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, salmon Zooplanktivore - fish that consumes zooplankton Herbivore - zooplankton that consume phytoplankton Phytoplankton - 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 poisoning Zooplankton increase Phytoplankton and chlorophyll a decline Secchi disk transparency increases
Case Studies Additions of piscivores Wisconsin 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 biomass 80 percent increase in algae biomass Adding nutrients to the bottom of the food webs resulted in: 180% increase in algae 24% increase in zooplankton.
Research Results The bottom- up processes had a greater impact on algae growth than the topdown 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