Exploitation specifically predation

1
Exploitation specifically predation

• Predator kills and eats victim
• snake, wolf, lion, spider, seed weevil, etc.
• Parasite lives intimately with victim and
  usually does not necessarily kill victim
• tapeworm, flea, louse, aphid, malaria, etc.
• Herbivore/Carnivore distinction not that
  important for dynamics

2
Exploitation

• How does the presence / absence of a predator
  affect
• species populations
• assemblages of prey species
• evolution of prey
• Does predation contribute to community patterns?

3
Predation population dynamics

• Predators eat prey prey die
• How does this affect population dynamics?
• Lotka-Volterra predator-prey model
• N number in prey population
• P number in predator population

4
Lotka-Volterra predator-prey

• Without predation, prey grow exponentially
• dN / dt r1 N
• Predation is an increasing function of N P
• Effect of predation on prey population C1 NP
• C1 is the capture efficiency
• So, with predation
• dN / dt r1 N - C1 NP

5
Lotka-Volterra predator-prey

• Without prey, predators starve to death
  exponentially
• dP / dt - r2 P
• Predation is an increasing function of N P
• Effect of predation on predator populationC2 NP
• C2 product of capture conversion
  efficiencies
• So, with prey
• dP / dt C2 NP - r2 P

6
Lotka-Volterra predator-preyEquilibrium
predictions

• At equilibrium
• dN / dt 0 and dP / dt 0
• there is a specific, constant density of
  predators, above which prey cannot increase
• there is a specific, constant density of prey,
  below which predator cannot increase

7
Lotka-Volterra predator-prey isoclines
8
Lotka-Volterra predator-prey isoclines
Predator (P)
Prey (N)
9
Lotka-Volterra predator-prey isoclines
10
Lotka-Volterra predator-prey isoclines
11
Lotka-Volterra predator-prey dynamics
12
Predator-prey cycles in real data

• Hare Lynx
• Other examples
• Driven by predator-prey dynamics?
• What assumptions are built into Lotka-Volterra
  predator-prey models?

13
Simplifying Assumptions

• Simplifying Environmental
• Constant in time
• Uniform or random in space
• Simplifying Biological
• Individuals are identical constant in time
• Exponential prey growth
• Prey limited only by predation
• Predator growth dependent only on predation

14
Explanatory Assumption

• Predators and prey encounter each other at
  random, like bimolecular collisions
• Frequency of encounter proportional to product of
  densities
• Individual predator feeding rate increases
  linearly as N increases
• No limit on increase in feeding rate

15
Unrealistic elements

• No limits on prey except predation
• expect real prey may be limited by food, space,
  etc. when abundant
• upper limit ( K ) for prey even with no predators
• Predators do not saturate with prey
• expect real predators to hit a maximum number
  eaten
• expect an upper limit for predators with maximal
  food (KP )

16
Gauses predator-prey experiments
Didinium Predatory ciliate
ParameciumPrey
17
Didinium - Paramecium predator-prey experiment
Paramecium
Density (N or P)
Didinium
Time (t )
18
Gauses Predator-Prey experiments

• Predator and prey in a simple environment
• No cycles (stable or otherwise)
• Predator exterminates prey
• Predator dies out shortly after
• Inconsistent with Lotka-Volterra predator-prey
  models

19
HuffakersPredator-Prey experiments

• Mites
• predator Typhlodromus
• prey Eotetranychus
• on oranges
• With oranges evenly spread on a tray
• no cycles
• prey extinction, then predator extinction

20
Huffakers modifiedPredator-Prey experiments

• Add barriers to dispersal
• rubber balls, vaseline
• cycles
• Confirms Lotka-Volterra prediction?
• NO
• violates simplifying environmental assumption

21
Utidas Predator-Prey experiments

• Bean weevil Callosobruchus
• Parasitoid Heterospilus
• Searching in petri dishes
• Produces population cycles

22
Predator-Prey models experiments Conclusions

• Lotka-Volterra models are largely inadequate
• lab systems meeting assumptions -- no cycles
• Stable oscillations when system is fixed
• Conceptual error
• Design experiments to meet assumptions, then test
  predictions
• Dont manipulate experiments until they confirm
  theory

23
Improved Predator-Prey models

• Self limitation of prey and predators
• Asymptotic prey consumption by predators
• Spatial refuges for prey
• graphical approach
• Rosezweig MacArthur (1963)
• mathematical approach
• Williams (1980) Grover (1997)
• Gilpin Ayala (1973) Populus 5.1

24
Rosenzweig MacArthur predator-prey isoclines
25
Rosenzweig MacArthur predator-prey isoclines
26
Predictions of R. MacA.

• 1. Inefficient predator
• isoclines dont cross
• predicts predator extinction

• 2. Intermediate predator efficiency 1
• isoclines cross to right of hump
• predicts stable coexistence with damped
  oscillations

27
Predictions of R. MacA.

• 3. Intermediate predator efficiency 2
• isoclines cross near hump
• predicts stable oscillations

• 4. Highly efficient predator
• isoclines cross to left of hump
• predicts expanding oscillations extinction

28
Predictions of R. MacA.
29
Further improvements A refuge for prey

• If prey have a refuge, then a certain proportion
  can escape predation
• Prey population in the refuge tolerates
  infinitely large predator population
• Makes stable coexistence more likely

30
A prey refuge stabilizes the system
31
Implications of graphical predator-prey models

• Many different patterns of dynamics are possible
• Stable oscillations are only one special case
• Prey may be exterminated (efficient predators)
• Prey may be reduced to stable populations below K

32
? logistic model prey dynamics

• dN / dt r N 1 - (N / K)? - f P
• r prey intrinsic rate of increase
• K prey carrying capacity
• ? quantifies form of density dependence
• ? 1 yields ordinary logistic
• f functional response
• function relating number eaten per predator to N

33
? logistic model predator dynamics

• dP / dt sP( f - D)
• f the number of prey eaten per predator
  (functional response)
• D minimum feeding required for dP / dt gt0
  (predator efficiency)
• s constant relating predator rate of increase
  to amount eaten

34
Resource models of predator-prey interactions

• Prey consume resources and prey population grows
• Predator eats prey and population grows
• Chemostat system

35
Community effects of predation?

• Basic effect - reduce prey abundance
• Increase likelihood of prey elimination?
• Reduce diversity?
• Single predator - single prey
• Smith 1983
• Pseudacris tadpoles
• Anax dragonfly nymphs

36
Temporary ponds

• Isle Royal - small rock islands off main island
• Lake Superior
• Depressions in rock form temporary ponds

37
Temporary ponds

• Vary in size
• 1 liter to 1000 liters
• influences pond duration, drying
• Vary in height above L. Superior
• 0 to 1 m
• influences probability of wave washout

38
Pseudacris distribution

• Might expect larger, higher ponds to have most
  Pseudacris
• Absent in low ponds, absent-rare in high ponds
• Predator Anax greatest in high ponds near forest

39
Experiment

• Four large ponds near forest
• Remove everything, stock with known densities of
  Pseudacris, Anax
• Pond Original Experimental Final
• P A P A P
• P1(746) 229 0 297 0 166
• P2(330) 102 0 132 33 0
• P3(283) 0 92 112 7 57
• P4(583) 0 50 234 95 0

40
Experiment

• Elimination in as little as 1 week
• Pseudacris in P3 grew better than in P1
• physical environment is OK
• Predation can yield reduction or local extinction
  of prey
• Environmental heterogeneity
• prey persist if a refuge is present

41
Other effects of predation

• Basic predator effect reduce diversity
• Keystone predator A predator whose removal from
  a community results in reduced species diversity
  in that community
• therefore keystone predators increase community
  diversity
• Keystone predator effect requires both
  interspecific competition and predation

42
Keystone predation in the Rocky intertidal zone

• Predator
• Pisaster sea star
• Nucella snails
• Grazers
• limpets snails
• chitons snails

43
Keystone predation in the Rocky intertidal zone

• Sessile species
• Mytilus Mussels
• Pollicipes Goose-neck barnacle
• Chthamalus, Balanus acorn barnacles

44
Pacific Northwest Intertidal(Paine 1966)

• Competition for space
• Mytilus the competitive dominant species
• Pisaster preys on all speces, prefers Mytilus
• Natural intertidal community 15 species
• Exclude Pisaster with cages
• 1 to 2 years 8 species
• Without Pisaster, Mytilus dominates

45
The Keystone effect
46
Pisaster is a keystone predator

• Keeps competitive dominant (Mytilus) from
  eliminating other species
• Other predators do not have this effect (e.g.,
  Nucella)
• Disturbance (e.g., storms, wave action, scouring)
  can have a similar keystone effect
• Create open space, allow poorer competitors to
  survive

47
Models of keystone predation

• Leibold 1996
• What environmental conditions promote keystone
  effects?
• 3 tropic levels
• resource R
• prey N (consumes resource)
• predator P (consumes prey)

48
Keystone predation isocline
49
Keystone predation isocline
50
Keystone predator effect

• Most likely at intermediate levels of
  productivity
• high productivity favors predator resistant sp.
• low productivity favors best competitor
• Predicts unimodal diversity - productivity
  relationship

51
Keystone predator gt2 prey

• For any given productivity (S ), there is a
  stable equilibrium with up to 2 prey spp.
• Across a gradient of productivity, prey species
  replace each other
• low productivity best competitors
• high productivity least vulnerable to predator
• May create large-scale unimodal
  diversity-productivity relationship

52
Keystone predator spatial heterogeneity

• With spatial heterogeneity in productivity, gt2
  species of prey can coexist locally
• Strong unimodal diversity-productivity
  relationship
• local patches of different productivities have 2
  prey
• regionally gt2 species coexist at intermediate
  average productivities

53
Related concept Intermediate disturbance
hypothesis
54
Intermediate Disturbance or Intermediate
predation

• Disturbance disruption of community progress
  toward competitive equilibrium
• Predation or physical disturbance
• Diversity maximal at intermediate disturbance
• Keystone effect may be a special case of
  intermediate predation

55
Intermediate Disturbance

• Low disturbance (frequency, intensity)
• Competitive dominant excludes other spp.
• low diversity, low S
• High disturbance (frequency, intensity)
• few species can endure disturbances
• low diversity, low S
• Intermediate disturbance (frequency, intensity)
• disturbance doesnt elimnate species
• reduces or eliminates competition among prey
• maximal diversity, maximal S

56
Intermediate predation Temporary pond
amphibians

• Woodland ponds, SE United States
• Fill with spring rains later dry up
• Up to 17 spp. amphibian larvae in one pond
• Up to 25 spp. present locally
• Morin 1983, 1981 Wilbur 1983 and many more
  recent papers

57
Temporary pond amphibians

• Predators salamanders
• Newts (Notophthalmus)
• adults and larvae
• Prey on larvae of anurans
• (frogs toads)

58
Temporary pond amphibians

• Common anurans
• Spadefoot toad
• (Scaphiopus holbrooki)
• Leopard frog
• (Rana sphenocephala)
• Southern toad
• (Bufo terrestris)
• All filter feeders scrapers

59
Temporary pond amphibians

• Other common anurans
• Spring peeper
• (Hyla crucifer)
• Barking tree frog
• (Hyla gratiosa)
• Grey tree frog
• (Hyla crhysocselis)
• Also filter feeders scrapers

60
Experiment 1 Artificial ponds

• Cattle tanks
• Stock with leaf litter, plants, invertebrates
• 1200 newly hatched larvae of a mix of the 6
  anuran species (150 to 300 each species)
• Predators 0, 2, 4, 8 adult newts

61
Effect of newt predation

• 0 newts
• Scaphiopus dominates, Hyla rare
• 2 newts
• Scaphiopus dominates, Hyla crucifer increases
• Maximal mass of anuran adults Maximal evenness
• 4 newts
• Hyla crucifer Scaphiopus equally abundant
• 8 newts
• 60 Hyla crucifer, all others rare

62
Supporting data

• Scaphiopus most vulnerable to newt predation
• Most active, moves, forages most
• Best competitor
• Hyla crucifer poorest competitor
• Moves very little
• General tradeoff -- high vs. low activity
• High activity, effective foraging, good
  competitor, vulnerable to predation
• Low activity, lower foraging success, poor
  competitor, less subject to predation

63
Temporary pond amphibians

• Newt predation concentrated on competitive
  dominant species
• Intermediate predation yields maximal diversity
• Both competition and predation are necessary for
  the keystone predator or intermediate predation
  effect

64
Temporary pond amphibians

• Predators salamanders
• Tiger salamanders (Ambystoma)
• larvae
• Prey on larvae of anurans
• (frogs toads)

65
Experiment 2 Artificial ponds

• Ambystoma as a predator, same prey species
• With any Ambystoma present, anuran larvae are
  exterminated
• No intermediate predation effect
• No keystone effect
• Effect on diversity specific to the predator prey
  combination

66
Beyond the keystone predator effect

• Predation is a pairwise interaction
• Interference competition is a pairwise
  interaction
• Effects on the two species involved
• There can be effects beyond the pair of species
• Indirect effect An effect of one species on
  another that occurs via an effect on a third
  species

67
Indirect effect
Increase predator ? Decrease Herbivore
? Increase Plant
TROPHIC CASCADE effects produced 2 or more
trophic levels down from top predator
68
Indirect effect
Decrease prey 1 ? Decrease Predator
? Increase Prey 2
APPARENT COMPETITION negative effects caused via
a shared enemy
69
A surprisingIndirect effect
RESOURCE COMPETITION negative effects caused via
a shared victim
70
Indirect effect
Decrease predator 1 ? Increase Prey
1 ? Decrease Prey 2
? Decrease Predator 2
INDIRECT PREDATOR MUTUALISM positive effects of
one predator on another via competing prey
71
Indirect effects

• Possibilities are complex
• Become more complex with more species
• Two problems
• 1. How do you detect indirect effects?
• 2. How important are indirect effects in
  determining community composition?

72
Detecting indirect effects

• You must
• know something about the pairwise direct
  interactions within the community
• Do experiments, typically species removals and
  additions
• If you dont know which pairwise interactions are
  present, indirect effects may be interpreted
  incorrectly even in an experiment

73
Misinterpreting an indirect effect in an
experiment

• Remove predator 2
• Predator 1 increases
• Prey 1 decreases
• Prey 2 increases
• If you dont know the interactions, it looks like
  Predator 2 might prey on Prey 2

74
The importance of indirect effects

• Commonly assumed that
• direct effects are strong
• indirect effects are weak
• Indirect effects may be stronger, more important
  determinants of species composition and diversity
• Data? (Wootton 1994)

75
Intertidal invertebrates (again)
76
Interactions in intertidal

• Observation Exclude bird predation (cages)
• Nucella decreases relative to control (2 - 4 X)
• Pollicipes increases relative to control (5 X)
• Semibalanus decreases relative to control (3 -
  7 X)
• Mytilus decreases relative to control (to 70)
• Excluding predator
• 2 prey species decrease
• 1 non-prey species decreases
• 1 prey species increases

77
Understanding this effect

• A hypothesis to explain this result
• Which direct interactions are strong?
• affect numbers of individuals
• Which direct interactions are weak?
• do not affect numbers of individuals

78
Hypothesis 1 strong weak interactions
79
Hypothesis 2 strong weak interactions

Sea star Leptasterias
-
Birds (crows, gulls)
-



-

Predatory snail Nucella

-
-


-
Goose Barn. Pollicipes
-
-
-
-
-
-
Mussel Mytilus
Acorn Barnacle Semibalanus
-
-
80
Hypothesis 3 strong weak interactions
81
Hypotheses ? new predictions

• Remove Pollicipes with birds excluded
• H 1 Mytilus, Semibalanus, Nucella all increase
• H 2 Mytilus, Semibalanus increase
• H 3 Mytilus only increases
• vs. birds excluded only

82
Hypotheses ? new predictions

• Exclude birds after removing Pollicipes
• H 1 no effects
• H 2 Nucella decreases, Leptasterias increases
• H 3 Semibalanus, Nucella decrease,
  Leptasterias increase
• vs. removing Pollicipes only

83
Experiment 1Manipulate Pollicipes without birds
84
Experiment 2.Manipulate birds without Pollicipes