Lecture 9: Interspecific Competition
EEES 3050
Competition
In the past chapters, we have been discussing how populations grow and what factors determine that growth.
What happens when you put populations of more than one species together?
How do species interact?
Competition
Predation
Herbivory
Parasitism
Disease
Mutualism
Interspecific Competition
Competition When two species use the same limited resource to the
detriment of both species. Assessment-some general features of interspecific
competition Competitive exclusion or coexistence Tilman’s model of competition for specific resources
(ZINGIs) Coexistence: reducing competition by dividing
resources
Assessment
mechanisms consumptive or exploitative — using resources (most
common) preemptive — using space, based on presence overgrowth — exploitative PLUS preemptive chemical — antibiotics or allelopathy territorial — like preemptive, but behavior encounter — chance interactions
Modeling coexistence?
Can we model the growth of 2 species? Remember logistic model?
What is K?
Now we add another factor that can limit the abundance of a species. Another species.
K
NKrN
dt
dN
Freshmen and donuts: an example There is a room with 100 donuts – what does a
typical male freshmen do? First – eat several donuts. (A male freshman can eat
10 donuts) Second – rapidly tell friends
But not too many! Third – Room reaches carrying capacity at 10 male
freshmen. So K=10 for male freshmen.
K
NKrN
dt
dN
Freshmen and donuts: an example What happens if a male and female discover the room at
the same time? First – eat several donuts. (A female freshman can eat 5
donuts) Second – rapidly tell friends
But not too many! Third – Room reaches carrying capacity at ? males and ?
females. What is the carrying capacity?
It depends…
1
1111
1
K
NKNr
dt
dN
2
2222
2
K
NKNr
dt
dN
Lotka-Volterra
Need a way to combine the two equations. If species are competing, the number of species A decreases if number of species B increases.
Such that:
Where alpha is the competition coefficient Lotka-Volterra: A logistic model of interspecific competition of intuitive factors.
21 NN
2
2222
2
K
NKNr
dt
dN
1
21111
1
K
NNKNr
dt
dN
Freshman Example
In a room we have 100 donuts. Need 10 donuts for each male freshmen. So K1 = 10 Need only 5 donuts for each female freshmen. So K2 = 20
If room is at K1 and 1 male leaves, how many females can come in? So, , where α = 0.5 And, , where B = 2
1
1111
1
N
NKNr
dt
dN
21 NN
1
21111
1
N
NNKNr
dt
dN 12 NN
2
12222
2
K
NNKNr
dt
dN
Possible outcomes when put two species together. Species A excludes Species B Species B excludes Species A Coexistence
1
21111
1
K
NNKNr
dt
dN Changes in population 1:
1
21111
1
K
NNKNr
dt
dN Changes in population 1:
Yellow: both increase
White: both decrease
2
12222
2
K
NNKNr
dt
dN Changes in population 2:
Yellow: both increase
White: both decrease
Yellow: both increase
White: both decrease
Green: Sp 1 increase
Brown: Sp 2 increase
Tilman’s model
Problems with Lotka-Voltera model? No mechanism
Dr. Tilman developed a model based on resource use.
1 – no species can survive
2 – Only A can live
3 – Species A out competes B
4 – Stable coexistence
5 – Species B out competes A
6 – Only B can live
Lab Experiments?
Gause using yeast and Birch using beetles. Results show both exclusion and coexistence
It was hypothesized that the yeasts had enough differences to allow coexistence. i.e. the requirements of the 2 species are slightly
different.
Gause’s hypothesis:
As a result of competition two similar species scarcely ever occupy similar niches… Also called the competitive exclusion principle:
“Complete competitors cannot coexist.”
Niche – still controversy about the definition. 1) The role of a species in the community – Elton
1927. 2) a subdivision of the habitat. - Grinnell 1917.
Back to Competition Coefficient Competition coefficient:
Intensity of competition from species. In our original donut example:
α = 1/ β Also, can read about Gause’s yeast populations in
book.
1
21111
1
K
NNKNr
dt
dN Changes in population 1:
K2
K2/β
K1 = 10
K2 = 20
α = 0.5
β = 2
Back to Competition Coefficient Competition coefficient:
Intensity of competition from species. In our original donut example:
α = 1/ β
However: in systems that are more complex the coefficients are not necessarily reciprocals. And carrying capacity may not be purely determined
by resource being competed for.
1
21111
1
K
NNKNr
dt
dN Changes in population 1:
K1 = 10
K2 = 17
α = 0.5
β = 3
K2
K2/β
Niche: Hutchinson
Redefinition in 1958.
Niche: Hutchinson
Redefinition in 1958. Two environmental variables, can produce an
environmental space or a species niche. Can add many other environmental factors. n-dimensional hypervolume
Or a species Fundamental Niche However, because competition can limit this
fundamental niche, what we witness in nature is the:
Realized Niche
Can 2 species exist in the same niche?
Can 2 species exist in the same niche?
Can 2 species exist in the same niche?
Observation: Several types of warblers live in the same tree
species. Hypothesis based on competition theory:
Warblers will use different parts/areas of the trees. Experiment:
No experiment conducted, but observations can be made to test hypothesis.
Can 2 species exist in the same niche?
McArthur suggested competition to explain warbler patterns Ghost of competition past. So how do species coexist?
Different food resources, i.e. diet specialization
What about plants? Plants usually need same resources, water,
nutrients, light. What about phytoplankton?
How do phytoplankton live in the same location? Phytoplankton
Common pool of nutrients Often large number of species Same environment, i.e. amount of light,
temperature. In many bodies of water, nutrients are limited.
Reasons? Environmental instability Non-equilibrium system.
Assumptions of competition theory life history characteristics of species are adequately
summarized by the per capita growth rate of species;
deterministic equations are sufficient to model population growth, and environmental fluctuations need not be considered;
the environment is spatially homogeneous and migration is unimportant;
competition is the only important biological interaction; and
coexistence requires a stable equilibrium point.
How to determine if interspecific competition has occurred (or is occurring)? From Wiens (1989)
1. Need a checkerboard distribution
2. Species overlap in resource use
3. Intraspecific competition occurs
4. Resource is limited
5. One or more species is limited
6. Other hypotheses do not fit.
Example where criteria 1 and 2 fit.
Feeds far from shore
Feeds near shore
Test theory with plants
Observation: Plants all require light, nutrients and water. Plant often found together.
Hypothesis: Competition between plants ought to be common. Plants do worse with other plants.
Experiment:
Results:
Experimental design. Compare the growth of annuals and shrubs in the
Mojave Desert. 2 experiments
Effects of annuals on shrubs Effects of shrubs on annuals
How?Effects of annuals on shrubs
Effects of shrubs on annuals
Results:
Annual had positive benefits from shrubs.
Shrubs had negative benefits from shrubs.
Facilitation: Where one or both species benefit (have a
positive effect) due to the presence of the other species.
Resource utilization curves
Species may evolve to minimize the impact of competition.
Character Displacement
Definition: In areas where species overlap, there has been a
divergence between the two species, supposedly as a result of competition.
Example: Galapagos finches Theory:
According to displacement theory, species that are sometime found together will have a character that has changed compared to when the species are found by themselves.
Observation: There are three species of finches in the Galapagos that are
sometimes found together and sometimes separate. Hypothesis:
There will be differentiation in bill size when species are on the same island:
Test (Not truly an experiment): Examine the bill size of three species.
Results:
Results:
*Could we actually test this?
Four criteria for determining character displacement Change in mean value of the character in
areas of overlap should not be predictable from variation within areas of overlap or areas of isolation.
Sampling should be done at more than one set of locations
Characters need to be heritable. Species must actually be competing for
resource.
r vs. K selected species
What do r and K refer to? r – growth rate K – carrying capacity
r – selected Species that remain in the growth rate stage for
most of their existence. K – selected
Where organisms remain near the carrying capacity
Influenced more by competition.
Is it really competition?
How else could these ideas be framed? Conflict avoidance. Not survival of fittest, but perhaps least
noticeable. Best hider.
Path or least resistance