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Lecture 12

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					Lecture 11: Grazers & Mutualisms

            EEES 3050
The Green World Hypothesis

   The world is green
       because herbivores are held in check by their
        predators, parasites, and diseases such that
        they cannot consume all the plant biomass.
       Briefly discussed on pg. 551 of text.
       Proposed by Hairston, Smith and Slobodkin in
        1960.




                                                        2
The Green World Hypothesis

   Not everyone was convinced that predators
    can limit herbivore numbers.
   Why else could cause the world to be
    “green”?
       Plant defenses
       Nutrients limit herbivores, not energy.
       Abiotic factors limit herbivores
       Spatial and temporal heterogeneity reduce the
        availability of plants.

                                                        3
The Green World Hypothesis

   Not everyone was convinced that predators
    can limit herbivore numbers.
   Why else could cause the world to be
    “green”?
       Plant defenses
       Nutrients limit herbivores, not energy.
       Abiotic factors limit herbivores
       Spatial and temporal heterogeneity reduce the
        availability of plants.

                                                        4
Plant defenses

   What types of defenses do plants have?
       Structural adaptations, i.e. thorns.
       Secondary plant substances
           Acetogenin, juglone
               remember the apple tree/grass relationship.
           Phenylpropanes
               E.g. cinnamon and cloves
           Terpenoids
               Peppermint oil
           Alkaloids
               Nicotine, morphine, caffeine.


                                                              5
6
Secondary defense example

   Oak trees
       Produce tannins
       Toughness




                            7
8
Secondary defense example

   Oak trees
       Produce tannins
       Toughness
   Herbivores have compensated by concentrating feeding
    in the early spring on young leaves or altering their life
    cycle.
       Some moths overwinter as larvae.




                                                          9
The Green World Hypothesis

   Not everyone was convinced that predators
    can limit herbivore numbers.
   Why else could cause the world to be
    “green”?
       Plant defenses
       Nutrients limit herbivores, not energy.
       Abiotic factors limit herbivores
       Spatial and temporal heterogeneity reduce the
        availability of plants.

                                                    10
Herbivores on the Serengeti Plains

   Several herbivores
       Zebra, wildebeest,
        Thompson’s gazelle




                                     11
Temporal differences in foraging.




                                    12
Temporal differences in foraging.




                                    13
Temporal differences in foraging.




                                    14
Differential eating of plant parts.




                                      15
Grazing facilitation: Does the grazing of one species
increase the availability of food for following species?
   Observation: Gazelles concentrated their
    feeding on areas that wildebeest ate.
   Hypothesis: Gazelles will increase in # if
    wildebeest increase in #.
   Test/Experiment: Examine time periods when
    wildebeest #’s go up.




                                                           16
Results




  No evidence of facilitation   17
Herbivores on the Serengeti Plains

   We have looked at only 3 of the herbivores
    on the Serengeti Plains
       38 species of grasshoppers
           In some communities, grasshoppers consumed almost
            half as much grass as the ungulates.
       36 species of rodents




                                                                18
Can grazing benefit plants?
   Observations: Overgrazing is clearly detrimental,
    but some have observed mutualistic relationship
    between grasses and grazers.
   Hypothesis: Biomass of grass will increase with
    moderate levels of grazing.




                                                        19
   Experiment: Measure biomass in grazed vs.
    ungrazed areas.



                                                20
Results




          21
The Green World Hypothesis

   Not everyone was convinced that predators
    can limit herbivore numbers.
   Why else could cause the world to be
    “green”?
       Plant defenses
       Nutrients limit herbivores, not energy.
       Abiotic factors limit herbivores
       Spatial and temporal heterogeneity reduce the
        availability of plants.

                                                        22
Herbivores can be selective (snowshoe hares)
   Observation: Some
    herbivores select
    certain plants over
    others.
       How does this relate to
        the “Green World”?
   Krebs suggests:
    “selectivity is a major
    reason why the world is
    not completely green
    for a herbivore”
       Is this a valid statement?

                                               23
Herbivores can be selective (snowshoe hares)


   I would suggest…
       A herbivore being selective is only relevant if food
        is a limiting factor.
       With the snowshoe hares, they still eat the other
        plants, suggesting that if Dwarf birch disappears
        the hares can still survive on spruce and willow.
       What could be limiting the population of snowshoe
        hares?
           Lynx, harsh winters, …



                                                           24
Testing the ‘green world’ hypothesis.

   Vegetation dynamics of predator-free land-
    bridge islands
       By Terborgh et al. Journal of Ecology 2006.


   Green world hypothesis =
       Predators keep herbivores in check allowing
        plants to grow.
   How to test this?


                                                      25
Testing the ‘green world’ hypothesis. –
Terborgh et al.
   Studied 14 sites in an area where a new reservoir
    was constructed in the 1986’s.
       Small islands – no predators, herbivores = leaf-cutter ants,
        howler monkeys, iguanas
       Medium islands – no vertebrate predators, but armadillos
        are present and are a predator of leaf-cutter ants.
       Large islands – had vertebrate predators, including snakes,
        raptors, and ocelot.
   Hypothesis: Islands without predators will have a
    decline in plant production:


                                                                   26
Fig. 2 Per-capita mortality plus
growth out of a sapling class (left
bars) vs. per-capita recruitment
into the class (right bars) of small
(top) and large (bottom) saplings
at small, medium and large
landmasses at Lago Guri,
Venezuela. Values shown are the
means of the proportions dying
and recruiting at each site. Black
portions of bars = mortality;
white = growth out of the size
class into the next larger;
grey = recruitment into the size
class from the next smaller. Post
hoc analyses were performed on
t-tests from model contrasts and
Bonferroni corrected for multiple
comparisons.



                                   27
28
   Further comments:
       Strong herbivore effects have been found many
        times when herbivores were introduced to a
        previously herbivore free location.
   What did this study account for that others
    may not have?
       All species were native.
       On herbivore free islands there is a tendency for
        plants to lose their secondary chemical defenses.

                                                            29
Interspecific Interactions
   Herbivory: one species benefits at the other’s
    expense
       Interactive herbivore systems
   Mutualisms: both species benefit
   Commensalism: one benefits, one unaffected
       Noninteractive herbivore systems
Spruce budworm irruptions (outbreaks)

   Spruce budworms have
    a 35-40 year cycle.

   During an irruption they
    kill many trees in a
    stand.




                                        31
Emerald ash borer: an invasive species




                                     32
33
How did EAB arrive in North America?

                     • Arrived in solid
                       wood packing
                       material from Asia
                       10 -12 years ago.
                     • First detected in
                       Detroit/Windsor
                       area in July 2002.


                                            34
35
Impacts




          36
37
38
Are there any ash trees in Asia?

   Mongolian ash trees have different secondary
    chemicals.
   Ash borers in North America could have
    escaped a disease.




                                               39
Evaluating the economic costs and benefits of
slowing the spread of the emerald ash borer in
Ohio and Michigan

   Objectives:
       1) to provide estimates of the regional economic
        impact emerald ash borer will potentially inflict upon
        the ash forestry in Ohio and Michigan;



       2) to provide policy-makers with quantitative
        guidance for cost-effective alternative strategies to
        control, prevent, or slow the spread of emerald ash
        borer.


                                                                 40
Objective 1: estimate regional economic
impact emerald ash borer.
   Estimate the current distribution of ash trees and
    emerald ash borer

   Predict the spread of emerald ash borer

   Estimate value of ash in spatially explicit manner

   Determine the regional economic consequences of
    emerald ash borer spread through the development
    of a CGE model
                                                         41
Objective 1: to provide estimates of the regional
economic impact of an invasive species.

   Estimate the potential habitat - A
                                         A
   Predict the spread - B

   Estimate economic impact in a
    spatially explicit manner - C        B       C
   Determine the regional economic
    consequences of spread through
    the economy - D                          D


                                                     42
Estimate the current distribution of ash
trees and emerald ash borer




                     From Dr. Louis Iverson and Anatha
                                                         43
                     Prasad - USFS
Predict the spread of emerald ash borer

   Natural Dispersal

   Human-mediated dispersal




                                          44
  Spread Model                         Insect Ride
                                                                    Roads

                                                                 Wood products
                                          Model
                                          (GIS)              Population Density

                                                                 Campgrounds
                                                Ash
                     Ash Abundance
                      (Basal Area)
                                             Abundance
                                              Multiplier   Weighted            Gravity
                                                           Random               Model
                                                           Seeding


 Maturity            Insect Flight
                                                       Outlier
of Infection            Model
   (EAB                                               Generator
Abundance)           (EAB SHIFT)



                                                       Random
                   Spatially Explicit                  Seeding
               Probability of Colonization

                                                                                    45
Predict the spread of emerald ash borer:
Natural Dispersal
                                                        270-m cells
   SHIFT model for EAB
    spread
    (spatially explicit cell-based model)         EAB occupied zone
   Calculates the
    probability of
    colonization of
    currently unoccupied cells
    based on abundance of
    EAB, habitat
    availability of ash,
    and distance between
    all cells.
                                                  Unoccupied zone



                                            From L. Iverson           46
 Predict the spread of emerald ash borer:
 Human-mediated dispersal
        K
U ij   Ai OiW j Dij
                         d

       i 1




 Campers with Firewood




                                            47
Model Needs
                                       K
                                U ij   Ai OiW j Dij
                                                        d

                                       i 1


   Number of campers
   Attractiveness of campgrounds
   Distance
   Distance coefficient
       Data for parameterization and validation.
   Primary Question –
       How many campers from areas with emerald ash
        borer are traveling to Ohio and Michigan
        campgrounds?

                                                             48
Number of campers
traveling to each
campground from
EAB region.




                    49
Why does long distance dispersal matter?




                      From. Spatial Ecology 1997   50
Estimate value of ash in spatially explicit
manner
   Forest products:
       The USDA Forestry Service maintains Forest
        Inventory Analysis (FIA) data to maintain
        inventories of forest resources in rural areas of the
        United States.
       Dunn & Bradstreet financial data for wood
        products industries.
   Impact on Communities
       Tree City USA program – to estimate urban
        values.
                                                            51
The Potential Economic Impacts of Emerald Ash Borer
(Agrilus planipennis) on Ohio, U.S., Communities
T. Davis Sydnor, Matthew Bumgardner, and Andrew Todd
Arboriculture & Urban Forestry 2007. 33(1):48–54.




   “The worst case scenario… the complete loss of Ohio’s
   urban ash, is a staggering $7.5 billion loss for a single
   pest in a single state.”

                                                               52
Conclusions:

   We are just getting started…

   Next steps:
       Developing spatially explicit economic models.
       Integrate natural dispersal model with human-
        mediated spread models.
       Determining if prevention methods are effective
           Includes education, removal of outlier populations, etc.



                                                                       53
Bottom Line
Attempting to develop a framework to
 balance the urgency of action and the
 complacency of neglect.

Or…

Does it matter if we do anything given
 the indications that the emerald ash
 borer isn’t stopping.
                                         54
Commensulism: Noninteractive herbivore
systems
   Foraging on seeds by finches does not affect
    the density or fitness of plants.
       What about ultimate vs. proximate issues.




                                                    55
Commensalism: other examples




                               56
Mutualism

   Definition: Both species benefit
   Examples:
       Seed dispersal by birds
       Ants and acacia




                                       57
Ants and acacia

   Benefits to ants?
       Food


   Benefits to acacia?
       If ants are removed,
        acacia are quickly out
        competed by herbivores.




                                  58
Mutualism: other examples
   Termites and intestinal
    flagellates: Although termites
    can physically chew and ingest
    wood, they are incapable of
    chemically digesting cellulose
    into sugars. They rely on
    intestinal flagellates, e.g.
    Pyrrsonympha spp. and
    Trichonympha spp. which are
    capable of producing
    cellulose. These genera of
    flagellates reside in the hindgut
    of termites and provide
    nutrition for them. They are
    not found anywhere else in
    nature.

                                        59

				
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