Lecture Outline: Predation

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Lecture Outline Predation

• Top-down, bottom-up, trophic cascades

• Lotka-Volterra predator-prey models
• Basic model with cyclic predictions
• Model with victim carrying capacity
• Assumptions and limitations

• Numerical functional responses and predation
  rates

• Case study wolves and moose
• Regulation and ratio-dependent predation

• Hyperpredation

• Intraguild predation

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Predation

• Broad definition consumption of one organism
  (prey) by another organism (predator) in which
  the prey is alive when the predator attacks it
  (excludes scavenging and detritivory).

• Functional classification
• True predators often consume prey in entirety
  (lions, hawks, carnivorous plants)
• Grazers remove only part of each prey individual
  (deer, cattle, leeches)
• Parasites consume part of host, rarely lethal,
  attacks concentrated on few individuals during
  lifetime, specialize (tapeworms, ticks, viruses)
• Parasitoids insects that lay eggs on or near
  host that is then consumed by larvae

• In wildlife ecology, when we speak of predators
  we are generally referring to true predators
  (i.e., carnivores), but many concepts and models
  apply to all types.

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Bottom-up vs. top-down control

• The importance of resources/prey (bottom-up)
  versus predators (top-down) in controlling
  populations and structuring communities is an old
  discussion in ecology.

(Hairston, NG, FE Smith, and LB Slobodkin. 1960.
Community structure, population control, and
competition. American Naturalist 94421-425.)
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• In time, ecologists recognized that both
  bottom-up and top down forces potentially are
  important for any trophic level.
• Key is to determine if patterns exist regarding
  relative roles of these structuring forces.

Hunter, MD, and PW Price. 1992. Ecology
73724-732.
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Trophic cascades

• Interactions between trophic levels that result
  in inverse patterns in abundance or biomass
  across more than one trophic link in a food web.

• For instance, top-down control by predator that
  creates indirect effects two or more links down.

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Trophic cascades

• Recent meta-analysis concluded that most
  terrestrial studies reported effects of
  carnivore removal on plant biomass, damage, or
  reproduction.
• Evidence for trophic cascades in 45 out of 60
  studies.
• Vertebrate carnivores (birds, lizards) had
  stronger effects than invertebrate carnivores
  (mostly ants).
• Concluded that terrestrial trophic cascades might
  be as common as those found in aquatic
  environments.

Schmitz et al. 2000. American Naturalist
155141-153.
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Lotka-Volterra Predator-Prey Model

• Model provides historical perspective and
  foundation for more realistic models

• Coupled interactions between two populations
  (predator and victim)

• Differential equations (Nicholson and Bailey
  developed discrete versions)

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Equilibrium solutions
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• Predator and victim populations track an ellipse
  in state space.
• Unless populations are exactly at intersection of
  isoclines, trajectories continue to move in
  counterclockwise trajectory.

Predator
Victim
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Growth curves translated from the ellipses in
state space graph

• Both populations cycle periodically

• Exceptions (1) populations exactly at isocline
  intersection, (2) starting point is too extreme
  in which case either predator or prey crashes

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• Although it is tempting to explain predator-prey
  cycles with this simple model, it is unlikely
  that any real populations behave like this in
  nature. The basic Lotka-Volterra model has many
  unrealistic assumptions.

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Adding a victim carrying capacity

• Without predators, equation is equivalent to a
  logistic model with a carrying capacity r/c

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Can predation limit or regulate prey numbers?
1. Predation Rate no. prey killed/prey
abundance x 100
3. Who gets killed?
Most studies investigating predation as a
regulatory factor have been correlative and not
experimental. Although evidence often is
compelling, cause-effect relationships cannot be
demonstrated unequivocally, and alternate
hypotheses for patterns should be evaluated.
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Example of compelling pattern

• Epizootic of mange was prevalent among
  Scandinavian red foxes during late 1970s and 1980s

• Substantially reduced fox population and created
  natural experiment on the importance of fox
  predation on prey density.

• Not really experiment because there was no
  spatial or temporal control (or replication).

Lindstrom et al. 1994. Disease reveals the
predator sarcoptic mange, red fox predation, and
prey populations. Ecology 751042-1049.
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Hazel grouse
Black grouse
Grouse
Hares
Fox
Voles
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Numerical and functional responses

• Numerical response relationship between density
  of predator population and prey abundance
• Functional response relationship between
  consumption rate by predator and prey abundance

• Type I simple Lotka-Volterra model (linear)

• Type II consumption rate reaches plateau
  because handling time remains constant as victim
  abundance changes

• Type III sigmoidal increased rate due to
  switching to more common prey or increased
  capture efficiency (develop search image)

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Predation Rates
(Mills)
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Case studies wolves and moose

• Empirical evaluation of whether wolf predation
  can regulate moose numbers
• Reviewed wolf-moose interactions over broad range
  of moose densities
• Estimated numerical and functional responses of
  wolves based on 27 studies in which moose were
  main prey species (gt75 of biomass consumed)
• Analyzed four parameters moose density, wolf
  density, per capita killing rate, and pack size.

Messier, F. 1994. Ecology 7548-488.
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Functional response

• Fit Michaelis-Menten model that is equivalent to
  a Holling Type II response.
• Kill rate asymptote at 3.4 moose per wolf per 100
  days

Messier, F. 1994. Ecology 7548-488.
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Numerical response

• Wolf density plateaued at 59 wolves/1000 km2

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• Predation rate integrates numerical and
  functional responses

• Density dependence and potential for regulation
  at low moose densities
• Above threshold, inverse density dependence and
  moose reach densities set by food regulation.

Messier, F. 1994. Ecology 7548-488.
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Intraspecific density dependence at high moose
densities
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Ratio-dependent predation

• Kill rates might depend not only on prey
  densities, as in the Lotka-Volterra model or
  Hollings Type II functional response, but also
  on predator densities.

• One approach for incorporating predator
  dependence is ratio-dependent predation in which
  kill rate depends on the ratio of predatorprey.

Vucetich JA et al. 2002. Ecology 833003-3013
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Hyperpredation

• Hyperpredation is a form of apparent competition.
• An introduced prey species that is well adapted
  to high predation pressure indirectly facilitates
  extinction of native prey species by enabling a
  shared predator to increase in population size.

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Hyperpredation
Eagle sightings
Fox survival
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Intraguild predation

• Potentially competing species may also engage in
  predator-prey interactions (e.g, carnivore
  species involved in resource competition but
  larger species also preys on smaller species).
• Hence, there is a direct and indirect link
  between species.
• Recent review1 suggests that intraguild predation
  is common situation in nature.

1Arim and Marquet. 2004. Ecology Letters
7557-564.