Download ECOLOGY-2

ECOLOGY-2
Interactions
In
Communities
An Ecological Community is…
• All populations of organisms…
• …inhabiting a common environment…
• …and interacting with one another.
Competition
• Individuals of the same species which
interact for resources are said to show
intraspecific competition
• Individuals of different species which
interact for resources are said to show
interspecific competition
• Both may limit the supply of resources
• Affects the reproductive success of the
population
Types of Competitors
• Producers – plants
– Compete with other plants for sunlight & water
• Herbivores – animals that eat plants &
algae
– Compete with other herbivores for food, etc.
• Carnivores – animals that eat other
animals
– Compete with other carnivores for food, etc.
Competitive Exclusion
• G.F. Gause formulated the competitive
exclusion principle in 1934:
– “if two species are in competition for the same
limited resource, one or the other will be more
efficient at utilizing or controlling access to
this resource and will eventually eliminate the
other in situations in which the two species
occur together.”
In Other Words…
• No two similar species can
occupy the same niche at the
same time!
Competitive
Exclusion
• Where two species
“overlap” in the
same niche, one
will exclude the
other
What’s a Niche?
• The total environment and way of life of all
members of a particular species of
organism in the community
• An ecological niche is the role that an
organism plays in its environment
• By analogy, a niche is roughly equivalent
to an organism’s profession, as opposed
to its address.
Niche
• Sum total of all
biological and
physical factors
which define the
“space” in which a
particular species
serves its function in
the ecosystem
Environmental Niche
• Description includes:
– Physical factors
• Temperature range
• Moisture requirements
– Biological factors
• Nature & amount of required food sources
• Pattern of movement & behavior
• Seasonal & daily activity cycles
Gause’s Experiments
Experiment Analysis
• Fastest growing species is not always the victor:
• Lemna gibba grows more slowly, but always
replaces L. polymorpha because of its flotation air
sacs – it forms a mass over the other species and
cuts off its supply of light!
Resource Partitioning
• When two similar species inhabit the same
area and have similar ecological
requirements
• Close examination shows that they “divide
up” the resources to avoid competition
• Exact cause of resource partitioning is
subject to debate
Resource Partitioning in Warblers
Resource Partitioning in Lizards
Resource Partitioning in Plants
Resource Partitioning in Seabirds
Experimental Studies of Competition
• Barnacle competition studied along the
Scotland coast
• Chthamalus is usually found only in upper
intertidal (smaller – slower growing)
• Balanus usually found in lower intertidal
(larger – grow faster)
• If Balanus removed, Chthamalus will move
into the lower intertidal zone
• Balanus however doesn’t invade the upper
zone, because it cannot survive the drier
conditions
Barnacle Competition
Fundamental vs. Realized Niche
• Fundamental niche – describes the
physiological limits of the organism
(maximum tolerance for temperature,
desiccation, etc.)
• Realized niche – that portion of the
fundamental niche actually used;
determined by physical factors and
interactions with other organisms
Winner Takes All
• A successful
competitor
may actually
displace its
rival
completely!
• Example:
introduced
starlings
replace
bluebirds
Predation
• Defined as the consumption of live organisms
– Plants by animals
– Animals by animals
– Animals by plants or fungi
Computer Model
Lynx – Hare Predation
Predation & Species Diversity
• Predation holds competition down and
more resources remain, more species can
coexist
• R.T.Paine did an experiment to test the
hypothesis that predators actually
INCREASE the community DIVERSITY!
Paine's work in intertidal
communities of the Pacific Northwest
In the undisturbed (control) areas:
15 prey species coexist
In the starfish removal areas:
8 prey species remain and
community dominated by
mussels
Community Interactions
Concept of Feedback Loops
Direct and Indirect Interactions Control Populations
Healthy oaks
produce
more acorns.
Gypsy
moths
Oak trees produce
Oak
large crops of acorns
every few years.trees
Oak
trees
Gypsy moth
infestations defoliate
oak trees, reducing
acorn population.
Acorn
production
Deer
mice
Oak trees produce
large crops of
Gypsyevery few
acorns
years.
moths
Deer mice eat
relatively few
gypsy moth
pupae; moth
population
Predators
on
mice reduce
Dense
mouse populations
keep
Mouse populations
increase
remains
high.
mouse
populations,
gypsyofmoth
populations
low allowing
by eating
greatly in years
heavy
recovery
of
gypsy
gypsy moth pupae. moth
acorn production.
populations.
Acorn
production
Deer
mice
Mouse
populations
increase
greatly in
years of
Predators
heavy
acorn
production.
Symbiosis
• Parasitism
– One species benefits, the other is harmed
• Commensalism
– One species benefits, the other not affected
• Mutualism
– Both species benefit
Symbiosis
• HOST
• SYMBIONT
PARASITISM
COMMENSALISM
MUTUALISM
Examples of Symbiosis
• Parasitism
– Usually smaller
than the host
– May be animal on
animal; animal on
plant; plant on
plant
Examples of Symbiosis
• Parasitism
– Usually smaller
than the host
– May be animal on
animal; animal on
plant; plant on
plant
Examples of Symbiosis
• Parasitism
– Usually smaller
than the host
– May be animal on
animal; animal on
plant; plant on
plant
Examples of Symbiosis
• Parasitism
– Usually smaller
than the host
– May be animal on
animal; animal on
plant; plant on
plant
– Nest parasitism
Examples of Symbiosis
• Parasitism
• Commensalism
– Scale worms live
in the grooves of
starfish arms
Examples of Symbiosis
• Parasitism
• Commensalism
– Scale worms live
in the grooves of
starfish arms
– Barnacles on a
scallop shell
Examples of Symbiosis
• Parasitism
• Commensalism
– Scale worms live
in the grooves of
starfish arms
– Barnacles on a
scallop shell
– Anemone fish
Examples of Symbiosis
• Parasitism
• Commensalism
– Scale worms live
in the grooves of
starfish arms
– Barnacles on a
scallop shell
– Anemone fish
– Epiphytes ( plants
which grow using other plants
or objects for support
)
Examples of Symbiosis
• Parasitism
• Commensalism
• Mutualism
– Ants & aphids
Examples of Symbiosis
• Parasitism
• Commensalism
• Mutualism
– Ants & aphids
– Ant & Acacia
Examples of Symbiosis
• Parasitism
• Commensalism
• Mutualism
– Ants & aphids
– Ant & Acacia
– Insects & plants
Examples of Symbiosis
• Parasitism
• Commensalism
• Mutualism
–
–
–
–
Ants & aphids
Ant & Acacia
Insects & plants
Mycorrhizal fungi
Examples of Symbiosis
• Parasitism
• Commensalism
• Mutualism
–
–
–
–
–
Ants & aphids
Ant & Acacia
Insects & plants
Mycorrhizal fungi
Cleaner shrimp
Community Composition:
Community Stability
And
Equilibrium
Island Biogeography Model
• R. MacArthur & E.O. Wilson (1963)
• Used small islands as models to study
species composition & stability of
communities
• Findings:
– The NUMBER of species was relatively
CONSTANT
– The SPECIES COMPOSITION CONSTANTLY
CHANGES
• Known as the Equilibrium Hypothesis
Equilibrium Hypothesis
Intermediate Disturbance Model
• Tropical rainforests & coral reefs long
thought to be stable, equilibrium
communities (diversity thought to be a
function of stability)
• Evidence now suggests that diversity is a
function of the frequency and magnitude of
the disturbances a community is subject to
Intertidal boulder fields on the California coast
Mode for
large
Mode for
medium
boulders
Wayne Sousa noticed that small and large boulders tended to have fewer
species of algae on them than boulders of intermediate size. Sousa
guessed that that small boulders were more likely to roll during storms
(“scouring” the algae off them) than medium boulders. Medium boulders,
in turn, were more likely to roll than large boulders.
Sousa tested this hypothesis by cementing small boulders to the
substrate so they could not roll.
The figure above shows that small rocks (“unstable small rocks”) are
normally dominated by a single species of alga (Ulva, sea lettuce).
Similar rocks that are cemented to the substrate (“stabilized small
rocks”) eventually develop a richer algal community.
Ecological Succession
Relatively long, gradual changes
in community composition
following initial colonization
Temporal Succession
• Temporal distributions of
plant species remains in
pack rat nests
(“middens”) in Texas
since the last glaciation.
• Each horizontal graph is
a different plant species.
• Time before present is
plotted left (past) to right
(present).
• Plant community changes
almost continuously
Spatial Succession
• First colonists gradually crowded out by
successive groups of organisms.
Theories of Succession
• Facilitation model
– Each stage “prepares the way” for next
• Inhibition model
– Each stage prevents colonization of next
• Tolerance model
– Existing species neither inhibit nor promote
the colonization of next
Succession Terminology
• Pioneer species
– Early colonists; grow rapidly (“weedy”) and
fully occupy available area
• Seral stages (or communities)
– Intermediate assemblages of species;
populations may vary significantly in size
• Climax community
– Group of organisms likely to remain in the
area until a disturbance
Climax vs. Continuum
• The classical concept of successional climax communities
has been superceded by the continuum concept, in line
with the acceptance that most communities are relatively
“open”: each species responds individually to climatic
conditions.
The key feature to
remember is that
communities are
DYNAMIC in their
COMPOSITION; their
makeup will change as
conditions change!