Interactions Within Communities (III)

December 3, 2010

Text p. 676-680

Re-Visit: Mullerian Mimicry• Predators and prey can protect themselves through

mimicry

• Unrelated species that resemble each other and are all poisonous or “distasteful”

• Why Mullerian? If predator is confused between 2 species, means both are more likely to survive

• To understand competition, must look at the ecological niche

Ecological Niche

• The use of, and interaction with, abiotic and biotic resources in its environment

Habitat = organism’s “address”

Ecological Niche = organism’s “occupation”

Ecological Niche• Can include:

– Prey– Predators– Reproductive methods– Temperature range– Habitat– Behaviour

Fundamental Niche

• Biological characteristics of the organism and the set of resources individuals in the population are theoretically capable of using under ideal conditions

• Ideal conditions– Abundant resources– No competition from other species

Realized Niche

• What the organism actually uses under prevailing environmental conditions

• Because of competition with other organisms for similar resources, a species can only use a portion of its fundamental niche

Competition• Interaction within communities• May be detrimental to one or both species

• Two Types:– Interspecific – interactions between individuals of

different species for an essential common resource that is in limited supply

– Intraspecific – interactions between individuals of the same species or population competing for resources in their habitats

Interspecific Competition

• Purpose: to restrict population growth

• 2 Types:– Interference competition– Exploitative competition

Interference Competition

• Aggression between individuals of different species, fighting over the same resource

• Typically detrimental to both individuals of the different species involved

• Asymmetric – one species has an advantage over the other

Exploitative Competition

• Consumption of resources by two species, but one species may limit resource availability to other species

• One species more efficiently uses a resource

Reduction in Survival

Principle of Competitive Exclusion• THEORY: if resources are limited, no two species

can remain in competition indefinitely

Species A Niche

Species B Niche

Competition Heavy competition leads to competitive exclusion – one species must go

Gause• Russian Ecologist G.F. Gause• Competitive Exclusion aka Gause’s Principle

– Two species with similar requirements could not coexist in the same community

Paramecium sp.

Red Squirrel Grey Squirrel

Introduced between 1876-1929

Related Types of Interspecific Competition

• Apparent competition– 2 species share a predator– The two prey can compete with each other to

make themselves less available for consumption

• Scramble competition– Inadequate resource for both species– Receive equal amounts, but not enough

Results of Interspecific Competition

• Population size of weaker competitor could decline

• One species could change behaviour to survive using different resources

• Individuals in one population could migrate where resources are more plentiful

Resource Partitioning

• Avoidance or reduction of competition for similar resources by different species through the use of non-overlapping ecological niches

Schoener, 1968

Anolis sp.

A. sagrei- Partly

terrestrial- Small and

large low perches

A. angusticeps-Small twigs at great heights

A. distichus-Trunks and large branches

A. carolinensis- Leaves and adjacent twigs

Foxtail

Indian Mallow

Smartweed

Evolutionary Adaptation

• Interspecific competition is driving force for species to evolve adaptations for continued survival

Classic Interspecific Competition Example:Connell’s Field Experiments

The Keystone Species

• Plays a critical role in the community

• Impact is greater than expected, based on its size or abundance

• Predator-Prey – usually is a predator acting on prey with no other predators

• Good target species for conservation

Intraspecific Competition

• Individuals of same population (and species) competing for same resources– Food – Shelter– Mates

• As population density increases there is more competition among individuals for resources and growth rate slows

• Classic territorial behavioural

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