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

Integrated (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-Parasitism

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 in picture (tick, leech)

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)

Parasitoidism: Insect lays eggs on or in a

host. The eggs feed on the host . . . eventually killing it

Disease

Pathogens are similar to parasites (typically bacteria, viruses or fungi)

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.

Chemical Defenses• Poisons

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 eat 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

Competition

Interspecific Competition

(-/-) Competition between organisms of different species

for a particular limited resource 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.• Niche: sum total of a species’ use of the biotic

and abiotic resources in an environment

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.

Character Displacement

Evidence for competition can sometimes be determined by looking at closely related species Allopatric (geographically separate): are

morphologically similar and use similar resources

Sympatric (overlapping geography): show different morphology and use different resources

Character displacement: the trend of sympatric species to be more divergent than allopatric species

Symbiosis

A close relationship between two organisms.

Symbiosis

Parasitism (+,-)Already discussed

Commensalism (+,neutral) Mutualism (+,+)

Commensalism (+0) relationship One partner benefits, the other

is not affected Examples:

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

SUMMARY

Relationship Organism #1 Organism #2

Commensalism + 0

Mutualism + +

Parasitism + -

Predator Prey + -

Competition - -

Evolutionary component

Many of the relationships discussed could be a result of coevolutionEach species influences the heritable

traits of another,closely associated, species

Community Structure

Community structure

Community structure describes the make up and interactions of the species in a community.

Many times this is a result of 2 factors:Species diversityFeeding relationships

Species diversity

Species diversity if made of two components:Species richness is the total number

of different species in a communityRelative abundance is the proportion

of each species that makes up the community

Who eats who?

A trophic structure describes the feeding relationship between organisms in a communityFeeding relationships always start with some

sort of primary producer (generally a photosynthetic organism)

Then you will have primary consumers (herbivores) and various secondary and tertiary consumers (carnivores)

Eventually, the cycle ends with decomposers

Food chains

Food Chains- A single pathway of energy relationships among organisms in an ecosystem

Energy transfer

The arrows DO NOT merely show what gets eaten The purpose of the arrows is to show where

the energy is going Scientists refer to eating as an energy

transfer, because when one organism eats another, the main goal is to get energy from the organism.

SO, the arrow points at the organism that GETS the ENERGY (the organism doing the eating)

Limits of food chain length

A food chain is usually only a few links longCan be as few as 2 to as many as 5 OR more

Why?One hypothesis is the energetic hypothesis: the

length of a food chain is limited by the inefficient transfer of energy from one organism to the next (only about 10%)• Food chains with more photosynthetic

organisms should be longer because you have MORE starting energy

Limits of food chain length

Second hypothesis is the dynamic stability hypothesis: long food chains are less stable than short food chainsThe more organisms involved in a food

chain the more potential for variation• Extinction, migration, climatic changes• With more species, you have more

chances to disrupt the food chain

Food web

Food chains are a very inaccurate depiction of feeding relationships in an ecosystem…Food webs are more accurate

Food webs are interrelated food chains of an ecosystem

Large impact

In communities, certain species may have a larger impact on the community structure than otherThey may be highly abundantPlay a pivotal role in maintaining the

balance in a community

Dominant Species

Dominant Species:Species in a community that have the highest

abundance or highest biomassThese species have a powerful effect on the distribution

and eating patterns of all other species in a communityPossible reasons for a dominant species

• Dominant species is most competitive in acquiring limited resources

• Dominant species is most successful at avoiding predators OR disease

• This may be the reason invasive species can take over a community that lack their natural predators and pathogens

Keystone Species

Keystone Species:Important to a community because of

their ecological roles (niches), not by numbers

When these species become extinct, or scarce, the entire community changes and usually many other species are affected

Sea otters and sea urchins

Keystone Species- Pisaster

When a species of starfish (pisaster) that feeds on mussels was removed from an intertidal zone, the mussel began to dominate and eat other species (decreasing biodiversity)

Foundation Species

Some organisms exert their influence by altering the environmentThis changes the landscape and alters

the structural dynamics of the environment

• They may act as facilitators that have a positive affect on the community (protect from salt variations, maintain soil cohesion)

• They may also be harmful . . . What species do you think has been altering the environment the most?

Species interactions

Two simple ways to explain the effects of organisms in a communityBottom-up model – lower levels of the food

web influence the levels above them (producers are the most influential and the higher levels, top consumers, have the least influence)

Top-down model – the opposite of aboveThere are also many intermediates of these

two

Ecological Disturbances

Disturbance Classic view of communities:

Communities are in a state of equilibrium unless seriously disturbed by outside influences (they are stable)• Constant composition of species

Newer model is the nonequilibrium modelCommunities are constantly changing in response to

disturbances• Disturbance: anything that changes the community,

removes organisms, or alters the natural resources (shocker: humans have the highest impact)

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 (fire, tidal wave, natural disaster)

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

Biodiversity

Biodiversity

Two factors are usually associated with species diversity:Geographic locationGeographic size

• In general, fewer organisms on islands, than on main continent

Tropical diversity

In general, there is a larger diversity of species in tropical regions than in temperate or polar regions

Possible reasons:Evolutionary history – in general,

tropical regions are generally older than other biomes (more consistent climate means fewer major disturbances)

Tropical diversity

Climate is another reason for more diversity in the tropicsFairly consistent solar inputAbundant rainfall

• Evapotranspiration measures how much water remains in a biome as opposed to the loss due to transpiration

• In the tropics, much of the water remains in the biome