Competition
Competition
 The
effects of
competition in nature
can be observed
through observation &
experimentation.
 Many
organisms compete
for limited resources and
may defend territories to
protect their access to
these resources.
Hypothetical Example
 Land



Iguanas
3 species identical in nearly every way.
Color (green, blue, yellow)
Green ones find food faster
Unlimited Resources
 If
resources are
plentiful, organisms
have as much as
they need and do
not compete.

Fast
growing/reproducin
g cacti.
Limited Resources

When food resources
are limited, some
organisms may have
an advantage over
others.


When cacti grow
slowly, not enough
food for all.
Green iguanas find
food faster,
outcompete other
colors.
Resources
 Resources






that can be limited include:
Food
Specific nutrients
Specific amino acids
Shelters
Nest sites
Water
Resources
 If
resources are unlimited, populations can
grow rapidly.


 In
Exponential growth
J shaped curve
reality, resources are limited.
Competition
 When
their life-sustaining resources are
limited, organisms are forced
to compete with each other.
 Sometimes those competitive interactions
will be direct and obvious, while other
times they will be indirect and subtle.
Niches
 Niche:
Summarizes environmental factors
that influence growth, survival, and
reproduction of a species.

An organism’s role within the environment.
 Hutchinson

defined niche as:
n-dimensional hyper-volume
n
equals the number of environmental factors
important to survival and reproduction of a
species.
Niches
 Fundamental
niche – the entire area
where a species could survive in the
absence of biotic interactions.
 Realized niche includes interactions such
as competition that may restrict
environments where a species may live.
Niche Overlap and
Competition Between Barnacles
 Connell
examined the relationship between two
kinds of barnacles – Balanus balanoides and
Chthamalus stellatus.

Chthamalus is found in the upper intertidal.


Limited by competition with Balanus.
Balanus is found in the mid & lower intertidal.

Limited by exposure to air in upper intertidal.
Niche Overlap and
Competition Between
Barnacles
 The
fundamental
niche of Balanus is
equal to its realized
niche.
 The fundamental
niche of Chthamalus
is much broader than
it’s realized niche.
 Both are limited by
predation in the
lowest part of the
intertidal.
Environmental Tolerance
 All
organisms have specific tolerances for
a variety of conditions like temperature,
pH, moisture level etc.
 Mosquitos, for example, have a range of
temperatures at which they survive and
reproduce.
Niche and Environmental
Tolerance
 Where
in Florida can
the invasive tiger
mosquito live?

Important abiotic
factors:
 Rainfall
 Temperature
Fundamental Niche
 The
fundamental niche is everywhere the
species could physically live.
Competitive Exclusion
 Gause:

Principle of Competitive Exclusion
Two species with identical niches cannot
coexist indefinitely.
 One
will be a better competitor and thus
have higher fitness and eventually exclude
the other.
Competitive Exclusion


Gause developed a
simple experimental
system of two single-celled
species
of Paramecium growing in
test tubes.
He found that both
species were able to grow
just fine if they were alone.
However, as shown in the
graph, one of the
species, P. aurelia, would
always out-compete the
second species,
P. caudatum, when they
were grown together.
Competitive Exclusion
 The
yellow fever mosquito can also survive
across all of Florida.

However, the Asian tiger mosquito has
displaced it throughout most of the state.
 Thus,
the yellow fever mosquito's
fundamental niche includes the whole
state, but its realized niche is now
restricted to small areas of the state, due
to competition with the invading Asian
tiger mosquito.
Classifying Competitive
Interactions

Example – desert shrubs use water & nutrients
in a halo around themselves.



Small plants can’t survive in this area.
Resource Competition – Exploitative
Competition
While individuals are not actively fighting
each other off, they indirectly compete
because each individual obtains less of the
resource due to the presence of other
individuals.

Self-thinning
Resource Competition



Resource competition is not limited to plants.
The simulated iguanas also compete through
resource competition, as do real mosquito
larvae.
In both cases, the animals do not directly
interact with each other, but because they
use the same resource (food for iguanas,
breeding sites for mosquitoes),
they indirectly compete.
Interference Competition

Interference competition
occurs when organisms
compete directly for
resources.


Aggressive interactions
Interference competition
does not require a direct
fight over a resource.

Some birds will attack
and destroy eggs in nests
near their own,
presumably to reduce
competition for
resources.
Interference Competition Allelopathy
 Plants
can interfere with each other
directly, and a common way they do this
is through releasing poisons that prevent
other species from growing.

Allelopathy
Interference Competition Allelopathy
 Examples



of Allelopathy
Black walnut
Eucalyptus
Bracken fern
Interference Competition Territoriality
 Some
organisms
defend limited
resources by
actively excluding
others that would
potentially use that
resource.

Territoriality
Interference Competition Preemption
 Preemption
is interference
competition that
occurs when an
individual prevents
other individuals from
occupying a location
by occupying the
space first.
Direct or
Indirect
Competition?
Intraspecific Competition
 The
strength of competition is greatest
when individuals share all of the same
limited resources. This occurs most often
among individuals of the same species
within a population.

Intraspecific Competition
Intraspecific Competition
 The
strength of intraspecific
competition factors strongly into whether
a population will grow or shrink.

But how strongly?
 Ecologists
use population growth models
to quantify the strength of intraspecific
competition and to predict how
competition will influence changes in
population size over time.
Intraspecific Competition
 When
resources are unlimited,
populations grow exponentially.
 dN/dt = r N
Intraspecific Competition

Resources are never unlimited for long.



As populations grow, they use more resources.
Ultimately, resources become limited and
individuals in the population must compete with
each other to obtain the resources they need to
survive and reproduce.
As a population grows, intraspecific competition
reduces birth rates and increases death rates,
and the population growth rate drops below its
maximum possible growth rate.
Survival of the Fittest (Fattest)
 Individual
mosquito larvae
grow slower at high densities,
they also die at a faster rate
(presumably because they
do not get enough to eat),
and those that survive take
longer to pupate and hatch
into adults.
Carrying Capacity




Increased population density results in slower
growth for individual larvae.
The slower individual growth rates translate
into slower growth of the population as a
whole.
Eventually, the density of larvae in the tire
limits individual growth to the point that
population growth completely stops.
A population's carrying capacity is the
maximum size it can reach when constrained
by limited resources.
Carrying Capacity
 At
carrying capacity, the resources
available to a population are exactly
equal to the resources required to sustain
that population.
Limiting Resources
 Population
growth is not always limited by
food.
 The limiting resource can be space,
chemical nutrients, sunlight availability, or
some other factor.
Logistic Growth
 The
growth of the
mosquito population,
with its carrying
capacity (K)
determined by
intraspecific
competition, can be
represented using
the logistic growth
equation.
Interspecific Competition
 If
the individuals
are different
species, it is known
as interspecific (b
etween species)
competition.
Interspecific Competition

The different strains of bacteria in your body
form their own ecological communities.



The bacteria living in your gut compete with
each other via exploitation for food (sugars,
fats, and proteins from the food you eat).
They also compete preemptively for space,
since the surface on the inside of your intestines
where they adhere is limited.
And they compete directly by releasing
chemicals that act as poisons.
Interspecific Competition
 Some
strains of bacteria benefit us
directly, such as those that aid digestion.
But there are also strains that benefit us
indirectly, including those that
outcompete pathogenic (harmful) strains.

What attributes would make a strain of
probiotic bacteria an effective competitor?
Probiotics vs Pathogens
 In
the simulation, probiotics use twice the
resources of pathogens.
 The average effect of a probiotic
bacterium on a pathogenic bacterium
was twice as strong as the effect of a
pathogen on another pathogen.
Competition Coefficients
 Ecologists
quantify the relative
competitive strengths of different versus
same species competitors with
competition coefficients.

A competition coefficient (α) is the per
capita effect of one species on the
population growth of another species.
Lotka Volterra
 Lotka-Volterra
competition models are
based on the logistic equation of
population growth – the s-shaped curve.
Lotka Volterra
 Four
possible outcomes of the LotkaVolterra competition equations:




Species 2 eliminated.
Species 1 eliminated.
Either species 1 or species 2 eliminated,
depending on starting conditions.
Both species coexist.
Lotka Volterra
 In
general, LV predicts coexistence of two
species when, for both species,
interspecific competition is weaker than
intraspecific competition.
Paramecia Lab Experiments
 Gause
demonstrated resource limitation
with Paramecium caudatum and
Paramecium aurelia in presence of two
different concentrations of food - Bacillus
pyocyaneus.
Paramecia Lab Experiments
 When
grown alone,
carrying capacity
was determined by
intraspecific
competition.
 When grown
together, P.
caudatum quickly
declined.

Reduced resource
supplies increased
competition.
Flour Beetle Experiments
 Tribolium
beetles infest stored grain
products.

Park studied interspecific competition
between T. confusum and T. castaneum
under varied environmental conditions.
Flour Beetle
Experiments
 Two
species of flour
beetle grown in
containers with and
without parasites.
 With
parasites,
T. c
onfusum has a
competitive advantage
over T. castaneum, but
this is reversed when
parasites are removed.
Competition and Niches
 Competition
can restrict species to their
realized niches.

But if competitive interactions are strong
and pervasive enough, they may produce
an evolutionary response in the competitor
population.
 Changes
fundamental niche.
Competition in a Complex
Environment
 The
logistic growth and Lotka-Volterra
competition equations provide a logical
framework to study intraspecific and
interspecific competition.
 The equations are particularly good
models for predicting results of
competitive interactions under tightly
controlled laboratory conditions.
Competition in a Complex
Environment
 However,
natural environments change
over time and across space.
 This inherent variability impacts species
interactions, making it difficult to describe
natural populations using a small number
of variables (e.g., N, K, α).
 Species often coexist in nature where
simple models of competition would
predict competitive exclusion.
Field Study Examples
 Studying
competition outside of the lab
can be challenging.
 Chance events, weather, and even
experimenters themselves can
add noise to the results, obscuring
patterns.
 Competitive relationships can also
change over time and from place to
place.
Field Study Examples
A
simple way to document competition is
to identify places where two species
seem to compete, remove one of them,
and then examine what happens to the
other.
Field Study Examples


Rodents and ants
compete for annual
seeds in Arizona
deserts.
The results indicate
that ants and rodents
are the primary
consumers of grass
seeds in these deserts,
and that they
compete with each
other for the seeds.
Common Garden Experiments
 One
theory for coexistence within niches
is that our interpretation of species' niches
is often wrong.
 Without experimentation, it can be
difficult to tell exactly how species are
using local resources.
Common Garden Experiments
 Heath
bedstraw is found growing in acidic
peat soil, while a related species, slender
bedstraw is found in more basic limestone
soils.
 Both plants grew in both kinds of soil,
although they both grew better on the soil
in which they are found in nature.
Common
Garden
Experiments


If seeds from both species
were planted in a pot of peat
soil, heath bedstraw grew
faster than slender bedstraw,
eventually blocking all the
light, and the slender
bedstraw died.
Conversely, if he planted both
seeds in limestone soil, he got
the opposite result—slender
bedstraw outgrew and
excluded heath bedstraw.
Common Garden Experiments
 Two
species of lizard, Sceloporus
merriami and Urosaurus ornatus occur in
each of the three isolated plots of land.
 Both species are small diurnal insectivores,
and they appear to compete with each
other.
Common Garden Experiments
 The
top plot contains S.
merriami only, the middle
plot contains U.
ornatus only, and the
bottom (control) plot
contains both species.
 Initial lizard densities
reflect naturally occurring
densities when
temperature and
humidity are average.
Common Garden Experiments
 Is
there evidence for competition
between these two species?


Rainy?
Drought?
 Environmental
factors can shift the
competitive interaction so it favors one
species under certain conditions and
another species under different
conditions.
Hypotheses for Competitive
Coexistence
 Variability
within and between local
patches can also favor one species over
another in a very small area or time
window.

Spatial and/or temporal variability (or
heterogeneity)
Hypotheses for Competitive
Coexistence

Two competing
species may evolve
in ways that reduce
the strength of
competition
between them
through opposing
shifts along some
trade-off.

Character
Displacement
Hypotheses for Competitive
Coexistence
 Another
way that competing species can
coexist is if competition is weak due to
predation or other ecological interactions
in the community.
 If predation reduces population sizes for
multiple competing species in an area,
then resources might never become
limiting and competition might not be
important.
Mosquitos in a Complex
Environment
 What
might explain the patches of
coexistence for these competing
species?
 Both species can survive in Florida’s
temperatures.
 Perhaps there is an interaction between
temperature or rainfall and the
competitive interaction between the two
mosquito species.
Mosquitos in a Complex
Environment
 Common
Garden
Experiment



Hot/Wet
Hot/Dry
Cool/Wet
Mosquito Abundance &
Competition
 Asian
tiger mosquitos have a competitive
advantage.
 The two species still coexist in some areas.
 Incomplete picture…
Mosquito Abundance &
Competition
 Another
possible explanation for the
pattern of competitive exclusion in some
areas and coexistence in others is a
trade-off between competitive ability and
colonization ability.
Mosquito Abundance &
Competition
 Colonization
is important because the
stagnant water habitats appear and
disappear as rain falls and then
evaporates.
Mosquito Abundance &
Competition
 Lotka-Volterra
coexistence occurs where
the yellow fever mosquito is still found: in
hot, dry, and urban areas of Florida.
 It turns out that the eggs of the Asian tiger
mosquito are more susceptible to
desiccation in hot, dry environments than
the eggs of the yellow fever mosquito.