Trophic Cascading
Trophic Interactions
Trophic Interactions
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.
z
Ominvores are relatively scarce. Typically food
chains have one omnivore per top-down
predator.
z
Omnivores feed on species in adjacent trophic
levels.
Properties of Food Webs
z The
ratio of prey to predators is relatively
constant. For freshwater invertebrate
communities we find that there are 0.36
predators per prey.
z The
ratio of basal, intermediate and top
predators is relatively constant
(0.19:0.52:0.29).
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.
Trophic Interactions
Management Implications
z
Stocking of piscivores or harvest of
zooplanktivorous fish may be useful for
rehabilitating eutrophic lakes
z
Represents a blend of fisheries biology,
limnology, and water quality management
z
In some cases, may substitute for engineering
solutions or chemical control of algae
z
z
z
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.
z Mysis
relicta is an opportunistic feeder
with both predatorial and filter feeding
habits.
z 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).
z Declines
in the number and size of game
fish have been documented since the
introduction of M. relicta
z This
is the Native foodchain before the
Mysis Shrimp were introduced into the
Bull Trout habitat.
z At
this point, the Lake Trout have been
introduced and are eating the smaller
Bull Trout and Cutthroats.
z The
Kokanee Salmon are introduced.
They are eating the Plankton. The Bull
Trout and the Lake Trout eat the
Kokanee Salmon.
z The
Mysis Shrimp are then introduced.
They eat the Plankton, same as the
Kokanee Salmon, and the Bull Trout eat
the Salmon.
z 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.
z In
this final example, the Mysis Shrimp
have eaten all the Salmon's food
(Plankton).
z The Kokanee Salmon die off because
they have no food.
z The
Lake Trout are eating the shrimp,
and thriving because of it.
z The Bull Trout's numbers are also
decreasing because of a lack of food (no
salmon and decreasing numbers of
Cutthroats).
Chaoborus
Chaoborus