Download Workshop on Predation – Thomas Herbert, Ph.D. I. Basics of the

Workshop on Predation – Thomas Herbert, Ph.D.
(revised by Dana Krempels in 2013)
Pre-Workshop Preparation: http://www.bio.miami.edu/dana/dox/lynx_hare.html and
http://www.bio.miami.edu/dana/dox/symbiosis.html
I. Basics of the predator-prey relationship
1. What is the name of the food source for a predator?
prey
2. What is the food source for prey?
Any organism at a lower trophic level than the prey.
3. Are all prey species herbivores? Give examples of some that are not, and what eats them.
Prey:
Prey:
Prey:
Prey:
Prey:
small carnivorous fish
Predator:
juvenile carnivores of any species Predator:
Venus flytrap plant
Predator:
Predator:
...ETC. Be creative!
Predator:
larger carnivorous fish
larger carnivores
herbivorous insects
4. Whenever an animal eats a food item, only about 10% of the energy in that food item is
converted into the biomass of that animal. About 90% is lost as entropy as the animal uses
the energy in the food for cellular work and homeostasis. Discuss the implications, in terms
of energy transfer from one feeding (trophic) level to another, of predators that feed on
herbivores, versus predators that feed on secondary (and higher) consumers.
The higher the trophic level, the lower the total biomass in that trophic level. For
example, there are fewer wolves in a tundra ecosystem than caribou.
If animals feed at more than one trophic level, they will have more food available if they
feed on a lower trophic level. For example, you can feed more humans if everyone eats
plant material than if you feed that plant material to chickens or cattle, and the humans eat
those. You lose about 90% of the energy of the producers (plant matter) by feeding it to
hervbivores (first trophic level), so there's that much less biomass left to feed the second
trophic level (small carnivores) and still less to feed the top carnivores. See below.
Figure 1. Representation of the reduction of biomass with each
trophic level. An ecosystem will have more primary producer
biomass, than herbivore biomass, more herbivore biomass than
small carnivore biomass, and so on up the pyramid of trophic
(feeding) levels.
DO NOT MAKE THE
COMMON MISTAKE OF
THINKING THAT YOU
NEED TO EAT MORE
SMALL
CARNIVORES
THAN HERBIVORES TO
GET THE SAME AMOUNT
OF ENERGY! NOT TRUE!
Protein has the same energy
content per unit mass, no
matter whether it comes from
a mouse or a raccoon (or a
cow or a human). We are
talking
strictly
about
NUMBERS and BIOMASS in
the ecosystem, not calories
consumed by each organism.
For the next section, be sure to review the information at:
http://www.bio.miami.edu/dana/dox/symbiosis.html
5. When is a human a predator? When--if ever--is a human prey?
Humans are predators any time they feed on a whole organism of any kind.
Humans can be prey when they are fed upon by other species (e.g., sharks, bears, etc.)
What about mosquitoes? Ticks? Fleas? Are they predators or parasites? Sometimes
it's hard to draw the line.
6. What the key similarity or difference between the predator-prey relationship and the
host-parasite relationship? What about the host-parasitoid relationship?
A parasite does not kill its prey outright or eat all of it, as a predator does.
A parasitoid kills its host as it metamorphoses, but does not consume it completely.
7. What the key similarity or difference between the predator-prey relationship and a
competitive relationship between two species?
Two species in competition are vying for the same resource. In predator/prey
relationships, the prey species is the resource of the predator. No competition for resources
between the predator and prey exists.
8. In terms of evolutionary results, what is the difference between competition between two
different species (i.e., interspecific competition), and competition between members of the
same species (i.e., intraspecific competition),?
Competition between two different species can lead to resource partitioning, in which
the two species begin to utilize different resources. This can result from natural selection,
in which individuals of the competing populations who do not use the most commonly
sought-after resources have a selective advantage (because there's more food!). If there is
complete overlap between the two species' niches, one will go extinct if it cannot shift its
resource use.
When members of the same species compete for resources, some individuals will have
traits that make them better suited to compete for those resources, and they will have more
energy and—all other things being equal—leave more offspring. This means that if the
situation stays the same, the population's will gradually shift to contain the genes that
encode traits that make the organisms better suited to exploit the desired resource.
II. Canadian Lynx and Snowshoe Hare: Fact or Myth?
A. Background
In 1937, MacLulich published a paper analyzing data collected by fur trappers selling pelts to
The Hudson Bay Company over a period of nearly 100 years. From these data, a "classic" Lynx
vs. Snowshoe Hare population fluctuation phenomenon emerged, as shown below. MacLulich
noted that the "boom" and "bust" of hare and lynx population seem to mirror each other, with the
lynx peaks and valleys coming slightly after those of the hares. (Figure 2)
An overview of this hypothetical interaction can be found at
http://blogs.britannica.com/2011/06/rise-fall-canada-lynx-snowshoe-hare/
Figure 2. Population density fluctuations of lynx and hare, as calculated from pelts taken
by trappers from 1845 through 1935.
1. How did the MacLulich and other earlier students of the population fluctuations of lynx and
hare explain the population fluctuations?
They claimed that as the season progressed, hares became more scarce because the lynx
were hunting them so effectively. At a certain critical point of low hare population density,
there would no longer be enough hares for all the lynxes to prey upon, and the population
of lynxes would also "crash" because lynxes who could not find enough hares would starve.
Once the lynxes became scarce, this allowed the snowshoe hare population to rebound,
and the cycle continued.
2. What are some other, more recent hypotheses that could be used as alternate explanations?
For some ideas, visit http://www.bio.miami.edu/dana/dox/lynx_hare.html.
Keith (1983) suggested that it was actually the lack of sufficient forage for hares in the
winter that caused their decline—not hunting by lynxes. He did agree that the lynx
population decline could be due to a shortage of hares.
Other authors have cited such factors as parasitic infections, bacterial infections and
other environmental stresses as being responsible for hare population declines.
Even sunspot activity was proposed as one possible driving force of the lynx/hare
fluctuations, though this was later discredited.
For more references on the hare/lynx cycle, find articles at:
http://www.jstor.org/action/doBasicSearch?Query=lynx+hare+popul
ation&wc=on&dc=Biological+Sciences
B. Exercise: Becoming a Predator or Prey
Each person in the group should choose to be a Lynx or a Snowshoe Hare. Imagine how
you would be living, feeding, foraging, hiding, etc. Picture yourself in the winter boreal
forest, and imagine what challenges you would face.
Go around the circle of predator and prey and have each lynx or hare answer one of the
following questions. As you discuss these questions, think about how your lynx or hare
behavior might be more complicated than first imagined.
1. (Lynx) The number of hares is decreasing rapidly because you, your conspecifics, and
predators of other species are eating them. Simple predator-prey theory describes a
relationship between the predator and prey. But what is the relationship between
you and your conspecifics?
Intraspecific competition.
What is the relationship between you and predators of other species?
Interspecific competition.
How might intraspecific interactions differ in their evolutionary impact compared with
interspecific interactions?
Interspecific competition: Competition between two different species can lead to
resource partitioning: over evolutionary time, natural selection will drive populations of
the two species to utilize different resources. Individuals of the competing populations who
do not use the most commonly sought-after resources may have a selective advantage
(because there's more food!). If there is complete overlap between the two species'
ecological niches, one will go extinct if it cannot shift its resource use.
Intraspecific competition: When members of the same species compete for resources,
some individuals will have traits that make them better suited to compete for those
resources, and they will have more energy and—all other things being equal—leave more
offspring. This means that if the situation stays the same, the population’s gene pool will
gradually shift to contain the genes that encode traits that make the predators better suited
to exploit the desired resource.
2. (Hare) You have to eat, too. Being a member of Order Lagomorpha (rabbits, hares, and
pikas), you don't usually eat Lynx. (Though a Knight of the Round Table sometimes makes
a tasty snack.) What do you eat in the spring and summer? Fall? Winter?
Spring and summer: new growth grass, leaves, twigs
Fall: dried grasses, remnants of plants going dormant, seeds, fruit
Winter: dried grasses, if you can dig them up; bark, twigs.
What type of symbiosis do you share each of the different types of things you eat?
If you devour the entire plant, you are a predator (even though you are an herbivore).
If you don’t kill the entire plant, you are…what? A parasite? Check out the definition
of parasite and decide.
Essentially, you're an herbivore of these food items. If you disperse their seeds in your
fecal matter, you’re also a seed disperser (mutualism).
3. (Lynx) What will happen to the predator and prey populations if the predator population size
is reduced to one individual - you?
Prey population will be able to recover without stress of predation, though other factors
such as food shortage and pathogens will also affect population size.
Predator population will either disappear or migrate to an area where there are other
conspecific predators for breeding.
4. (Hare) What will happen to the predator and prey populations if the prey population size is
reduced to one individual - you?
The predators will either have to switch to a new type of prey, or starve.
Unless you're a pregnant female, your population is history when you die, unless you
migrate to an area where there are others of your species.
5. (Lynx) Recall the data on the relationship between lynx and hare populations as analyzed by
MacLulich. What is your relationship to the trappers? (Consider all possibilities.)
Predation: The trappers are your predators when they trap you.
Competition: They are your competitors when they trap hares.
Commensalism? When the trappers kill predators that are not lynxes, they may actually
be helping the lynx population by reducing interspecific competition.
Side note: If you've avoided the traps because you are particularly smart, and your
intelligence is hereditary, you may pass on your smart genes to future generations, and
future populations of lynxes might be harder to trap. Natural selection in action! But is
commensalism in effect on an individual-organism basis? Not really.
6. (Hare) Now it's your turn to recall the data on lynx and hare populations collected by
Hudson Bay Company fur trappers. What is your relationship to the trappers? Again, be
sure to consider all possibilities.
Predation: The trappers are your predators when they trap you.
Commensalism? Trappers benefit the hare population by removing predators, though the
trappers are not directly benefitted in this regard by the hare population.
As with the lynxes: If you've avoided the traps because you are particularly smart, and
your intelligence is hereditary, you may pass on your smart genes to future generations,
and future populations of hares might be harder to trap.
7. (Lynx) More recent research suggests that snowshoe hare population numbers fluctuate even
when lynxes are excluded from the environment.
a. What might be responsible for this "natural" fluctuation in your population numbers?
Pathogens, food shortage, competition between hares, migration to areas with better
resources, all can occur when population density becomes very high. This can cause a
population crash even without predation by lynxes or other predators.
b. What are the implications of this phenomenon, in terms of population numbers for lynxes
and snowshoe hares over time, even when the populations do not interact?
Population density may still fluctuate due to factors other than lynx/hare interactions.
8. (Hare) Would you expect to see the same type of "natural" fluctuations in the lynx population
in the absence of snowshoe hare prey? Discuss! Why or why not?
The same population-density changes should be true for lynxes, though these solitary
predators are not as gregarious as snowshoe hares, and so would be less likely to share
pathogens. However, territorial fighting might cause lynx deaths, or cause subordinate
lynxes to migrate out of the habitat in search of friendlier hunting grounds.
III. Predator/Prey Relationships: Graphical Representations
Review the Hudson Bay data from Figure 2.
Figure 2. Population density fluctuations of lynx and hare, as calculated from pelts taken
by trappers from 1845 through 1935.
1. Choose any time point in the figure above. With a straight-edge, draw a vertical line
intersecting the number of predator and prey living at that time. Do this ten times, drawing ten
vertical lines at ten time points in Figure 2. Label each of your lines.
The figure should look something like this:
1. Choose any time point in the figure above. With a straight-edge, draw a vertical line
intersecting the number of predator and prey living at that time. Plot a point at these two
coordinates on Figure 3, and label that point “a”.
Repeat this process ten times, labeling each line alphabetically.
2. In the table below, record the numbers of lynx and hares at each point on the vertical lines
you’ve drawn on Figure 2.
For my example, the numbers are (approximately) in thousands:
point
# lynxes
# hares
point
# lynxes
# hares
a
10
50
f
20
15
b
12
22
g
20
20
c
58
60
h
50
75
d
8
45
i
10
15
e
35
100
j
32
75
3. If McLulich’s hypothesis is correct, what should you predict about the relative numbers of
lynx and hares over time?
a. When hare density is low, lynx density should be
low
.
b. When hare density is high, lynx density should be
high
.
c. When lynx density is low, hare density should be
high
.
d. When lynx density is high, hare density should be
low
.
4. In the space below, plot the number of lynx (y axis) by the number of hares (x axis). LABEL
YOUR X AND Y AXES APPROPRIATELY, and give your figure an appropriate title.
Figure 3. The relationship of predator abundance as a function of prey abundance.
5. What do the points in Figure 3 tell you about the relationship between the two populations?
If there is a relationship between predator and prey numbers, then this figure should show
the hypothetical change in predator and prey densities, with predators increasing in
number as prey numbers increase, and decreasing together, as well. Does it? Nope. These
points are all over the map, and do not form a function.
6. Connect the dots. Draw a line from point a to b, b to c, c to d, etc. Do the data allow you to
predict population numbers over the course of time? (Do the populations vary predictably
with each other as time progresses?)
If you draw lines between sequential points in time, here’s what you get (at least in this
random example):
Your lines may vary, depending on which points you picked. But it’s pretty clear that
there is no relationship between predator and prey numbers over time. This suggests that
the populations’ numbers may not be correlated with each other. (Could you devise a
different method to sample the data to see if there is a relationship between lynx and hare
population densities?)
Time isn't included in Figure 3 axes. But if you try to connect the dots via time
progression, you can see that lynx and hare population densities change without respect to
time or, apparently, to each other.
7. To clarify your answer to the previous question, redraw the Figure 3, placing point a (from
the earlier time) in the same position as where you previously had point e (later time). Place
point e where point a used to be. (That is: reverse the positions of points a and e from Figure
3, and plot them in Figure 4, below. (Don’t forget to label your axes appropriately every time).
Figure 3. The relationship of predator abundance as a function of prey abundance,
irrespective of time progression.
8. Connect the dots again. Is the function any different?
Not appreciably different. Still no function here.
9. What do these plots (Figures 3 and 4) imply about the actual effects of the species we are
calling the "predator" and "prey" on each other's population numbers?
The numbers of the populations do not fluctuate together in easily predictable ways,
especially when time progression is not an axis.
10. What factors besides predation by Lynxes might affect Snowshoe Hare populations?
Discuss.
Pathogens, overcrowding, intraspecific competition, predation by predators other than
lynxes, lack of food, migration to search for new forage, etc. Snowshoe hares are somewhat
gregarious, and more likely to interact when crowding occurs. Hence, greater chance for
territorial spats (injuries), transmission of pathogens and parasites, etc., that can have an
effect on population density.
11. What factors besides a shortage of Snowshoe Hare prey might affect Lynx populations?
Discuss.
Pathogens, overcrowding, intraspecific competition, competition with predators other than
lynxes, Monty Pythonesque rabbits that prey on lynxes, lack of food, migration to search
for new forage, etc.
Suggested Quiz Questions
1. Define predation.
Ans: One population benefits at the expense of another population by killing and eating
members of that other population.)
2. Define competition.
Ans: Two populations both require the same resource(s), and their interaction is
disadvantageous to both populations.
3. What is the predator in today’s workshop?
Ans. Lynx
4. What is the prey animal in today’s workshop?
Ans. Snowshoe Hare (or just Hare is okay)
5. What is meant by “trophic levels” in an ecosystem?
Ans. This refers to how many degrees away from primary producers a particular species feeds.
Primary producers to primary consumers comprise the first trophic level, (i.e., producers to
herbivores), primary consumers (herbivores) to secondary consumers (carnivores) the second
level, and so on.
6. Give an example of a “top carnivore”
Ans. Any large predator, including humans, that eat high on the food web, and are not usually
eaten by other species.
7. Define “symbiosis”.
Ans. Two populations in an ecosystem engaging in an interaction that affects both populations.
8. What is meant by intraspecific competition?
Ans. Competition for resources between members of the same species.
9. McLulich used data from the Hudson Bay Company to estimate population densities of
lynxes and snowshoe hares. What did the Hudson Bay Company provide?
Ans. Numbers of pelts of each species sold to them by trappers.
10. Give one reason that hare populations might undergo a “boom and bust” cycle?
Ans. Any of the reasons in the workshop, including predation by lynxes.
Or you can think up your own.