Communities & Ecosystems Chapters 53-54. Communities A community is an assemblage of populations in an area or habitat. Are species in a community independent.

Communities & EcosystemsChapters 53-54

Communities

• A community is an assemblage of populations in an area or habitat.

• Are species in a community independent of one another?

• Are some species locked in association?

• Species in a community do interact.

Populations May Be Linked In Several Ways

• Ecological Niche: The sum total of a species’ use of the biotic and abiotic resources in its environment.

• Competitive Exclusion Principle (Gause’s Exclusion Principle, 1934). Two species that compete for the same resources (or occupy the same niche) cannot stably coexist. One will become extinct.

• Resource partitioning – Species occupy slightly different niches. Extinction didn’t occur.

• Character Displacement (niche shift) may occur over time.

Resource Partitioning by 7 Species of Anole Lizards

Direct competition is avoided when each species occupies a different micro-habitat

La Palma, Dominican Republic

One species seeks sunny places

Another species seeks shady branches

Character Displacement

The two species have similar beak size when on different islands

Beak sizes have become different on the same island and they now eat different size seeds

Niches

• Fundamental niche – Niche that an organism occupies in the absence of competing species.

• Realized niche – That part of their existence where niche “overlap” doesn’t occur and competition is avoided.

Fundamental vs. Realized Niche

Three Forms of Symbiosis

Mutualism between the ants and acacia trees - The ants and trees both benefit

Commensalism between Remora and shark

Mutualism between Clown fish and anemones

ParasitismThe host is harmed

Mosquito

Tapeworm – parasitic flatworm

Ascaris lumbricoides – parasitic round worm

Coevolution may explain why two species seem to be

reciprocally adapted.

A change in one species acts as a selective force on another species, and

vice versa.Examples: Mimicry; secondary

compounds in plants (tannins, nicotine, latex

Animal Defenses Against Predators

Camouflage or cryptic coloration

Protective Coloration SchemesAposematic (warning) Coloration

Batesian Mimicry – A palatable species mimics an unpalatable one

Hawkmoth larva Snake

Mullerian Mimicry: Two unpalatable species resemble one another

Poison Dart Frogs

Trophic Levels in Ecosystems

Terrestrial Marine

Food Webs

Dominant vs. Keystone Species

• Dominant Species: Highest abundance - Oak, Hickory forests

• Keystone Species exert control by niche not by abundance

Disturbances of the Community

Fires may be part of the ecology

Disturbances by Human Activity

• Destruction of habitats• Extinction of species• Salinization of streams• Toxic Wastes• Fires that are not part of the natural order• Lack of fires where needed• Etc., etc., etc.• TOO MUCH CHANGE – TOO FAST

Primary Succession

Occurs on substrates that never previously supported living things. For example, on volcanic islands, on lava, and on rock left behind by retreating glaciers

Succession on Rock or Lava • Begins with establishment of lichens• Lichens hold moisture and break down rock

into soil• Bacteria, protists, mosses and fungi appear

and soil continues to improve• Insects and arthropods become established• r-selected species become established• K-selected species may replace r-selected

ones

Succession on Sand Dunes(A very long process)

• Organic matter washes ashore and decays at tide line adding nutrients

• Grasses stabilize the fore-dune sand and hold moisture. They also add nutrients

• Shrubs colonize stabile sand

• Cottonwood trees become established

• Pines and black oaks further develop soil

• Beech-maple climax forest develops

Primary Succession at the Indiana Dunes

Dune Succession

Changes in the amounts of soil nutrients, moisture, and the creation of new microclimates, encourages new species to replace old ones.

Primary Succession in the Arctic

Bare rock

Mosses

Lichens

Shrubs and stunted trees

Fir and pinesClimax boreal forest

Lake Succession

• Eutrophication• A slow process taking

thousands of years.• A continuous process

of change.• What will be the end

stage of succession?

Secondary Succession of Farmland

Established soil nutrients and organisms make colonization easier

Secondary Succession in a Cornfield

http://www.micro.utexas.edu/courses/levin/bio304/ecosystems/succession.2.gif

1st year Stubble

2nd year Wild grasses invade

5th year Mature grasses and sedges

10th year Prairie plants and shrubs, few juniper

20th year Mature juniper, birch and maple

Generalized Secondary Succession to Forest

2yr. 5yr. 10yr. 20yr. 50yr. or more

Summary of Succession• Slow process taking many years (100’s or 1,000’s).• Primary succession begins on bare ground or rock.• Physical and chemical processes break down the

substrate to provide basic living conditions.• Organisms colonize the substrate and enrich the

substrate with humus (decayed matter).• Soil and microclimates develop allowing new

species to colonize and replace the old ones.• A stable climax community eventually develops

depending upon climate.• Secondary succession begins with a fertile

substrate.

Ecosystems Approach To Ecology

• Ecosystem ecologists study the flow of energy and nutrients in an ecosystem

• Species are grouped by trophic levels

• All ecosystems are dependent upon the primary producers for energy

• Nutrients can be recycled by decomposers, but energy is eventually lost as heat

Ecosystem Dynamics

Primary Production

• The amount of light energy converted to chemical energy (organic compounds) by autotrophs in a given time period

• 1% of light striking plants is stored

• Producers create 170 billion tons of organic matter per year (mostly cellulose)

• Gross Primary Production (GPP)

• Net Primary Production (NPP)=GPP - R

Comparing Ecosystems

Aquatic Ecosystems• Light and nutrients limit aquatic systems

• Nitrogen limits phytoplankton growth in enrichment experiments with algae

• Fertilizer run-off can cause algal blooms

• Iron may limit production in some oceanic systems

Terrestrial Ecosystems

• Temperature and moisture are the key factors controlling primary production here

• Locally, soil minerals may control GPP

• Primary production removes soil nutrients

• Decomposition restores nutrients

• Farming may deplete the soil of nutrients

Secondary Production…

• is amount of chemical energy in consumers’ food converted to biomass in a given amount of time.

• Only small amounts of primary production by plants are converted to biomass in animals.

• Production Efficiency: the fraction of food energy that is used for growth and reproduction – not respiration.

• 1-40%

Trophic Efficiency

• Also know as ecological efficiency.

• Proportion of energy at one trophic level that is passed on to the next level.

• Averages around 10%.

• Limits the number of possible trophic levels in a food chain to about 3.

Pyramid of Energy

Pyramid of Biomass

Pyramid of Numbers

The Cycling of Chemical Elements in Ecosystems

Consider the reservoirs, assimilation, and release of

nutrients in each cycle

A General Model of Nutrient Cycling

Water Cycle

The Carbon Cycle

Huge amounts of carbon are locked up in limestone and peat deposits

http://www.eia.doe.gov/oiaf/1605/ggccebro/chapter1.html

The Nitrogen Cycle

Hubbard-Brook Forest Experiment

The Phosphorus Cycle

Human Impact• Deforestation results in mineral and water

loss• Slash and burn agriculture releases CO2 and

removes nutrients as crop harvest• Accelerated eutrophication of bodies of

water• Toxic chemicals concentrate in the food

chain• Loss of biodiversity destroys ecosystems• Disruption of natural cycles

Acid Rain From Burning Fossil Fuels

Global Warming

http://www.whrc.org/carbon/index.htm

Ozone Layer Depletion by CFC’s

Ozone layer hole