Transcript Herbivory and predation - Nevada Agricultural Experiment

Allelopathy and herbivory
Additional readings:
– Hawkes CV, Sullivan JJ . 2001. The impact
of herbivory on plants in different resource
conditions: A meta-analysis. Ecology
82:2045-2058.
Seminars
Outline
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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
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Aspects of herbivory in range and forest
Herbivore functional responses
Sublethal effects and compensation (example)
Allelopathy
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Could be considered “interference
competition” for plants...
Definition:
Often mentioned, but hard to prove.
Why?
Allelopathy
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Inferred by:
1. Spatial pattern/arrangement of plant
community
2. Presence of chemicals in leaves, roots and/or
soil
3. Demonstration that chemicals cause decline
in growth or survival of surrounding
vegetation
Allelopathy
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Difficult to prove because:
1. Low [chemicals] in the field
2. Mediated by 3rd party (e.g. microbes and
litter)
3. Trophic interactions similar to apparent
competition: shrub harbours seed predator –
causes “allelopathic” spatial arrangement.
Allelopathy
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Recent work on Centauria maculata
(spotted knapweed) provides good
evidence for allelochemicals.
Knapweed is an important rangeland
weed; what are implications of
allelopathy?
Amensalism
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Negative to one species, inconsequential to
other.
Often is actually very asymmetric
competition/pathogenesis
Example: allelopathy
“Plus-Minus” relationships
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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/dt=V[b-aP]=f1(V,P)
dP/dt=V[kaV-d]=f2(V,P)
Where: V=#prey, P=#predators, b=prey growth rate,
a=prey consumption rate by predator, k=rate of
increase of predator per unit prey, and
d=predator death rate
Functional response
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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…
Herbivory
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What is the importance of herbivory?
– Small amount of biomass removed: 10%
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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.
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Top down control (predators)
Bottom up control (plant forage quality)
Herbivory
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But – trophic cascade models too
simplistic; herbivory has more dramatic
effects than they predict...
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Defensive compounds (coevolution)
Community composition
Productivity
Seedling survival and demography
Seed predation
Herbivory
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What are some of the effects of
herbivores on plants and plant
communities?
Effects and issues
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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
Forests
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Mortality of seedlings
Insects
Outbreaks (insects especially)
Sublethal effects and herbivory escape
Defensive compounds?
Rangelands
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Productivity
Compensation/overcompensation
Herbivore functional responses
Toxicity and herbivore resistance
Selectivity/preference
Diversity and coexistence
Herbivore selectivity
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Plant stress VS plant vigour hypotheses:
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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
Compensation
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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?
Sublethal effects of herbivory
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Herbivores don’t often cause mortality of adult
plants.
Can affect plants in other ways:
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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 138:231-241.
Grazing experiment
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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)
Grazing experiment
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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.
“Life history” transition diagram
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Transitions between 5 size (height) classes for
trees; final stage is “escape from herbivory”.
Models created for each habitat and treatment
combination.
P
P
P
P
P
S
1
P
11
P
P
S
S
[<25 cm]
S
1
P
21
P
0
P
S
[25 - 49 cm]
0
S
[50 - 74 cm]
0
S
52
0
0
[ >100 cm]
0
Matrix Two
5
S
0
[<25 cm]
S
0
S
0
[25 - 49 cm]
0
S
0
0
0
P
0
0
P
P
P
P
S
[ >100 cm]
5
P
P
15
P
P
53
P
44
P
35
52
43
34
P
25
P
P
P
51
42
33
24
P
P
P
5
41
32
23
14
P
P
P
S
4
31
22
13
[75 - 100 cm]
P
P
P
S
3
21
12
[50 - 74 cm]
S
2
11
3
S
S
1
4
0
25
55
53
2
0
P
P
54
42
1
0
P
P
44
43
P
S
4
24
5
P
23
[75 - 100 cm]
5
31
3
0
P
4
S
33
22
3
S
45
41
0
P
2
S
P
21
1
P
S
4
18
P
51
S
2
34
3
P
P
Matrix One
P
S
23
22
15
24
13
2
P
P
P
S
12
25
P
P
14
35
54
P
45
55
Proportion reaching escape height (100 cm)
Grassland
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Woodland
Callitris glaucophylla
Eucalyptus albens
-r -k -s
+r -k -s
+r +k -s
+r +k +s
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
years since planting
Results
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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
herbivory…tradeoff.