Download Herbivory review

Herbivory review
Defense theory has emphasized probability of being eaten and cost
of herbivory.
Investment in defense invokes a trade-off between growth and
mortality. Herbivores can therefore influence species distributions
across resource gradients.
Most herbivore damage occurs on young leaves. Plants development
can limit exposure of young leaves to herbivores or limit resource
loss.
Plants may also engage a third trophic level as a defense against
herbivores.
Growth and defense characters of tropical trees (from Coley 1983 and subsequent work)
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 Rapid leaf expansion
Develop whole leaves (or
branches in a few days)
Brownea claviceps Herbivory and the third trophic level
“Inviting friends to feast on foe”
Many ways that plant harness the third trophic level to defend
themselves:
- Acacia, Cecropia, Macaranga ant mutualisms
Sunshine Van Bael (2003) Birds defend trees from herbivores in tropical forest canopies
Enclosed branches
in mesh to exclude
birds/mammals but
not insects. Then
measured herbivory rates Open bars are for
exclosure treatment
Hatched bars are for
bird-accessible
treatment
Three tree species
Cecropia spp +
Anacardium
Why no effect in the
understory?
Quantitative defenses slow down insect feeding and/or
digestion rates
‘Quantitative’ defenses (tannins, fiber and toughness) do not
present an absolute barrier against herbivores. Hypothesis: Defensive effectiveness depends on making
herbivores more susceptible to predation
Slowing herbivore growth rates lengthens the time that larvae are
exposed to predators and parasitoids (‘slow-growth-highmortality’ SG-HM hypothesis)
Slowing grazing rates is important because most damage occurs
in the last instars of insect development
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 or have
a lower probability
of being found instar
Trophic cascades
Alosa (planktivorous fish)  selects small zooplankton
?? phytoplankton growth??
Alosa selects for
smaller-sized
zooplankton species
Do predation effects cascade down through a
food chain?
Two viewpoints:
Errington’s hypothesis (1946)
Predators have little impact on total prey abundance
because…______________________
Many species are highly territorial (eg blackbirds,
oystercatchers), removing birds does not affect
growth rate.
Support for Errington’s hypothesis:
Allan (1982): Studied effect of trout removal on stream invertebrate
abundance
- Reduced trout abundance to 10 % pre-experiment level by electroshock fishing a 1.2 km stretch of Colorado stream
- Stomach contents analysis of fish showed that they fed intensively
on a few taxa of invertebrates but that populations of these taxa showed no significant change in abundance in response to trout
removal
Why no effect in this study?
Alternative…
Hairston, Smith and Slobodkin (1960)
HSS Hypothesis or “Why the world is green”
Why don’t herbivores become so abundant that they consume
all the plants? (world is brown)
- Divide terrestrial communities into 4 compartments: primary
producers, herbivores, carnivores, and detritivores
(decomposers)
Trophic levels
3
Carnivores
2
Herbivores
1
Producers
?
Detritivores
No predators, must be food limited
(competition)
Lots of food?? Must be predation
Limited?
Resource limited (light, water
nutrients). (competition)
Food limited? Detritus doesn’t
accumulate?
Why does HSS represent a trophic cascade?
Because the third trophic level (carnivores) exerts a significant
influence on the abundance of organisms in the first level
(producers) by limiting consumption of producers by the second
level
Also known as ‘top-down’ regulation
Arguments against HSS
Availability of plants to herbivores may be illusory if most plants
are unsuitable for food (too well defended).
Effects of carnivores may not cascade directly (Polis and Strong
1996)
2 trophic level systems There are no carnivores to regulate
herbivore populations, therefore competition should regulate
herbivores and producer populations could be driven down to very low levels
Example: Gypsy moth (Lymantria dispar) - introduced from Europe to USA during the Civil war to start a silk industry. Moth lacks natural enemies in introduced range. http://www.fs.fed.us/ne/morgantown/4557/gmoth/spread/
Outbreaks of moths can completely defoliate their hosts
(oak and aspen trees). Sustained
defoliation over 4 yr will kill
the host.
4 trophic level system: occurs when a second carnivore level feeds
on the third trophic level. Limited to productive habitats where there is sufficient energy flow to support secondary carnivores.
Bottom-up versus top-down!
Bottom-up!
Top-down!
What would happen if you add another level?!
Hairston and Hairston (1993)
Some of the differences between terrestrial communities
(green) and aquatic communities (blue) might be because
freshwater ecosystems tend to have four trophic levels. Terrestrial food chain (mostly):
Producers -- herbivores -- carnivores
Aquatic food chain (mostly):
Producers -- grazing zooplankton -- planktivorous fish (e.g.,
Alosa) -- piscivores
Carpenter et al. (1985) Cascading trophic interactions and
lake ecosystems
Observation:
Lakes that have very similar supply rates of limiting nutrients (P)
have up to a thousand fold difference in productivity…
Carpenter provides a framework linking demography of piscivores
(eg pike and salmon) in lakes to the abundance of plankton and
primary productivity:
How might the effects of high abundance of a top-predator fish
cascade through a community?
Lake systems often have four trophic levels
Phytoplankton - Zooplankton - planktivores - piscivores
(little fish)
(big fish)
Top down? Lakes with similar levels of limiting nutrients
have 1000-fold variation in productivity Lake
Manipulation
Phytoplankton response
Lillestockelidsvatten
Planktivores removed
Primary production reduced by
92 %
Michigan
Stocked with piscivores reduced
Planktivores to 20 % initial
Summer chlorophyll reduced to
20 % of initial level
Tuesday
Piscivores stocked to eliminate
Planktivorous fish
Chlorophyll reduced to 20 % of
Initial level
Round
70 % of planktivores removed
Chlorophyll reduced by 50 %
St George
50 % removal of planktivores
No change detected
Lake systems often have four trophic levels
Phytoplankton - Zooplankton - planktivores - piscivores
(little fish)
(big fish)
Top down? Lakes with similar levels of limiting nutrients
have 1000-fold variation in productivity Lake
Manipulation
Phytoplankton response
Lillestockelidsvatten
Planktivores removed
Primary production reduced by
92 %
Michigan
Stocked with piscivores reduced
Planktivores to 20 % initial
Summer chlorophyll reduced to
20 % of initial level
Tuesday
Piscivores stocked to eliminate
Planktivorous fish
Chlorophyll reduced to 20 % of
Initial level
Round
70 % of planktivores removed
Chlorophyll reduced by 50 %
St George
50 % removal of planktivores
No change detected
Solid line : response to
increased recruitment of
large fish. Note: young
piscivorous fish start off as
‘vertebrate planktivores’ when
they are small…
Dashed line: response to a reduction in piscivorous
fish biomass - eg exploitation
or high over-winter
mortality
Wootton and Power (1993) Opposite approach to Carpenter to
manipulate bottom-up indirect effects:
Manipulated amount of light available to algae by differentially
shading small portions of a natural stream
Varying light availability either increased or decreased the primary
productivity and biomass of algae
Predictions:
- If the consumption rate of predators is a simple function of
prey density (αV) then community structure should be
determined by predators at the top of the chain and by
producers at the bottom of a 3 level chain.
- In a 4 trophic level chain, increasing productivity should
increase the biomass of secondary carnivores, but not primary
carnivores, allowing biomass of herbivores to increase
Results:
Productivity did respond to light (but lots of variance)
In 3 trophic level chains predator and algal biomass did respond
to light; grazer biomass was not affected:
3 versus 4 trophic levels showed expected effects...
The case of the cownose ray
Myers et al. (2007) Cascading effects of loss of predatory shark
Upsurge in shark fishing in last 20 yrs (bycatch, meat and finning)
has cleaned the oceans of large >2 m shark
Loss of top predator has weakened top down control of shark prey:
These are elasmobranchs (rays, skates, small sharks). Elasmobranchs
are large - even as juveniles. Their only predators are large sharks
Meta-analysis of time series data - surveys of shark and
elasmobranch populations
Shark populations have plummeted in 30 years…
Populations have declined by 87 % (sand bar shark) to >98 %
(the rest)…
Sharks are now functionally extinct
Concurrently, mesopredators (prey of great sharks) have
increased:
Most elasmobranch predtors have increased in population size by
and order of magnitude over the last 30 years Cownose ray: Range SE Florida to New Jersey
Population size estimate: 40 million!!
(rays and sharks notorious for low fecundity and
late maturity, therefore not a population ‘spike’)
What are all these rays eating?
Bay scallops
Oysters
Soft-shell clams
If rays consume 200 g of shell fish per day, and spend 100 days/yr
in Chesapeake, then the population will consume 840,000 tonnes/yr
Commercial scallop catch Virginia and Maryland in 2003: 300 tonnes
Ray exclosure
experiments
show that rays
cause 100 %
scallop
extinction
Commercial
fishery is now
closed