Transcript Food webs & top-down effects

Relationships between fish predators and prey
Bottom up
•Richer systems have higher productivity at all trophic levels
•Enrichment usually increases the biomass of the top trophic level in the web
and their prey’s prey.
Top down
•Predators usually reduce the biomass of their prey
•And cause changes in the structure of prey communities
•Lake Michigan example
Bottom-up effect: Reductions in fish biomass usually accompany reductions in nutrient loading
Original Lake Michigan Food web
Lake trout Trophic position 4-4.5
“Once upon a time”
Benthos& zooplankton
sedimentation
Phtoplankton
Offshore food chain
Benthic algae
Aquatic
macrophytes
&detritus
Inshore food chain
Changes in the Lake Michigan Food web during the 60’s
Top-down cascade
Lake trout Trophic position 4-4.5
Lamprey wipes out lake trout
Alewife invades and outcompetes other zooplanktivores; becomes very abundant
Mysis very abundant
Benthos& zooplankton
Large zooplankton decimated
sedimentation
Phtoplankton
Algal blooms
Transparency drops
Offshore food chain
Benthic algae
Aquatic
macrophytes
&detritus
Inshore food chain
Reduction of littoral zone
Test of the top-down cascade theory: introduce pacific salmon
Biomanipulation experiment
Alewife declines
Benthos& zooplankton
sedimentation
Large zooplankton recover
Phtoplankton
Benthic algae
Aquatic
macrophytes
Algal blooms stop
Transparency increases&detritus
Offshore food chain
Inshore food chain
Littoral zone expands
Zebra mussel invading a compartmentalized food web:
a combination of top-down & bottom-up effects
Prior to the
zebra mussel
invasion, the
rich nutrient
regime
allowed the
phytoplankton
to shade out
the littoral
zone
vegetation
A
H1
A2
H3
H2
As water
clears
light
reaches the
bottom and
plants
& benthic
algae
grow
F1
F2
P1
P2
Top-down effects.
Predators selectively remove vulnerable prey, and make it
possible for species and varieties that have better defense
mechanisms to win out over faster growing competitors that
lack defenses.
Prey defense mechanisms
•Reduced detectability
Smaller size, transparency, less turbulence
•Defensive behaviour
Vertical migration and night time activity, and avoidance
responses
•Unpalatability
Spines, toxicity
•Altered life-cycle
Diapause and speeding up life-history
Small size can be an effective defense
Effects on size structure of
prey communities
Hrbacek
Brooks and Dodson
•Generally in lakes where
zooplanktivorous fish are
the top trophic level there is
a reduced zooplankton
biomass and a shift in
community compositon
toward smaller species and
species with more effective
defenses
•Similar effects have been
noted in benthic
invertebrate communities.
Why do large herbivorous
zooplankton dominate
communities when there are no
zooplanktivores?
The size efficiency
hypothesis
Which Daphnia can deplete its
food supply the most and still
survive on it?
Why are larger Daphnia more
efficient than smaller Daphnia at
filtering even tiny algae?
Reduced visibility/ less pigmentation also works
In fishless lakes zooplankton are strongly pigmented,
mostly with carotenoid pigments that they obtain from
algae
In lakes with zooplanktivorous fish, zooplankton are
usually nearly transparent and thus very hard for fish to
see
Why do you think that pigmented zooplankton species and
varieties win out over transparent ones in fishless lakes?
Defensive behaviour
In fishless lakes many invertebrates swim about freely in
the water column of both lakes and streams during the
daytime
Where fish are present, they usually confine such
behaviour to the night hours and hide in the bottom during
the day.
Defensive behaviour: vertical migration
The effect of zooplanktivorous fish onvertical migration of herbivorous zooplankton
Defensive behaviouir: escape responses
McPeek’s studies on the escape response
of damselflies
Damselflies in fishless lakes are preyed
on heavily by dragonflies
The species that live in lakes with fish
usually respond to a nearby fish by
remaining motionless
The species that live in lakes without fish
respond to dragonflies and other
invertebrate predators by rapidly moving
a short distance.
Morphological Defenses
Spines and other extensions of the body
are a good defense against
zooplanktivorous fish
Daphnia with and without helments
Successful species invasions often involve unpalatable species
Fish predators generally avoid zooplankton with large spines
Sticklebacks are small fish that are extremely well defended against
piscivorous fish—large dorsal spines, pelvic spines, and armoured plates
Sticklebacks in fishless lakes have much smaller spines and much fewer
Armoured plates
Sunfish have both spines and deep body shape that can exceed most predator’s
gape..
As a result, most pumpkinseeds older than 1 or 2 years are rarely preyed upon by
pike or bass.
Top-down effects.
Predators selectively remove vulnerable prey, and make it
possible for species and varieties that have better defense
mechanisms to win out over faster growing competitors that
lack defenses.
Prey defense mechanisms
•Reduced detectability
Smaller size, transparency, less turbulence
•Defensive behaviour
Vertical migration and night time activity, and avoidance
responses
•Unpalatability
Spines, toxicity
•Altered life-cycle
Diapause and speeding up life-history