Community Ecology

Communities

A community is a group of organisms of different species that live in a particular area

Individualistic Hypothesis vs. Interactive Hypothesis Individualistic Hypothesis:

A community is a chance group of species found in the same area because they have similar abiotic requirements

Interactive Hypothesis: A community is a group of closely linked

species locked together in mandatory biotic interactions that cause the community to function as an integrated unit

Interspecific Interactions

Interspecific interactions are interactions that occur between populations of different species living together in a community

There are 4 major interspecific interactions: Predation (and parasitism) Competition Commensalism Mutualism

Predation (and Parasitism)

(+ -) The interaction is

beneficial to one species and detrimental to the other

Predation: When a predator

eats its prey Example in

picture:

Predation (and Parasitism)

Parasitism: Predators that live

on or in their hosts, usually feeding off their body tissues or fluids

• Usually do not kill their hosts

Examples:

Plant Defenses Against Herbivores

“Plants Fight Back!” Plants have 2

major mechanisms by which they defend themselves against being eaten Mechanical

Defenses• Thorns, hooks,

etc.

Plant Defenses Against Herbivores Chemical Defenses

Produce chemicals that are distasteful or harmful to an herbivore

Morphine (opium poppy)

Nicotine (tobacco)

Animal Defenses Against Predators

Animals defend themselves against predators passively (hiding) or actively (fleeing)

Cryptic coloration (camouflage) makes prey difficult to spot

Aposematic coloration (warning coloration)warns predators not to each animals that may be toxic or may sting.

Animal Defenses Against Predation Mimicry

When one species “imitates” or “mimics” another Batesian mimicry

• When one edible or harmless species mimics an bad-tasting (unpalatable) or harmful species

• Example: hawkmoth mimics a snake

Animal Defenses Against Predation

Mimicry Mullerian mimicry

• Two species, both of which are unpalatable (taste bad) or harmful, resemble each other

• Example: monarch butterfly (unpalatable) and queen butterfly (unpalatable) resemble each other

Symbiosis- close relationship between two organisms.

Parasitism (+,-) Commensalism (+,neutral) Mutualism (+,+)

Parasitism

One organism (the parasite) gets its nourishment from another organism (the host), which is harmed in the process

Endoparasites: Live within host

tissues (tapeworms) Ectoparasites:

Feed on external surfaces (mosquitoes)

Interspecific Competition

Competition between organisms of different species

The Competitive Exclusion Principle:Two species with similar needs for the

same limiting resources cannot coexist in the same place

Niches may overlap but they may not be identical.

Ecological Niches An organism’s niche is the

specific role it plays in its environment All of its uses of biotic and

abiotic resources in its environment

Example: oak tree in a deciduous forest

• Provides oxygen to plants, animals, etc.

• Home for squirrels• Nesting ground for blue jays• Takes water out of the soil• Etc., etc.

Fundamental v. Realized Niche

Fundamental Niche includes resources an organism could theoretically use (if no competition)

Realized Niche includes resources it actually does use given competition from other species.

Resource Partitioning

Similar species develop ways to partition/divide resources in order to coexist.

Commensalism

(+0) relationship One partner benefits, the

other is not affected Examples:

Cattle and cattle egret (birds)

Sea anemone and clownfish

• Clownfish gets a place to live, sea anemone is not affected

Mutualism(++) relationshipBoth partners benefit from the

relationship“You scratch my back, I’ll

scratch yours”Examples:

Ants & acacia treetree provides high

protein food in beltian bodies & habitat for nests inside thorns; ant protects against predators

Mycorrhizae-fungal extentions on plant roots

Plant gets increased water/nutrition, fungi gets food

Hummingbirds & flowersHummingbirds get food,

flowers can reproduce

Dominant & Keystone Species Dominant Species:

Species in a community that have the highest abundance or highest biomass

Sugar maple in eastern North America Keystone Species:

Important to a community because of their ecological roles (niches), not by numbers

Sea otters control sea urchin population, which controls kelp population

Sea stars are keystone predators in many aquatic environments.

Ecological Succession

Ecological succession is a change in the species that live in a given area over a period of time One community replaces another

Primary succession = occurs in places where soil is not yet formed (bare bedrock)

Secondary succession = occurs in places where there is soil, but where some disturbance has eliminated the previous community

Ecological Succession

Ecological Succession

The first organisms to inhabit an area undergoing succession are known as pioneer organisms These are usually small organisms (bacteria,

lichens, algae, etc.) The ecosystem goes through a number of

stages, with each new stage usually consisting of larger organisms than the last one

Once a community has become stable and is not changing much, it is known as a climax community

Causes of Ecological Succession

There are 3 major causes of ecological succession:1. Human Activities

- logging, mining, development, etc.2. Natural Disasters/Disturbances

- fires, volcanic eruptions, etc.3. Natural Competition Among Species

- Fictitious example:- turtles and frogs both eat crickets- frogs are faster, turtles are slower- frogs eat more crickets, turtles starve- turtle population dies out, frog

population gets bigger