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Allelopathy and herbivory

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Title: Allelopathy and herbivory


1
Allelopathy and herbivory
  • Additional readings
  • Hawkes CV, Sullivan JJ . 2001. The impact of
    herbivory on plants in different resource
    conditions A meta-analysis. Ecology
    822045-2058.

2
Seminars
3
Outline
  • Amensalism
  • What is allelopathy and how is it inferred?
  • plus-minus interactions
  • Herbivory and importance in range and forest
  • Trophic relationships top down VS bottom up
  • Effects of herbivory
  • Aspects of herbivory in range and forest
  • Herbivore functional responses
  • Sublethal effects and compensation (example)

4
Allelopathy
  • Could be considered interference competition
    for plants...
  • Definition
  • Often mentioned, but hard to prove. Why?

5
Allelopathy
  • Inferred by
  • Spatial pattern/arrangement of plant community
  • Presence of chemicals in leaves, roots and/or
    soil
  • Demonstration that chemicals cause decline in
    growth or survival of surrounding vegetation

6
Allelopathy
  • Difficult to prove because
  • Low chemicals in the field
  • Mediated by 3rd party (e.g. microbes and litter)
  • Trophic interactions similar to apparent
    competition shrub harbours seed predator
    causes allelopathic spatial arrangement.

7
Allelopathy
  • Recent work on Centauria maculata (spotted
    knapweed) provides good evidence for
    allelochemicals.
  • Knapweed is an important rangeland weed what are
    implications of allelopathy?

8
Amensalism
  • Negative to one species, inconsequential to
    other.
  • Often is actually very asymmetric
    competition/pathogenesis
  • Example allelopathy

9
Plus-Minus relationships
  • Include disease, parasitism, predation and
    herbivory.
  • Effects on population (or biomass) can be
    modelled using Lotka-Volterra equations to
    predict population of prey and predators.
  • dV/dtVb-aPf1(V,P)
  • dP/dtVkaV-df2(V,P)
  • Where Vprey, Ppredators, bprey growth rate,
    aprey consumption rate by predator, krate of
    increase of predator per unit prey, and
    dpredator death rate

10
Functional response
  • Relationship of predator (herbivore) consumption
    of prey (plants) to density of prey (plants)
  • 3 types unsaturating (I), saturating(II), and
    sigmoidal (III)
  • Type of functional response has implications for
    community structure and stability
  • Discuss further in lab

11
Herbivory
  • What is the importance of herbivory?
  • Small amount of biomass removed 10
  • Tundra/alpine 3
  • Forest 4
  • Grassland 10-15
  • Rangeland/grazing systems 30-60
  • world is green hypothesis (who?) there is more
    plant biomass than herbivores can eat.
  • Why? Trophic interactions.
  • Top down control (predators)
  • Bottom up control (plant forage quality)

12
Herbivory
  • But trophic cascade models too simplistic
    herbivory has more dramatic effects than they
    predict...
  • Defensive compounds (coevolution)
  • Community composition
  • Productivity
  • Seedling survival and demography
  • Seed predation

13
Herbivory
  • What are some of the effects of herbivores on
    plants and plant communities?

14
Effects and issues
  • Mortality of seedlings
  • Insects VS vertebrates
  • Herbivore outbreaks (insects especially)
  • Sublethal effects and herbivory escape
  • Compensatory growth and overcompensation
  • Productivity
  • Herbivore functional responses
  • Resistance/defence

15
Forests
  • Mortality of seedlings
  • Insects
  • Outbreaks (insects especially)
  • Sublethal effects and herbivory escape
  • Defensive compounds?

16
Rangelands
  • Productivity
  • Compensation/overcompensation
  • Herbivore functional responses
  • Toxicity and herbivore resistance
  • Selectivity/preference
  • Diversity and coexistence

17
Herbivore selectivity
  • Plant stress VS plant vigour hypotheses
  • Herbivores attack already stressed plants
  • Herbivores favour plants in high resource areas
    and with larger plant modules
  • Evidence for both
  • Also influenced by herbivore defenses
  • Can alter species composition and lead to
    coexistence IF favoured species is best
    competitor
  • Differs among herbivore species, therefore
    management of different species can change
    community composition

18
Compensation
  • Response of plants to defoliation can vary
    widely positive, negative or neutral
  • Compensation means plant growth increases after
    herbivory to compensate for lost tissue
  • Overcompensation much discussed this means
    plants are stimulated to grow MORE after
    grazing/browsing.
  • Is overcompensation possible?
  • Is compensation over extended periods possible?

19
Sublethal effects of herbivory
  • Herbivores dont often cause mortality of adult
    plants.
  • Can affect plants in other ways
  • Reduced seed set/fruit abortion
  • Reduced size/growth rate
  • Change in architecture
  • Delay or prevention of maturity
  • These may all affect plant fitness (contribution
    to next generation)
  • Example population consequences of herbivory on
    three Australian native plants. Allcock and Hik
    2004. Oecologia 138231-241.

20
Grazing experiment
  • Three groups of grazing animals domestic stock
    (sheep and cattle), native macropods (kangaroos
    and wallabies), and introduced rabbits.
  • Four treatments control, stock fence, kangaroo
    fence, rabbit fence.
  • Two habitats woodland (intact Eucalyptus canopy)
    and grassland (cleared native pasture)
  • Three target species kangaroo grass (Themeda
    australis), cypress pine (Callitris glaucophylla)
    and white box (Eucalyptus albens)

21
Grazing experiment
  • Plants placed in experimental plots in April 1998
  • Monitored until April 2001.
  • Generalized linear modelling used to analyze
    factors affecting survival (habitat, grazing
    animals, competition)
  • Survival and growth data used to parameterize
    stage-based population models for trees.

22
Life history transition diagram
  • Transitions between 5 size (height) classes for
    trees final stage is escape from herbivory.
  • Models created for each habitat and treatment
    combination.

23
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24
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25
Results
  • Different herbivore species affected different
    plant species (rabbits/kangaroos cypress stock
    Eucalyptus)
  • Plants in productive environment better able to
    compensate (more rapid growth)
  • High herbivory rate in unfenced low productivity
    habitats prevented escape.
  • This could have population consequences even
    though mortality was fairly low.
  • Interaction between competition and
    herbivorytradeoff.
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