Community Ecology Interactions: Competition

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Community Ecology Interactions Competition
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Learning objectives

• discuss intra-specific competition and
  inter-specific competition
• describe how inter-specific competition has led
  to the evolution of the niche and competitive
  exclusion

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Community ecology

• A population of different species living close
  enough to interact is called a biological
  community
• What kind of interactions occur in communities???
• Some key relationships in the life of an organism
  are its interactions with individuals of other
  species in the community

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Competition

• Definition an interaction among individuals
  utilizing a limited resource
• What are these resources????
• Between species utilizing a shared resource
  INTER-SPECIFIC COMPETITION example????
• Among members of a species INTRA-SPECIFIC
  COMPETITION example?

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Types of competition

• Chemical prod. of toxin or deterrent chemical to
  exclude compeditors
• Consumptive competitive use of resource, e.g.
  food
• Encounter physical defence of resource
• Overgrowth overwhelms in size or number
• Territorial defence of territory, breeding,
  feeding areas

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Resources

• elements of an organisms habitat it needs to
  survive reproduce, e.g. Food, water, light,
  space, mineral nutrients, mate???
• Some abundant, e.g. oxygen some scarce, e.g.
  energy has to be allocated for best effect, e.g.
  reproduction, growth or further energy
  acquisition

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When to compete?

• Any time during life cycle
• E.g. between seedlings for light and space,
• between offspring in the same litter,
• between adults, between adults and young, e.g.
  trees and seedlings on the woodland floor

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Natural selection?

• Competition is central to theory of natural
  selection struggle for resources determines
  which individuals are able to reproduce
• Fittest survive and secure the resources to
  produce offspring

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Intra-specific competition inds. of same species

• Think back.limited resources result in
  density-dependant pop. Growth and stabilisation
  of pop. Around K
• Direct combat over a resource, e.g. mate, food is
  costly and carries risk
• Mechanisms to diffuse confrontation size,
  colouration, ornamentation used to signal status
  of owner or position in hierarchy
• Major overlap in resource use between same
  species SO major force in ecology

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Intra-specific comp. cont.

• Major force in ecology may depress fitness of
  inds. in crowded pops SO..influence processes
  e.g. fecundity, mortality
• SO..regulates pop. Size
• Also leads to behavioural adaptations to
  overcome, cope with comp.
• E.g. dispersal, territoriality

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Back to density-dependence!

• Density-dependence relationship between fitness
  and pop. Size
• Key pop. Regulatory factor is ve density
  dependence declining fitness occurs as pop.
  Density increases within species due to
  intraspecific comp.

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Dispersal

• Disperse away from area of high pop. Density
• E.g. plant species seeds carried by wind, water
  currents, e.g. coconut palm
• Sessile marine have mobile larval stage
• ALSO avoids costs of inbreeding
• BUT ..involves location of suitable habitat where
  pop. Densities low

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Competitive ability reproductive value

• Tendency to disperse also depends on spp.
  competitive ability, e.g. some pigeons quick to
  colonise Pacific islands but easily ousted by
  native species
• Dispersal risky SO undertaken by inds. With low
  reproductive value, i.e. younger inds. , e.g.
  plants, sessile marine, birds, small mammals

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Territoriality

• Competition for areas of space, known as
  territories, e.g. mating pair, social animals
  wolfs defend common territory
• General purpose e.g. many songbirds feeding,
  mating, rearing of young
• Leks closely aggregated males for breeding which
  are short lived, e.g. fallow deer

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Territoriality cont.

• Female red-winged blackbirds (Agelaius
  phoeniceus) choose males on basis of territory
  quality
• Territories in short supply so you get floaters,
  satellites. E.g. male black grouse fail to
  establish territory rarely successful in mating

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Territory size

• Determined by balance of costs and benefits
• Both increase as territory size increases
• At some critical size costs of maintaining
  outweigh benefits
• When resources abundant optimal territory size
  will be smaller
• E.g. golden-winged sunbird (Nectarina reichenowi)
  territories vary in size and flower density but
  each provides nectar supply

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Gauses competitive exclusion principle

• The notion that no 2 species can occupy the same
  niche
• If 2 species compete in a stable env. 2 possible
  outcomes
• One species is excluded
• Both species coexist
• The competitive exclusion principle states that
  coexistence can only occur in a stable env. If
  the species niches are differentiated (because if
  2 species have identical requirements 1 would
  dominate and outcompete the other)

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Niche

• Distinction between habitat and niche
• Habitat where organism is found
• Niche precise way in which organism fits into
  its env. ..how it relates to its physical and
  biological env.
• Ecology has few laws. One of them is that each
  species has its own unique niche (Grinnell, 1924)

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Resource partitioning

• BUTecologically similar spp. Can coexist in a
  community where there are one or more significant
  differences in their niches
• Evolution by natural selection can result in one
  of the species using different set of resources
• Differentiation of niches that enables similar
  species to coexist in a community resource
  partitioning

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Resource partitioning

• Where 2 or more species divide out a resource
  such as food or nesting sites
• E.g. Mojave desert plants (Cody 1986). Plants
  dividing up access to water. Some specialise on
  ephemeral sources of water which may be trapped
  by surface roots, others rely on relatively
  permanent sources of deep water

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Testing.

• How do ecologists identify the fundamental niche
  of a species????
• Test the range of conditions in which it grows
  and reproduces in the absence of competitors..
• Remove a competitor and see if first species
  expands into newly available space.

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Inter-specific competition cont.

• Barnacles Semibalanus balanoides and Chthamalus
  stellatus
• Chthamalus upshore, Semibalanus low shore. Young
  Chthamalus settling low shore do not survive but
  in an experiment when protected from smothering
  by Semibalanus survived and grew well
• Upper shore Chthamalus do not have to compete as
  Semibalanus cannot survive desiccation
• COMPETITION AND ENV. TOLERANCE

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Question???

• Other observations showed that Semibalanus cannot
  survive high on the rocks because it dries out
  during low tides
• Would Semibalanuss niche change if Cthalamus was
  removed????

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Character displacement

• Closely related species whose pops. Are sometimes
  allopatric (geographically separate) and
  sometimes sympatric (geographically overlapping)
• Allopatric - species morphologcally similar and
  use similar resources
• Sympatric diffs. In body structure and
  resources they use

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Character displacement

• Niche contraction due to comp. results in
  morphological character change
• e.g. Galapagos 2 of Darwins finches (classic
  instance of evolution of several species from a
  common ancestor)
• Geospiza fortis and G. fuliginosa when occur
  alone on islands have similar beak sizes
  (allopatric)
• When occur together G. fuliginosa has narrower
  beak. natural selection has caused the 2 species
  diverge which allows the birds to specialise on
  diff. foods. (sympatric)

G. fulignosa
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Inter-specific competition

• Where 2 species utilize same limited resource
• E.g. Darwins finches Geospiza fortis and
  Geospiza scandens
• Small Isla Daphne, Galapagos, drought late 70s
  reduced production of seed (food of both
  species).
• Both species survived but changed their diets G.
  fortis small cactus seeds, G. scandens larger
  seeds
• COMPETITION RESULTING IN COEXSISTENCE VIA A NICHE
  SHIFT

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Community structure of coral reef fish

• Huge diversity of fish
• What allows coexistence of so many species?
• 1. Traditoinal, equilibrium hypothesis coral
  reef communities at equilibrium showing precise
  resource partitioning in response to comp.
  between reef fish
• 2. Stochastic hypothesis number of fish species
  on reefs kept high by stochastic processes, ie.
  unpredictable and operate in relation to
  density-independent factors
• Refers to assemblages of fish denying that fish
  really coexsist as a community where actions of
  one affects another

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Spatial and temporal heterogeneity

• Natural envs. Are not constant or homogeneous
  patchworks of habitats which vary in quality and
  resource levels in both time and space
• In time and space env. Is highly heterogeneous
• Competitive battles may not be completed before
  env. Changes and balance of play alters in favour
  of weaker competitor
• Heterogeneity therefore plays a very imp. role in
  fostering ecological diversity

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Summary

• Competition an interaction among individuals
  utilizing a limited resource
• Between species utilizing a shared resource
  INTER-SPECIFIC COMPETITION
• Among members of a species INTRA-SPECIFIC
  COMPETITION
• Competition is central to theory of natural
  selection struggle for resources determines
  which individuals are able to reproduce
• Major force in ecology may depress fitness of
  inds. in crowded pops
• Tendency to disperse also depends on species
  competitive ability
• Competition for areas of space, known as
  territories
• Competitive exclusion The notion that no 2
  species can occupy the same niche
• Resource partitioning Where 2 species divide out
  a resource such as food or nesting sites
• Character displacement Niche contraction due to
  comp. results in morphological character change
• Natural envs. Are not constant or
  homogeneous!!!!!!

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Recommended Reading

• Campbell Reece, Biology, 8th Edition, Pearson
  pp1198 1200
• Mackenzie, A, Ball, AS and Virdee, SR (1998)
  Instant Notes in Ecology.Oxford BIOS Section I
  pp93-102