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 Benthic algae Aquatic macrophytes &detritus sedimentation Phtoplankton Offshore food chain 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 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 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.

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?

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.

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 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

McPeek’s 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 don’t 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 predator’s gape.. As a result, most pumpkinseeds older than 1 or 2 years are rarely preyed upon by pike or bass.