Module 9S1 Ecological Interactions

4.9.1S1 page 2

Explore

Predator-Prey Relationships

The success of the predator means the death of the prey. Predators are selecting agents on prey populations because they’re more likely to kill the sick or poorly adapted prey individuals, removing those deleterious alleles from the prey gene pool, leaving superior alleles and improving the quality of the gene pool.

 

Predators also control prey population numbers so that they can not exceed the capacity of the environment. When rabbits were introduced into Australia, there were no predators to keep rabbit populations in check. Rabbits are now considered a nuisance and threat to native species in Australia.

 

The population numbers of predators and prey follow each other in predictable cycles. Because more successful predators select for more successful prey, the two species co-evolve.

 

High predators → low prey → low predators → high prey

 

Producer-Consumer relationships follow the same patterns as predator-prey. When rabbits were introduced to Australia in the absence of predators, their unhindered reproduction resulted in many habitats being completely denuded of grasses.

 

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 favors 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 also competing with coyotes and hawks in interspecific competition. If two species with the same niche (are competing for the same resources, in the same place, using the same methods), typically only one species survives – a principle called competitive exclusion.

 

In symbiotic relationships, the lives of two species are locked together in co-evolution; success of one limits the success of the other. Like human relationships, symbiotic relationships can be defined by the effect on each participant species:

1. + - One species benefits at the other’s expense.
2. + + Both species benefit.
3. + 0 One species benefits but the other is unaffected.

Mutualism ( + + ) increases survival of both species;the products of one, provide the needs of the other. The same cannot be said for commensalism (+ -), where one species unwittingly boosts the survival of another without obtaining any benefit. Parasitism ( - - ) allows one species to expend very few resources in staying alive, but at the expense of its host.

To further explore these concepts:

Read

Read p 717-4 of your text.

 

Watch and Listen

Parasitism, Mutualism, Commensalism.

 

Predator, Prey Relationships

 

Self-Check 1

SC 1. Re-read the Explore section on predator-prey relationships. Although we can argue that the relationship is positive for the predator and negative for the prey, support the argument that predator/prey relationships are in fact +  + relationships when we consider populations rather than individual organisms.

 

SC 2. Scientists have found that the number of predators in a stream influences the natural selection of guppies.

 

Conduct the on-line simulation of evolution in guppy populations at

1. Guppy Simulation
2. In what ways do the populations change over time?

SC 3. Fill in the empty boxes in the table using examples from your text.

Example	Relationship	Population that increases?	Population that decreases?	Population that remains stable?
	commensalism
Tapeworm and human
Plant species that produces root toxins that kill plants of the same species nearby
	Interspecific competition
	Predator/prey
Acacia tree and stinging ants of Latin America
Viceroy and monarch butterflies