Download Community Ecology 1 2

Community Ecology
Community - a group of organisms, of different species, living
in the same area
Community ecology is the study of the interactions between
species
The presence of one species may affect the ability of another
species to live in the same area positively, negatively, or have
no effect.
1
Classification of types of interspecific interactions:
Type of
interaction
Competition
Predator-Prey
Parasite-Host
ConsumerConsumee
Mutualism
Commensalism
Effect of
Effect of
Species 1 on Species 2 on
Species 2
Species 1
+
+
+
+
+
+
0
2
Competition
• among individuals of a single species is called intraspecific
competition - this results in density dependent effects on
population growth
• between members of different species is called interspecific
competition
Two species can have negative effects on each other through
mutual consumption of a limited resource
A resource is anything that if decreased will decrease the
numbers or growth rate of a species in the area.
Competition for resources occurs if consumption by one
individual reduces availability of the resource to others.
3
Two types of interspecific competition:
• Contest or Interference competition - one or both species
actively defend a resource against consumption by the other
species
• Exploitative competition - both species use the resource but
neither defends it
Contest competition - involves active defense of a resource
examples: Red cockaded woodpeckers defend nest holes from
occupation by squirrels
Damselfishes defend mats of algae from consumption by other
fishes
Hummingbirds in tropics defend nectar producing trees
from other nectar feeding birds and insects
4
Exploitative competition - competition through consuming
activities of an animal or plant
examples: Canopy trees consume light and limit its availability
to understory plants
Many fish species feed on the same invertebrates
Exploitative competitors may not even encounter each other
because they feed at different times or on different life stages
of prey
Example: in the deserts the principal consumers of seeds are ants
and birds
5
The limiting resource concept In natural systems it is common to find that one or at most a
few resources are in limited supply for a species
In plants, it is commonly just one soil nutrient that is in limited
supply.
Plant nutrients: N, P, K and micronutrients - all are required for
plant growth. If any one becomes reduced below a critical value
then growth of the plant or the plant population is limited.
Leibig’s Law of the Minimum - each population increases until
the supply of some resource no longer satisfies the population’s
requirement for it.
Which nutrient becomes limited first often depends up the rate of
consumption of the nutrient by the species using it.
6
dN/dt > 0
P
Resource utilization
will reduce both N
and P, but N will be
reduced to limiting
level before P.
P*
limiting
level of
P
dN/dt < 0
N
N* limiting level of N
7
Competition Experiments - exclusion and coexistence
Early experiments (by Gause) with cultures of more than species
of Paramecium showed that one species can cause the extinction
of another
Each species had
logistic growth
when cultured
alone
When cultured in pairs
either exclusion or
coexistence occurred
8
Gause’s Competitive Exclusion Principle - when two
competitors utilize the same required resource that is available in
only limited quantities, one species will cause the extinction of
the other - “complete competitors cannot coexist”
Exclusion doesn’t happen if the two species are able to exploit
portions of their niche that don’t overlap with the other’s
Gause found P. caudatum was able to
feed on bacteria at the top layers of
the culture tube and excluded P.
bursaria from that zone. P. bursaria
fed on yeast in the bottom layers of
the culture and excluded P. caudatum
from that zone.
9
The outcome of competition often
depends on the environment
If one environment is more
favorable to one species allowing faster growth or a higher
carrying capacity - it may be able
to out-compete a species that
would replace it in another
environmental situation.
10
Competition in nature - many field experiments have shown that
competition is an important process in natural communities.
Fenced
enclosures
constructed to
exclude
Kangaroo rats
but not
exclude
smaller rodents
A larger number of smaller rodents could live inside the
enclosure when Kangaroo rats were excluded They were each exploiting some of the same resources - after
removal, the small rodents did not have Kangaroo rats as
competitors
11
Field experiments have demonstrated that the effects of
competition are often not symmetrical.
Sceloporus merriami has a negative effect on Urosaurus ornatus
but the reverse is not true. S. merriami is limited more by abiotic
factors. U. ornatus is limited by biotic factors (competition).
12
Classic example of competition in nature - interaction between
barnacles - Chthamalus and Semibalanus
Chthamalus lives in the intertidal
zone
Semibalanus lives in the subtidal
zone
If Semibalanus is removed
Chthamalus also utilizes the
subtidal zone
Semibalanus is unable to live in
the intertidal zone due to
desiccation
Semibalanus excludes Chthamalus from the subtidal zone through
its rapid growth - it undercuts or crushes Chthamalus
The realized niche is smaller than the fundamental niche for
13
Chthamalus
Mathematics of Competition - the Lotka-Volterra equation
The logistic equation describes the growth of a single species
when there is a limited supply of resource.
dN
K−N
= rN (
)
dt
K
This equation can be modified to incorporate the effect of a
second species:
dN1
K − N1 − α N 2
)
= rN1 ( 1
dt
K1
14
The Lotka-Volterra equation incorporates the effect of a second
species on species 1.
α represents the effect of an individual of species 2 on species 1
relative to the effect of an individual of species 1 on species 1.
1
1
2
2
2
1
K
1
Here the effect of species 2 on species 1 is less than the effect of
species 1 on species 1: α < 1.
15
dN1
K1 − N1 − α N 2
= rN1 (
)
dt
K1
K1/α
dN1/dt < 0
N2
dN1/dt = 0
dN1/dt > 0
N1
K1
Any combination
of N1 and N2 that
falls on the green
line represents a
point where
species 1 will not
grow or decline
16
dN 2
K − N 2 − β N1
= rN 2 ( 2
)
dt
K2
The effect of species 1 on species 2 can also be expressed by the
Lotka-Volterra Equation
Here, β represents the effect of species 1 on species 2 relative to
the effect of species 2 on species 2
17
dN 2
K − N 2 − β N1
= rN 2 ( 2
)
dt
K2
K2
dN2/dt < 0
N2
dN2/dt = 0
dN2/dt > 0
N1
K2/β
18
When species 1 and species 2 are considered together 4
different outcomes are possible
Case 1: Species 2 wins
K2
N2
dN2/dt = 0
K1/α
dN1/dt = 0
N1
K2/β
K1
19
Case 2: Species 1 wins
K1/α
K2
N2
dN2/dt = 0
dN1/dt = 0
N1
K2/β
K1
20
Case 3: Stable coexistence
Equilibrium population sizes
K1/α
K2
N2
dN2/dt = 0
dN1/dt = 0
N1
K2/β
K1
21
Case 4: Unstable coexistence
Unstable equilibrium
K2
K1/α
N2
dN2/dt = 0
dN1/dt = 0
N1
K2/β
K1
22
Predation is an important factor
in community organization and
can reduce the effects of
competition. Predators can
keep each species below its
carrying capacity - allowing
coexistence, so long as the
predator is present.
Environmental disturbances and
disease can have similar effects
- keeping species below the
limiting level of resources.
23
When species coexist through utilization of different types of
resources or different ranges of a limiting resources it is called
“Resource Partitioning” or “Niche Partitioning”
Anolis lizards in Cuba live in the same area but exploit different
microhabitats
24
Species under continual competition will sometimes evolve
differences because of selection against those individuals that
suffer the most severe competition
Finches in the Galapagos
Islands feed on seeds and
their beak size determines
the size seeds they can eat
When found on different
islands (allopatric) the beak
sizes are similar
When found on the same island (sympatric) they have divergent
beak sizes - called “Character Displacement”
Competition for seeds is highest on intermediate sized seeds - one
25
species specializes on large seeds, the other on small seeds