Trophic Cascading

Trophic Interactions

Trophic Interactions

Trophic Interactions

Control of Primary Production

Only about one-half of the variation in primary production among lakes worldwide can be explained by nutrient (N, P) supply.

Nutrient control is known as Bottom-Up Effect

Properties of Food Webs

Few food webs seldom have more than 3 or 4 levels. Webs are not too complex.

Connectance (C) usually declines with species richness (S). This is consistent with theoretical models.

Ominvores are relatively scarce. Typically food chains have one omnivore per top-down predator.

Omnivores feed on species in adjacent trophic levels.

Properties of Food Webs

The ratio of prey to predators is relatively constant. For freshwater invertebrate communities we find that there are 0.36 predators per prey.

The ratio of basal, intermediate and top predators is relatively constant (0.19:0.52:0.29).

Cascading Trophic Interactions

Principles of fishery management can be used to help explain differences in primary productivity among lakes with similar nutrient supplies but different food webs

Four Trophic Level System

Piscivore - fish that consumes other fish, e.g. bass, pike, salmonZooplanktivore - fish that consumes zooplanktonHerbivore - zooplankton that consume phytoplanktonPhytoplankton - primary producers

Bottom-up Control & Top-Down Control

Bottom-up control - structure depends upon factors, such as nutrient concentration and prey availability, from lower trophic levels.

Top-down control - structure (abundance, biomass, diversity) of lower trophic levels depends upon the effect of consumers from higher trophic levels.

Top-Down Control

Rise in piscivore biomass initiates “cascade”

Planktivore biomass declines

Large herbivore biomass increases

Phytoplankton biomass declines

Predator Influence on Food Webs

Decline in piscivore biomass can have opposite effect

Predator control is known as a top-down effect

Interactions

Trophic Interactions

Trophic

Trophic Interactions

Interactions

When It Doesn’t Work That Way

Food webs typically are more complex than simple four-level systems with one representative in each level

Time lags in response may occur after change in piscivore biomass or reproduction

Fish can change from zooplanktivory to piscivory with age, thus reversing the cascade

Case Studies

Removal of zooplanktivorous fish from lakes, usually by poisoningZooplankton increasePhytoplankton and chlorophyll a declineSecchi disk transparency increases

Case Studies

Additions of piscivoresWisconsin lakes (experimental purposes)Lake Michigan salmon (sport fishery)

Research Results

Analysis of 54 studies provided support for the trophic cascade hypothesis.

Data reported in eight papers from 11 experiments testing the impact of adding fish versus nutrients to food webs and comparing them with controls.

Research Results

Adding small fish, such as minnows, to the top of the small food webs in the studies caused:

75% decrease in zooplankton biomass80 percent increase in algae biomass

Adding nutrients to the bottom of the food webs resulted in:

180% increase in algae24% increase in zooplankton.

Research Results

The bottom- up processes had a greater impact on algae growth than the top-down processes.

Trophic Interactions

Management Implications

Stocking of piscivores or harvest of zooplanktivorous fish may be useful for rehabilitating eutrophic lakes

Represents a blend of fisheries biology, limnology, and water quality management

In some cases, may substitute for engineering solutions or chemical control of algae

Within its native range it has been shown to be an important prey item for freshwater fishes. However, when introduced into what was considered to be an "empty" niche, its impact on the aquatic community was significant. Dramatic changes and species extinctions of native zooplankton communities have been attributed to this opportunistic lifestyle.

Mysis relicta is an opportunistic feeder with both predatorial and filter feeding habits. When zooplankton are abundant they serve as the primary food source; when scarce M. relicta will feed on suspended organic detritus or from the surface of benthic organic deposits (Pennak 1989).

Declines in the number and size of game fish have been documented since the introduction of M. relicta

This is the Native foodchain before the Mysis Shrimp were introduced into the Bull Trout habitat.

At this point, the Lake Trout have been introduced and are eating the smaller Bull Trout and Cutthroats.

The Kokanee Salmon are introduced. They are eating the Plankton. The Bull Trout and the Lake Trout eat the Kokanee Salmon.

The Mysis Shrimp are then introduced. They eat the Plankton, same as the Kokanee Salmon, and the Bull Trout eat the Salmon. By bringing in the shrimp, the food source is being depleted for the Salmon, thus the food source (i.e. Salmon) is being depleted for the Bull Trout.

In this final example, the Mysis Shrimp have eaten all the Salmon's food (Plankton). The Kokanee Salmon die off because they have no food.

The Lake Trout are eating the shrimp, and thriving because of it. The Bull Trout's numbers are also decreasing because of a lack of food (no salmon and decreasing numbers of Cutthroats).

Chaoborus

Chaoborus