Transcript Trophic Level - Springer Static Content Server

Supplementary Material for Chapter 13
Exploring Trophic Cascades in Lake Food Webs with a
Spreadsheet Model
This chapter is published as:
Emery KA, Gephart JA, Wilkinson GM, Besterman AF, Pace ML. 2016. Exploring trophic cascades in lake
food webs with a spreadsheet model. In: Byrne L (ed) Learner-Centered Teaching Activities for
Environmental and Sustainability Studies. Springer, New York. DOI 10.1007/978-3-319-28543-6_13
Kyle A. Emery, Jessica A. Gephart, Grace M. Wilkinson, Alice F. Besterman, Michael L. Pace
Department of Environmental Science, University of Virginia, Charlottesville, VA USA
Corresponding Author Email: [email protected]
This file contains the following supplementary material:
 E: Introductory presentation
This chapter also has the following supplementary material, available on the chapter’s website:
• A: Instructor guide
• B: Student worksheet
• C: Glossary
• D: Food web spreadsheet model (Excel file)
Exploring Trophic Cascades in
Lake Food Webs
Note to instructors: This file does not contain photos
due to copyright and reproduction limitations. Users
are encouraged to add photos throughout the slides
to increase their visual appeal.
Organizing Ecosystems
We must first understand our system and its
components to understand how interactions
between components may be changed
Ecosystem: The living and non-living components of
a spatially defined area of the environment where
the components interact as a complex system
We can organize the components of ecosystems
and their interactions in many ways…one is through
energy production and transfer
Primary Production
Primary Producers: Plants and some bacteria fix
C02 (or sometimes other compounds) into
sugars, both the process and sugar provide
energy for survival, growth and reproduction
Original source of energy that fuels all other
organisms in the ecosystem
Consumers
Consumer: organisms that cannot create their
own energy; they must eat other organisms to
acquire energy for survival, growth, and
reproduction
Trophic Level: A group of species that all feed on
the same “level” (ex. Herbivores all feed on
primary producers, they are trophic level 2)
Lake Trophic Dynamics
Trophic Pyramid: A numerical representation of
all the trophic levels in an ecosystem; may depict
trophic levels in biomass, or number of
individuals.
Level
4
Top Predator
Level 3
Secondary Consumer
Level 2
Primary Consumer
Level 1
Primary Producer
Trophic Cascades: Top-Down Control
When a top predator is added to the ecosystem, there are cascading, ALTERNATING
effects on the biomass of the lower trophic levels due to predation
L4: Top
Predator
L3: Secondary
Consumer
L2: Primary
Consumer
L1: Primary
Producers
A 4th trophic level, a top
predator is added to the
system
Predation by the new top
predator reduces 2°
consumer biomass
The decrease in 2°
consumer biomass
releases 1° consumer
from predation,
increasing their biomass
An increase in 1° consumer
biomass decreases the 1°
producer biomass as
grazing pressure increases
Trophic Cascades: Bottom-Up Control
When biomass is added at the lowest trophic level (either from energy or nutrient
input) the effects work their way up to the highest trophic level.
L4: Top
Predator
L3: Secondary
Consumer
L2: Primary
Consumer
L1: Primary
Producers
The effect of increased
biomass at the bottom of
the pyramid cascades
upwards
The increased 1°
producer biomass spurs
an increase 1° consumer
There is an increase in
biomass at the lowest
trophic level
Common Impacts to Lakes
Nutrient Loading: The mass flux of nutrients (N
and P especially) entering a system over time,
often increases due to human use of fertilizer
Eutrophication: The result of excessive nutrient
loading that leads to phytoplankton blooms and
impairment of water bodies
Invasive species: A non-native species that is
introduced to a system it previously did not
inhabit, which then reproduces and spreads
Modeling Trophic Cascades
We will be using a spreadsheet with
programmed formulas to look at how trophic
cascades work in lakes!
We will do this by simulating real-world
scenarios that would perturb the food web of a
lake
A little background about our model system…
Modeling Trophic Cascades: Lake Pace
Lake Pace is located in a farming community in Minnesota.
It is 30 acres in size with a four trophic level food web.
Level 4: Piscivores are fish that primarily
eat other fish
Level 3: Planktivores are fish that
primarily feed on plankton
Level 2: Zooplankton, which are
microscopic animals that drift in the
water and feed on plankton; in Lake
Pace they are primarily grazers that
eat phytoplankton
Level 1: Phytoplankton, which are
photosynthetic microorganisms that
drift in the water
Modeling Trophic Cascades: Logarithms
In this case, the biomass of phytoplankton is
so much larger than the biomass of the toppredator that if we use a linear scale, they
barely fit on the same graph!
y = 10x
is the same as
Log-scales are commonly used
to graphically represent values
that vary dramatically in
magnitude
log10(y) = x
Modeling Trophic Cascades: General Instructions
1. We will read and consider each scenario
together. Follow-along on your worksheet.
2. Hypothesize what will happen to each
trophic level after the described
perturbation.
3. Modify the biomass in the model of the
trophic level of interest to a high, middle, and
low level within the value range. (Details
follow…)
Example: Scenario 1- Initial Conditions in the Excel Spreadsheet
The possible
range of values
to enter into cells
H26/27 are
provided here.
Figure of biomass at each
trophic level. The blue bars will
remain unchanged while the
yellow bars will change to
reflect the biomass changes
induced by entering a new
number into cell H26/27.
Based on the scenario,
enter a new value for
piscivore biomass here.
Hit Enter.
To reset the
simulation, enter
the initial value
provided here
into cell H26/27
Example: Scenario 1- Changing Piscivore Biomass
The possible
range of values
to enter into cells
H26/27 are
provided here.
The yellow bars in the figure of
biomass in each trophic level
changed. Note that the blue
bars which display the initial
biomass before the reduction in
piscivore biomass was entered
remain unchanged.
Based on the scenario,
the user entered a lower
value of piscivore
biomass
To reset the
simulation, enter
the initial value
provided here
into cell H26/27
Scenario 1: Overfishing
Many active fishermen live in the community near
Lake Pace. The community has no fishing
regulations in place; therefore, fishermen are able
to take as many fish as they like year round.
Eventually, this leads to declines in the population
of the top predator.
What do you hypothesize will happen?
Is this a form of top-down or bottom-up control?
Scenario 1: Overfishing
Follow-up Questions
1. Do your observations agree or disagree with
your hypothesis? If not, what differed and
how might you explain the differences
2. What does a value of 1 imply about the
fishing regulations? What about a value of
50?
3. If you were managing the lake, what factors
would you take into account when setting
fishing regulations?
Scenario 2: Stocking Fish
After the numbers of top predators decline,
fishermen petition the Lake Pace Association to
stock the lake with more piscivorous fish. They
hope this will improve their own catches, and
boost tourism to the lake. This leads to an
increase in piscivore biomass above normal
levels.
What do you hypothesize will happen?
Is this a form of top-down or bottom-up
control?
Scenario 2: Stocking Fish
Follow-up Questions
1. Do your observations agree or disagree with your
hypothesis? If not, what differed and how might you
explain the differences?
2. What are the positive and negative consequences of
stocking a lake based on your observations?
3. If you were managing the lake, what factors would
you take into account when determining the stocking
level?
4. What are other potential consequences of stocking
the lake that are not represented in this model?
Scenario 3: Fertilizer Runoff
Farmers in the local community apply fertilizer to
help improve crop yields. Farmers grow a variety of
crops including: corn, soybeans, apples, potatoes and
carrots. Each spring farmers apply large amounts of
fertilizer after planting seeds. However, since no
plants have grown yet to take-up the fertilizer, there
is excess fertilizer on the ground. After a
precipitation event significant loads of fertilizer run
off the fields into Lake Pace.
What do you hypothesize will happen?
Is this a form of top-down or bottom-up control?
Scenario 3: Stocking Fish
Follow-up Questions
1. Do your observations agree or disagree with
your hypothesis? If not, what differed and
how might you explain the differences?
2. How does the biomass change over the
entire range of nutrient loading? Explain this
pattern.
3. Farming is necessary for food production, but
how might we prevent the negative impacts
of eutrophication?
Scenario 4: Invasive Species
After a summer vacation to Lake Huron, a boater
returns with zebra mussels attached to his boat. He
does not thoroughly check for “hitch-hikers” on the
boat, or organisms that attach themselves to boats,
equipment, and clothes. Then, when he puts the boat
in Lake Pace the zebra mussels that attached are able to
release into the water and establish a population. These
mussels are very efficient filter feeders, and decrease
the phytoplankton biomass drastically.
What do you hypothesize will happen?
Is this a form of top-down or bottom-up control?
Scenario 4: Invasive Species
Follow-up Questions
1. Do your observations agree or disagree with
your hypothesis? If not, what differed and
how might you explain the differences?
2. How might the spread of invasive zebra
mussels be reduced or prevented?
3. What would you expect the effects to be of a
different invasive species, such as the
Chinese mystery snail, the rusty crayfish, or
the Asian carp?