Relationships between fish predators and prey Bottom up
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Transcript of Relationships between fish predators and prey Bottom up
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 preys prey. Top down Predators usually reduce
the biomass of their prey And cause changes in the structure of
prey communities Lake Michigan example Bottom-upeffect: 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 Benthic algae Aquatic macrophytes &detritus
sedimentation Phtoplankton Offshore food chain Inshore food chain
Changes in the Lake Michigan Food web during the 60s
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 Benthic algae
Aquatic macrophytes &detritus sedimentation Algal blooms
Transparency drops Phtoplankton Offshore food chain Inshore food
chain Reduction of littoral zone Test of the top-down cascade
theory: introduce pacific salmon
Alewife declines Benthos& zooplankton Benthic algae Aquatic
macrophytes &detritus Large zooplankton recover sedimentation
Algal blooms stop Transparency increases Phtoplankton Offshore food
chain Inshore food chain Littoral zone expands Light is a key
physical factordetermines 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 withlittoral vegetation
(macrophytes, epiphytic, and benthic algae) and enrichment by
nutrients usually leads to a reduction in the extent of the
littoral zone community. 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 A2 As water clears light reaches the bottom and plants &
benthic algae grow H3 H2 H1 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. The size efficiency hypothesis
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? Defensivebehaviour 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. In completely fishless streams thereis 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 Epinette Creek Invertebrates that commonly occur in the
drift
Some common mayfly larvae (Ephemeroptera) Net-spinning caddis
larvae
(Trichoptera) The effect of zooplanktivorous fish onvertical
migration of herbivorous zooplankton McPeeks studies on damselflies
in littoral
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. Spines and other
extensions of the body are a good defense against zooplanktivorous
fish
Daphnia with and without helments Unpalatibility: predators dont
like spines 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 predators gape..
As a result, most pumpkinseeds older than 1 or 2 years are rarely
preyed upon by pike or bass.