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