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Lecture Outline: Predation

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Empirical evaluation of whether wolf predation can regulate moose numbers ... Study used 30 years of data on wolf-moose interactions on Isle Royal National Park. ... – PowerPoint PPT presentation

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Title: Lecture Outline: Predation


1
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

2
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.

3
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.)
4
  • 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.
5
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.

6
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.
7
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)

8
Equilibrium solutions
9
  • 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
10
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

11
  • 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.

12
Adding a victim carrying capacity
  • Without predators, equation is equivalent to a
    logistic model with a carrying capacity r/c

13
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.
14
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.
15
Hazel grouse
Black grouse
Grouse
Hares
Fox
Voles
16
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)

17
Predation Rates
(Mills)
18
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19
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.
20
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21
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.
22
Numerical response
  • Wolf density plateaued at 59 wolves/1000 km2

23
  • 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.
24
Intraspecific density dependence at high moose
densities
25
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
26
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.

27
Hyperpredation
Eagle sightings
Fox survival
28
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.
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