Interactions Within Communities

December 1, 2010Text p. 684-686

Mortality (a) and growth (b) in steelhead trout (Keeley et al. 2001)

Species Interactions

• Populations do not live in isolation• Interact with other species in the community• Individual interactions within populations can

shape communities• One type of interaction: symbiosis

Symbiosis

• Relationship where individuals of two different species live in close (usually physical) contact

• At least ONE organism benefits• 3 categories of symbiosis:

1. Mutualism2. Commensalism3. Parasitism

Mutualism• Both organisms benefit, neither are harmed • A (+/+) relationship

Lichens

2 Main Types of Mutualisms

• Obligatory– Neither organism could grow or reproduce

without the other

• Facultative– One or both organisms can grow or reproduce

without the other

Obligatory Mutualism

Facultative Mutualisms

Ecological Classifications of Mutualism

1. Trophic partnerships– To obtain energy and nutrients

2. Defensive partnerships– Provides protection against herbivores,

predators or parasites

3. Dispersive partnerships– Disperse pollen or seeds, generally for food

reward

Trophic Partnership

Defensive Partnerships

Dispersive Partnerships

• Mutualistic relationships do not have to be symmetric – One organism may be obligated to the mutualism,

while the other can live without its mutualistic partner

Commensalism

• A symbiotic relationship where one organism benefits and the other is unaffected

• A (+/0) relationship

• Is there really such a thing?• Not well understood, and difficult to classify

Commensalism

Parasitism

• One organism benefits at the expense of another organism (the host)

• The other organism is usually never killed but is often harmed

• A (+/-) relationship

• Come from almost all taxonomic groups

4 Types of Parasitism

1. Microparasites2. Macroparasites3. Endoparasites4. Ectoparasites

Microparasite• Microscopic, rapid reproduction rate• Blood protozoans• Often requires a vector

Plasmodium Trypanosoma

Macroparasites

• Larger parasites that are readily visible

Endoparasites

• Live and feed within the host’s body

Ectoparasites

• Live and feed on the outside of the host

Social Parasites

• Complete their life cycle by manipulating the social behaviour of their hosts

Effects on Host• May kill • Reduce fitness• Sterilize• Alter the phenotype– E.g. alter behaviour of host to benefit parasite but

at host’s expense

Herbivory• The eating or consumption of plants (primary

producers) by animals known as primary consumers– Heterotrophs– Autotrophs

• Like parasitism & predation, is a (+/-) interaction

• Herbivory is important because plants are the ultimate source of energy for food webs in most environments.

Are all herbivores equal?Feeding Specialization Specialists:

Koalas Monarch caterpillars

Generalists: Deer Cows Leaf-cutter ants

Herbivory

• Alters species dominance patterns

Control of Herbivores?

• Predators and parasites control herbivore abundance

• Plants defend themselves (particularly terrestrial plants)

Amensalism

• One species is hurt, but the other does not benefit

• A (-/0) relationship

Species Interactions

• Interactions can be complex and varied• Next class... Predators, prey, cycles, defense

mechanisms