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Ecological Communities
Chapter 45
Characteristics of Communities
• Community: all species that coexist and
interact with one another in a given location
• Characteristics:
– Species richness and composition
• Communities are assembled thru the gain and
loss of individual species
– Species compositions do change over time and
space!!!
Communities Change Over Space and
Time
• Species composition changes along
environmental gradients
– Remember the range or dispersal of a species is
due to abiotic and biotic factors
• Some species of organisms only live in specific
temperature, precipitation, soil type, light levels, etc.
• Presence of one species may have a direct affect on the
presence or absence of another!!
– Often certain animal species are only associated with certain
vegetation – which in turn is only associated with certain
biomes or geographical locations!
Several Processes Cause Communities
to Change Over Time
• Three major processes contribute to changes
in community composition:
– Extinction and colonization
– Disturbance
• Ex: sudden environmental change – earthquake,
volcanic eruption; fire
• Succession – characteristic pattern of change that
occurs in a community after an environmental
disturbance
– Primary vs secondary succession
– Climate Change
Dung Beetle Species Composition
Changes Over Times
Each species
directly
affects/contributes
to changes in
various abiotic or
biotic factors that
can then promote
or inhibit other
species!!
Species Composition Changes with Climate Change
Succession
• Characteristic pattern of species change after a disturbance
– Each species alters abiotic/biotic factors that promote/inhibit other
species
• Primary
– No soil present; often associated with NEW islands, etc.
• Secondary
– Soil still present, but serious damage to species on environment has
occurred
• Ex: clear cutting forest; forest fire
• Ecological transition – distinctly new community recolonizes
after a disturbance
Succession
• Change involving a series of species replacements in a
community following a disturbance
• Primary succession
– Occurs in area where there is no soil - no preexisting ecosystem
• Lichens (pioneer - break down rock to soil)
• Secondary succession
– Soil present - prior community present
• Old-field
• Fire
• Pioneer species
– First species to take hold - usually a producer
– Climax community - mature, established community
Secondary Succession
(a) Soon after fire
(b) One year after fire
Primary Succession
1941
1907
2 Dryas stage
1 Pioneer stage, with
fireweed dominant
1860
0 5 10 15
Kilometers
Glacier
Bay
Alaska
1760
4 Spruce stage
3 Alder stage
Soil nitrogen (g/m2)
60
50
40
30
20
10
0
Pioneer
Dryas
Alder
Successional stage
Spruce
Trophic Interactions In Communities
• Energy flow and nutrient cycling in ecosystems
– Nutrients and materials cycle
– Energy is a one-way street
• Ecological efficiency
• Trophic interactions can change the species
composition of communities
– Ex: trophic cascade – wolves!!
• Energy flow and Chemical cycling
– While nutrients (C, N, P, water) are
recycled - Energy is NOT recycled in an
ecosystem
• Relationships
– Trophic levels
• Primary producers (input energy into system;
convert inorganic to organic compounds)
• Primary consumers
• Secondary and tertiary consumers
• Detrivores/detritus (release nutrients
back as inorganic substances)
• Food chain
A Food Web in Yellowstone
The Major Trophic Levels
Trophic Cascade
removal of one species
(keystone species)
dramatically alters trophic
levels or species
diversity/richness
Diversity = species richness + evenness
Energy Flow: Primary Productivity
•
Primary productivity - amount of light energy converted to chemical energy
by autotrophs
– Gross (GPP): total energy converted
– Net (NPP): represents the storage of energy available to consumers
– R: respiration (amount of energy used by the primary producers for own energy
needs)
•
•
•
•
NPP = GPP - R
Biomass: primary productivity reflected as DRY weight of organic material
Secondary productivity: rate at which an ecosystem’s consumers convert
chemical energy of the food they eat into their own new biomass
Overall, terrestrial ecosystems contribute about 2/3 of global NPP
– Temperature and moisture influence PP
– PP generally lowest in high-latitude tundras and deserts and highest at equator in
tropical rain forests
• High productivity provides varied niches and increased biodiversity
•
Marine ecosystems contribute 1/3
– In aquatic ecosystems, both light and nutrients control primary productivity
• Depth of light penetration affects primary production in the photic zone
– Often more limiting factor in photic zone is nutrient level (N, P)
• Pyramid of net
production
• Pyramid of biomass (dry
weight)
• Pyramid of numbers
Energy Transfers Between
Trophic Levels Less than 10%
Efficient
– These all represent the loss of
energy with each transfer in a
food chain!!
• Ecosystems are limited
to number of trophic
levels they can
support!!!
• Limits the number of
individuals at any
particular trophic level!!
– Remember Malthus - while
reproduction rates exponential,
resources are limited!!!
Tertiary
consumers
10 J
Secondary
consumers
100 J
Primary
consumers
1,000 J
Primary
producers
10,000 J
1,000,000 J of sunlight
Trophic level
Tertiary consumers
Dry weight
(g/m2)
1.5
Secondary consumers
11
Primary consumers
37
Primary producers
809
Most biomass pyramids show a sharp decrease in biomass at successively
higher trophic levels, as illustrated by data from a
bog at Silver Springs, Florida.
Trophic level
Tertiary consumers
Number of
individual organisms
3
Secondary consumers
354,904
Primary consumers
708,624
Primary producers
5,842,424
Energy Transfers
Between Trophic
Levels Less than
10% Efficient -WHY??
Species diversity affects community
function
• Species diversity = species richness + species
evenness
• NPP and GPP can vary greatly with species
richness
– Diverse plant communities may use soil nutrients
more efficiently
– Output of seeds and fruit by plants is a function of
insect diversity
– Retention of nutrients in ecosystem increases with
more diversity of decomposers
– Insert fig 45.10
Species Richness Increases Toward Equator
Structurally Complex Habitats Support Greater Diversity
NPP Varies Among Ecosystem Types
Which ecosystems have
highest net primary
productivity and why?
Why is open
ocean so low?
Often limiting factor
in photic zone is
nutrient level (N,P)
Marine NPP is Highest around Coastlines
Aquatic Ecosystem NPP Generally Limited by N
or P Availability
• Too much of a
good thing may
cause problems
• High nutrient
levels can lead to
eutrophication
and dead zones.
Terrestrial NPP Corresponds to Climate
Terrestrial NPP Varies with Temperature and
Precipitation
Energy is a One-Way Street But Nutrients Cycle
• Biogeochemical cycles - nutrients cycle between both biotic
and abiotic components in an ecosystem
–
–
–
–
Water
Carbon
Nitrogen
Phosphorus
• In studying cycling of water, carbon, nitrogen, and
phosphorus, ecologists focus on four factors:
1. Each chemical’s biological importance
2. Forms in which each chemical is available or used by organisms
3. Major reservoirs for each chemical
4. Key processes driving movement of each chemical through its cycle
Chemical Elements Cycle among Compartments of the Biosphere
Global Water Cycle
Global Carbon Cycle
Global Nitrogen Cycle
N2 in atmosphere
Assimilation
Nitrogen-fixing
bacteria in root
nodules of legumes Decomposers
Ammonification
NH3
Nitrogen-fixing
soil bacteria
NO3–
Nitrifying
bacteria
Nitrification
NO2–
NH4+
Nitrifying
bacteria
Denitrifying
bacteria