ecology lecture 12: competition

what do organisms compete for?

- resources: food, water, light (among plants), space

- mates (apparent in animals + plants with pollen grain "racing" down the style to the ovary

intraspecific competition

- competition between individuals of the same species

- leads to the density-dependent logistic growth curve

interspecific competition

- competition between individuals of two different species

exploitation (indirect) competition

- occurs when one competitor uses resources and in doing so reduces the resource availability to the other competitor

- the most common form of competition in all organisms!

interference (direct) competition

- occurs when two competitors physically challenge or harm each other to obtain a resource

- reasonably common in animals

- also seen in plants, when one plant releases toxins to harm the competing plant (allelopathy)

example of competition with field experiments

- large acorn barnacles (Semibalanus) and small acorn barnacles (Chthalamus) are both found within intertidal zones

- large acorn barnacles are found throughout the lower + middle intertidal

- small acorn barnacles are found only in the upper intertidal

- when large barnacles are removed, the small acorn barnacles colonize all of the intertidal zone

- when small barnacles are removed, the large barnacles remain out of the upper intertidal

conclusion of competition experiment

- conclusion: competition influences distribution + tradeoffs

- large barnacles are superior competitors, keeping small barnacles out of the lower and middle intertidal

- large barnacles are susceptible to desiccation, which prevents them from surviving in the upper intertidal

competitive exclusion principle (asymmetrical competition)

- established by G. F. Gause in 1934

- two species cannot coexist in a community if they use the exact same resources in the exact same way

- one species will persist, while the other will be outcompeted and driven to local extinction

niche

- when, where, and how a species makes its living

- all of the environmental factors necessary for a species' existence (its survival, growth, reproduction)

- a species' role in its environment

- an n-dimensional hypervolume

fundamental niche

- the full set of resources, plus other biotic + abiotic requirements of a species

realized niche

- the restricted set of resources that a species is limited to, due to species interactions

redefining the competitive exclusion principle

- two species cannot occupy the exact same niche within a community

- very similar species can coexist by way of resource partitioning

- using the same resources but in slightly different ways to avoid complete competitive overlap

example of resource partitioning

- cyanobacteria: one species absorbs green wavelengths, another, red

- they are able to coexist because they are partitioning the light source

Lotka-Volterra competition equations

dN1/dt = r1N1 ((K1-N1-αN2)/K1)

dN2/dt = r2N2 ((K2-N2-βN1)/K2)

- α and β: competition coefficients

- α indicates the effect of an individual of species 2 on an individual of species 1

α and β = 1

- if a competitor of species 2 decreases the survival, growth, and reproduction of species 1 by the same amount as another individual of species 1 would, then α = 1

- intraspecific and interspecific competition have the same effect in this case

α and β = 2

- if a competitor of species 2 decreases the survival, growth, and reproduction of species 1 twice as much as another individual of species 1 would, then α = 2

- interspecific competition is then stronger than intraspecific competition

- if both α and β > 1, the two species could not likely coexist

equilibrium solutions modeling competition

- N1 = K1 - αN2

- N2 = K2 - βN1

- the equilibrium size for species 1 (N 1) is its carrying capacity (K1) reduced by the presence of species 2 (αN2)

- when there are no individuals of species 2 present to compete, species 1 equilibrates at its carrying capacity

- the same interpretation applies to the equilibrium

size for species 2 (N2)

isoclines

- equilibrium solutions can be demonstrated graphically as zero-growth isoclines

- isoclines show equilibrium sizes for one species in combination with different numbers of individuals of the other species

- isoclines can also be used to determine the conditions under which each species will increase or decrease in size

- view pages 36-39 for graphs*

coexistence

- occurs when: α < K1/K2 < 1/β

- if α and β are equal and close to 1, the species are equally strong competitors and have similar effects on each other

if α = β = 0.95

- 0.95 < K1/K2 < 1.053

- coexistence is predicted when the two species also have similar carrying capacities

if α = β = 0.1

- 0.1 < K1/K2 < 10

- coexistence is predicted within a much broader range of carrying capacities

does competition only occur between closely related species?

- no, guilds consist of different (potentially distantly related) species that acquire nutrition in the same way

- competition will occur between members of the same guild

- many rodents + ants belong to the granivore (seed-eating) guild, so these very different species compete for seeds

amensalism (-, 0)

- a special form of competition (-, -)

- asymmetric or one-sided competition

- all competition lies somewhere between completely equal and completely one-sided