Trophic Cascading
Trophic Interactions
Control of Primary Production
z Only
about one-half of the variation in
primary production among lakes
worldwide can be explained by nutrient
(N, P) supply.
z Nutrient
Effect
control is known as Bottom-Up
Properties of Food Webs
z
Few food webs seldom have more than 3 or 4
levels. Webs are not too complex.
z
Connectance (C) usually declines with species
richness (S). This is consistent with theoretical
models.
Cascading Trophic Interactions
z 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
z Piscivore
- fish that consumes other
fish, e.g. bass, pike, salmon
z Zooplanktivore
- fish that consumes
zooplankton
z Herbivore
- zooplankton that consume
phytoplankton
z Phytoplankton - primary producers
Bottom-up Control &
Top-Down Control
z
Bottom-up control - structure depends
upon factors, such as nutrient
concentration and prey availability, from
lower trophic levels.
z Top-down
control - structure
(abundance, biomass, diversity) of lower
trophic levels depends upon the effect of
consumers from higher trophic levels.
Top-Down Control
z Rise
in piscivore biomass initiates
“cascade”
z Planktivore
z Large
biomass declines
herbivore biomass increases
z Phytoplankton
biomass declines
Predator Influence on Food
Webs
z Decline
in piscivore biomass can have
opposite effect
z Predator
effect
control is known as a top-down
Interactions
Trophic Interactions
Trophic
Trophic Interactions
Interactions
When It Doesn’t Work That Way
z
Food webs typically are more complex than
simple four-level systems with one
representative in each level
z
Time lags in response may occur after change
in piscivore biomass or reproduction
z
Fish can change from zooplanktivory to
piscivory with age, thus reversing the cascade
Case Studies
z Removal
of zooplanktivorous fish from
lakes, usually by poisoning
z Zooplankton increase
z Phytoplankton and chlorophyll a decline
z Secchi disk transparency increases
Case Studies
z Additions
of piscivores
z Wisconsin lakes (experimental purposes)
z Lake Michigan salmon (sport fishery)
Research Results
z Analysis
of 54 studies provided support
for the trophic cascade hypothesis.
z 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
z
Adding small fish, such as minnows, to the top
of the small food webs in the studies caused:
z
z
z
75% decrease in zooplankton biomass
80 percent increase in algae biomass
Adding nutrients to the bottom of the food
webs resulted in:
z
z
180% increase in algae
24% increase in zooplankton.
Research Results
zThe
bottom-up processes
had a greater impact on
algae growth than the topdown processes.