14.4 Interactions Within Communities

• The theory that two species with similar requirements cannot coexist in the same community was proposed by Gause

- Habitat: the food, water, shelter and space requirements determining where an organism lives, its "address“

- Niche: the role of the organism in its environment; where it lives, and how it "fits in" to the community structure, its "job"

• Human populations are theoretically capable of living anywhere on this planet with abundant resources and no competition. This is a statement of our fundamental niche

• Humans do not make use of the total space available to live on the planet and find themselves crowded into smaller areas than theoretically possible. This is a statement of our realized niche.

• In many ways, a science classroom can be considered a model of an ecosystem. The students rely on the teacher for knowledge and guidance; the teacher would not have a job without the students. Neither can exist without the other. Ecologically, this relationship demonstrates mutualism.

• In many ways, a science classroom can be considered a model of an ecosystem. A student copies another student's work, contributing nothing. The student who did the work feels hurt and used. Ecologically, this relationship can be described as parasitism.

• An ecological relationship between two species, where one species benefits and the other is harmed is either parasitism or predation

resource partitioning may increase the chance of species success.

- the occupying of non-overlapping niches by organisms requiring similar resources

- reduces competition, making resources available

character displacement may provide a survival advantage

- traits of species occupying the same geographic range are more different than traits in the same species geographically separated

- reduces competition by letting species sharing a region to occupy different niches

It can be argued that, while not good for the victim, predation has a positive

effect on the prey species.

- predators remove the old, weak, and unhealthy, raising the general strength of the population

- predators help prevent the prey species from exceeding their food supply

Prey species often develop passive defense mechanisms that reduce the

chance of them being eaten.

- morphological (e.g., spines, hooks, needles, etc.)

- chemical ( taste, toxic, smell, etc.)

- camouflage

- hiding

- mimicking dangerous species

• The coral snake and the king snake are very difficult to tell apart. One is poisonous, the other is not. This is an example camouflage

• When attacked, the sea cucumber ejects its intestines. The predator attacks these allowing the animal an opportunity to escape. It will grow a new digestive system. This is an example of active defence

Commensalism - some biologists argue that it does not exist

- a relationship in which one organism benefits, and the other is unaffected

- very difficult to determine if "unaffected" species benefits, is harmed or is truly not affected without direct observation

Hermit crabs live in shells left when snails die. The shell provides protection for the crab. This is an example of commensalisms and a defence mechanism

• Termites eat wood, but cannot digest cellulose. In their gut live a protist that can digest cellulose, but is unable to survive outside the termite. If the protists are removed, the termite will starve. This is an example of obligatory mutualism

• Organisms introduced into a region that is not their natural habitat and in which they have few natural predators are known to ecologists as exotics or nonindigenous species

• Nonindigenous species presently causing concern by affecting naturally-occurring species in Ontario include purple loosestrife and zebra mussels.

Purple loosestrife (Lythrum salicaria., a European perennial, is establishing populations in aquatic ecosystems in North America, where it is disrupting interactions between native species. Researchers at the University of Guelph are studying a European beetle as a method of biological control.

a. What general assumptions are being made concerning the European beetle?

- beetles eat purple loosestrife

- beetles do not eat native plants

- beetles are not harmful to native animals

b. What will happen to the beetle population if it is successful in

controlling loosestrife?

- as loosestrife population declines, beetle population declines

- beetles will not start to eat native plant species

c. Describe the ecological concerns if the assumptions prove to be incorrect.

- beetles will eat native plants, interfering with interactions between native species

- beetles will over-populate, there being no natural predators

- one problem will have been traded for another