Community Interactions

• Habitat is the environment in which an organism lives.

• A population's niche is its role in the community– How it uses the biotic and abiotic resources of its

habitat

Communities

• There are five main types of relationships among species within communities– Competition– Predation– Parasitism– Commensalism – Mutualism

Community interactions

Community interactionsInterspecific competition occurs between two populations if they both require the

same limited resource

Intraspecific competition occurs between organisms of the same species.

• The competitive exclusion principle

– Populations of two species cannot coexist in a community if their niches are nearly identical

Figure 36.2

Hightide

Chthamalus

Balanus

Lowtide

Ocean

• Competition between species with identical niches has two possible outcomes

– One population will eventually eliminate the other

– Natural selection may lead to resource partitioning (division)

• Predation is an interaction where one species eats another– consumer = predator– food species = prey

Predation

• Prey gain protection against predators through a variety of defense mechanisms

1. Mechanical defenses, such as the quills of a porcupine

Adaptations are driven by these relationships…COEVOLUTION!

2. Chemical defenses

– Animals are often brightly colored to warn predators

– Example: the poison-arrow frog

Physical/Chemical Combat

3. Camouflage

– Example: gray tree frog

Figure 36.3C

Camouflage

4. Batesian mimicry occurs when a harmless species mimics a harmful one

– mimicry can involve behavior

– hawkmoth larva puffs up its head to mimic the head of a snake

Figure 36.3D

Trickery/Mimicry

• Eliminates weaker individuals• keystone predator maintains

diversity by reducing numbers of the strongest competitors in a community

- Ex. sea star is a keystone predator

How does predation affect the community?

Figure 36.4A

• Predation by killer whales on sea otters, allowing sea urchins to overgraze on kelp–Sea otters represent

the keystone species

Parasitism

• Parasitism is a form of predation– Parasite, host– Not immediately lethal– Example: mistletoe on oaks, tapeworm in human

intestine

• Commensalism - one partner benefits and the other is unaffected

• Examples

- Algae that grow on the shells of sea turtles

– Barnacles that attach to whales

– Birds that feed on insects flushed out of the grass by grazing cattle

• Mutualism:both partners benefit

Examples:

- Nitrogen-fixing bacteria and legumes

– Acacia trees and the ants of the genus Pseudomyrmex

Figure 36.5B

Symbiosis• Any long term biological interaction is known

as symbiosis or a symbiotic relationship• Some biologists only characterize mutualism

and commensalism as symbiosis• Others include parasitism as well• Endosymbiosis is the theory that several

eukaryotic organelles are the result of a symbiotic relationship between specialized prokaryotic cells.

Your task…• Community Interaction comic• Pick one of the types of interactions we discussed.

(competition, predation, parasitism, mutualism, commensalism)

• Draw a comic showing the interaction.• Comic must have at least three panels• No words are necessary, but it must clearly show the

type of interaction• Do not write the type of interaction on your comic or

your name• When it is finished, bring it to me for a number and

further directions.