Download Population Ecology Simulation

Population Ecology – A Simulation
Computer Simulations on Food webs – BiologyLabs On-line
The following was modified from http://www.biologylabsonline.com/protected/PopEcoLab/
Objectives: PopEcoLab will allow you to study how food usage influences the
population dynamics of individual species and the co-existence of species in a
community. You can manipulate several aspects of the ecology of these
species, and then track how these variables alter the population dynamics of
a single species and, in multi-species simulations, how they alter the ability
of these species to co-exist for a century. Note that the species are not
evolving in this simulation.
This simulation mimics the kind of research that conservation biologists,
population biologists, and community ecologists frequently do because largescale ecological manipulations are difficult or impossible to undertake with
most species. When you have completed this lab, you should:
1. Understand the link between food availability and single-species
population density (carrying capacity)
2. Understand how competition influences the carrying capacity of
individual species where they co-occur and compete
3. Recognize the importance of predation in maintaining species
diversity
4. Realize the difficulty of predicting ecological outcomes and the
importance of mathematical modeling in these fields of biology
Background Information
For any population (a group of actually interacting individuals of a particular
species), a wide range of conditions can influence the number of individuals
and result in changes in population size. Some of these conditions include
environmental changes such as weather, habitat quality, and food availability.
Other conditions involve interactions with competing populations of other
species, and predator-prey relationships.
One reason population size is influenced by environmental factors is that
most populations exist within defined boundaries of habitat. A population's
1
boundaries may be vast, covering large areas of forest or ocean, yet the
overall growth and health of the population's individuals is directly
influenced by the geographical boundaries of the population. This is true in
part because the boundaries of habitat in which a population resides define a
finite carrying capacity (the maximum population size that a geographical
area can support) dictated by available resources such as food and shelter.
If a population exceeds the carrying capacity of a geographical area, there
may be diverse responses by its members. Sometimes responses result in
increased carrying capacity, such as the addition of alternative food
sources. Most responses result in a decline in the population size.
Individuals may leave the area (migrate) in search of a richer food supply,
they may reduce their reproductive rate (number of offspring per individual)
or suffer an increased death rate. In some cases, exceeding carrying
capacity can lead to extinction of a species. Often, exceeding the carrying
capacity leads to a decline in the quality of the habitat so that when the
population recovers, it has a new, lower carrying capacity.
In many communities (groups of interacting populations inhabiting the same
geographic region), competition for a common food supply will influence the
population size of all the competitors. Theoretically, two species competing
for exactly the same limited resources cannot coexist. This concept is known
as the competitive exclusion principle. When two species have partial
overlap of resource requirements, each will lower the carrying capacity of
the other.
Even under conditions of adequate population size, adequate habitat and
food supply, populations must deal with predation, which will lower the
population numbers below the carrying capacity (although some biologists
consider carrying capacity of an environment to include the impact of
predation, most consider carrying capacity to refer only to the resources
available to the species being considered). Predator-prey relationships
influence both the prey and the predator; the prey species form part of the
resource base that determines the carrying capacity of the predator
species. Predation is often a mechanism that prevents prey populations from
exceeding the carrying capacity of the habitat, and may be important in
maintaining diversity of prey species if the predators are generalists that
eat prey in proportion to their availability.
2
Biologists study these relationships by carefully designed observational
studies over extended periods of time, documenting resource availability,
population growth rates, and competitive and predatory relationships.
Experimental studies are more difficult to do with complex communities
because of the number of potentially important interactions. For this
reason, theoretical models and simulations are very important for our
understanding of community structure and habitat management. This
laboratory will help you to better understand the problems that confront
conservation biologists, as you seek to establish a conservation program that
will allow a rare sparrow species to survive in the face of strong competition
and predation.
Experiment 1: Single species population dynamics
A. Run the experiment with the default settings; only one of the two
sparrow species is present. The output we are concerned with in this lab is
the plot of population size versus time. In table 1, note which sparrow you
are working with, the input food densities and the average population size
after the population reaches the carrying capacity. We expect population
size to be tightly regulated by food availability. To test this hypothesis, run
the simulation a second time with food availability (Resource Densities)
reduced by 50%, and a third time with food availability increased by 50%.
Table 1a: Single species:___________ Initial population size:______
Food availability: seeds
Food availability:
insects
Carrying capacity
B. These sparrows do not rely equally on both food sources. Look at the
statement of competition; which food is your sparrow primarily dependent
upon?
Restate the hypothesis above by considering only one of the two food
sources (do not alter the food usage by your species):
3
Rerun the experiment varying only the food source upon which you based
your hypothesis, entering the results in Table 1b. Do the results support or
refute your hypothesis?
Table 1b: Single species __________Initial population size:_______
Food availability: seeds
Food availability:
insects
Carrying capacity
Experiment 2: Competition without predation
2a. Return the food levels to 100 seeds (or insects) per square meter, and
leave the competition at 0.50. Turn on “multiple run mode” and choose to run
the experiment 5 times. A table will appear that allows you to set the initial
population size for each species for each run: set the population sizes for
blue and brown sparrow to 100 (total initial sparrow density of 200); leave
the hawk population at 0.
Examine the results for the first run: What is the carrying capacity for
each of the two species?
Examine the results for the other runs. How consistent are the results
across runs?
2b. Propose an hypothesis for how a shift in the intensity of competition
influences the carrying capacity of each species (in other words, if both
species used the same food at the same rate, how would their population
dynamics change?). Write your hypothesis here:
Keeping all other inputs the same (and keeping it to 5 runs), shift the
competitive interactions on both species to 1.0. Write your predicted
outcome here:
4
Run the experiment and observe the output for run 1. Do these results
agree with your prediction and therefore support your hypothesis?
Examine the results for each of the 5 runs. Are they consistent with what
you saw for run 1? Briefly compare the different runs:
2c. Propose an hypothesis for how a shift in relative competitive abilities
influences the carrying capacity of each species (in other words, if one
species was better able to use both food sources and the other species was
more dependent upon only one food, how would their population dynamics
shift?). Write your hypothesis here:
Test your hypothesis by setting the competitive interaction of one species
to 1.0 and of the other species to 0.5. Briefly describe what you expect to
see happen (your prediction) for this run of the experiment; include in this
prediction a statement of how consistent you expect the results to be
across the multiple runs.
Examine the results for each of the 5 runs. Do they support or refute your
hypothesis?
How are these results different from what you observed in Experiment 2b?
Experiment 3: Coexistence of species. The above results reflect what
conservation biologists are often faced with, as habitat patches become
small and species that previously subdivided their habitat geographically are
forced to co-exist. It is also the situation that biologists must manage when
a strong competitor invades the habitat (such as the invasion of the US by
5
starlings, which had a strong negative effect on many cavity-nesting birds
like blue martins). One of the common short-term solutions is to manipulate
food sources to try to preserve the rare species while other conservation
methods are devised. Examining your results from Experiment 2c, identify
each species as “rare” and “dominant”. Your goal will be to manipulate the
habitat and the biological community in a way that will maintain the rare
species in the face of the competition by the dominant species.
In the short term, we will attempt to maintain the rare species by providing
extra food. Hypothesize here which of the two food sources must be
augmented in order to allow both species to survive:
Leaving all other settings the same, and keeping 5 runs, double the amount
of the food you hypothesize to be important. Enter your results in table 2;
persistence is the number of years to extinction. If a species does not go
extinct, enter “infinity”.
Table 2: Rare species_____________Initial population size 100
Dominant species____________ Initial population size 100
Food level seeds_________ insects________
run
blue sparrow persistence
brown sparrow persistence
1
2
3
4
5
How good were you at choosing the “right” food to supplement? How reliable
were the results from run to run (what proportion of the runs did the rare
species go extinct?).
6
Experiment 4: The importance of predators.
One long-term solution to maintaining species diversity among species that
compete is to reintroduce a predator (assuming that one occurs there and
has gone locally extinct). Such situations are referred to as “top-down”
community regulation, in contrast to the food supplementation which is
described as “bottom-up” community regulation. It is believed that a
predator that consumes various prey will attack the most common prey item
more frequently simply because it encounters this species more frequently.
Theoretically, this should reduce the population of the strong competitor
enough to allow the rare species to persist.
In this simulation, we have the ability to introduce a hawk that will eat both
sparrows; if the settings for flight speed are left equal then the hawk will
capture both species equally well. We are going to test the hypothesis that
introducing this hawk will allow the rare species to persist.
Start by returning food availabilities to 100 units/m2 (why?). In the table
for “multiple runs”, increase the number of runs to 10 and add hawks to each
run. Choose two different initial densities of hawks, running 5 replicates of
each. Record in Table 3 the number of hawks you introduce in each run.
When you examine your results, note that the red line is the hawk population
size. If you see no red line then the hawks failed to successfully establish a
population.
7
Table 3
run
1
N. hawks
blue persistence brown persistence
hawk persistence
2
3
4
5
6
7
8
9
10
Examine your results. What percentage of the runs for each hawk
introduction level have all three species surviving?
How did the introduction of the hawks influence the persistence of the rare
species?
Compare your results with results of your classmates that ran different
initial hawk population sizes. How did the number of hawks introduced alter
the likelihood that the hawk became established in the community?
How did the establishment of hawks alter the likelihood that both sparrow
species persisted 100 years?
8
Experiment 5: Allowing evolution
In all of these simulations, we have prevented evolution from happening:
there have been no shifts in physiological factors such as clutch size, life
span, or flight speed that in real life might increase fitness in the rare
species and thus increase the likelihood that the species would persist.
Choose one of these parameters, and predict how an evolutionary change
(increase or decrease) in that parameter would increase the likelihood of
persistence of the rare species in the community:
Choose an initial setting for this input parameter, and record it in table 4. Be
sure to only alter one parameter only in the rare species, and to leave all
other parameters as they were set in Experiment 4 (after choosing one hawk
population size to use). Adjust the input parameter accordingly and run 10
replicates.
Hawk initial population________ Rare species initial population 100
In table 4, record the initial settings for your run, and the proportion of
runs that end at 100 years with the persistence of all three bird species.
Repeat the simulation with four other settings of this parameter.
Table 4: Varying ___________ in the rare sparrow
Parameter setting
% runs with 3 species persisting
How strong a change in the physiological parameter is required for the rare
species to persist?
9
At any settings, did the relationship between the two sparrow species
reverse (with the “rare” species becoming the dominant one)?
If you did observe a reversal in the competitive relationships, can you
determine what the “break point” was where this occurred?
Finally, what is the strategy that is most likely to result in the maintenance
of all three species (two sparrows and the hawk) in the community?
References
1. Bush, M. B. Ecology of a Changing Planet, 2nd ed. Upper Saddle River, NJ:
Prentice Hall, 2000.
2. Caswell, H. Predator-Mediated Coexistence: A Non-Equilibrium Model."
American Naturalist, 112 (1978).
3. Rosenweig, M. L. Stability of Enriched Aquatic Ecosystems. Science, 175
(1972).
4. Smith, R. L., and Smith, T. M. Elements of Ecology, 4th ed. San Francisco:
Benjamin/Cummings, 2000.
10