utilization

See finches on Galapogos islands Beaks of same species are different when they compete

Fundamental Niche –Where species can exist with no competion Realized Niche –Niche utilized under competition See cattail in water depth experiment See two models for species competition –Compression –Splitting to suboptimum periphery

See effect of environmental gradients Note effect of competitive release