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Chapter 8 Competition |
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Harned Hall 302
(615) 963 - 5782 |
Sections:
Interactions
between organisms:
The book uses a pictorial
scheme common to many presentations that compare the types of species interactions,
so I will use it here. Note that the scheme below does not correspond
exactly to that in the book
- Ammensalism
has been added for the interaction in which one species is hurt, but the other
does not benefit
- As wild pigs forage,
they often disturb the upper layer of soil and many organisms may be taken
from their burrows and exposed to predation by the action of the pigs,
although the harm that the burrowers suffer does not improve the pig's
situation at all
- Allopathy
refers to the release of toxins in to the environment
- can be a situation
in which a species is hurt by the release of toxins but the species that
released the toxins gains no benefit (in which case it is ammensalism)
- can be that the
species that releases the toxins does gain by their release, in which
case it becomes a mechanism for competition or predation avoidance
Hurt(-), helped(+), or not affected(0)
| Name of interaction |
Species A |
Species B |
| Mutualism* |
+
|
+
|
| Commensalism* |
+
|
0
|
| Competition |
-
|
-
|
| Allelopathy |
+
|
-
|
| Herbivory |
+
|
-
|
| Predation |
+
|
-
|
| Parasitism* |
+
|
-
|
| Ammensalism |
-
|
0
|
* symbiosis
(living in intimate contact) can occur in several of these interactions
- Mutualism, Commensalism, and Parasitism can all involve symbiosis
Competition
defined:
Competition is the
outcome of shared, limiting resources.
- Sharing
implies that the species share resources that are
important to their ecological success
- Not all
individuals of each competing species will receive
sufficient resource to maximize their survival and
reproduction (thus the use of the term
"limiting")
- Notice that
they do not need to die directly, only their overall
performance must be reduced
Interspecific
versus intraspecific
Modeling
competition:
Intraspecific
Competition
We have already modeled
the effect of intraspecific competition on population growth when we discussed
the Logistic model of population growth
- K, the carrying
capacity, assumes that there is a limited resource that sets the maximal size
of the sustainable population.
- The drag term (K-N/K)
is a measure of the intensity of interspecific competition.
We can also model the effect
of interspecific competition on individual growth (as opposed to population
growth)
- Mobile species are
hard to model, as they can leave (the population) in order to search for more
resources
- Sessile organisms don't
have the option to move, and they often become smaller when they overexploit
an environment. Yoda found that the loss of average individual biomass in
an area was predictable.
- Predicting the size
of plants that result from a given population density is useful for farmers,
foresters, and others who depend on harvesting plants (indirectly, we all
depend on this).
- if w is the average
weight of an individual, and N is total plant density (measured as total
biomass of the population), then w is predicted by (c is a constant that
fits the equation to different plant species):
- this equation can
be made linear if you take it's log. Linear equations are easier to analyze
(we seem to think linearly)
Thus, we can see the effect
of intraspecific competition. As the population size increases, the size of
individuals is predicted to decrease in a regular fashion (I hope our plant
lab agrees with this!).
Lotka-Volterra
model of Interspecific competition
- Based on the Logistic
model
- Next, we modify the
logistic by including the effect of the presence of one species on another,
using the relationship defined above
and
- So, what are the possible
outcomes predicted by these equations?
- Depending on the
values of K's and a's, they can predict either:
- Trivial equilibria
- Called trivial because they
predict an outcome that is obvious
- One species drives the other
out and you get the K number of the winning species
- either Species A or B remains
and the other goes locally extinct
- Stable equilibrium
- Displacing the system (adding
or removing individuals of one or both species) takes you back
to the same equilibrium point
- this predicts that both species
will continue to coexist in this environment indefinitely
- Unstable equilibrium
- Displacing the system takes
you to one of two possible outcomes:
- Species A wins
- Species B wins
- Which one occurs depends
on what sort of displacement takes place
- How can you tell what
is predicted?
- When the system
gets to any of the equilibrium outcomes above, it hits a condition of
no change
- Any species which has lost is at
N = 0 and the winner is at the predicted logistic equilibrium, so
its dN/dt is 0
- The equilibria with both species
present are still both states of no net change, which implies that
dN/dt for both species is 0
- So, in the cases where
both species are predicted to coexist, if dN/dt = 0, then for species 1:
- or (dividing by r2N1,
and then multiply by K1)
- We can analyze the situation
graphically
- each of the last
two equations is the equation of a straight line, with one species as
x, the other as y, K as the intercept, and a as the slope
- Graph is in "species
1 and species 2 space" (see diagram axes) below
- the idea of a zero isocline is a
line that represents all of the combinations of species 1 and 2 (which
are points in 2 dimensional space) that result in one species having a
growth rate (dN/dt) of zero
- make sure you are comfortable
with this idea and also the fact that the lined region is where dN/dt
is greater than 0 (where the population of species 1 will increase
in number) and the checked region is where the growth rate is negative
- to do this look at where
K1 is and imagine that there
are no species 2 present (so you are along the x axis) and then
go beyond K1 to the right
- this is obviously where
the population must get smaller and it is also where the checked
region is!
- Now, do the same
for species 2
- Using these two expressions
as a linear prediction of where each species will have a growth rate of 0,
we can see under which conditions exclusion and coexistence are predicted.
- We must superimpose
the two graphs above, so that both isoclines appear on the same axes,
then we can judge graphically what outcome is predicted
- When one species has a zero growth
isocline greater than another at all points, that species will be
able to grow under some conditions where the other decreases.
- Here, region B is where the population
of Species 1 can get larger and Species 2 will get smaller
- This growth and shrinkage will continue
until Species 2 hits the x axis (where its population size is 0 and
it has gone extinct)
- Species 1 will then grow to its K1,
which is its carrying capacity, and it is the winner
- The situation is reversed
if the isocline of Species 2 is farther from the origin than Species
1 (Species 2 is the winner when it hits K2)
- and are equilibrium points, as the
population sizes will not change unless something disturbs the system
- Now, lets look at the
situation where both are present and the zero growth isoclines do intersect.
- Depending on the
relative sizes of K1, K2, a1 and a21, either stable or unstable equilibria
are predicted
- the important areas
here are 1 and 2
- to understand why the top is unstable,
go to these areas and predict how the populations will change
- to do this, look at the zero
isoclines to decide what will happen to each species
- you will get a similar situation
as when the lines don't intersect (one species will go extinct
and the other will go to its carrying capacity
- thus, when the equilibrium
is unstable, one species wins and the other loses when the system
is displaced from the equilibrium point represented by the intersection
of the zero isoclines
- in the lower diagram, the regions
1 and 2 are reversed (you can see why if you look at where they are
with respect to the zero isoclines)
- now if you predict where
each species will go, you see that you will head back towards
the intersection of the zero isoclines
- thus, any disturbance will
just result in return to the equilibrium point with both species
present (unless the disturbance is so severe that one species
is eliminated, which is a point from which it can not recover)
Tilman's
R* model of competition
- Tilman did not like
the Lotka-Volterra competition models as they did not include the mechanism
by which organism's compete. He developed a model that is specific for organisms
that compete because they use the same resource or resources (works nicely
for plants, which were his interest).
- A graphical model that
predicts the outcome based on increasing biomass for each species as resources
increase, with loss of biomass from other sources. Where loss and increase
are equal, the species reaches its equilibrium, which Tilman called R*, the
equilibrium resource level (it's like K, or carrying capacity, although here
it is a dynamic outcome of loss and gain, not just the total resource divided
by the amount each individual needs and is expressed in units of resource,
not in population size units)
- competition here
is exploitative, in that each species takes what it can, reducing the
amount available for the other species
- the winner will
be the species with the lowest R*, the one that can maintain its population
at the lowest level of resource
- Versions of this model
allow competitors to compete for more than one resource
- Notice that the resource
must be limiting, so that the resource taken by one species actually reduces
the amount the other species can get!
Studies
of competition:
Laboratory
studies
Saccharomyces
and Schizosaccharomyces
Interaction mediated
by production of alcohol, not by exploitation of a resource (interference
competition)
Fit Lotka-Volterra
model very well
As expected, because
L-V is based on the logistic, and the logistic predicted the growth of
each yeast well
Tribolium
- Two species of beetle
living in stored grain and flour (important pest species)
- Almost always, one
species ousts the other, but not always the same species wins
- mechanism of competition
is predation of eggs and pupae by larvae and adult beetles
- Some see this
not as competition, rather as mutual predation, but the outcome is the
same
- Sporozoan infection
first altered the outcome (T. confusum more resistant and won)
- Abiotic conditions
affected outcome
- T. confusum
won when flour dry, T. castaneum when wet
Field
and Natural Studies
- Before we get into this area, I have
noticed that the book uses some terms that are specific to ecology but that
have not yet been defined. Of course, you can look in the book for definitions,
so I will just mention some briefly here:
- Trophic structure
- the mode of feeding or capture of energy and resource
- usually seen as a hierarchy
in the following fashion (going from the basal species to the top
species):
- primary producer
- an autotroph
- primary consumer
- herbivores - eat primary producers
- secondary consumers
- several levels - carnivores - eat primary consumers or other
secondary consumers
- other ways of classifying feeding
are also mentioned:
- deposit feeders - detritivores
- eat dead materials, important in decay process
- filter feeders - filter food
from fluid medium, such as water or air
- Green World Hypothesis
(Hairston, Smith and Slobodkin, 1960)
- Plants are green,
although all have herbivores that could eat them up
- Herbivores limited by predators
- Plants compete for light, mineral
resources, water
- Predators have predators
- Litter does not
accumulate on the ground
- Decomposers use up all resource,
so it is limiting and competition ensues
- Will not work under
some circumstances
- Introduced herbivores can reduce
plant species to level that they can not find all of them, and stable
situation results without predation (Cactus system)
- Fire can remove litter, so that it
only appears that litter is limiting
- Agriculture and plant
competition
- Farmers know that
one must thin crops to maximize production
- This implies that plants are competing
with one another in fields, although it is not always evident what
resource is limited
- However, do they
do this in natural situations?
- Allelochemicals,
bacteriocins, and mycocins
(killer factor)
- No reason to kill
others than the reduction of competition
- Experiments, for
most part, have not been done
- Barnacles in the Rocky
Intertidal
- Chthalamus
upper limit set by desiccation, lower limit by Balanus
- Remove Balanus and Chthalamus grows
- Remove Chthalamus and Balanus does
not invade
- Balanus upper
limit set by desiccation, lower by starfish predation
- Remove predators and Balanus invades
- Schoener and Connell
- Reviewed competition
studies and found:
- Competition demonstrated in many
or most studies (Schoener), but in only 40% of studies reviewed by
Connell
- differed in their criterion
for accepting that competition had been shown by field studies
- also looked at different
studies
- Reasons to believe that competition
was too often found
- negative results (no competition)
may not make it into the literature as it is harder to publish
- ecologists choose a system
to study partly because they suspect that the phenomenon in which
they are interested is likely to occur there
- Reasons to suspect that competition
may be more common or important than field studies can demonstrate
- If competition is important,
then we may not see it because one species has won or the species
have evolved to reduce its importance (Ghost of Competition Past)
- Competition may be present
only in years of low resource, yet may still be dominant force
determining distribution of a species
- Schoener did classify
the mechanisms by which the competition occurred
- Consumption
(Exploitative)
- Preemptive
- Overgrowth
- Chemical (Allelopathic)
- Territorial
- Encounter (fights)
- which type is more
common is linked to the kind of organisms (plants don't fight, motile
animals don't overgrow)
Competitive
Exclusion
- When two
species are limited by the same resource or resources and
one is better able to gain access to the resource, then
one species will exclude the other from that locality
- Related to the idea
of a niche, a "space" describing
all of the needs and abilities of a species
- Restatement
of idea is that no two species can occupy the
same niche
- some
see a niche as a geometric space with each
important abiotic factor or resource as an axis
(so this space has many more dimensions than the
three normal space has)
- Used to be a
principle, but it must be proven in each case, and so is
not a principle but a hypothesis
- Niches can be
altered by presence of competitors or predators that
reduce the total "space" occupied by a species
- fundamental
niche - maximum niche when no competitors, predators, parasites,
etc. present
- realized
niche - actual space occupied by the species when other species
are present
Coexistence
when competition is present:
"Homage to Santa
Rosalia or why are there so many kinds of animals?" Hutchinson, 1959
- Asked an important
question - if competition has the power to exclude all but the best competitors,
why then are so many environments full of similar species
- looked at a kind
of bug found in ponds and found that more than one species of these
bugs, which all look alike and feed in the same manner, occurred in
the same ponds
- question was why did
not the best competitor force the other species out?
- found that bugs
in ponds would not coexist if their feeding apparatus was too similar
(idea of Limiting Similarity)
- expressed as a
ratio between the size of two species (or the size of some body part
important in dealing with the limiting resource)
- Hutchinson measured this ratio
as somewhere around 1:1.28 for his bugs in the ponds near Santa
Rosalia
- many went out and found the ratio
and concluded that competition was the cause
- criticized for
non-experimental nature of findings
- Character
displacement
- Best evidence for
limiting similarity
- When species are
allopatric (look at chapter 3 on modes
of speciation). their utilization patterns (as reflected by their size
or the size of their mouth parts) overlap (ratio is below 1:1.1 or so)
- When they are sympatric,
the overlap is reduced (indicated by larger ratios of the magnitude from
1.3 and 2) due to Character Displacement
- so character displacement is the
evidence of past competition forcing species that were too similar
to become more dissimilar in order to coexist
- Has been shown for
Darwin's finches in the Galapagos islands
- Measuring
niche dimensions
- These ideas about
competition all assume that there is a limiting resource (usually 1, but
two or more have been considered)
- many have modeled
resources as resource niche axes:
- Resource
Axis -- a line representing change in a resource, such
as size, or sugar concentration, or concentration of toxins
- Species
Utilization Curve -- the degree to which a species can
utilize a resource at some point on the resource axis
- Different species
can be represented as humps along the axis
- humps (usually bell shaped, but not
necessarily so) give each species range of resources sizes utilized
and its optimum resource utilization point
- Ideally, if limiting similarity is
correct, the overlap between species resources will not be larger
than the limiting similarity ratio
- Species
Packing -- when all of the species on a resource axis are
spaced so that each shows maximal overlap allowing coexistence (therefore,
no additional species can be added without causing more overlap than
limiting similarity would allow) the resource axis is called "packed"
- What happens when
the niche dimension is not a continuous variable, so that an axis can't
be used to describe it?
- this situation might
describe a resource that is uniform (like suitable space) but occurs as
patches separated by unusable space
- here we can use
a different measure of niche and a different measure of overlap
- Niche breadth
- Niche breadth is the degree
to which a species utilizes all available patches or resources
- there are many ways to measure
this, but all are related to the simplest given below (Levin's
niche breadth)
- here the symbols are
- S = number of resources
or patches of resource
- pi = proportion
of a species utilizing the ith resource or patch
- will range from 1.0 when
the species is equally distributed across each resource or patch or 1/S
when all members of a species are found on one patch or resource
- the pi's are often measured
as the proportion of different food types in the guts of the species of
interest
- Overlap between the breadth of
two species (here designated species j and species k) may be as Proportional
Similarity (between the two species)
here, pij and Pik
are the proportions of the least-abundant species found on the ith
patch or resource
As a rule of thumb, a PS of
over 70% is considered to indicate active competition between the species,
and lower values allow coexistence of the competitors
This concept has been applied
to comparing two different communities of species as well as two different
species (as presented here)
chapter 16 on community metrics
has index of similarity between two communities (go to page on community metrics)
Competition
and Evolution revisited:
r
and
K selection
Remember that K selected
species are thought to experience competition but r selected species simply
move on to new, unexploited resource rather than compete for resource in a
crowded patch
r-selected
species are species that experience local patch extinction and
live by colonizing new patches
they rarely experience
carrying capacity, and often grow near to the maximal rate (~r when N is
low for the logistic)
K-selected
species occupy more long-lived patches, which can become saturated
and so they experience carrying capacity (K)
When one characterizes
species by this dichotomy, then "suites" of characters are associated
with each type of selection
| Character |
r-selected species |
K-selected species |
| Mortality |
density-independent |
density-dependent |
| Survivorship |
type III |
types I, II |
| Population Size |
variable |
constant |
| Competition |
minor |
keen |
| Life expectancy |
short |
long |
| Fecundity |
high |
low |
| First reproduction |
early |
late |
| Type of reproduction |
semelparous |
iteroparous |
| Body size |
small |
large |
Terms:
Competition, Interspecific
competition, intraspecific competition, Scramble (Resource) competition, Exploitative,
Preemptive, Interference (Contest) competition, Lotka-Volterra model. Interaction
term, Stable equilibrium, Unstable equilibrium, Tilman's R* model of competition,
Leibig's Law, Saccharomyces and Schizosaccharomyces, Tribolium,
Allelochemicals, bacteriocins, and mycocins, Chthalamus, Balanus, Mechanisms
of Competition (Consumption, Preemptive, Overgrowth, Chemical, Territorial,
and Encounter), Competitive
Exclusion, niche, fundamental niche, realized niche, Limiting Similarity,
Resource Axis, Species Utilization Curve, Species Packing, Niche breadth,
Proportional Similarity, Character displacement, allopatric, sympatric, r
and K selection, r-selected species, K-selected species, semelparous, iteroparous
Literature Cited:
Connell, J. H. 1961a. Effects of competition, predation by Thais
lapillus and other factors on natural populations of the barnacle Balanus
balanoides. Ecological Monographs 31:61-104
Connell, J. H. 1961b.
The influence of interspecific competition and other factors on the distribution
of the barnacle Chthamalus stellatus. Ecology 42:710-723.
Connell, J. H. 1983
On the prevalence and relative importance of interspecific competition:
evidence from field experiments. American Naturalist 111:1119-1144.
Hairston, N. G. Sr., F.
E. Smith and L. B. Slobodkin. 1960. Community structure, population control,
and competition. American Naturalist 94:421-425.
Hutchinson, G. E. 1959.
Homage to Santa Rosalia, or why are there so many kinds of animals. American
Naturalist 93:145-159.
Schoener, T. W. 1983. Field experiments on interspecific competition.
American Naturalist
122: 240-285
Schoener, T. W. 1985. Some comments on Connell's and my reviews
of field experiments in interspecific competition.
American Naturalist 125: 730-740.
Last updated on September 12, 2006
