Download Types of competition

Competition
What is competition?
• Competition: use or defense of a
resource by one individual that
reduces the availability of that
resource to other individuals.
• Competition among individuals may be:
– intraspecific (within species)
– interspecific (between species)
Types of competition
Competition occurs when use of a needed resource
(food, water, space, light, nutrients, etc.) by one
individual reduces the availability of that resource
to another - whether of the same or another
species – Ricklefs
• Intraspecific competition most intense for
common species
• Interspecific competition most important for
sparse species
What is a resource?
• Resources include foods that are eaten,
but also include:
– open space for sessile organisms
– hiding places and other safe sites
• Conditions are not resources:
– for example, temperature is not a resource:
it may affect growth and reproduction, but is
not consumed by organisms
Competition?
Types of competition - 1
Exploitation (scramble, diffuse) Competition
• Consumptive - use of renewable resource
• Preemptive - based on occupation of open space
Interference (contest, direct) Competition
• Overgrowth - when one grown on or over another,
depriving it of resources
• Chemical - toxin that acts at a distance
• Territorial - defense of space
• Encounter - transient interactions over a
resource
Types of competition - 2
• Symmetric competition: ability to
capture resources is proportional to
size.
• Asymmetric competition: the large
organisms capture a disproportionate
amount of the resource
Intraspecific competition
Relationship
between density and
yield in herbaceous
plants.
The “Law of
constant final yield”
Note that after a critical
density is reached, yield is
independent of density.
Plant weight as a
function of plant
density, and its
change with
plant growth
y=w.d
1/w = d(1/y)
1/w = Ad + B
Survivorship in 7
populations of
Loblolly Pine
established in
1925 in the Duke
Forest.
G= { ΣiΣj |yi-yj| }/(2n**2)ybar
all plants equal --> 0.0 ; all plant but one infinitely small --> 1.0
The -3/2 thinning
rule
w= c.p-3/2
area proportional to L2
weight proportional to
volume proportional to
L3
Therefore, area
proportional to w2/3
and area proportional to
1/density
Thus, weight = c.p-3/2
Natural thinning
in loblolly pine
populations of
differing initial
density.
Limiting Resources
The potential of a resource to limit a
population depends on availability relative
to demand:
This concept is embodied in Liebig’s law of
the minimum:
• each population increases until the supply of some
limiting resource becomes depleted
• this law applies strictly to resources that do not
interact to determine population growth rate
The Competitive Exclusion
Principle
Results from many studies were summarized
by Gause and others in the competitive
exclusion principle:
• two species cannot coexist indefinitely on
the same limiting resource
• although similar species exist, careful study
usually reveals that they differ in their
habitat or diet requirements
Lotka - Volterra model
Incorporates interspecific competition
into the logistic equation as follows:
• Two models are needed to describe the
joint effects of species upon one another.
• For two species to coexist, each population
must reach a stable size greater than zero:
dN1/N1dt = 0 when:
N1 = K1 - αN2
dN2/N2dt = 0 when:
N2 = K2 - βN1
Species 2
Species 1
K1
K1/a
Species 2
Species 1
K1
K2
Species 2
K2/β
Species 1
G.F. Gause conducted early experimental studies of
competition: he grew Paramecium aurelia and P.
caudatum alone and in mixture in nutritive media:
• each species grew well alone, but in mixture only P.
aurelia persisted
• similar experiments conducted on a wide variety of
species have tended to show the same thing - one
species persists and the other dies out, usually
within 30-70 generations
24%
29%
34%
70 F
confusum castanum
71 - 29
14 - 86
castanum
0 - 100
30 F
confusum confusum
100 – 0
87 – 13
confusum
90 - 10
1. Relative intrinsic growth rate
L minor > S. natans > L. gibba > L. polyrhiza
2. Arithmetic growth rate when crowded
S. natans > L. polyrhiza > L. gibba > L. minor
3. Asymptotic yield / culture
L. polyrhiza > L. minor > > S. natans > L. gibba
4. Succession, mixed
S natans & L gibba > L. polyrhiza >= L. minor
Resource Ratio Model
• Dave Tilman constructed a model of
competition based on differential depletion
of two resources.
• When there is an abundant supply of
resources, the population of each species
increases and the supply of resources per
unit competitor declines until growth
becomes zero (ZNG = Zero Net Growth)
Tansley’s Classic Study
British ecologist A.G. Tansley was the first to
experimentally demonstrate competition between
closely related species:
• He selected two species of the plant species Galium:
G. saxatile is normally found on acid, peaty soils
G. sylvestre is normally found on limestone hills and
pastures
• These two species were grown alone and in mixture
with the other species on both soil types in a common
garden
Tansley’s Results
• When grown alone in common garden experiments,
each species performed better on its preferred
soil, although each could grow on the other soil
type.
• When grown in mixture, each species overgrew and
shaded the other on its preferred soil type.
• Tansley concluded that each species was at a
disadvantage in competition when grown on the
other soil type; this helped explain the observed
distributions of the two species in nature.
Tansley’s Conclusions
Tansley’s conclusions have far-ranging
implications for all competitive situations:
– the presence or absence of species can be
determined by competition with other species
– conditions of the environment affect the
outcome of competition
– competition may be felt very broadly
throughout the community
– the present segregation of species may have
resulted from past competition
Competition among barnacles
Experimental studies of competition among intertidal
barnacles by Joe Connell enhanced our understanding
of how competition can affect spatial distributions
of species:
Barnacles are not food-limited, but compete intensely
for limited space on rocks in the intertidal zone:
• The barnacle Chthamalus is more tolerant of
desiccation and thrives in the upper intertidal zone
• The barnacle Balanus is less tolerant of desiccation
but can displace Chthamalus in the lower intertidal
zone
Competition may occur through
exploitation or interference.
• Exploitation competition occurs when individuals
compete indirectly through their mutual effects
on shared resources:
– a common mechanism, typical of examples we’ve
considered thus far
• Interference competition occurs when individuals
defend resources through antagonistic behaviors:
– less common, requires that resources can be
profitably defended