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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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