Download Last lecture Foodweb interactions

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.
•(example: Fertilization to enhance sockeye salmon)
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
Top-down effects of zooplanktivorous fish on zooplankton communities
Hrbacek (1964)
Brooks and Dodson (1966)
•Generally in lakes where
zooplanktivorous fish are
the top trophic level there is
•reduced zooplankton
biomass, &
•shift in community
compositon toward smaller
species and species with
more effective defenses
•Similar effects have been
noted in benthic
invertebrate communities.
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
Phylum Chordata—animals with a notochord, gill clefts and a dorsal nerve cord at some
stage of their life, most species with sexual reproduction (dioecious)
SubPhylum Vertebrata—animals with a vertebral column
Class Agnatha—jawless vertebrates
Family Petromyzontidae—lampreys
•Naked (no scales) eel-like body, no bones
•Jawless mouth with a circular sucking disc with rasping teeth
•Dorsal and caudlal fins but no paired fins,
•7 gill openings, no gill covers, single median nasal opening between eyes.
Adult lampreys 0.3-0.8 m long
Life cycle—many species anadromous and parasitic—spawn in freshwater, develop into a blind
and toothless ammocoetes larva that burrows into mud in streams and lakes--detritivore.
Larva 5-30 cm
Metamorphosis into a an adult with 1-7 yrs, followed by migration to sea. Adult of most species parastic on
marine salmonids—ruptures skin and sucks blood and fluids. Some freshwater species that are not parasitic
spawn soon after metamorphosis and die. All adults die after spawning.
Lake trout with parasitic marine lampreys—Petromyzon marinus
http://www.aquaticcommunity.com/news/Lamprey-fish.jpg
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
Light is a key physical factor—determines the boundaries within which
photosynthesis (primary production) can take place
Rooted plants cannot grow at depths beyond the light limit.
In offshore regions where the bottom is below the photic zone suspended
phytoplankton are the main photosynthetic organisms
Photic zone
Light limit
Phytoplankton compete for light with littoral vegetation (macrophytes, epiphytic,
and benthic algae) and enrichment by nutrients usually leads to a reduction in
the extent of the littoral zone community.
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
Example
Brooks and Dodson study
•Shifts toward smaller size
in predation impacted
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
Defensive behaviour: night-time drifting in streams
Effect of brook trout on the drift response of benthic invertebrates
response = drift density (#/m3) / abundance (#/m2)
no f ish
0.030
f ish (0.5/m2)
Drift response
0.025
0.020
0.015
0.010
0.005
09-10
13-14
16-17
18-19
20-21
21-22
23-24
Time of day
•In completely fishless streams there is usually no difference between day and night
drift of invertebrates, but where drift feeding fish are present there is usually a sharp
increase in drift at night.
•The differences seen here (fish/no fish) are a result of consumption depleting the #/m3
of drifting inverts.
Drift net in a small Creek
Invertebrates that commonly occur in the drift
Some common mayfly
larvae (Ephemeroptera)
Net-spinning caddis larvae
(Trichoptera)