Transcript Trophic level

Food webs and trophic cascades in
lakes
How to represent trophic relationships?
(Paine 1980)
1.
Connectedness
Based on observations
2. Energy flow web
Based on literature and
measurements
3. Functional web
based on controlled
manipulations
1. Connectedness food web
• Depicts observed feeding
links between organisms.
• Indicates ‘structure’ of
system.
Food web of the North Atlantic
• Simple food chain
Another food web of
the North Atlantic
• Complex food web
2. Energy flow food web
• Trophic level – number of energy transfers between
primary producers and consumer (trout).
• Assumes discrete food chain, simple trophic
structure
Energy flow
• Trophic level concept
useful for predicting
contaminant levels
• Bioaccumulation: the
accumulation of
substances that are
not excreted and are
subsequently passed
from prey to predator
3
4
5
Estimated
# of trophic levels
Energy flow: more complex pathways
Use diet information to describe energy flow pathways
Trophic position – trophic height of species. Considers diet and energy flow
pathways. Species can fall between trophic levels.
Omnivory – feeding on more than one trophic level
3. Functional food web
• Depicts dynamically
important interactions
• Impact of predators
on prey populations
Summary
• Many different ways to view food
webs.
• Basis for food web ecology and
and ecosystem ecology.
• Ecosystem management and many
applied problems (contaminants,
fisheries management) require
understanding of complex species
interrelationships.
• Species do not exist in isolation –
all linked through the food web
and predator/prey relationships
Trophic Cascade Hypothesis
1.
Nutrient supply limits algae biomass (chl-a) and production
2.
Nutrients explain only some of the variability in algal biomass
Does food web structure explain
remaining variation in algae
biomass and production???
Concern with functional web:
Impact of predators on prey
Trophic Cascade Hypothesis
Piscivore biomass in relation to biomass (solid line) and production
(dashed line) of vertebrate zooplanktivores, large herbivores,
and phytoplankton.
(Top-down) trophic cascades
• Nutrients and predation explain variation in primary production
• Trophic levels are alternately limited by resources and predation
Conditions for
Trophic Cascades
1. Consumers can
control
biomass of
prey
Conditions for
Trophic Cascades
If Cpredator > Pprey
Prey controlled by
predator
If Cpredator < Pprey
Prey not controlled by
predator
P = production
C = consumption
Bioenergetic budget
G=growth (~ production)
C=consumption
R=respiration
F=feces
E=excreta
Conditions for
Trophic Cascades
2. Simple and wellknown food web
Conditions for Trophic Cascades
3. Sizeselective
predation
alewife
From Brooks and Dodson 1965
Experimental tests of trophic cascades
• Stream food webs
• Small lake
• Lake Mendota
Trophic cascades in
California streams
Number of trophic levels (3
or 4) determines
abundance of predators,
grazers, and algae!
Algae
Invertebrate
grazers
Invertebrate
predators
Fish
Trophic Cascade Hypothesis: Peter and Paul Lake, MI
Piscivore
lake
Planktivore
lake
Peter and Paul Lake, MI
Food web biomanipulation of Lake Mendota
(1976-2001): Stocking piscivores to improve
fishing and water clarity
• Algal blooms a major problem
• Piscivore populations were low
• Biomanipulation had reduced algal levels in
other lakes (Peter and Paul)
• Lots of long-term data
• Partnership between DNR and UW Madison
Lake Mendota pelagic
food web
• Food web of
intermediate
complexity
• Simplification of
reality
• Captures major
species and
interactions
• Lumps species into
trophic groups
• Piscivore stocking
1987-1999:
– 2.7 million walleye
– 170,000 northern pike
From Lathrop et al. in press
Walleye fingerlings
(1 + year old hatcheryraised)
Harvest regulations
• Size limits
• Daily bag limit changed
5 to 1 for pike
5 to 3 for walleye
From Lathrop et al. in press
Total consumption
estimated using
bioenergetic model
From Lathrop et al. in press
Impact of changing planktivore
community on zooplankton?
Zooplankton response
Small-bodied Daphnia galeata mendotae
Large-bodied Daphnia pulicaria
From Lathrop et al. in press
Impact on water clarity??
Dotted = large-bodied zooplankton
Solid = small-bodied zooplankton
Impact on water clarity??
From Lathrop et al. in press
Major cisco dieoff 1 year before
stocking began
From Lathrop et al. in press
From Lathrop et al. in press
From Lathrop et al. in press
Must consider nutrient loading as well!
Biomanipulation of Lake Mendota:
Summary
• Success: Daphnia pulicaria have dominated
for over a decade
• Changes in fisheries management affected
water quality. Links fields of limnology and
fisheries.
• Management considered the human
dimension in an ecosystem context
• Managers learned how food webs can be
manipulated to improve water quality
Biomanipulation of Lake Mendota:
Summary
• Importance of
reducing nutrient
loading
• Nutrients set the
potential algal biomass,
food web structure
can affect realized
biomass
Planktivore
systems
Piscivore
systems
Energy flow
• Energy and nutrients are
transferred up the food chain
• Bioenergetic approach –
calculate energetic budget
for organism or population
• I= ingestion
• NU= lost through feces
• A= assimilation
• R= respiration
• G= growth
• B= biomass
B
Energy flow
• Link up bioenergetic budgets for multiple
trophic levels
Trophic
level 1
Trophic
level 2
Trophic
level 3
Trophic cascade
• Stream food webs in
California
Algae
Invertebrate
grazers
Invertebrate
predators
Fish
Food web of Lake
Mendota
• Intermediate complexity
• Simplification of reality
• Capture the most species
and interactions
• Lump species into trophic
groups
What about benthic-pelagic linkages?
• Is the benthic food chain linked to the
pelagic food chain?
?????
Benthic-pelagic linkages
• Fish get much of their
energy from benthic
food chains
Importance of benthic food webs
• Reliance on benthic
food chains varies
among lakes
• Decreases with lake
area