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 Phtoplankton Benthic algae Aquatic macrophytes &detritus Benthos& zooplankton Lake trout Trophic position Offshore food chainInshore food chain sedimentation “Once upon a time”

Changes in the Lake Michigan Food web during the 60’s Top-down cascade Phtoplankton Benthic algae Aquatic macrophytes &detritus Benthos& zooplankton Lake trout Trophic position Offshore food chainInshore food chain sedimentation Lamprey wipes out lake trout Alewife invades and outcompetes other zooplanktivores; becomes very abundant Large zooplankton decimated Algal blooms Transparency drops Mysis very abundant Reduction of littoral zone

Test of the top-down cascade theory: introduce pacific salmon Phtoplankton Benthic algae Aquatic macrophytes &detritus Benthos& zooplankton Offshore food chainInshore food chain sedimentation Alewife declines Large zooplankton recover Algal blooms stop Transparency increases Littoral zone expands Biomanipulation experiment

A F1F1 H1H1 H3H3 A2A2 H2H2 F2F2 P1P1 P2P2 Zebra mussel invading a compartmentalized food web: a combination of top-down & bottom-up effects As water clears light reaches the bottom and plants & benthic algae grow Prior to the zebra mussel invasion, the rich nutrient regime allowed the phytoplankton to shade out the littoral zone vegetation

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

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. Small size can be an effective defense

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? Why do large herbivorous zooplankton dominate communities when there are no zooplanktivores?

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

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. McPeek’s studies on the escape response of damselflies Defensive behaviouir: escape responses

Spines and other extensions of the body are a good defense against zooplanktivorous fish Daphnia with and without helments Morphological Defenses

Fish predators generally avoid zooplankton with large spines Successful species invasions often involve unpalatable species

Sticklebacks in fishless lakes have much smaller spines and much fewer Armoured plates Sticklebacks are small fish that are extremely well defended against piscivorous fish—large dorsal spines, pelvic spines, and 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