Community Ecology: predation and other interactions

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Community Ecology predation and other
interactions

• BS 111
• Introduction to Ecology

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Learning objectives

• describe the interactions between predators and
  prey and the concept of the doomed surplus
• explain the significance of keystone predators,
  and how predators are used in biological control

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PRS multiple choice question revision

• Which of these is NOT an abiotic factor
• a. climate
• b. intra-specific interactions
• c. topography
• d. latitude
• e. altitude

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• What does phenotype describe?
• genetic composition of an individual
• the interaction between genotype and the
  environment.
• variation due to environmental influences on the
  genotype
• the regulation of genes passed on to the next
  generation
• e. the organisms ability to adapt to the
  environment

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3. The graph below represents which type of
survivorship curve? Log (population
size) a. Type I relatively constant
death rate throughout life. Death, e.g. hunting,
disease. E.g. corals, squirrels, many
reptiles b. Type II high survival young, live
most of expected life span and die in old age,
e.g. humans c. Type III large numbers of
offspring most die before maturity, e.g. plants,
oysters, sea urchins
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The nature of predation

• Definition consumption of all or part of another
  individual
• Eating and avoiding being eaten are prerequisite
  for reproductive success
• Adaptations of both predator and prey refined
  through natural selection
• encompasses
• i. true predators kill prey soon after
  attacking them
• ii. grazers consume only part of prey
  individual
• iii. parasites live in close association with a
  single prey individual (the host), often inside
  the hosts tissues.

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Carnivores and herbivores

• Herbivores plant tissue-eating (some special
  digestive systems to digest all kinds of plants,
  incl. grasses). need a lot of energy to stay
  alive. Many e.g. cows/sheep, eat all day
• Carnivores animal tissue-eating (eat
  herbivores/omnivores occ. other carnivores.
  Carnivores/omnivores imp. to any ecosystem, keep
  other species from overpopulation)
• Omnivores consume both (Some hunt like
  carnivores, eating herbivores other omnivores.
  Some scavengers. Many eat eggs. Cant eat all
  plants digestion. Fruit veg. some insect
  omnivore pollinators imp. To life cycle of some
  plants

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Defence against predators

• Structural, e.g. shell tortoise, coconut
• Chemical defences, e.g. plants. Vertebrates
  generally lack except e.g. amphibians, fish
• Behavioural strategies animals, e.g. run, fight,
  hide, mimicry
• 1. generally immobile chemical defence
• 2. mobile behaviourally complex

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Predator adaptations

• Acute senses that enable them to locate and
  identify potential prey
• Also claws, fangs, stingers, poison
• Pit vipers, rattlesnakes
• Predators that pursue prey generally fast and
  agile, ambush predators disguised in environment

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Generalists and specialists

• Variations in diet
• exclusively one prey type specialist
• Number of prey species generalist
• Herbivores usually more specialised than
  carnivores monophagous one prey type, e.g.
  koala, oligophagous few prey types. Polyphagous
  range of plant and animal species but avoid toxic
• Parasites generally specialists. E.g. aphids
  (plant parasites). 80 of 55 UK spp. restricted
  to 1 host genus
• .
• Larger carnivores and herbivores more varied diet
  

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Predators Prey

• Most direct effect of predator on prey is to
  reduce its numbers
• BUTif prey species is only food source then prey
  numbers will in turn govern abun. Of preds.
• Models based on log. Equation used to examine
  pred-prey relationships

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Predators Prey

• 2 pop. Models where capacity for growth in 1
  determined by pop. size of the other vice versa
• Prey and predator control each other
• Real ecosystems not models????
• Predator and prey numbers not so closely linked
  as simple oscillations suggest.

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Impact of predators on prey pop. Size

• 1. effect of any 1 pred. may be small component
  of total mortality causes affecting prey species,
  i.e. removal of pred. only small effect
• E.g. experiment ants excluded from bracken
  fronds,
• majority of bracken herbivores no sig. change in
  nos.

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Keystone predators

• Predators moderate competition among prey species
• Heavy predation can reduce density of a strong
  competitor SO allowing weaker competitors to
  exist in community

The sea otter is a keystone species of coastal
ecosystems. Without sea otters as predators, sea
urchins increase in numbers and devour kelp
forests.
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Robert Paines experiments

• 60s 1st to provide clear picture of complex
  interaction
• Sea star Pisaster removed from experimental areas
  within intertidal zone, Washington.
• Prey Mytilus (mussel)
• Dominant competitor over other tidepool orgs.
• When Pisaster removed Mytilus increased. Spp.
  richness went from 15 to 8.

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Keystone predators cont.

• Keystone predators maintain higher species
  diversity in a community by reducing densities of
  strong competitors, competitive exclusion does
  not occur.

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Kelp Forestskelp - sea urchin - sea otter

• In mature kelp forests, the urchin population is
  kept in check by high predation pressure from sea
  otters and starfish.
• In these forests, sea urchins feed on fragments
  of seaweed.
• In barren areas, the supply of kelp fragments is
  low and urchins actively graze on developing
  seaweeds.
• In barren areas, otters and starfish are subject
  to high predation pressure that removes this
  top-down control on the urchins.

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Kelp Forests kelp ? sea urchin ? sea otter ?
killer whale

• In Alaska, in recent years, killer whale
  predation on sea otters has increased
• (likely due to decline of seal and sea lion
  populations).
• Kelp forests are declining as sea urchin
  population increases.

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Summary

• Predators, grazers, parasites..diff.?
• Herbivores, carnivores, omnivoresdiff.?
• Defences structural, chemical, behavioural
• Generalists or specialists??
• predator prey relationships
• Impact of predators on pop. Size..doomed surplus?
• Keystone predators with detailed examples

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Recommended reading

• Campbell Reece, Biology, 8th Edition, Pearson
  pp1201-1204
• Mackenzie, A, Ball, AS and Virdee, SR (1998)
  Instant Notes in Ecology.Oxford BIOS Section J
  pp107-111