Biology of Competition
Reading; Smith and Smith, Chapter 14
Question; Why are we so quick to blame
the zebra mussel?
• 19th century ecologists documented that closely
related species living in the same general area seem to
prefer slightly different habitats.
• The role of interspecific competition in the formation
of communities was suspected-after all, there seemed
to be no reason why small differences in environment
would prevent either species from growing on both
types of habitat.
• Tansley’s (1917) common garden experiment;
– worked with 2 closely related bedstraw, Gallium
saxatile and G. sylvestre.
– Planted each alone in both types of habitat, then
planted both together in each type of habitat.
ResultEach species
competes best
on its “own”
type of soil.
G. saxatile-acid
soil
G. sylvestrecalcareous soil.
Tansley’s study
illustrates
competition as a
valid mechanism
for organizing
communities
Competition
• Competition is the use of a resource by one
organism that reduces the availability of that
resource to other organisms.
– Competition is thought to be ubiquitous in nature,
both as an agent of natural selection and a factor
structuring communities.
– For competition occur:
• both organisms must use a common resource that is
important to their survivorship and reproduction
• that resource must be limited-use by one individual must
decrease what is available to others in a meaningful way
• Not a limiting resource, example-Oxygen is
essential to the metabolism of most animals, yet in
terrestrial environments, the use of oxygen by
individuals of one species does not significantly
depress the amount of oxygen available to other
species (or other individuals of the same species)although essential for animal life, oxygen diffuses too
fast/regenerates too fast to become an important
limiting resource.
• Some possible limiting resources– plants-light, water, nutrients, space, pollinators
– animals-prey, nesting sites, territories, water, host
organisms, space (sessile organisms), mates
(intraspecific only)
Limiting resources, example-within the appropriate intertidal
region, space is a very important limiting resource. Once it is
used up, individuals can only settle and grow at the expense of others
• Intraspecific Competition: Competition among
members of the same species for an important,
limiting resource
• Interspecific Competition: Competition among
members of different species for an important,
limiting resource
Types of Competition
• Exploitation Competition-one species denies
another access to a resource simply by
consuming it first.
• Interference Competition-one species actively
inhibits the foraging, survival, or reproduction of
the other species
– I.e., chemical, behavioral
• Preemptive Competition-one species denies
another access simply by getting there first.
– Also, overgrowth competition
– territoriality
Consequences of Competition
• Coexistence
• Exclusion of one species
Exclusion
• The phenomenon of competitive exclusion
was first documented experimentally by the
Russian biologist C. F. Gause.
– Gause’s experiment is now quite famous
– P. caudatum is larger than P. aurellia, but has a
slower reproductive rate. Both species
consume bacteria via a “funnel” lined with
cilia.
– Gause grew each species alone, in a culture
where a fixed amount of food (bacteria) was
added each day.
– He then grew the two species together.
His result was the
exclusion of P.
caudatum by P. aurellia.
He hypothesized that
the two species compete
for the same foodultimately P. aurellia is
ultimately able to
multiply under
conditions where P.
caudatum can no longer
gain enough energy to
divide.
This is called
competitive exclusion
• Gause’s experiment was tremendously influential.
Based on this, and other experiments, ecologists
arrived at the competitive exclusion principle,
which is now firmly established.
– Two species cannot exist on the same limiting resource
indefinitely-ultimately, even a slight reproductive
advantage to one of them will result in their displacing
the other.
– In terms of the niche-if the niches of two species
overlap completely, only the superior competitor can
survive.
• Ironically, this experiment gives different results, depending
upon which strains of Paramecium are used-some strains
coexist, presumably by partitioning the limiting resource
(bacteria).
Example of Competitive
• Aphytis vs. Aphytis
Exclusion
– The California red
scale insect attacks
citrus trees.
– It is a serious
economic pest in
Southern California,
and has evolved
resistance to
pesticide.
– Adults live under
waxy sheaths, and
are protected from
many generalist
predators as well
• Parasitoids of the genus Aphytis attack
scale insects
– Aphytis chrysomphai was
accidentally introduced to CA.
• Despite imposing severe mortality, it did
not control the population-especially in
dry valleys.
– A linganensis was introduced from
China in 1950.
• It replaced A. chrysomphai within a
decade-its higher reproductive rate may
have been a factor.
– In interior valleys, scale insects were
still a problem
• Cold temps kill A. linganensis.
Example of Coexistence via Niche Partitioning
• A cold tolerant species, A. melinus
was introduced from Pakistan in
1957
– A. melinus quickly spread
throughout the valleys, but did
not displace A.linganensis from
coastal regions.
– The two species coexist today,
providing very good protection
against red scale-they have
partitioned the habitat based on
winter temperatures.
The Lotka-Volterra Model of Competition
• The Lotka-Volterra model of competition starts with
the logistic equation, and adds a term to account for
interspecific competition as well as the intraspecific
competition inherent in the original model.
– It has some interesting dynamics, and makes
predictions about the conditions necessary for
species to coexist.
– Thus:dN1/dt=r1N1(K1-N1-a12N2)
• dN2/dt=r2N2(K2-N2 -a21N1)
– where
• N1=Population of Species 1
• N2=Population of Species 2
• K1=Carrying Capacity of Species 1
• K2=Carrying Capacity of Species 2
a12=Effect of Species 2 on Species 1
a21=Effect of Species 1 on Species 2
The presence
of an interspecific
competitor lowers
the equilibrium
density of a
species below its
original carrying
capacity
– Consider the
populations of two
species plotted on the X
and Y axes respectively.
• For the species on the X
axis, (Sp 1) a diagonal
line can be drawn. This
is an isocline.
• To the right of it, species
1 will decrease in
number, to the left of it,
species 1 will increase in
number.
Interspecific competition alone stops growth
of species 1
Some combination of
the two stops population
growth of species 1
Intraspecific competition alone
stops population growth of sp 1
– Likewise, for
the species on
the Y axis (Sp
2), an isocline
can be drawn.
– Above it,
species 2 will
decrease in
number, below
it, species 2
will increase in
number.
K2
K2/a21
– Notice, this
particular pair
of isoclines,
there is one
area where
species 1
increases and
species 2
decreases, and
one area where
the reverse is
true
K2
here
here
K2/a21
– Thus, a stable
equilibrium
exists where
the two species
can coexist
K2
here
K2/a21
In all these cases, interspecific
competition is less severe than
intraspecific competition
I.e., K1/a12>K2 and K2/a21>K1
• If the reverse is true-I.e., interspecific competition is more
severe than intraspecific competition, then the equilibrium is
unstable-only 1 species survives, but it can be either one
• Or, the contest can be totally unequal. If the isocline of 1
species lies above the isocline of the other, then that species
wins-it excludes the other.
Predictions
• Alhough somewhat abstract, and very simple, the
Lotka-Volterra model makes interesting predictionssome of which are testable.
– In theory, both alpha and K are measurable.
• Case 4
• Case 3
• In cases where one
competitor excludes the
other, the dominant
competitor should be the
species that can grow
under conditions where
the limiting resource is
too scarce for the other
species to grow
– -e.g.-it is not the
species with the higher
r that wins, it is the
species with the higher
isocline based on
carrying capacity.
Case 2
Laboratory Experiments
• Competition has been studied in laboratory
experiments on a wide variety of plants, animals,
and protists.
• For practical reasons, these experiments have
been carried out in small, simple environments,
and on small, r-selected organisms (the kind
least likely to suffer high levels of competition
under natural conditions)
Tribolium
Competition
• Thomas park worked on
competition in two closely
related species of flour beetles,
Tribolium castaneum and
Tribolium confusum.
– Both species infest stored
flour products.
– This is a very challenging
environment, with unlimited
carbohydrates, but limited
protein and extremely
limited moisture.
• When cultures were started with equal
numbers of founders from each species;
– T. castaneum always displaced T. confusum
under moister, or warmer conditions.
– T. confusum always displaced T. castaneum
under colder, or drier conditions.
– Under intermediate conditions, the outcome
could not be predicted-only 1 species persisted,
but it could be either one.
• Park referred to this as the indeterminate zone.
– Other experiments showed that an intercellular
parasite, Wolbacchia sp., could reverse the
outcome of competition.
Starting number
was also important
if cultures were
started with
unequal numbers
of beetles, it predisposed that
species to outcompete
the other.
• In the case of flour beetles, both species
compete strongly for moisture, and one of the
most important mechanisms for competition is
egg cannibalism.
– The larvae of both species eat the eggs of their own
species, as well as the other.
– This is a form of interference competition which
exerts very strong interspecific and intraspecific
effects.
– Different strains differ in their propensity for
cannibalism-and thus differ in competitive ability
• kin selection on some strains has actually decreased
cannibalism-decreasing their competitive ability in mixed
cultures
Field Experiments on Competition
• Some of the best studies of competition have been “field”
studies. These are conducted under more natural
conditions, with actual populations of wild animals.
– Although much more representative of the evolutionary
processes that actually occur in nature, field studies of
competition have their own drawbacks.
• They are difficult to replicate-weather, local
conditions, and genetic features of the populations
in question might influence the outcome.
• Philosophical issues-nobody sets out to look for the absence
of something-there might be a selection bias toward choosing
scenarios where competition is especially intense.
• Geko vs. Geko in an Aircraft Hangar “Cage
Match”
– Many species of nocturnal gekos eat essentially the
same insect prey.
– In Polynesia, introduced, sexual species of geko tend
to displace the native, parthogenic species.
– Petren and Case studied competition among
native and introduced gekos in aircraft hangars
on Oahu, left over from WWII.
• These aircraft hangars were quite similar-in some
ways they were as close as possible to experimental
replicates.
• Different hangars were fitted with arrays of lights
(gekos forage for insects near lights at night) and
barriers, to create variable amounts of
environmental complexity
– “simple hangars”-one fixed light source, few barriers
– “complex hangars”-many light sources, many barriers
• Researchers measured the behavior, and the body
condition of the gekos.
– In the absence of habitat structure, their niches
overlap a great deal, and the house geko
Hemidactylus frenatus outcompetes the others.
– When habitats are structured by vegetation, rocks,
etc., space (and the food within) is partitioned, and
interspecific competition is reduced enough to
allow possible coexistence
H. frenatus
H. mabouia
Predation and Competition
• The presence of predators can have enormous
potential effects on the outcome of competition.
• This effect is best known from the experiments of
Paine et al., who studied intertidal communities in
California.
– Rocky coasts in California harbor an enormous
number of plant and animal species, including
mussels, gooseneck barnacles, barnacles, limpets,
chitons,and various algae, all of which compete for
space
– The starfish, Pisaster sp. is the dominant predator.
• Paine chose 2 areas
– in one area, 8m long and 2 m deep, he removed
all sea stars
– in the other area (the control), he did not
remove sea stars.
Whelks
Pisaster ochreu
Mytillus sp. mussels
• Result-in the sea star removal area, the number of
species decreased rapidly, till eventually a single
species of mussel dominated-the number of
invertebrate species dropped from 15 to 8, and
most of the rock surface was covered with
mussels.
Competition, Niche Breadth and Evolution
• Competition is thought to be an important force in
organizing biological communities, and an important
cause of natural selection.
• In the “assembly” of natural communities, only groups
of competing species that can coexist by resource
partitioning can coexist-sometimes this coexistence is
mediated by a predator.
– The addition of a new species may cause a series of
extinctions, as competitive relationships differ, and
food sources for higher trophic levels disappear.
• In evolutionary terms, many pairs of conspecifics are
expected to have evolved to minimize interspecific
competition.
An example of niche partitioning; Mojave Desert flora
• Body Size
– Size differences between closely related sympatric
species have been implicated as being necessary for
coexistence
– In the "assembly" of communities, the most likely
species to coexist will generally differ in body size,
and thus exploit different habitats/prey/resources.
– There may be a definite limit on how similar two
competitors can be and still avoid competitive
exclusion;
• character displacement in average mouthpart sizes is often
about 1.3, and the length ratio of 1.3 has been suggested as
a crude estimate of just how different two species must be
to coexist syntopically.
Lizards
• Pianka and others have studied assemblages of
lizard species.
– Most lizards are generalized predators, and lizards of
similar size might be expected to compete for the same
prey.
• The comparative method was used-this is common
in evolutionary biology-an actual group of
already-extant lizards is compared with the
expectations of theory, as an implied experiment.
• 24 species of Australian Varanus were estimated
for head length.
Australia's largest lizard, Varanus giganteus
(2 meters long).
Varanus brevicata
• Ratios of larger/smaller for all possible pairs of
species were computed (N = 276) and a
cumulative frequency distribution assembled.
– This represents a null model of expected size ratios
against which distributions of ratios in observed
assemblages can be compared.
• In real assemblages, there are many more high
Hutchinsonian ratios than expected in random
subsamples drawn from the species pool of all
Australian varanids
– Such high Hutchinsonian ratios suggest that either size
assortment or character displacement has resulted in
extant assemblages that differ in size.
Australian Varanids
Character Displacement and Ecological Release
• Character displacement is an evolutionary response
to competition.
• Populations of a species with an interspecific
competitor often tend to evolve in such a way as to be
different in their resource utilization.
– Some evolutionary biologists think of competition as a
“transient phenomenon” -evolution tends to lead to its
disappearance, although ecological studies tend to indicate
that there is plenty of competition going on right now.
• The opposite of character displacement is called
“ecological release”-in an isolated environment with no
competitors, a species will frequently evolve a broader
range of resource use than before.
Ecological Release on Islands
• Evidence for ecological release comes from studies on
so-called "incomplete" biotas, such as islands, where all
of the usual species are not present.
– Those species that invade such areas often expand
their niches and exploit new habitats and resources
that are normally exploited by other species on areas
with more complete faunas.
– On the island of Bermuda, considerably fewer
species of land birds occur than on the mainland,
with the three most abundant being the cardinal,
catbird, and white-eyed vireo.
• Crowell found that, compared with the mainland,
these three species are much more abundant on
Bermuda and that they occur in a wider range of
habitats.
• In addition, all three have somewhat different feeding
habits on the island, and one species at least (the
vireo) employs a greater variety of foraging
techniques.
Bermudan Birds