Population Ecology

Population Ecology • What kind of ‘competition’ are we considering here? • Exploitative – depression of shared resources • Examples? Food, nutrients • Interference – actions reduce the exploitation efficiencies of others • Examples? Birds, ants, allelopathy

Population Ecology • Another category of competition is ‘pre-emptive’ competition • Irrespective of the type of competition, the result should be a lowering of all species in question, however, they should be modeled differently

Population Ecology • One problem with ‘interference’ competition is modeling it..why? • Because its very nature suggests it is a density dependent phenomena

Population Ecology

Population Ecology • Competitive exclusion of species 1

Population Ecology • Competitive exclusion of species 2

Population Ecology • Coexistence in a stable equilibrium;

Population Ecology • Competitive exclusion in an unstable equilibrium

Population Ecology • Competitive Exclusion • The worst scenario for species 1 will be if its own abundance is close to 0 (N1 ≈ 0) and the abundance of its competitor is close to carrying capacity (N2 ≈ K2)

Population Ecology • If species 1 can achieve positive growth, then it should be able to invade (e.g. [dN1/dt][1/N1] > 0) • Since r1 is always positive, it must be that (K1 – αK2)/K1 > 0 or K1 /K2 > α • For species 2 it is K2/K1 > β

Population Ecology • These numbers help explain the principle of competitive exclusion • If 2 species are close in resource use and body size, then α and β will be close to 1 • 1/β > K1/K2 > α for stable coexistence • 1/0.9 (1.1) > K1/K2 > 0.9 • 1/0.2 (5) K1/K2 > 0.2

Population Ecology • Populations are highly variable and differences may be the result of different r and starting population size

Population Ecology • It is counterintuitive that a small r is more stable. Why?

Population Ecology • Relationship between pop(n) variability and risk of extinction

Population Ecology • Adding a competitor or predator generally makes population dynamics less stable

Population Ecology • It is counterintuitive that a small r is more stable. Why?

Population Ecology • If the competitive or predatory relationship is particularly strong, we might see an ecological response (change in resource use or range shift) or an evolutionary response (e.g. character displacement or habitat shift)…more later

Population Ecology • Parasitism and herbivory rarely threaten a population extinction, but both can significantly depress a populations numbers

Population Ecology • Other factors that can influence population dynamics:

Population Ecology • Empirical demonstrations of competitive exclusion • Most successful examples are relatively simple cases in species-poor systems

Population Ecology • Growth of Paramecium separately and in the presence of one another

Population Ecology Given that one species is almost always more competitive than others, why don’t we see more extinctions? Variable competitive landscape

Population Ecology Park studied two species of Tribolum (flour beetles)

Population Ecology When conditions were altered to a cooler climate with less humidity, Tribolium confusum was the winner

Population Ecologycompetitive coexistence

Population Ecology Similar results have been shown with a series of fruit fly experiments (Ayala 1970) where Drosophila serrata would outcompeted D. pseudoobscura at temperatures >25o C but would win when temperatures were <22o C

Population Ecologycompetitive coexistence Competitive advantages vary with changing environmental conditions Coexistence can be possible even amongst strong competitors in a variable (spatially or temporally) environment Another option may be competitive advantages occur at different stages of the life cycle

Population Ecologycompetitive coexistence Hutchinson (1953) pointed out… periodicity of environmental fluctuation ecologically similar species generation time

Population Ecologycompetitive coexistence Huston (1979) has shown that introducing occasional episodes of strong density-independent mortality can almost indefinitely delay competitive outcomes

Population Ecologycompetitive coexistence Sale (1977, 1979) has examined the influence of ‘chance’ into competitive outcomes Some systems (e.g. corals) occupancy is completely random Consequently, no one consistently wins and all species are able to coexist

Population Ecologycompetitive coexistence Fig 2.2

Population Ecology • What about empirical demonstrations of predator-prey extinctions

Population Ecology • Huffaker’s (1958) classic mite study

Population Ecology simple predator-prey dynamics

Population Ecology