Biology 213 Chapter 54
Community Ecology
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You will be able to…
• Define and give examples of niches
• Describe various types of interactions within a
community
• Compare & contrast how dominant and
keystone species influence a community
Dining In
• Wasps and Pieris caterpillars form an
unusual three-step food chain
• The 4-mm-long wasp Apanteles
glomeratus stabs thru skin of
Pieris rapae caterpillar
& lays her eggs
• caterpillar is destroyed
when wasp larvae hatch
& nourish themselves
on its internal organs
• Ichneumon wasps can detect when a Pieris
caterpillar contains Apanteles larvae
Female ichneumon will pierce caterpillar
and deposit her own eggs inside of
Apanteles larvae
• Finally, another wasp, a chalcid, may lay its
eggs inside the ichneumon larvae
• Usually, only the chalcids will emerge from
the dead husk of the caterpillar
• Biological community
interactions &
interdependence of
organisms living in it.
• Ecosystem functioning depends on
complex interactions btwn its community of
organisms and the physical environment
A community = all organisms inhabiting
a particular area
• Several factors characterize every
community
– Biodiversity
– Prevalent form of
vegetation (& its levels)
– Response to disturbances
– Trophic structure
(feeding relationships)
Biodiversity = variety of different kinds of
organisms that make up a community
two major components:
1. Species richness, or total # of different
species in the community
2. Relative abundance of different species
Interspecific interactions: (+), (-), neutral
• Competition (-)/(-) occurs btwn 2 populations if
they both require same limited resource
Interspecific interactions: (+), (-), neutral
• Predation (+)/(-) one species kills & eats other
species
Symbiosis
• A symbiotic relationship is an interaction
between two or more species that live
together in direct contact
• 3 main types of symbiotic relationships
within communities
• Symbiosis
1. Mutualism
• Both partners benefit
2. Commensalism
• One partner benefits and the other is
unaffected
3. Parasitism
• One partner benefits while the other is
harmed
Interspecific interactions: (+), (-), neutral
• Parasitism (+)/(-) one species is nourished by
another species; harms other species.
• Parasitism: type of predator-prey
relationship
– parasite benefits and
host is harmed
– parasite obtains food at
expense of host
– Parasites are typically
smaller than their hosts
Worldwide, 25% of all deaths result
from parasite infections
Interspecific interactions: (+), (-), neutral
• Mutualism (+)/(+) both species benefit
• Examples of mutualism
–Nitrogen-fixing
bacteria and legumes
Mutualism can be
• Obligate:
one species cannot
survive without the
other
• Facultative:
both species can
survive alone
Mutualism:
Algae in coral polyps give C and N compounds,
get N in return (NH3) from coral waste.
Obligate or Facultative?
Pollination is mutualism: e.g. Milkweed
(Asclepias) is pollinated by butterflies
mutualism often demonstrates
coevolution
Each party
evolves features
that take advantage
of the association,
but also provides them with
an encouraging reward.
Interspecific interactions: (+), (-), neutral
• Commensalism (+)/(0) only 1 species
benefits; other species is not harmed
• Commensalism - symbiotic
relationship where one partner
benefits and other is unaffected
• e.g. of commensalism
–Algae growing on shells of sea turtles
–Barnacles that attach to whales
–Birds that feed on insects flushed out
of grass by grazing cattle
Commensalism
Spanish “moss”
and southern
trees
(e.g. Golden Larch)
One benefits, the
other isn’t helped
or harmed
Competitive exclusion principle
Populations of 2 species cannot co-exist
in a community if their niches
are nearly identical
Resource
partitioning or
character
displacement
occurs
(or extinction of
one species)
High
tide
Chthamalus
Balanus
Ocean
Low
tide
• Competition btwn species with identical
niches has two possible outcomes
1. One population survives:
• more efficient using resources
• reproductive advantage,
• may eventually eliminate other (difficult
to observe).
2. resource partitioning
• Ecological niche
– Distinctive lifestyle & role of an organism
in a community
– all abiotic and biotic aspects
– E.g. organism’s habitat is one parameter
used to describe the niche
Various community niches interact: How does a
bumper crop of acorns relate to a human disease?
Increase in human
exposure to Lyme
bacterium
Public health
hazard
Deer attracted to forest
Tick population
increases
Healthier forest
Oaks thrive; bumper
crop of acorns
White-footed Mouse
population increase
Decrease in gypsy
moth pupae
• Fundamental niche
–Potential ecological niche for an
organism
• Realized niche
–Niche an organism actually occupies
• Limiting factors
–Environmental resources and
components that restrict an organism
to a realized niche
Competition effect on organism’s realized niche
Brown anole: introduced species
Out-competes native green anole
where 2 niches overlap
Fig. 54-3a
EXPERIMENT
Chthamalus
Balanus
High tide
Chthamalus
realized niche
Balanus
realized niche
Ocean
Low tide
Fig. 54-3b
RESULTS
In terms of their niches,
what do the
results tell you
about Chthamalus?
And about Balanus?
Ocean
High tide
Chthamalus
fundamental niche
Low tide
• Competition
–Two or more individuals attempting
to use same resource
–Intraspecific competition
• Among individuals within a
population
–Interspecific competition
• Between different species
Interspecific
competition
between protists
Grown together:
P. aurelai excludes
P. caudatum
• Some species reduce competition by
resource partitioning
• Competition among species is reduced
by character displacement:
–Structural, ecological, & behavioral
characteristics diverge
where ranges overlap (sympatric)
Spatial
Temporal
Food
Character Displacement
Characteristics tend to be divergent in
sympatric populations of 2 species than in
allopatric populations of same 2 species
Fig. 54-4
G. fuliginosa G. fortis
Character
Displacement
Percentages of individuals in each size class
Beak
depth
60
Los Hermanos
40
G. fuliginosa,
allopatric
20
0
60
Daphne
40
G. fortis,
allopatric
20
0
60
Sympatric
populations
Santa María, San Cristóbal
40
More
divergent
20
0
8
10
12
Beak depth (mm)
14
16
Resource partitioning
Galapagos finches
Ancestral finches
diverged to
partition
limited resources:
insects, seeds, etc.
Character
displacement:
structural changes
occur
Resource partitioning: different life stages
Agraulis vanillae
Adults feed
differently from
larvae.
Different Adult
species avoid
direct competition:
feed on different
parts of plants,
& select different
microhabitats
Heliconius charitonius
• Predation
–Consumption of one species
(the prey) by another (the predator)
• Coevolution
–Predator and prey both evolve more
efficient ways to interact.
–Prey changes to escape predation
–Predator becomes more efficient at
predation
Coevolution in sea shell studies
• Modern shells show scars from molluscivore
crab predation
• Ancient shells from similar environment show
scars too, but crab claws have evolved to be
stronger with bigger “teeth”
• Shells have evolved more thickness, larger
size, rigid calcification, quick scarring, and
variations in shape.
• As predators adapt to prey, sometimes natural
selection also shapes prey's defenses
• process of reciprocal
adaptation is known
as coevolution
Eggs
– Example: Heliconius
& passionflower vine
females avoid phony eggs.
Sugar
deposits
Ants & wasps also attracted to sugar deposits
& prey upon butterfly’s eggs.
• Defenses
– Mechanical & chemical defenses
– Associating in groups
– Cryptic coloration
– Aposematic (warning) coloration
– Müllerian & Batesian mimicry
• Prey gain protection against
predators thru variety of defense
mechanisms
–Mechanical
defenses,
e.g.quills of a
porcupine
Associating in groups
Chemical defenses –
widespread & very effective
– Animals with effective chemical defenses
often brightly colored to warn predators
– e.g.: poison-arrow frog
Skunk
• Camouflage - very common defense in
animal kingdom
– Example: gray tree frog
Cryptic
coloration
• Batesian mimicry: when a palatable or
harmless species mimics an unpalatable or
harmful one
– Mimicry can even involve behavior
– This hawkmoth larva puffs up its head to
mimic the head of a snake
These butterflies are not Batesian
mimicry but are Müllerian mimicry
• Müllerian mimicry: 2 unpalatable species
that inhabit same community mimic each
other – Example: cuckoo bee and yellow jacket
Predation can maintain diversity in a
community
• keystone species exerts strong control on
community structure due to its ecological role
• A keystone predator may maintain community
diversity by reducing #’s of strongest
competitors in a community
– e.g. sea star is a
keystone predator
• Seals over-hunted –
normal food for Orcas
• Predation by Orcas
(Killer whales) on sea
otters, allowing sea
urchins to overgraze on
kelp
– Sea otters represent the
keystone species
Fig. 54-16
Otter number
(% max. count)
100
80
60
40
20
0
(a) Sea otter abundance
Grams per
0.25 m2
400
300
200
100
Number per
0.25 m2
0
(b) Sea urchin biomass
10
8
6
4
2
0
1972
1985
(c) Total kelp density
1989
Year
1993 1997
Food chain
• Keystone species
– Present in small numbers but are crucial
in determining species composition and
ecosystem functioning
• Dominant species
– Affect the community because they are so
commonly found
– Largest biomass
Dominant species
• One hypothesis: dominant species most
competitive in exploiting resources
• Another hypothesis: dominant species most
successful at avoiding predators
• Invasive species, introduced to a new
environment by humans, often lack
predators or disease.
– What are some invasive species in our region?
Foundation Species
(Ecosystem “Engineers”)
• ecosystem “engineers”
cause physical D’s in environment
affecting community structure
Biology 213 Chapter 54
Community Ecology
Part 2
You will be able to…
• Compare and contrast various types of
symbiosis
• Explain the concept of biodiversity and its
importance in studying ecosystems.
• Compare & contrast 1o & 2o succession
• Explain types of “disturbances” (man-made &
natural ) that can cause succession.
Biodiversity
• Ecological measurement of an ecosystem’s
“health”
• Measures interactions within a community
and interdependence of various species.
• Describes a community’s ability to survive
various types and degrees of disturbance
• Species richness
–# of species within a community
• Species diversity
–Relative importance of each species
within a community
Effect of community
complexity on species
richness
Complexity of
chaparral
communities based
upon density and
height of foliage.
Higher complexity is
reflected in larger #
& variety of bird
species:
more variety in food
and shelter.
Reflects drier habitat
in Chile.
Species richness
• Related to solar E & H2O (autotrophs)
• Evolutionary history & age of an area
• Related to isolation
– “islands,” mountain tops
– replacements vs extinction
Fig. 54-27
Equilibrium number
Number of species on island
(a) Immigration and extinction rates
Rate of immigration or extinction
Rate of immigration or extinction
Rate of immigration or extinction
“Islands” can be any isolated region
Small island
Large island
Number of species on island
(b) Effect of island size
Far island
Near island
Number of species on island
(c) Effect of distance from mainland
Species richness
• One major dominant species reduces
richness (think city fauna)
• Community margins overlapping
– Ecotones or Edge Zones
– Enrich a community’s diversity
Edge Zones = Ecotones
Species richness = stability
• Older, moderately disturbed communities often
richer in species
– Glaciated regions vs rain forests
• Less diverse habitats more prone to
devastation by one event or agent
– Planted field vs diverse “natural” field responding
to pests
Older moderately
disturbed
communities
richer in species
Frequently or recently
disturbed area
shows less
diversity in
some cases.
But is that always true?
• Intermediate disturbance hypothesis
–Disturbance affects succession and
species richness
–Species richness is greatest at
moderate levels of disturbance
Intermediate disturbance hypothesis
The nature of communities
– Organismic model
• Views a community as a “super-organism”
that goes through stages of development
(succession)
The nature of communities
– Individualistic model
• Abiotic environmental factors are primary
determinants of species composition
• Organisms are separate from each other;
communities can rearrange independently
of each species
• Ecological succession
–Primary succession
• Occurs in an area not previously
inhabited
–Secondary succession
• Occurs where there is a preexisting community and
well-formed soil
• Ecological succession is a transition
in species composition of a community
following a disturbance
– Primary succession
is gradual colonization of barren rocks,
gravel, or sand by living organisms
– Secondary succession
occurs after a disturbance has removed
vegetation but left soil intact
Primary succession
Starts without soil.
No organic matter,
only mineral material:
sand,
bare rock,
gravel
from glacial outwash,
volcanic ash
lava,
man-made structures
or severe erosion
Primary succession
Volcanic regions:
Pioneer plants must
establish themselves in
tiny crevices that
collect some moisture
and infinitesimal bits of
organic detritus
Secondary succession
Starts with soil.
Pioneer plants start from roots or seeds in soil or
transported seeds (by wind or animals) from
surrounding areas. Faster than primary succession.
Some regions are periodically
“disturbed” by natural phenomena
• Floods
• Blizzards & avalances
• Hurricanes
Are these “disturbances” actually part of the
area’s ecosystem?
What happens when these disturbances are
eliminated? e.g. flooding on damned
rivers / fires in our region?
Some regions are periodically
“disturbed” by man-made phenomena
• Logging
• Agriculture – land clearing, pests “encouraged”
• Alien species introduced accidentally &
purposefully
• Grazing
• Building
How do communities react to disturbances?
How can humans minimize impact on local
communities?
Succession study:
Abandoned Field to Oak Forest
• Dwight Billings
in 1930s
• Succession of
plant species on
abandoned
agricultural
fields in
N. Carolina.
• Deserted from a
few years to
~ 150 years.
First stage of secondary
succession
• Pioneer plants
– Annual species & lichens
– Colonize bare ground and nutrient poor soils
Second stage
• Annuals quickly replaced in dominance
the next year by biennial plants and
grasses
• Two year life cycle
• Usually reproduce in 2nd year
Perennials and Shrub stage
• After about 3 to 4 years:
– biennial & grass species
gave way to perennial
herbs and shrubs
• Perennial plants
– live for many years
– reproduce several times
over their life spans.
Softwood tree species take over
• After about 5 to 15 years:
– Loblolly, Shortleaf, and Virginia pines &
Sweet Gum trees colonize the area
(Note: S.E. U.S.)
Softwood tree species take over
• Forest canopy starts to form
– canopy reduces light reaching forest floor
– shaded understory excludes light loving
perennial herb and shrub species
– low light perennial herb and shrub species
take over ground cover
Canopy changes habitat
• Canopy changed
microclimate of habitat
near ground level.
– more humid
– moderated temps
– less wind
– rain drops cushioned
Canopy changes habitat
• Low light conditions
inhibited germination of
pine seedlings.
• Development of a soil litter
layer & humus
• Soil chemistry changes
Forest composition changes slowly
• Shade, humidity, humus and leaf
litter, low light
– germination of hardwood species:
• oak (Quercus spp.)
• hickory (Carya spp).
• 50 to 75 years after pioneer species
hardwoods start to replace softwood
species
– pines max heights ~ 25 meters
– oaks & hickories average ~ 10 meters tall
Climax Community
• Softwoods: shorter life span
~ 50 yrs
• Gap created filled by
subdominant hardwood
trees
• Oak and hickory, can live for
more than 100 years
• Sites more than 100 years
old were found to be
dominated by mature oak
forests
This succession takes ~ 120 yrs
from pioneer stage to
climax community.
• With infrequent disturbance: a stable
climax community consisting of
plants and animals that can
reproduce themselves in the existing
conditions will become established.
• Disturbance of the ecosystem will
start the process of succession anew.
Remember:
succession isn’t usually uniform
• In one area - usually small patches in different
stages of succession
• Depends on time & severity of last disturbance
• Adds diversity in types of vegetation and
animals living in the greater region.
Agriculture, overgrazing, and
logging affect a region’s succession
Fig. 54-24
Disturbance of ocean
floor by trawling:
removes ~ 5 to 25%
of an area's seabed
life on a single run.
The role of fire in ecosystems
–key abiotic factor in many
ecosystems
–Grasslands so dependent on fire its
absence is considered a disturbance
–Allelopathic chemicals burnt off
–Accumulated debris reduced and
nutrients released
Fire Ecology
1. Fire Dependence: effects of fire make environment
more hospitable for regeneration & growth.
2. Fire History: fire frequency occur in an area.
– Record: fire scars or charcoal layer in tree rings
3. Fire Regime (characteristics): intensity, severity,
frequency, and vegetative community.
Fire Ecology
4.
Fire Adaptation: special traits to
survive fires at various stages of plant
life cycles:
• serotinous cones
• fire resistant bark
• fire resistant foliage
• rapid growth and development
Fire maintains ecotones:
small fires create many “edges”
Clicker Question #2 on succession:
A small fire burns through a Douglas-fir old
growth forest. What type of succession would
occur on the forest floor, and what would the
dominant plant species be several months after
the fire?
A. 2o succession; mature Douglas-fir trees
B. 1o succession; grasses
C. 2o succession; oak and madrone trees
D. 1o succession; mosses & lichens
E. 2o succession; grasses
Rogue valley succession
Depends upon:
Climate
Local physical factors
Slope – what does that do?
* Water run-off; angle & direction of sun
Altitude
Fires, storms, & floods
Rogue valley succession
Depends upon:
Communities
•
Composition of plants: type, age, &
distribution
•
Animals – grazing, soil
compaction/fertilization, others?
Rogue valley succession
Depends upon:
Human activities:
•
Alien plant & animal species introduced
•
Logging, agriculture, grazing,
building houses & roads
•
Fire suppression
Successional stages in our area:
•
•
•
•
•
•
Grasses
White oak savanna
Chaparral
Black oak and Madrone
Ponderosa pine (may be mixed)
Douglas-fir and other trees
(e.g. Incense cedar)
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