Biology of Competition

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Biology of Competition

• Reading Smith and Smith, Chapter 14

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Question Why are we so quick to blame the zebra
mussel?
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• 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.
• Tansleys (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.

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Result- Each species competes best on its
own type of soil. G. saxatile-acid soil G.
sylvestre- calcareous soil. Tansleys
study illustrates competition as a valid
mechanism for organizing communities
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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

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• 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)

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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
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• 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

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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

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Consequences of Competition

• Coexistence
• Exclusion of one species

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Exclusion

• The phenomenon of competitive exclusion was first
  documented experimentally by the Russian
  biologist C. F. Gause.
• Gauses 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.

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His result was the exclusion of P. caudatum by P.
aurellia. He hypothesized that the two species
compete for the same food-ultimately 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
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• Gauses 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).

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Example of Competitive Exclusion

• Aphytis vs. Aphytis
• 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

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• 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.

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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.

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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.

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• ThusdN1/dtr1N1(K1-N1-a12N2)
• dN2/dtr2N2(K2-N2 -a21N1)
• where
• N1Population of Species 1
• N2Population of Species 2
• K1Carrying Capacity of Species 1
• K2Carrying Capacity of Species 2
• a12Effect of Species 2 on Species 1
• a21Effect of Species 1 on Species 2

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The presence of an interspecific competitor
lowers the equilibrium density of a species
below its original carrying capacity
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• 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.

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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
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• 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
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• 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

here
K2
here
K2/a21
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• Thus, a stable equilibrium exists where the two
  species can coexist

K2
here
K2/a21
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In all these cases, interspecific competition is
less severe than intraspecific competition
I.e., K1/a12K2 and K2/a21K1
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• 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

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• 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.

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Predictions

• Alhough somewhat abstract, and very simple, the
  Lotka-Volterra model makes interesting
  predictions-some of which are testable.
• In theory, both alpha and K are measurable.

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• Case 4

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• Case 3

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• 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
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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)

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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.

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• 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.

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Starting number was also important if cultures
were started with unequal numbers of beetles, it
pre- disposed that species to outcompete the
other.
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• 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

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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.

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• 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.

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• 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.

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• 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
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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.

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• 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 ochreus
Mytillus sp. mussels
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• 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.

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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.

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An example of niche partitioning Mojave Desert
flora
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• 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.

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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.

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• 24 species of Australian Varanus were estimated
  for head length.

Australia's largest lizard, Varanus giganteus (2
meters long).
Varanus brevicata
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• 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.

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Australian Varanids
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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.

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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.

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• 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.

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Bermudan Birds