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When does competition occur?

When organisms utilize or seek out a common resource that is in short supply


Name the 2 types of competition.

1. Resource competition:
a. intraspecific (among population of the same species). Ex: plants and nitrogen soil
b. Interspecific

2. interference competition: long-legged ant vs red harvester ant. Long-legged ant blocks exit of red harvester nest.


Define the Lokta-Voltera model of competition. What are 3 key things the model suggests?

Predicts coexistence of 2 species when, for both species, interspecific competition is weaker than intraspecific competition.

1. Competition can lead to one species winning and the other going extinct
2. Some competitive interactions can lead to coexistence.
3. We can understand competitive interactions only by knowing the resources involved and the mechanisms by which species compete.


Give formula for Intraspecific term



Give forumlas dN/dt population interspecific competition.

dN1/dt = r1*N1 * (K1-N1- 𝛼 *N2)/K1
𝛼: competition coefficient. Influence of species 2 on species 1

dN2/dt = r2*N2 * (K2-N2- 𝛽 *N1)/K2
𝛽 : influence of species 1 on species 2.


How can we tell from the dN/dt formula when the population stops growing?

dN1/dt = r1*N1 * (K1-N1- 𝛼 *N2)/K1 = 0
K1-N1-𝛼 N2=0
N1=K1- 𝛼 N2

similar for N2


Define the three Isoclines of zero population growth. When does each happen?

1. No coexistance
when K1 > K2/beta and K1/alpha > K2

1. Unstable coexistance
when K1 > K2/beta and K2 > K1/alpha

1. Stable coexistance
when K1 < K2/beta and K2 < K1/alpha
*both beta terms bigger.


Under conditions of extreme crowding in laboratory experiments, it is possible for two species to live together indefinitely if they differ even slightly in their requirements

Yes, ex:
Grain Beetles experiment
Beetle type 1: live and feed inside the grain
Beetle type 2: live and feed outside the grain.
Same feeding behaviour.


Define the Competitive exclusion principle (by Gause)

Complete competitors cannot coexist.
* Competitors can only coexist if their requirements are slightly different.


What does a niche include? Hutchinson(1958)

Includes all of the environmental parameters (n) important to the survival and growth of a species → n-dimensional hypervolume.
Fortunately (for ecologists) only a few environmental factors are of overriding importance –these are not necessarily the same for everyorganism
Feeding niche for animals
For plants temperature and precipitation niche.


Give an example of organisms live in the same habitat but occupy different niches.

Organisms live in the same habitat (ex: Boreal forest) but occupy different niches (ex: sunnier areas of Boreal forest vs more humid areas).


Define Fundamental Niche and Realized Niche.

Fundamental niche: ecological space (conditions, resources, time) that the organism could occupies in the absence of competition and predation by other species.

Realized niche: The observed ecological space used by a species in the presence of competition and predation.


What are the two defining views on competition?

1. Competition rare in nature:
No competitive exclusion –because the species occupied separate niches?

2. Competition very common:
Among closely related species – did competition occur in the past? Selection pressure caused adaptation to specialize which minimized competition?


What did the Warbler study discover? What were the three niches he targeted?

All birds live in the same tree but no competitive exclusion. Turned out they occupied different niches. They fed in different parts of the trees.

Warblers’ niche: temperature, prey size and, foraging height


Why is it Hard to use Observational data to understand competition? How can this be remedied?

Hard to use observational data, because we don’t know if it’s competition has driven adaptation to use different parts of the tree or that they just naturally differed in those traits and so they wouldn’t compete.

Need to use also manipulation data to test outcome of competition.
Ex: Barnacles experiment.


What was the hypothesis in the Competition and Niches of Small Rodents experiment? What was the conclusion?

If competition among rodents is mainly for food,then
1) Small granivorous rodent populations would increase in response to removal of larger granivorous rodents(Dipodomys).
(2) Insectivorous rodents would show little or no response.

Conclusion→Strong evidence for competition occurs in nature


Give the SES (Effect size) formula. What do different values indicate?

SES = (avg no competition - avg w/ competition)/std deviation = (Xe-Xc)/S
SES >0 evidence of competition
SES = 0 no competition
SES<0 Facilitation


What are the outcomes of evolutionary adaptation on competition and fitness?

Adaptation should reduce the fitness of competing individual.
Greater the niche overlap, lower fitness.
On evolutionary timescale, interspecies competition is predicted to cause a directional selection to minimize the niche overlap.


How was Character displacement inferred?

Character displacement: inferred from looking at niche of organism when they live together and look at same niche when they live apart.

Ex: Galapagos finches → size of seed determined by the size of the beak.


What are the differences between species in r and K selection: unstable and stable environments

R-selected (exponential phase of growth)
Small-size organisms
Many small reproductive units (seeds, spores, offspring)
Little energy used per reproductive unit
Early maturity
Short expectation of life
Single reproductive episode (semelparous)
Type 3 survival curve

K-selected (stable environment near their carrying capacity) good competitors
Large-sized organisms
Few larger reproductive units
Much energy used to produce one reproductive unit
Long life expectancy
Long life expectancy
Many reproductive episodes (iteroparous)
Type 1 or 2 survival curve