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Terrestrial Trophic Cascades
In terrestrial systems trophic cascades tend to
be less well documented than in aquatic systems.
Why? Spiller and Schoener (1990) looked at the
impact of a top predator (Anolis lizards) on
small islands. How are producers affected by a
trophic cascade in this system?
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9 x 80 m2 enclosure plots on Staniel Cay (central
Bahamas) 3 treatments - enclosure with lizards,
enclosure without lizards, unenclosed plots with
lizards Measured herbivory on the sea
grape (Coccoloba uvifera) -dominant species
in the enclosures
Classified damage as scars (aphids and leaf
hoppers) holes (beetles, lepidopteran larvae),
or galls (midge)
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Anolis lizard diet determined by gape size and
prey body size
Beetlesholes Aphidsscars Midgesgalls
Lizards
Spiders
leafhoppers
midges
Beetles Lepidopt.
aphids
Mangroves
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Hole damage relatively low overall (1.7 times
higher in lizard removal treatment than control -
but not statistically significant) Scar damage
most abundant type of herbivory - highest in
lizard removal exclosures (2.2 times higher than
control) Gall damage also low, but 1.7 times
higher damage in control than lizard removal
treatment was statistically significant (3 times
as many spider webs found in lizard-free
enclosures than in controls) This is consistent
with observations of herbivory on mangroves on
small islands with and without lizards - damage
is 1.5 times higher on lizard-free islands
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Food web on Bahamian islands is a combination of
model A and B
Model A is equivalent to a 3-level trophic
cascade Model B is equivalent to Carpenters
4-level trophic chain Key factor in organizing
the food web may be differences in body size
between predators and prey
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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
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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?
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McLaren and Peterson (1994) Study on 500 km2
Isle Royale National Park in Lake
Superior Producer Balsam fir Herbivore Moose
(59 of winter diet of Moose is fir) Carnivore
Wolf (colonized island in 1959) Measure browsing
effects of moose by examining growth rings on fir
saplings Over 30 years fir trees from across
the island show cyclic intervals of ring growth
suppression that accompany elevated moose
densities. Periods of growth suppression show
no correlation with climatic data
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Wolf population fluctuation 1960 - 1990
Moose population fluctuation
Fir tree mean ring width (mm) western Isle Royale
Fir tree mean ring width (mm) eastern Isle Royale
Actual evapotranspiration
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Adding behavioural ecology to trophic
cascades Wolf behavioural response to climate
(Post et al. 1999) How would the change in
behaviour of a top predator cascade through a
community? Isle Royale Fluctuations in North
Atlantic Oscillation (NAO) results in changes in
winter snow accumulation (NAO oscillation in
atmospheric pressure between the Azores and
Iceland affecting jet-stream/storm tracks and
therefore snow fall) Annual aerial surveys of
wolves show close correlation between wolf pack
size and the state of the NAO (and therefore snow
depth)
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Lower NAO index greater snow depth correlates
with larger wolf packs
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Kill rate per pack (and per individual wolf)
increases with wolf pack size
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Climatic effects on hunting behaviour cascade,
influencing Moose population size...
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and finally fir growth...
2 effects of NAO on firs i) drop in
moose density reduces browsing intensity year
after heavy snow ii) additional effect in current
NAO year due to protection from browsing by snow
cover
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Climate has multiple effects - on sociality of
wolves - on vulnerability of moose to predation
(deep snow impedes moose locomotion and
susceptibility to predation) - on accessibility
of forage to moose Trophic cascade results from
higher predation rate by wolves Changes in wolf
behaviour have ecosystem level effects in this
community because moose influence primary
production, litter production and edaphic
nutrient dynamics (Pastor et al. 1993)
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Trophic cascade without prey consumption
(Beckerman et al. 1997) Carnivores can affect
the impact of herbivores on producers in 2
ways 1) Direct consumption of herbivores 2)
Behavioural mediation the mere presence of
predators could alter prey foraging behaviour by
reducing herbivore feeding time, or by inducing a
shift in herbivore diet selection Prey
behavioural responses to predators are well
established in aquatic systems (changes in
foraging locations in planktivorous fishes).
Fewer studies in terrestrial systems
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Study in old field community in NE
Connecticut Constructed 3 trophic levels
Grasses and herbs generalist leaf chewing
grasshopper (Melanoplus femurrubrum) and nursery
web hunting spider (Pisaurina mira) Problem
Need to create a predator that does not have the
capacity to consume prey, but, can display
hunting behaviour or signal risk to its
prey Solution Glue the spiders mouth parts
together! - No effect on spider hunting
behaviour. - Spiders can survive up to 2 months
this way Experiment Test whether indirect
effect of predators on prey arises from density
or behavioural responses in the herbivore
population
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Made mesh enclosures for 3 trophic levels -
plants only - plants juvenile or adult
grasshoppers - plants grasshoppers glued or
unglued spiders Experiment run from Aug to Oct
over 2 years Grasshopper densities with/without
glued spider treatment were not significantly
different (no predation) Significant effect on
grass biomass consistent with trophic cascade.
- Treatments containing spiders significantly
less herbivore damage than treatments with just
grasshoppers. - No significant difference
between glued and unglued spiders on grass
herbivory and biomass
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Are abundances and distributions of organisms
controlled by resources (bottom-up forces) or by
predation (top-down forces)? Bottom-up view
organisms at each trophic level are food
limited Top-down view top level is food limited,
lower levels are alternately predator then food
limited Hunter and Price (1992) bottom-down
view the removal of higher trophic levels
leaves lower levels intact (though modified) the
removal of producers leaves no system at
all Reconciling the two views Fretwell
proposed that productivity determines the number
of trophic levels that can be supported in a
community - plants therefore ultimately constrain
the top-down forces that in turn regulate their
standing crops
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More refined question What factors determine
when and where predators or resources dominate in
regulating populations? (Power 1992) -
Propagation of top-down effects through the chain
can be strongly influenced by consumer efficiency
- shape of the predator numerical response and
functional response Predators may not regulate
their food resources efficiently if they -fight
with each other (interference competition) -are
limited by or compete for resources other than
food, -cause prey to hide -induce defenses in
their prey or, if significant time lags exist
between prey consumption and predator population
response
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Additional feedback pathways across trophic
levels (unrelated to consumer efficiency 1.
Non-linear herbivore effects on primary
productivity - nutrient recycling from grazers
can increase productivity at intermediate levels
of consumption for terrestrial plant and
phytoplankton communities - Highly non-linear
relationships between grazing pressure and
productivity could decouple top-down effects 2.
Plants provide cover for predators and prey.
Changes in plant biomass could alter the
consumption rates of herbivores by predators
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Interference competition
(-)
Predators
(-)
(/-)
Induced defense, Retreat to refuges
Consumers
(/-)
Stimulation of Primary productivity
Cover from (for) predators
Plants
()
Mechanisms modulating top-down and bottom-up
forces in food chains (modified from Power 1992)
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Conclusions Plants have primacy in food webs
because productivity is a fundamental control of
higher trophic levels. Other plant attributes
such as architecture and defenses may also have
strong effects Many difficulties exist in
applying food web theories to real world data -
appropriate spatio-temporal scales - operational
definitions e.g. trophic levels and
connectedness of food webs - often cant study
the whole web (are you choosing the right
bits?) - integrating dynamic feedback mechanisms
into models