Lecture #17 Selection: Competition & Mutualism

Lecture #17 Selection: Competition & Mutualism

Lecture #17—Selection: Competition & Mutualism

The external environment consists of 1) biological factors (competitors, predators, prey, parasites, mutualistic partners, and members of your own species) and 2) non-biological factors ( =abiological) factors (rain, sleet, snow, temperature, humidity, O2 and CO2, soil nutrients, etc.).At any one time dozens of these factors affect a given species determining whether it survives. In this lecture we focus on how competition from other species has a negative affect and mutualism has a positive effect on survival.

Mutualism—two or more species provide a positive effect upon one another improving their chances for survival. The species might exchange resources (e.g. food) or services (e.g. cleaning parasites off of the skin; providing protection from predation).

Bee pollinationis an example of a Resource/Service relationship. The bee provides a service to the flower by carrying pollen between flowers; the flower provides a resource for the bee by providing food (nectar and pollen).

Nitrogen fixing by bacteria living in root nodes of peanut plants is an example of Resource/Resource relationship. The bacteria provides a resource by taking nitrogen gas from the air and combining it with hydrogen making NH3which the plant can then use to make amino acids and nucleotides, etc.; the plant provides a resource to the bacteria in the form of carbohydrates and other nutrients.

The sea anemone/clown fish is an example of a Service/Service relationship. The clown fish attracts prey to the poisonous tentacles of the sea anemone; the sea anemone protects the clown fish from predators because it is not affected by the anemone’s stinging cells on the tentacles among which it hides.

Some mutualistic relationships are obligate (the two species cannot exist alone—termites and their gastric protozoa); others are facultative (the two species can live together or separate—some blue green bacteria can live independently and also live with fungi as part of lichens.)

Competition—two or more species have a negative upon one another decreasing the chances for survival of the others. The competition is usually for resources that the species both need. The competition can be between individuals of different species (interspecific competition) or between members of the same species

Scramble Competition is when individuals consume the resource at different times. They are not in direct physical contact. E.g. moose and snowshoe rabbits eat mostly the same food. Who finds the food first is the beneficiary.

Contest Competition is when two individuals actually are competing for the resource at the same time—often involving fighting e.g. Lions and hyenas.

The results of competition between species can be Competitive Exclusion: one species is superior at gaining a resource and eliminating its opponent. The Russian biologist, Georgy Gause, stated that two species competing for a limited resource cannot co-exist (Gause’s Principle)

Co-existence can exist if there are shifting advantages between species so that one species is not always dominant or…

if the resources are not limited. This can occur if the competitors’ populations are keep low by disease or predation. E.g. along the tidal regions starfish are predators who eat many species of mollusk keeping their populations low. When they are removed, competition between the mollusks for living space on rocks becomes intense and some species are eliminated or….

ifspecies evolve in such a way as to divide up the resources—Resource Partitioning. They do so by becoming specialists, sharing resources, consequently reducing the intensity of competition by dividing up the habitat (habitat partitioning), feeding at only certain parts of the day (temporal partitioning) or season (seasonal partitioning), using different foraging strategies so attacking different food items (foraging partitioning). Usually when this happens the previous competitors become physically more distinct as they specialize: this is called Character Displacement. We see this clearly with the Galapagos finches, which have different beak sizes and specialize on different sizes of seeds.

Keystone Species are ones whose very presence tends to dominate an ecosystem.“It is aspeciesthat has a disproportionately large effect on itsenvironmentrelative to its abundance.” If it were to disappear the entire ecosystem would be drastically changed. So the starfish in the tidal community is an example as are beavers who transform environments from streams into ponds and swamps or elephants which destroy trees and transform woodland into a savanna.

In summary: Species have negative and/or positive effects upon the survival of other species. When two species are competing or benefiting from a mutualistic relationship both parties are acting as selective agents on the other; they are said to be co-evolving. In mutualism, any mutation that comes along in either partner that enhances the success of the partnership will have a selective advantage. We might expect the partners to become moredependent on one another.

In competition, any mutation that occurs in one species to improve its chances will be selected; thus a species might drive another to extinction in that habitat. Unless the other species develops a counter measure.But another type of mutation might occur which decreases the competition intensity, say by resource partitioning. This would be advantageous for both competitors leading to co-existence.

Terms/Concepts to Define:



Interspecific competition

Intraspecific competition





Resource mutualism

Service mutualism


Facultative vs. Obligate mutualism

Resource competition

Scramble competition

Contest competition

Competitive exclusion principle (Gause’s principle)

Keystone species

Resource partitioning: habitat partitioning; temporal partitioning; seasonal partitioning; foraging strategy partitioning.

Character displacement

Can you answer these questions?

  1. On their path to domestication, dogs were once probably simply commensal species. Please explain this logic.
  2. What does it mean to say that termites and their wood-digesting protozoans are obligate mutualists?
  3. Gause was the first to pronounce the Competitive Exclusion Principle, what was the experiment that convinced him of its truth?
  4. In spite of Gause’s principle, closely related species do co-exist, how is this possible?
  5. What does it mean if we declare a species to be a keystone species? Why should we take this into consideration when considering conservation efforts?
  6. What does it mean when we say that co-evolution is occurring between two species? Please explain giving an example.
  7. What evolutionary advantage do you see for species to have Character Displacement? How might Resource Partitioning be involved?