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
(No Transcript)
24
(No Transcript)
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.