Module 8 Lesson 7 - 2
Lesson 7 — Species Interactions and Symbiotic Relationships
Lesson 7 — Species Interactions and Symbiotic Relationships
Species Interactions
Read pages 717 - 722
Predator-Prey Relationship
Predation occurs when one organism consumes another. Predators are selecting agents on prey populations because they are more likely to kill individuals that are sick or poorly adapted. By removing those potentially disadvantageous alleles from the prey gene pool, the predators unwittingly preserve superior alleles and improve the quality of the prey gene pool.
Predators control prey population numbers so that they cannot exceed the capacity of the environment. When rabbits were introduced into Australia, no predators were there to keep rabbit populations in check. Now, rabbits are a threat to native species
in Australia.
The population numbers of predators and prey follow each other in predictable cycles. As more successful predators select for more successful prey, the two species co-evolve.
A common example of predator-prey dynamics is the relationship between the lynx and hare populations. The boom or bust cycle lasts for 10 years. The graph above shows the lynx population fluctuation followed the increases and decreases of the hare
population. As the prey population increases, more food becomes available for the lynx population. The lynx population, in turn, also increases. However, an increased hare population is not sustainable because eventually they deplete their food
resources, and the hare population size crashes. This crash is followed by a reduction in the lynx population because their food resource declines. When the lynx population decreases to a point where it relieves the predation pressure on the hare
population, the cycle begins again.
If the prey is a plant, the relationship is called the producer-consumer relationship. Producer-consumer relationships follow the same patterns as predator-prey relationships. When rabbits were introduced to Australia in the absence of predators,
their unhindered reproduction resulted in many habitats being completely bare of grasses.
Competition
Scarcity leads to competition, and competition leads to natural selection of the fittest phenotype/genotype. A plant that is a better competitor than its neighbors of the same species may make do with less water, cast its seeds further from the
mother plant, or produce more seeds per seed pod. This intraspecific competition within members of the same species favours one phenotype at the expense of the other and thus reduces diversity.
At the same time that a lynx is competing with others in its own species, it is competing with coyotes and hawks in interspecific competition. If two species are competing for the same resources, in the same place, using the same methods, they occupy the same niche. In this scenario, typically only one species survives - a principle called competitive exclusion. Normally, two species do not share the same niche for this reason. Their niches may overlap incompletely to avoid direct competition with one another. Do not confuse habitat for niche: a habitat is the location an organism lives, and a niche is a pattern of living.
Symbiosis
Symbiosis refers to the interaction between two or more unrelated species. In symbiotic relationships, the lives of two species are locked together in co-evolution; success of one limits the success of the other. Similar to human relationships, symbiotic relationships can be defined by the effect on each participant species.-
Parasitism: One species benefits at the expense of the other.
- Parasitism allows one species to expend very few resources to stay alive but at the expense of its host.
- Example: reed warbler and common cuckoo
The common cuckoo lays its eggs in the nest of a reed warbler so that the reed warbler takes care of the chick to its maturity. Cuckoo chicks often are much larger than an adult reed warbler and often push other eggs or hatchlings from the host nest.
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Mutualism: Both species benefit.
- Mutualism increases survival of both species through mutual benefit.
- Example: ant and acacia plant
The acacia plant offers food and protection to the ant colony. The ants provide protection for the plant by eliminating any herbivorous insects. They also help the acacia plant compete with other plants by cutting away any branches that touch the acacia. This helps them obtain more sunlight in a very competitive forest.
- Mutualism increases survival of both species through mutual benefit.
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Commensalism: One species benefits but the other is unaffected.
- One species unwittingly boosts the survival of another without obtaining any benefit.
- Example: clownfish and sea anemone
The clownfish obtains protection from sea anemones by staying in their tentacles and obtaining leftover scraps from their food.
Watch and Listen
Watch the following Biologix video on Interactions and Relationships among Organism.
Lab Simulation
Scientists have found that the number of predators in a stream influences the natural selection of guppies.
Conduct the online simulation of evolution in guppy populations:
- Be sure to click through the tabs at the bottom of the page to see the stages of the investigation. In what ways do the populations change over time?