Herbivory

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Herbivory
Top-down effects in communities Assumption that
top predators regulate lower trophic levels of
consumer organisms assumes that food is not
limiting (e.g lemmings and stoats) Is the
abundance of food an illusion?? Assess here -
costs of herbivory - plant defense theory -
trade-offs of defense with other life-history
traits and significance for species coexistence
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Cost of herbivory
Obvious costs when complete defoliation of plants
precludes reproduction or results in death (e.g.
Gypsy moth defoliation of oak trees) Less
conspicuous herbivores may have significant costs
that are difficult to assess without
experimentation (e.g. grazing of ovules or
undispersed seeds affecting reproductive output,
or partial defoliation resulting in decreased
carbon budget) Marquis (1984) Looked at the
effect of simulated leaf herbivory by a weevil
Ambetes on an understorey tropical shrub Piper
arieianum in Costa Rica
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Piper (Piperaceae) large genus of tropical and
sub-tropical shrubs (1400 spp) both pioneers and
understorey species Includes black pepper
Looked at herbivory to Piper arieianum in forest
understorey at La Selva in Costa Rica. Herbivory
rates were very variable among plants 1-6 lost
over 2-3 months, but leaves live up to 2.5 years
therefore total losses over the leaf life time
can be substantial - One time measure of missing
leaf area on entire plants ranged between 3 and
50
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- Cloning of plant material and transplanting to
understory indicated that variation in herbivory
rate has a genetic component - Experimentally
removed leaf area with a hole-punch to mimic the
pattern of natural damage - some leaves lots of
damage others remove little tissue - Treatments
of 0, 10, 30 and 50 of the plants total leaf
area removed, plus 100 removal of leaves
(mimicking leaf-cutter ant damage - Tracked
growth and reproduction over following 2
years Results Small and medium sized plants
showed a 50 reduction in growth with gt 30
defoliation measured over the two years Seed
production dropped in half for both first and
second years after defoliation
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• Large effects of damage on growth and
  reproductive output in Piper coupled with
  genotypic variation in susceptibility to damage
  suggests that defensive characters of Piper are
  under continuous selection
• Coley (1986) found similar effects in Cecropia
  peltata
• Measured growth and herbivory rates of seedlings
  grown from seeds of several parent trees
• Measured tannin levels in foliage as major
  chemical defense
• -Wide variation in tannin levels among plants
• Found that plants with high tannin levels had low
  herbivory rates, but also had lower growth rates.

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Plant defense theory - Under what conditions to
plants evolve different kinds of defenses? - What
are the predictors for the level of defense
exhibited? Biochemical coevolution theory
Ehrlich and Raven (1964) - Plant species
evolve secondary compounds in response to attacks
by insects, while insects evolve new
detoxification systems to over-come them -
Adaptation to a set of host plant chemicals
results in losing the ability to consume other
hosts - Chemical arms races eventually results in
plant families acquiring a complex of defenses
that exclude all but a fauna of related taxa of
specialist herbivores - Can explain patterns of
specialist herbivores (e.g. Berenbaum 1983), but
does not address wider issues
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Why do most vertebrates (and many insects) have
wide host preferences? Why do plants differ so
much in vulnerability to herbivores? Plant
apparency theory (Feeny 1976) Plants that are
easily found by herbivores (apparent plants)
should invest heavily in quantitative defenses
that are effective against all herbivores. Plants
that are difficult to locate (unapparent
plants) should invest smaller amounts in
qualitative defenses that are effective against
all but specialist herbivores Apparent plants
Trees and shrubs, and grasses from late
successional communities with long generation
times Unapparent plants Short-lived herbaceous
plants of early successional environments
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Ecological correlates of plant defenses according
to apparency theory (from Howe and Westley 1988)
Apparency theory arose out of Feenys studies on
Oaks (apparent) and mustard plants (unapparent)
in central New York Mustard very low
concentrations of a variety of glucosinolates,
toxic at extremely low doses to all but
specialist feeders
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Oaks defensive chemicals are primarily tannins.
Stunt larval growth and reduces fecundity of
insects when they reach maturity. Oaks only
suffer major outbreaks during early spring
bud-breaks before tannin concentrations in
expanding leaves reach toxic concentrations Limit
s to apparency theory Futuyma (1976) reviewed
literature on defenses - some correspondence but
some apparent plants had qualitative defenses as
well as quantitative defenses and some herbs had
high phenol concentrations. Apparency is
difficult to measure. Need more explicit
hypotheses linking plant traits to constituents
of defense
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Resource availability theory
Coley et al (1985) Proposed that plant defensive
capabilities are mediated by their capacity to
replace lost parts with resources at their
disposal. While apparency theory stresses
herbivore foraging efficiency Resource
availability stresses economics of growth
inherent growth rate, and nutrient availability
as determinants of the amounts and kinds of
defenses that plants use. Fast-growing plants in
well-lit environments with fertile soils can
easily replace leaves or other tissues lost to
herbivores (cost of herbivory is low).
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Resource availability theory predicts that these
plants should invest relatively little in
defense, and should use mobile resources that
can be moved out of quickly senescing tissue -
Why invest costly immobile defenses in tissues
that will be discarded after a few months
anyway? Slow growing plants, characteristic of
low resource environments (eg deserts, forest
understory) should invest more in defense because
tissue is costly to replace. Costly replacement
means tissues should be built to last and can
use more immobile defenses (lignin and tannins)
that are permanently employed in leaves and stems
and less expensive in the long run - Plant
structures in low resource environments can be
extremely long-lived (e.g 14 year old leaves in
tropical forest understory)
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Plants varying in intrinsic growth rate and
habitat preference should differ in the optimal
level of defense investment to maximise realized
growth rates Vertical arrows indicate optimal
defense investment to maximize growth rate
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Can argue the opposite allocation to defense is
part of the resource budget of the plant plants
that allocate a large proportion of resources to
defense have little left to invest in leaf
production and therefore have low intrinsic
growth rates Can therefore think of allocation
to defense as imposing a trade-off on plants that
limits the range of microsites in which
recruitment can occur Growth-defense (or
growth-mortality trade-off) High investment in
defense low growth rate and low mortality rate.
Plants grow in shade Low investment in defense
high growth rate and high mortality rate (in
shade). Plants constrained to sunny sites
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Kitajima (1994) highlighted this trade-off and
shifted a paradigm which stressed physiological
traits as determining shade-tolerance to one in
which allocation patterns are emphasized
Plants that grow fastest in high light (24 full
sun) also grow fastest in shade (2 full sun)
Individual points on graph represent species
(n13) varying in shade tolerance
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Growth rate in sun or shade is positively
correlated with mortality rate in the shade In
Kitajimas growing house experiment mortality was
attributable to fungal pathogens
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a)
Growth - mortality for pioneer species in small
gaps (10 Full sun)
Proportional seedling mortality
Dalling Hubbell 2002

b)
Mortality is attributable to browsing damage and
insect herbivores
Proportion of seedlings with apical damage
Maximum daily relative height growth mm/mm/day x
1000
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Growth-mortality trade-off driven by
herbivores/pathogens has important implications
for understanding species distribution
patterns Among site variation in the cost
of herbivory Among site variation in
intensity of herbivory Understanding species
invasions
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Resource availability theory arose out of
community wide studies of herbivory. Coley (1983)
measured herbivory rates and characterized plant
defenses of 46 tree species in lowland forest,
Panama - Multivariate analyses to determine what
traits correlated with damage leaf
toughnessgtfiber contentgtnutritive value -
Pioneer species have least tough leaves, lowest
phenolics and fiber concentration - Mature
leaves of pioneer trees were grazed six times
more rapidly than leaves of shade-tolerant
trees - In 70 of species, young leaves suffered
higher damage than mature leaves - young leaves
have not toughened but have 2-3 times phenolics
of mature leaves - Several common adaptations to
minimize damage to young leaves (rapid expansion,
synchronous leaf flush, delayed greening)
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Delayed greening
Young leaves are white or pink and do no net
photosynthesis Only observe delayed greening in
tropical forest understories, but is a common
trait across evolutionary lineages
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Growth and defense characters of tropical trees
(from Coley 1983 and subsequent work)
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Rapid leaf expansion Develop whole leaves (or
branches in a few days) Brownea claviceps
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Herbivory and the third trophic level Inviting
friends to feast on foe - Many ways that plant
harness the third trophic level to defend
themselves - fast growing trees are commonly
ant plants because abundant light allows them to
make sugar and lipid awards relatively
cheaply - mites are also common, but little
studied (Walter and ODowd 1992). Mites live in
domatia and feed on fungal spores and so might
be important in protecting plants against
pathogens?? In N. Queensland 15 of trees have
domatia (ODowd and Wilson 1989)
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Quantitative defenses slow down insect feeding
and/or digestion rates Quantitative defenses
(tannins, fiber and toughness) are clearly
effective anti-herbivore defenses. Yet they do
not present an absolute barrier against
herbivores. Their effectiveness may be due to
mediation by the third trophic level -Slowing
grazing rates is important because most damage
occurs in the last instars of insect
development -Slowing rates also lengthens the
time that larvae are exposed to predators and
parasitoids (slow-growth-high-mortality SG-HM
hypothesis)
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Evidence for SG-HM Benrey and Denno (1997) -
Several studies using free-living larvae show
higher incidence of mortality from parasitoids
for slow vs fast developing larvae. - Not
supported in cases where larvae are protected
(building shelters out of plant material or
inside galls)
Fast developing larvae are better able to defend
themselves against parasitoids
instar
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Some plants may also send out a distress signal
(see lots of neat work by Karban et al at UC
Davis on jasmonate signalling) Thaler (1999)
looked at the effect of Jasmonate a volatile
chemical that induces chemical defence in
plants. Compared parasitism of caterpillars in
induced vs non-induced plants