TRITROPHIC
INTERACTIONS
READINGS:
FREEMAN, 2005
Chapter 53
TRITROPHIC INTERACTIONS
• Eating (trophic) relationships often link
several species in a community through
herbivory, predation and/or parasitism.
• When the links go across three eating
(trophic) levels, they are called tritrophic
interactions.
• Species that play an special role in
trophic relations are called “keystone
species”.
A TRITROPHIC INTERACTION
• Hover flies sip
nectar from flowers
and in turn are
eaten by a spiders.
• This is a version of:
Predators eat
herbivores, and
herbivores eat
plants.
ANOTHER TRITROPHIC INTERACTION (I)
• Daphnia are
herbivores on a
unicellular algae and
the prey of damsel
flies.
• A decrease in algae
would soon result in
a decrease in
Daphnia and later a
decrease in damsel
flies.
TRITROPHIC INTERACTION (II)
• A sudden decrease in
Daphnia would result in
a decrease in damsel
fly larvae and an
increase in unicellular
algae.
• A sudden increase in
damsel fly larvae would
result in a(n) _____ of
Daphnia and a(n)
_____ of unicellular
algae.
FOOD CHAIN (I)
• A trophic interaction that links three or
more levels is called a food chain.
• Where the trophic levels are
dynamically linked, any change in
abundance of one population within the
chain can result in changes in
abundance of the other populations.
FOOD CHAIN (II)
• All food chains begin with producers
[green plants, green algae and bluegreen algae (cyanobacteria)].
• Herbivores are known as primary
consumers.
• Predators are known as secondary or
higher level consumers.
PREDICTED DYNAMICS OF TRITROPHIC
INTERACTIONS (I)
• Where eating relations exert control on the
abundance of populations within a food chain,
a change in the abundance of one population
can have an influence on the other
populations.
• A change in abundance of producers will first
change the abundance of primary consumers
and later the abundance of secondary
consumers.
PREDICTED DYNAMICS OF TRITROPHIC
INTERACTIONS (II)
• A change in abundance of secondary
consumers will first change the
abundance of primary consumers and
later the abundance of producers.
• A change in abundance of primary
consumers will change the abundance
of one or both of the other immediate
trophic levels and the timing of change
is difficult to predict (depends on life
history characteristics, etc.).
WILLOW-HARE-LYNX
• Willow is a primary food of
artic hare in the winter
when food is scarce.
• Although evidence
suggests that lynx control
hare cycles, the hare
population is subject to
both top down and bottom
up control.
• The interaction of food
and predation has a
strong influence on hare
abundance.
WOLF-MOOSE INTERACTION
REVISITED (I)
• There is no strong
evidence that wolves
control moose
populations on Isle
Royal.
• There is good evidence
that declining moose
populations may result
in wolf population
increases.
WOLF-MOOSE INTERACTION
REVISITED (II)
• Moose populations are known
to fluctuate dramatically -- from
a low of around 500 to a high of
around 2,500 in the last nearly
50 years.
• Prior to the immigration of
wolves, the moose population
showed similar fluctuations.
• What is a primary cause of
these fluctuations?
?
Moose-Balsam Fir Interaction
• The island had a high density of
balsam fir (a common Christmas
tree), estimated at 46% of overstory
prior to moose immigration. Today,
it is only 5%.
• Nearby islands, which have no
moose, have balsam fir as a large
component of their forests.
• Thus, the decline of balsam fir has
been attributed to moose browsing.
• Although not optimum forage, it can
be up to 59% of food for moose
during the winter.
Balsam Fir-Moose-Wolf
Interaction on Isle Royale (I)
• Current research is
focused on the tritrophic
interaction between wolfmoose-balsam fir.
• It attempts to use tree
ring data to account for
fluctuations in moose and
wolf populations.
Balsam Fir-Moose-Wolf
Interaction on Isle Royale (II)
• This research is being
conducted using a bottom
up and top down (trophic
cascade) model.
• It also uses data on
temperature and
precipitation in the form of
annual actual
evapotranspiration (AET).
Balsam Fir-Moose-Wolf
Interaction on Isle Royale (III)
• The bottom up hypothesis predicts
that plant growth is determined by
temperature and precipitation. This
is a version of the primary
production scenario.
• The top down hypothesis predicts
that changes in one trophic level
result in opposite changes in the
level below it.
• For example, a decrease in moose
abundance should produce
increased plant growth if moose
herbivory limits plant growth.
Balsam Fir-Moose-Wolf
Interaction on Isle Royale (IV)
• Future observations on Isle
Royale should provide a good
opportunity to test these
prediction. It will be sometime
before answers are available.
• Given the fragile nature of the
wolf population on the island,
experimental studies are not
being currently conducted.
EXPERIMENTAL STUDIES OF
TRITROPHIC INTERACTIONS
• Experimental studies of tritrophic interactions in
field settings are difficult, time consuming and
expensive to conduct; but some notable ones
have been carried out in more recent years.
• Field studies on insectivorous birds-plant eating
insects-oak saplings.
• Artificial pond studies with newts-frog tadpolesalgae.
• Sea star-mussel,chiton,limpet,barnicle - algae
manipulations in intertidal tide pools.
Effects of Predation by Birds on Herbivory
by Forest Insects (I)
• An experimental study
has focused on
insectivorous birds-leaf
eating insects-white oak
saplings.
• A number of bird
species and
insectivorous insects
were present during the
study.
• One species of oak.
Effects of Predation by Birds on Herbivory
by Forest Insects (I)
• Hypothesis: Birds reduce insect populations
and their damage to leaves of oak saplings.
• Method: 3 sets of 30 white oak saplings were
treated by:
* sprayed with insecticide.
* doing nothing (control)
* covering with white nylon mesh to
keep birds out, but not insects.
Insect density and missing leaf area was recorded
for all three set.
Effects of Predation by Birds on Herbivory
by Forest Insects (IIa)
• Results: Insect density in N per 10,000 cm2
INSECT DENSITY
12
10.75
10
8
6.45
6
4
2
0.25
0
Sprayed
Control
Birds Excluded
Data from ECOLOGY 75: 2007-2014 (1994).
Effects of Predation by Birds on Herbivory
by Forest Insects (IIb)
• Results: Missing leaf area (%)
MISSING LEAF AREA
40
35
30
25
20
15
10
5
0
Sprayed
C ontrol
Birds Excluded
Data from ECOLOGY 75: 2007-2014 (1994).
Effects of Predation by Birds on Herbivory
by Forest Insects (III)
• Conclusion: White oaks are important
trees in a number of deciduous forest
communities. The growth rate of oak
tree saplings is a function of leaf area.
Thus, insectivorous birds increase
growth rates of deciduous forest trees.
Note: One of the investigators teaches in Bio @ UIC..
Influence of Predation on Competition
in an Amphibian Community (1)
• The red-spotted
salamander is known to
prey on a number of
species of frog and toad
tadpoles.
• Tadpoles, the larval
stages of frog and toad
species, have similar
diets and thus show
strong interspecific
competition for food.
Influence of Predation on
Competition in an Amphibian
Community (2)
• Hypothesis: Increasing predator abundance
will benefit some competitors and harm
others.
• Method: Six competing populations of
tadpoles were subjected to different
intensities of predation by a species of
salamander. % survival was recorded at the
end of the experiment.
Effect of Predation on Competition (3a)
•Results: % survival for 3 species
DENSITY OF SALAMANDERS
TADPOLE SPECIES
0
2
4
8
Spadefoot Toad
93%
58% 43% 18%
Spring Peeper
4%
28% 27% 30%
63%
58% 24% 12%
S. Leopard Frog
Declining survival as predator density
increases
Increasing survival as predator density increases
Effect of Predation on Competition (3b)
•Results: % survival for the other 3 species
DENSITY OF SALAMANDERS
TADPOLE SPECIES
Southern Toad
Barking Tree Frog
Grey Tree Frog
0
2
4
8
5%
8%
2% 21% 12%
3%
38% 24%
48% 30% 12% 17%
Declining survival as predator density increases
Little consistent effect of predator density
Effect of Predation on Competition (3c)
• Summary of Results: In the absence of
salamander predation, spadefoot toads
were the most abundant species in the
amphibian community. But as the
number of salamanders increased, the
most abundant competitor was the
spring peeper. All but one of the other
species decreased as predator density
increased.
Effect of Predation on Competition (4)
• Conclusion: A predator population can
change the relative abundance of prey
populations by reducing the numbers of
superior competitors. If a predator
species is removed from a community,
the entire community structure can be
changed significantly.
AN EXPERIMENT THAT DEFINED
KEYSTONE PREDTOR
• An experiment with
an intertidal
community defined
the concept of
keystone predator.
• It involved sea starmussel,chiton,limpet,
barnicle - algae
interactions.
• See page 1230 and
Figure 53.17 in
Freeman (2005) for
details.
OTHER STUDIES OF KEYSTONE
PREDATORS
• Sea otters
promote growth of
kelp beds by
preying on sea
urchins.
• Lobsters keep sea
urchins from
overgrazing kelp
beds.
Of Acorns, Mice, Moths, Deer, Ticks,
Spirochetes -and Lyme Disease?(1)
• Gypsy moth outbreaks and Lyme disease
pose major problems for people who live in
deciduous forests of the Northeast and Great
Lakes. They are part of a web of eating
relations centered around acorns.
• Acorn production determines the abundance
of white-footed mice and deer.
• Abundant mice suppress gypsy moth
outbreaks, and mice and deer support tick
populations that harbor the Lyme disease
causing spirochete.
Of Acorns, Mice, Moths, Deer, Ticks,
Spirochetes -and Lyme Disease? (2)
• Lyme disease (LD) is an
infection caused by
bacterium that is carried by
deer ticks.
• An infected tick can
transmit the spirochete to
the humans and animals it
bites.
• Untreated, the spirochete
parasitizes tissues, and
can cause a number of
mild to severe symptoms.
Of Acorns, Mice, Moths, Deer, Ticks,
Spirochetes -and Lyme Disease? (3)
• The gypsy moth was
introduced into Massachutes
around 100 years ago and
has become the most
destructive pest in deciduous
forests.
• It has only one generation
per year and hatches from
eggs about the time that
leaves are emerging.
• The larval (caterpillar) stage
feeds on leaves and during a
major outbreak can almost
defoliate an entire forest.
Adult moths do not feed.
Of Acorns, Mice, Moths, Deer, Ticks,
Spirochetes -and Lyme Disease? (4)
• Red oaks produce large
numbers of acorns every 2
to 5 years (masting).
• White-footed deer mice eat
acorns and their
populations boom after
masting.
• These mice are important
predators on gypsy moth
pupa; so as their
population increases, more
moth pupae are eaten.
Of Acorns, Mice, Moths, Deer, Ticks,
Spirochetes -and Lyme Disease? (5)
• White-tail deer seek
acorns to eat, carrying
adult ticks that mate.
• Adult deer ticks drop off
the deer and overwinter
in the leaf litter on the
ground.
• The following spring,
adult females lay eggs
that hatch into larval
ticks.
Of Acorns, Mice, Moths, Deer, Ticks,
Spirochetes -and Lyme Disease? (6)
• Deer tick larvae
infest mice which
carry the Lyme
disease spirochete.
• The spirochete is
transmitted to the
larval tick during a
blood meal.
Of Acorns, Mice, Moths, Deer, Ticks,
Spirochetes -and Lyme Disease? (7)
• Tick larvae molt into
nymphs that
overwinter on the
forest floor.
• In the spring
infected nymphs
seek hosts such as
deer and humans.
Of Acorns, Mice, Moths, Deer, Ticks,
Spirochetes -and Lyme Disease? (8)
RED OAK
HUMAN
WHITE-TAIL
DEER
GYPSY
MOTH
LEAF
ACORN
WHITE-FOOTED
DEER MOUSE
Science 2-13-98, pp 1023-1026
DEER
TICK
LYME DISEASE
SPIROCHETE
Risk of Lyme Disease
TRITROPHIC
INTERACTIONS
READINGS:
FREEMAN
Chapter 53