COMMUNITY ECOLOGY: How do populations interact with one another?

All the populations of different species living in a particular area comprise that area's COMMUNITY--the living portion of the ecosystem. The number of different species found in an ecosystem comprise that system's SPECIES DIVERSITY. Diversity varies greatly among ecosystems, and hence, so do the interactions among populations in those ecosystems.

Symbiosis

The term symbiosis (from the Greek sym, meaning "together" and bios, meaning "life") refers to the members of two different species (i.e., two populations) having some sort of ecological interaction that affects both populations. Here are some of the theoretical types of interactions that can evolve over many generations. When two species evolve in response to each other's activities, the process is known as coevolution.

"+" means that the population benefits from the interaction

"-" means that the population is harmed by the interaction

"0" means that the population is not affected by the interaction

type of interaction

pop'n A

pop'n B

nature of effect

obligate mutualism

+

+

obligatory; both populations benefit

EXAMPLES:

 

  

 

 

 

protocooperation

+

+

NOT obligatory; both pop'ns benefit

EXAMPLES:

 

    

 

competition

-

-

populations inhibit one another

EXAMPLES:

 

    

 

neutralism

0

0

populations don't affect one another

EXAMPLES:

 

    

 

predation

+

-

predator (A) kills & consumes prey (B)

EXAMPLES:

 

    

 

parasitism

+

-

parasite (A) exploits the host (B), but

does not kill it outright

EXAMPLES:

 

 

 

 

 

 

 

parasitoidism

+

-

parasitoid (A) eventually kills host (B)

EXAMPLES:

 

    

 

commensalism

+

0

commensal (A) benefits; host (B) not

affected

EXAMPLES:

 

    

 

amensalism

-

0

A inhibited; B unaffected

EXAMPLES:

 

    

 

Examples of Symbiotic Interactions

Obligate Mutualism

The termite and its intestinal flagellate symbionts exemplify Obligate Mutualism: Neither organism can survive without the other.

Protocooperation

The Clown Fish and its Sea Anemone partner both benefit from the relationship: Nemo gets a safe home that protects him from predators, and he fiercely protects his sea anemone from predators. He also feeds the anemone. How cute is that?

Competition

The Green Anole (Anolis carolinensis) is native to the southern United States. In the 1960's, The Brown Anole (Anolis sagrei) was introduced from Cuba. The two species vie for habitat and food resources, and it appears that the exotic Brown Anole has displaced the native Green Anole in some physical spaces, such as lower shrubbery and grass. The Green Anole generally lives higher up in the trees and foliage than the Brown Anole does. This result of competition is known as resource partitioning.

Predation

This is possibly the most familiar type of symbiosis. The predator species (in the illustration below, the Lion (Panthera leo)) kills and consumes the prey species (in this case, a Cape Buffalo (Syncerus caffer). You can no doubt think of dozens of other examples of predation.

Predation has driven the evolution of some truly amazing phenomena, such as crypsis (camouflaging coloration), aposematism (warning coloration), mimicry, and other ways animals avoid being eaten.

Parasitism

A parasite is an organism that takes up residence in or on a host organism and feeds on the host's body without killing it outright.
An organism that is host to an adult parasite is known as the definitive host. An organism that is host to a juvenile parasite is known as an intermediate host. The definitive host is usually a predator of the intermediate host, and the life cycle is completed when the definitive host eats the intermediate host, freeing the larval forms to take up residence, as shown below in the tapeworm life cycle.

Commensalism

In this case, one species benefits from the presence of another, which is not affected by the presence of the first species. An example is the Cattle Egret. As large grazers move through the grass, they stir up insects. Cattle Egrets follow them and get a banquet. The large grazers are neither helped nor harmed by the presence of the birds.

Ecological niche

  • Defined by the species, this is everything that an organism eats, where it nests, sleeps, forages, etc. In short, everything that defines its natural history.
  • Two species can never occupy exactly the same ecological niche, or one will eventually become extinct. However, the niches of different species may overlap to some degree. When this happens, interspecific (i.e., between species) competition occurs.
Keystone species
    This is a species on which many other populations ultimately depend for survival.
    EXAMPLES:
Indicator species
    This is a species whose population density allows humans to gauge the health of an entire ecosystem.
    EXAMPLES:
Native species
    This is a species found in the place where it naturally evolved. Native populations often have complex and delicately-balanced interactions with one another that can be disrupted by the introduction of exotic species.
    EXAMPLES:
Exotic species
    This is a species living in a place where it did not originally evolved. These are often introduced by humans into ecosystems where they do not naturally belong. Some exotic species introduced into new environments are much better able than native species to compete for resources, and they can sometimes displace native species. This can have very profound effects on the other native species which used to rely on a native species driven extinct (or to great scarcity) by an invasive exotic.
    EXAMPLES:

Some interesting results of predation...

Crypsis - camouflaging coloration

Aposematism - warning coloration (poisonous or venomous species)

Mimicry - a species has evolved the superficial appearance of something else

Batesian Mimicry - a harmless mimic looks like a poisonous model.

Mullerian Mimicry - several poisonous/distasteful species resemble one another.

And note that there's an important difference between poisonous and venomous.
  • poisonous - toxic when eaten or touched
  • venomous - delivers poison/toxin with some sort of apparatus, such as a stinger or fang