PREDATION
• READINGS: FREEMAN Chapter 53
• Students who wish to observe their
religious holidays in lieu of attending
class must notify Dr. Molumby
(molumby@uic.edu).
CONSUMPTION
• The consuming of one living thing by
another.
• A basic eating relationship between
populations of different species.
• Must be evaluated on the basis of its
effects on populations, not on
individuals.
• A + (consumer) / - (consumed)
interaction.
MAJOR TYPES OF
CONSUMPTION
• Herbivory --- Eating of plants by animals. May not
result in death of individual plant.
• Parasitoidism --- Larvae of parasitoids consume
hosts.
• Cannibalism --- The eater and eaten belong to the
same species (intraspecific predation).
• Parasitism --- Host provides nutrition to one or
many individual parasites. Host may or may not
die.
• Predation --- Predator kills prey and consumes all
or part.
HERBIVORY
• Occurs when animals eat plants.
• Herbivores are those animals that exclusively
or primarily eat plant tissue.
• Generally restricted to specific parts of the
plant (leaves, flowers, fruits, roots, tubers,
sap); thus, leaving the rest to regenerate.
• Resembles predation when seed (which
contains plant embryo), seedling or whole
plant is consumed.
VERTEBRATE HERBIVORES
• Large ungulates are
the most conspicuous
native herbivores in
North America.
• Those that feed
primarily on grasses
and forbs are grazers.
Those that feed on
tree leaves are
browsers.
INVERTEBRATE
HERBIVORES
• Half of all insect species
are thought to be
herbivores. Groups
such as butterflies,
moths, weevils, leaf
beetles, gall wasps,
leaf-mining flies and
plant bugs are almost
exclusively plant eaters.
• Snails, slugs, mites and
millipedes are largely
herbivores.
HERBIVORY
Is thought to be ecologically important, but its
impact is still debated. Suggested positive
impacts include:
1. Increased production and nutrient uptake.
2. Increased quality of leaf litter and soil.
3. Increased chances of successful seedling
establishment.
4. Improved conditions for plant growth
(pruning effect).
Some Evolutionary
Responses of Plants to
Herbivory
• 1. Mechanical forms of protection.
Microscopic crystals in tissues, thorns,
hooks, spines.
• 2. Defensive chemicals. Strychnine,
morphine, nicotine, digitoxin, etc.
• 3. Fruits. Attractive and tasty tissues
surrounding seeds that promote
dispersal.
PARASITOIDISM
• Insects, usually flies and small wasps,
that lay their eggs on living hosts. The
larvae then feed within the body of the
host, eventually causing death.
• Recent experimental evidence suggests
that parasitoids locate their hosts by
responding to airborne chemical signals
from plants damaged by the host.
PARASITOIDS
• A tachinid fly lays
eggs on a hornworm
(moth larva). The fly
larvae develop by
consuming the
hornworm.
• Many species of
ichneumon wasps
are parasitoids.
CANNIBALISM
• An individual consumes another individual of
the same species.
• A form of intraspecific predation.
• Relatively common among insects when
density is high. Usually involves adults
consuming eggs and larvae.
• Demonstrated to be density-dependent factor
regulating experimental insect populations.
PARASITISM
• Occurs when a member of one species
(parasite) consumes tissues or nutrients
of another species (host).
• Parasites live on or in their hosts; often
for long periods of time.
• Parasites are most often much smaller
than their hosts.
• It is not necessarily fatal to the host.
A VERTEBRATE PARASITE
• The sea lamprey was
introduced into the
Great Lakes in 1921
through the Welland
Canal.
• Contributed greatly to
the decline of whitefish
and lake trout (shown).
• Chemical control
programs started in
1956 have reduced
lamprey populations.
INVERTEBRATE PARASITES
• Tapeworm is an
intestinal parasite in
many species of
vertebrates,
including humans.
• The deer tick (small
one) and wood tick
are common
external parasites
on mammals.
VIRAL PARASITES
• The common influenza
virus (top) has inhabited
every host in this room!
It has caused more
deaths than any other
pathogen.
• The bird flu virus
(bottom) is a potential
threat to humans.
Freeman Figure 52.9 Part 1
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Freeman Figure 52.9 Part 2
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this pi cture.
PREDATION
• The most conspicuous interaction is when an
individual of one species (predator) eats all or
most of an individual of another species
(prey).
• The most thoroughly studied consumptive
relationship between species.
• Of high ecological and evolutionary
significance.
• An everyday occurrence in nature.
Possible Outcomes of
Predation
• 1. Predator population has little effect
on abundance of prey population.
• 2. Predator population eradicates prey
population; this may contribute to
extinction of predator population due to
lack of food.
• 3. Predator and prey populations
coexist in dynamic equilibrium.
A Dynamic Equilibrium Model
of Predator/Prey Populations
• 1. Assume an exponential growth model for a
prey population living in the absence of
predators.
• 2. Assume an exponential decline model for a
predator population living in the absence of
prey.
• 3. Assume density of predators is a function
of density of prey and vice versa.
Prey Population Living Alone
18
16
16
14
Number (N)
• Assume a constant rate
of increase in absence
of predators.
• dN/dt = r1 N
where N = number of prey
t = time
r1 = reproductive
capacity of prey (births
exceed deaths)
12
10
8
8
6
4
4
2
2
1
0
0
1
2
3
Time (t)
4
5
Predator Population Living
Alone
18
16
16
14
Number (N)
• Assume a constant rate
of decline in absence of
predators.
• dP/dt = - r2 P
where P = number of
predators
t = time
- r2 = reproductive
capacity of predators
(deaths exceed births)
12
10
8
8
6
4
4
2
2
1
0
0
1
2
3
Time (t)
4
5
Predator and Prey
Populations Living Together
• Assume a constant rate of increase in prey
population is slowed by an amount depending
on the number of predators. dN/dt = (r1 - K1)
N ; where K1 = a constant related to the effect
of predation on prey.
• Assume a constant rate of decrease in
predator population is slowed by an amount
depending on the number of prey. dP/dt =
( -r2 + K2) P ; where K2 = a constant related to
the effect of predation on predators.
A Model Predator/Prey Cycle
Number of Individuals (N)
250
200
150
P re da to r
P re y
100
50
0
0
20
40
60
80
Time (t)
This graph shows a limit cycle of predators and prey.
Description of Dynamic
Equilibrium
• When predator numbers are low, prey
numbers increase rapidly.
• As prey numbers increase, predators
begin to increase.
• When predators numbers are high, prey
numbers decrease rapidly.
• As prey numbers decrease, predator
numbers fall.
The Hare & Lynx
Predator/Prey Relationship
• Snowshoe hare and
Canadian lynx show
classic population
cycles with a 10-11 year
periodicity.
• Hare are herbivores
and feed on twigs under
the snow in winter; lynx
feed primarily on
snowshoe hare.
The Hare/Lynx Cycle
Based on Pelt Sales
Similar data is provided in Figure 53.10 (Freeman, 2005).
Are Hare/Lynx Populations
Dynamically Linked?
• Evidence For: Lynxes usually have large
populations at the same time or just after
hares do. Prey abundance often has a
dramatic effect on predator abundance.
Snowshoe hare abundance has a strong
influence on lynx abundance.
• Evidence Against: Snowshoe hare
populations show cycles on islands where
lynxes are absent. Do lynx populations have
a strong influence on hare populations?
What is the impact of food and predation on the
snowshoe hare density?*
• Hypothesis: Food or predator or both will influence
hare density, thus contributing to the snowshoe hare
cycle?
• Predictions: 1. Food addition (rabbit chow) will
increase hare density. 2. Predator exclusion
(enclosure by electric fence that excludes lynxes) will
increase hare density. 3. Food addition and predator
exclusion will interact to increase hare density. 4.
Fertilizer (NPK plant nutrients) addition will stimulate
plant growth that will act as hare food and thus
increase hare density.
*Reported
in SCIENCE 8-25-95
What is the impact of food and predation on the
snowshoe hare density?
• Method: 1 kilometer 2 areas of boreal (coniferous) were
managed for 8 years by:
1. Food addition (rabbit chow)
2. Predator exclusion (mammals only, not birds)
3. Food addition and predator exclusion
4. Fertilizer (NPK plant nutrients) addition
5. Control areas (nothing was done in these areas)
The 5 different management areas were selected as random
from a larger area that had a relatively uniform community
structure.
Snowshoe hare density was monitored at various periods
throughout the 8 year study.
What is the impact of food and predation on the
snowshoe hare density?
• Result: Relative to control areas:
1. Food addition tripled (3x) hare density.
2. Predator exclusion doubled (2X) hare density.
3. Food addition and predator exclusion increased hare
density eleven-fold (11X).
4. Fertilizer addition had hare density equivalent to
control areas (no effect).
• Conclusion: The snowshoe hare population cycles results from:
FOOD - HARE - LYNX INTERACTION
• Also see: Figure 53.11 in Freeman (2005). He reports the results
of the study at the end of 11 years.
What Drives the 10-year Cycle of
Snowshoe Hares?*
I.
Food Hypothesis**
Test 1. Twig consumption increases as hare
density increases, but 60-80% of available
food is not consumed.
Test 2. Unlimited added rabbit chow does not
stop cycle.
Test 3. Added natural food does not stop hare
decline.
* Bioscience 1/01 ** HYPOTHESIS REJECTED
What Drives the 10-year Cycle of
Snowshoe Hares?*
I.
Predator Hypothesis**
Test 1. 95% of radio-collared hare deaths
were due to predation.
Test 2. There were few deaths of radiocollared hare where predators were excluded.
Test 3. Predator exclusion nearly eliminated
the decline phase of the snowshoe hare
cycle.
* Bioscience 1/01 ** HYPOTHESIS ACCEPTED
Moose and Wolf of Isle Royale
• The world’s longest running
predator/prey research
project. The 47th year of wolf
and moose monitoring was
completed in the winter of
2006.
• Winter provides the best
opportunities for aerial
surveying of the wolf and
moose populations, with
leaves off the trees and
snow on the ground.
Moose Population
(Early History)
• Prior to 1900 there were no moose on the island.
• Sometime between then and 1905 a moose
population was established.
• By 1929, the population was estimated to be around
2,000.
• During the early 1930’s the moose destroyed their
own food supply and numbers declined.
• A fire in 1936 burned browse over a quarter of the
island, and by 1937 the moose population was
around 400. Many predicted extinction of the
population.
• The fire stimulated sapling production (browse), so by
1948 the population increased to around 800.
Moose Population
(Recent History)
• First scientific
surveys of the
moose population
began in 1959.
• Since that time the
population has
fluctuated from a
low of around 500 to
a high of around
2500.
Wolf Population
• The first wolf tracks
on Isle Royale were
observed in 1949.
• Annual monitoring
began in 1959.
• Numbers have been
as low as 12 and as
high as 50.
Moose and Wolf Populations
of Isle Royale
• Significant fluctuations have
been observed in both the
moose and wolf populations
since 1959.
• The significant increase in
wolf population during the
1970’s corresponds to the
decline in moose. Wolves
prey on very young, very old,
sick or injured moose.
• Evidence that wolves impact
the moose population is
lacking.
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
Predators as Agents of
Biocontrol
• Predators have
been used in
attempts to control a
variety of plant and
animal pests. Often
called: biocontrol.
• Ladybird beetles
and ant lions
(lacewing larvae)
have been used.
Parasites as Agents of
Biocontrol
• European rabbits were introduced
into Australia in 1859 and became
a major pest.
• In late 1950, the myxoma virus,
spread by mosquitoes, began
killing rabbits in large numbers. By
1953, rabbit immunity was
detected. Today, the virus may kill
only 50 % of the rabbit population
during an epidemic.
• Another virus (calicivirus), native
to China, was found and testing as
a potential biocontrol agent began
in 1995 and continues to the
present.
PREDATION
• READINGS: FREEMAN Chapter 53
• Students who wish to observe their
religious holidays in lieu of attending
class must notify Dr. Molumby
(molumby@uic.edu).