3rd Exam

1
3rd Exam

• Monday 5 April
• Material Last exam -- Friday 3 April
• Multiple choice and essay
• Study guide later this week
• check course web page

2
Mutual altruism or mutual exploitation?

• Mutualism has costs and benefits to both parties
• Costs and benefits depend on the environment
• In many cases there is a fine line between
  mutualism and parasitism
• cost gt benefit parasitism
• cost lt benefit mutualism
• Example AM fungi and plants

3
VAM fungi and plant root

• Benefit Plant gets Phosphorus (P)
• Roots deplete P in soil

• Fungal hyphae extend plants ability to get P

• Cost Plant must give photosynthates to fungus

4
Soil conditions

• Low P soil
• Plants without mycorrhizal fungi cannot get
  enough P
• Mycorrhizal plants grow faster than
  non-mycorrhizal plants
• Benefit gt Cost
• Mutualism

• High P soil
• Plants obtain ample P even without mycorrhizal
  fungi
• Mycorrhizal plants grow more slowly than
  non-mycorrhizal plants
• Benefit lt Cost
• Parasitism

5
Above-ground conditions

• High light
• High photosynthesis
• High carbohydrate availability
• Plant easily afford the cost of AM fungi
• Mutualism

• Low light (shade)
• Low photosynthesis
• Low carbohydrate availability
• Plant less able to pay the cost of AM fungi
• Parasitism

True?
6
General point

• Mutualism depends on the balance of costs and
  benefits
• For plants, that balance may vary in space and in
  time
• Mutualism ? Parasitism
• For fungus, plant always provides net benefits
  (benefit gt cost)

7
Models of mutualism

• Lotka-Volterra type models of mutualism
• Will not be covered
• (applause)

8
Community Ecology

• Ch. 21
• Community all species living in one place and
  potentially interacting with one another
• Interactions Exploitation, Competition,
  Mutualism
• Communities have properties beyond those of the
  species in the community
• Species number
• Relative abundances (proportions)

9
Species number relative abundances

• Components of species diversity
• Consider 2 communities
• Community 1 Community 2
• n proportion n
  proportion
• sp. 1 50 0.25 197
  0.985
• sp. 2 50 0.25 1
  0.005
• sp. 3 50 0.25 1
  0.005
• sp. 4 50 0.25 1
  0.005

10
Species number relative abundances

• Consider 2 more communities
• Community 1 Community 3
• n proportion n
  proportion
• sp. 1 50 0.25 176
  0.88
• sp. 2 50 0.25 12
  0.06
• sp. 3 50 0.25 6
  0.03
• sp. 4 50 0.25 4
  0.02
• sp. 5 2
  0.01

11
Definitions

• Species number (S)
• Evenness (E)
• Quantification of relative abundance patterns
• Maximal when all species are equally abundant
• Species diversity indices combine E and S
• (pp. 416-418)
• High evenness, High species number High
  species diversity
• Species diversity is hard to quantify and to
  compare

12
Dominance-diversity plotspp. 414-415
13
Dominance-diversity plotspp. 414-415
Figure 21.19
14
Interactions affect S E

• Interspecific competition
• competitive exclusion ? reduced S E
• Predation
• Elimination of prey ? reduced S
• Reduction of some prey ? reduced E
• Mutualism
• greater population growth ? increased S E

15
End 8th Lecture
16
Species interactions combinedpp.407-413

• Effects of interspecific competition and
  predation together
• Competition leads to exclusion
• Predation reduces prey density and reduces impact
  of competition
• Net result can be that presence of a predator can
  increase S and E

17
Keystone predator effect

• Keystone predator A predator whose removal from
  a community results in reduced species diversity
  (usually S ) in that community
• Keystone predator effect requires both
  interspecific competition and predation

18
Keystone predation in the Rocky intertidal zone

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

19
Keystone predation in the Rocky intertidal zone

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

20
Pacific Northwest Intertidal

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

21
The Keystone effect
Predator (Pisaster)
Competitor 2 (other species)
Competitor 1 (Mytilus)
22
Pisaster is a keystone predator

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

23
Keystone predator effect

• Selective predation on competitive dominant
• Intense, nonselective predation that reduces prey
  density reduces interspecific competition
• Similar effect of frequent disturbance

24
Intermediate Disturbance / Predation

• Low disturbance (frequency, intensity)
• Competitive dominant reduces or 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 eliminate species
• reduces or eliminates competition among prey
• maximal diversity, maximal S

25
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

26
Temporary pond amphibians

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

27
Temporary pond amphibians

• 6 spp. of common anurans
• Spadefoot toad (Scaphiopus holbrooki)
• Spring peeper (Hyla crucifer)
• 4 other species
• All filter feeders scrapers

28
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

29
Effect of newt predation

• 0 newts
• Scaphiopus dominates, Hyla rare
• 2 newts
• Scaphiopus still 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

30
End 9th Lecture
31
Beyond the keystone predator effect

• There can be effects of interactions beyond the
  pair of species involved
• Indirect effect An effect of one species on
  another that occurs via an effect on a third
  species

32
Indirect effect
Increase predator ? Decrease
Herbivore ? Increase Plant
TROPHIC CASCADE effects produced 2 or more
trophic levels down from top predator
33
Indirect effect
Decrease prey 1 ? Decrease Predator
? Increase Prey 2
APPARENT COMPETITION negative effects caused via
a shared enemy
34
Indirect effect
Predator 2
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
35
Indirect effects

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

36
Detecting indirect effects

• Know the pairwise direct interactions within the
  community
• Do experiments species removals and additions
• If you dont know the pairwise interactions,
  indirect effects may be misinterpreted even in an
  experiment

37
Intertidal invertebrates (again)
38
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

39
Understanding this effect

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

40
Hypothesis for strong weak effects
41
Hypothesis ? new predictions

• 1. Exclude birds after removing Pollicipes ...
• Predict no difference compared to no exclusion
  of birds after removing Pollicipes.
• 2. Remove Pollicipes with birds excluded ...
• Predict Mytilus, Semibalanus, Nucella all
  increase compared to birds excluded only.
• Predictions can be tested in experiments

42
Experiment 1.Manipulate birds without Pollicipes
Birds (crows, gulls)
-

-
Predatory snail Nucella


REMOVE Pollicipes
-
-
Mussel Mytilus
Acorn Barnacle Semibalanus
-
-
43
Results of tests of hypothesisexperiment 1

• Exclude birds (cages) after removing Pollicipes
• Mytilus unaffected
• Semibalanus unaffected
• Nucella unaffected
• compared to no exclusion of birds after removing
  Pollicipes
• As predicted by the hypothesis

44
Experiment 2Manipulate Pollicipes without birds
Birds EXCLUDED
-
Predatory snail Nucella
-


-
Goose Barnacle Pollicipes
-
-
-
-
Mussel Mytilus
Acorn Barnacle Semibalanus
-
-
45
Results of tests of hypothesis experiment 2

• Remove Pollicipes in cages that exclude birds
• Mytilus increases (2 X)
• Semibalanus increases (7 X)
• Nucella increases (3.6 x)
• compared to cages with Pollicipes
• As predicted by the hypothesis

46
End 10th Lecture
47
Hypothesis supported(supplementary material)

• No measurable direct effects of birds on Mytilus,
  Nucella, Semibalanus
• Indirect effects of birds are more important than
  direct effects of birds, except for the effect on
  Pollicipes

48
3rd Exam
49
Supporting data

• Hyla crucifer
• moves little, forages less
• poorest competitor
• least vulnerable to predation

• Scaphiopus
• most active, forages most
• best competitor
• most vulnerable to predation

• General tradeoff -- high vs. low activity
• High activity
• effective foraging, good competitor, high risk of
  predation
• Low activity
• ineffective foraging, poor competitor, low risk
  of predation

50
Temporary pond amphibians

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

51
A surprisingIndirect effect
Predator 2
Predator 1


-
-
Prey
RESOURCE COMPETITION negative effects caused via
a shared victim
52
Misinterpreting an indirect effect

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

-
53
The importance of indirect effects

• Commonly assumed that
• direct effects are strong
• indirect effects are weak
• Data?