Warblers Coexist in Simple Habitat

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COMPETITION

• We wish to know
• What happens when two individuals, populations or
  species are sustained by the same resource
• Do populations compete for resources, or are
  their needs sufficiently different that they can
  be said to occupy non-overlapping niches
• What are the consequences of competition for the
  distribution and abundance of species with
  similar needs
• If there is any relationship between competition
  for a shared resource and evolution by natural
  selection

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Competition

• Competition occurs when two species each require
  a resource that is in short supply, so that the
  availability of the resource to one species is
  negatively influenced by the presence of the
  other species.
• A -/- interaction
• Competition is linked to two important ideas
• struggle for existence (Darwinian evolution)
• the uniqueness of a species niche

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Evidence of Competition

• bottle experiments grow two similar species in
  a simple environment
• observations of natural populations conduct
  detailed studies of similar species apparently
  sharing the same niche (over-lapping in resource
  use)
• field experiments with natural populations use
  cages, transplants, etc, to determine competitive
  outcome in nature

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bottle experiments

• place two similar species in a simple environment
  in a test-tube, bottle or garden
• competitive exclusion is always the outcome
• winner may be consistent over a wide range of
  environmental conditions, or vary with
  environment
• basis for competitive exclusion principle no
  two species can indefinitely occupy the same
  niche.

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The Niche Concept
And NUH is the letter I use to spell Nutches,
Who live in small caves, known as Niches, for
hutches. These Nutches have troubles, the biggest
of which is The fact there are many more Nutches
than Niches. Each Nutch in a Nich knows that
some other Nutch Would like to move into his
Nich very much. So each Nutch in a Nich has to
watch that small Nich Or Nutches who haven't got
Niches will snitch. Dr. Seuss -
On Beyond Zebra (1955)
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Test-tube Competition
Paramecium is a single-celled protozoan ciliate.
It feeds by engulfing bacteria and reproduces by
binary fission. It grows readily in test-tubes
of bacterial broth, making it an extremely simple
laboratory study organism. Classic studies of
competition examined the population growth
patterns of two species (P aurelia and P
caudatum), alone and together. Results led to the
competitive exclusion principle No two species
of similar requirements can long occupy the same
niche.
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Test-tube Competition
Grown separately, each species exhibits
S-shaped population growth. An initial rapid
increase slows as the population reaches the
carrying capacity of the environment (test
tube). Grown together, P caudatum invariably
loses, and P aurelia emerges as the winner. Only
one species can survive in this simple
environment.
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Observations of Natural Populations

• Field observations of similar, often
  closely-related species.
• Classic study of five warbler species in spruce
  forest -- very similar species, very uniform
  habitat
• many specific differences in foraging ecology and
  habitat use indicates that these five species
  occupy different niches
• suggest that competition is rare in nature
  because of niche differences among species and
  complexity of habitat

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Warblers Coexist in Simple Habitat
Bay-breasted warbler
Blackburnian warbler
These five species of warblers, similar in size,
all insectivorous, co-exist in a very uniform
habitat, the spruce forests on Maine, from spring
until autumn.
Black-throated green warbler
Cape May warbler
Myrtle (Yellow-rumped) warbler
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Warblers Competition or Coexistence?
The five warbler species differed in many aspects
of their foraging behavior in what part of the
spruce tree they frequented, whether they
captured insects in flight, Whether they foraged
from needles, under bark, etc. In the end, it was
concluded that enough differences could be found
to explain coexistence, essentially by arguing
that each species niche was sufficiently
different.
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The Niche Concept
The niche of a species includes all aspects of
its habitat, how it makes a living, and where it
is found. It appears that even superficially
similar species, on close inspection, really
differ in resource use how, when, where, and
what types of resources are utilized.
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Resource Partitioning
Resource Partitioning - similar species share the
same resources in different ways. Arises in 2
ways (1) Ecological differences b/w established
competing populations may increase through
natural selection (2) only species that are
dissimilar from established ones can succeed in
joining an existing community
Bristly foxtail
Indian mallow
Smartweed
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Experiments with Natural Populations

• Two barnacles on rock walls in inter-tidal
  Balanus occurs lower, and Chthamalus higher, in
  the inter-tidal zone.
• Is competition an on-going force determining the
  spatial distribution of these two species, or do
  they represent the ghost of competition past,
  so that today they occupy different niches and
  have no influence upon one another?

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Barnacles and Competition
Barnacles as larvae float in the plankton, before
settling on a rock face and transforming into
their adult form, which filter feeds on small
plankton. Finding open space on rock surfaces,
and holding their own against other species, is
critical to their survival. A classic study in
Scotland found one species, Balanus, in
inter-tidal and sub-tidal zones. Another,
Chthamalus, occurred only in the upper
inter-tidal.
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Barnacles and Competition
Further observation revealed that larvae of both
species settled throughout the inter-tidal zone.
Larvae of Balanus died out in the upper regions,
larvae of Ch. Dies out in the lower reaches. When
larvae that had settled on flat rocks were
transplanted (ie, the rocks were moved), and
other species prevented from colonizing, Balanus
still died out in the upper inter-tidal, but Ch.
thrived in the lower inter-tidal.
Observational evidence suggested that the
distributions of Balanus and Chthamalus only
partly overlapped. Were they each adapted to
live in different regions of the inter-tidal zone
(occupy different niches?)
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Competition in Barnacles
Many studies find competition to be asymmetrical.
One species (in this case, Balanus) appears
capable of excluding the other (Chthamalus) from
all regions that Balanus can occupy. Chthamalus
coexists by virtue of its ability to live in
physically harsher environments, where Balanus
cannot live.
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Mechanisms of Competition

• Individuals of two populations may compete by
  using more effectively and depleting the
  resources that are critical to both. Eg, nest
  sites, food, soil minerals. Indirect competition
• Individuals of one species may interfere with or
  directly harm individuals of a second species.
  Eg, combat, release of chemicals. Direct
  competition

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Competition and Specialization

• If we begin with two very similar species,
  competition is expected to be strong.
• Individuals whose resource use (character trait)
  differs, in the direction of low overlap, should
  be favored by natural selection.
• Over time, character traits in the two
  populations will diverge.
• Although thought to be a common phenomenon, it is
  difficult to distinguish character displacement
  from ecological release.

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Darwins Finches
The fourteen species of Galapagos finches provide
a classic link between theories of competition
and adaptive radiation. This group includes
seed-eating finches, insect-eating finches, and a
range of body sizes, and even a woodpecker finch
that uses a cactus spine to probe for insects in
crevices. Their differences have been cited as
evidence of character displacement
http//www.rit.edu/rhrsbi/GalapagosPages/DarwinFi
nch.htmlanchor725315
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Character Displacement in Galapagos Finches
Geospiza fortis and G. fuliginosa differ more in
beak size when they occur together on the same
island, compared to when they are sole occupants
of an island.
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On-going Evolution in Darwins Finches
1973 drought year 1977 wet year
Medium ground finch Geospiza fortis
Daphne major
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Summary Consequences of Competition

• Influences the distribution and abundance of
  natural populations.
• Favors evolutionary specialization, which can
  lead to reduced niche overlap
• affects biological diversity
• in the short run, competition is likely to reduce
  the number of co-occurring species
• in the long run, competition is likely to
  increase the number of species by encouraging
  evolutionary specialization.