Since a community comprises all the species that occur at a particular location, one of the most important things about communities is how the species interact with one another.
Four different types of interactions between different species (interspecific interactions) have been identified:
Competition
Two organisms mutually harm one another
Predator–prey or parasite–host
One organism benefits, the other is harmed
Mutualism
Both organisms benefit
Commensalism
One organism benefits, the other is not affected
Effect on organism 2
Harm
Benefit
No effect
Effect on organism 1
Harm
Competition (−/−)
Predation or parasitism (−/+)
—
Benefit
Predation or parasitism (+/−)
Mutualism (+/+)
Commensalism (+/0)
No effect
—
Commensalism (0/+)
—
The Ecological Niche Concept
Ecological niche
Definition: the sum total of an organism's use of the biotic and abiotic resources in an environment
It includes:
Space utilization
Food consumption
Temperature range
Moisture requirements
Niche versus habitat
Niche
Like an occupation
What an organism does
Habitat
Like an address
Where an organism lives
Fundamental Niche versus Realized Niche
Some species are not able to occupy their entire niche because of the presence or absence of other species.
Interspecific competition occurs when two different species attempt to utilize the same resource and there is not enough of the resource for both species.
Observation of this phenomenon in nature has led to the concepts of fundamental and realized niches.
Fundamental niche: the set of resources a population is theoretically capable of using under ideal conditions
Realized niche: the resources a population actually uses
The realized niche may be smaller than the fundamental niche because of interspecific interactions such as:
Competition
Predation
Joseph Connell's Famous Experiments
Two species of barnacles live in a stratified distribution in the intertidal region along the Scottish coast.
Observations:
Balanus is most concentrated in the lower intertidal area.
Chthamalus is most concentrated in the upper intertidal area.
The free-swimming larvae of each species can settle anywhere on the rocky shoreline, and presumably be able to grow to be an adult.
Question: why don't we see Balanus and Chthamalus growing together?
Experiment 1:
Connell removed Chthamalus from the upper area, and no Balanus replaced it.
Inference: Balanus could not survive in an area that experienced so much desiccation (due to low tides).
Conclusion: Balanus's realized niche was the same as its fundamental niche.
Experiment 2:
Connell removed Balanus from the lower area and Chthamalus replaced it.
Inference: Balanus was a more successful competitor in the lower intertidal zone.
Conclusion: the fundamental niche and realized niche for Chthamalus were not the same—its realized niche was smaller due to interspecific competition.
Conclusion: competition occurs in nature and can explain the distinction between a fundamental niche and a realized niche.
The biologists Lotka and Volterra theorized that no two species with similar requirements for resources (food, shelter, etc.) could coexist in the same niche without competition driving one to local extinction.
Gause's Experiment
A Russian scientist, G.F. Gause, tested the Lotka–Volterra theory.
He grew two different species of Paramecium alone, under identical conditions.
Their populations grew to their carrying capacity and leveled off—showing a logistic growth pattern (see graph).
He put them together and discovered that one species died out (it couldn't compete).
Conclusion:
Two species competing for limited resources cannot coexist in the same place at the same time.
This concept was named the competitive exclusion principle (or Gause's Principle).
A consequence of Gause's competitive exclusion principle
If competition for a limited resource is intense, there are two possible outcomes:
One species drives the other to extinction.
Natural selection reduces the competition between the species.
Robert MacArthur of Princeton University did a famous study of five species of warbler (small insect-eating songbirds) in the late 1980s.
It appeared that they all were competing for the same resources on spruce trees.
On closer inspection, he realized the five species were each feeding on different parts of the tree and therefore eating different insects (see illustration).
In essence, each species had evolved to utilize a different portion of the spruce tree resource.
They had subdivided the niche, partitioning the available resource to avoid direct competition with one another.
This process became known as resource partitioning.
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Character Displacement
A comparison of allopatric versus sympatric populations of species shows evolutionary evidence of competition in nature.
Example: Galápagos finches
When two species occur on the same island (sympatric populations), they tend to exhibit greater differences in morphology (shape of beak) and resource use than when found on different islands (allopatric populations).
According to your text, “this tendency for characteristics to be more divergent in sympatric populations of two species than in allopatric populations of the same two species is called character displacement.”
Character displacement allows the two species to avoid competition.
Interspecific Interactions and Community Structure
A huge question that has occupied ecologists is the influence of interspecific interactions (competition, predation, mutualism, etc.) on the structure of a community.
One of the most dramatic examples of the interdependence of species and community structure is in the concept of keystone species.
Robert Paine's Experiment
Robert Paine's (University of Washington) famous experiment involved two species of intertidal invertebrate: the sea star, Pisaster ochraceous, and the mussel, Mytilus californicus.
Normally these two different species live in a harmonious balance in their intertidal community.
The sea star is an important predator: when Paine removed this predator from experimental areas, what had been diverse communities of algae and invertebrates (like what we saw when we snorkeled at Catalina Island), became overgrown with solid stands of the California mussel!
So, even though the California mussel is a good competitor, its populations had been held in check by sea star predation.
With the predator gone, the species diversity and structural complexity of the habitat changed radically.
A term coined by Paine to indicate a species that has an exceptionally great impact on the surrounding species relative to its abundance.
Another example:
Sea otters are a keystone species.
They feed heavily on large herbivorous invertebrates such as sea urchins.
When sea otters keep sea urchin populations low, the huge algae (called kelp) can grow more readily and form forests that are home to a diversity of fish and invertebrates.
If sea otter populations are reduced, then the near-shore community is overrun with sea urchins and species diversity is reduced.
