SA Niche Wars 2019-1 - Professor Ghosh PDF

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SimBio Virtual Labs®: EcoBeaker®

Niche Wars Sample Workbook

This workbook, or any portion thereof, may not be reproduced or used in any manner without the express written permission from the copyright holder. Students purchasing this workbook new from authorized vendors are licensed to use the associated laboratory software; however, the accompanying software license is non-transferable. .

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SimBio Virtual Labs®: EcoBeaker®

Niche Wars

with the underlying models in our simulations so that the results they produce (i.e. the “right answers”) change, and we let instructors know h teresting experience and a fun way to learn.

Introduction The Ecological Niche The idea of an organism’s ecological niche is likely one that you’ve encountered before, as it is one of the most basic concepts in ecology, and one that has been the focus of research for nearly a century. Some ecology textbooks define it as the complete set of conditions that allow an organism to survive and reproduce, while others describe it as the role the organism plays in the environment. Historically, ecological research on niches focused on describing the ecological requirements of individual species. Current research uses the niche concept to make predictions about the relationships between organisms and their environments and the consequences of those relationships for community structure—especially species diversity within communities. This work is particularly important in the face of modern challenges such as the introduction of non-native species into new habitats and global climate change. The modern concept of the ecological niche integrates the work of many important ecologists. Joseph Grinnell used the term in 1917 to describe the combination of abiotic and biotic conditions that limit the distribution of California Thrashers, birds restricted to very specific habitats in California. In 1927, British ecologist Charles Elton used the term to describe the interactions that determine whether or not species can coexist within the same community. Although they focused on different questions, both ecologists recognized the explanatory power to be gained from understanding the ways in which species interact with their environments.

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SimBio Virtual Labs® | Niche Wars

At about the same time that Grinnell and Elton were developing their ideas based on direct observations of natural plant and animal communities, others were using the tools of mathematics to describe and predict patterns of population growth and competition among species for scarce resources. The Russian scientist G.F. Gause (1934) combined mathematical modeling and rigorous laboratory experiments to study what he called “the struggle for existence”—specifically, population growth, interspecific competition, and predation. Gause’s studies confirmed, both empirically and mathematically, the predictions of earlier investigators that two species requiring the same limiting resource cannot coexist; one species will inevitably be better adapted to its environment and outcompete the other. This idea forms the basis for the principle of competitive exclusion, which states that two species cannot persist in the same niche in the same habitat. In the late 1950s, a limnologist named G. Evelyn Hutchinson developed a conceptual model of the ecological niche that would allow scientists to develop specific, testable, falsifiable hypotheses about species interactions, distributions, and coexistence within communities. A good way to understand Hutchinson’s model is to perform a thought experiment. Imagine that you are describing the ecological niche of some animal and have determined that it can survive and reproduce across temperatures ranging from 10–80º C. You could represent that aspect of the animal’s niche as a line on a graph whose x-axis was temperature. Now suppose that you further discovered that the animal could survive and reproduce in environments with rainfalls ranging from 25–50cm per year. You could plot that aspect of your animal’s niche on the y-axis of your graph. At this point, you would have a graphical and mathematical expression of your species’ niche as a 2-dimensional shape. Next, imagine that you determine that your animal is active from 4 a.m. to 11:00 a.m. If you plotted that range of times as a third, z-axis on your graph, you would have represented your animal’s niche as a 3-dimensional volume. (a)

(b)

Figure 1. The graph on the left (a) shows how two dimensions of an organism’s niche (rainfall and temperature) can be visualized with an area. The graph on the right (b) shows how adding a third dimension (activity) to that niche can be visualized with a volume. In both graphs, the dark bar on the axis line represents the range of that condition that is tolerated. A graph with more than three dimensions would be difficult to depict, but forms the conceptual basis of Hutchinson’s “n-dimensional hypervolume”.

Hutchinson’s insight was that an organism’s ecological niche is defined by the range of all conditions— biotic and abiotic—that underlie its ability to survive and reproduce. Because many more than 3 sets of conditions are important, each organism’s niche will be a “hypervolume” with n dimensions, where n is the number of conditions involved. Hutchinson’s concept of the niche as an n-dimensional hypervolume is basically a complex formulation of Grinnell’s and Elton’s ideas. Hutchinson referred to this abstraction of an organism’s niche as its fundamental niche and noted that it describes the sum total of the organism’s basic (fundamental) ecological properties. Building on Gause’s concept of competitive exclusion, Hutchinson pointed out that two species sharing the same fundamental niche could not coexist. But he noted that the more interesting case occurs when two species’ niches overlap only partially (imagine two partially overlapping squares or cubes in Figures 1a and 1b). In that case, the two species could potentially coexist within the same community if one or both restricted its niche to minimize overlap. The restricted version of a species’ niche—restricted usually because of interactions with another species—is called its realized niche. When organisms coexist in environments with limited resources, we generally assume that the niches we observe are realized niches; we often find that their fundamental niches are broader. Hutchinson’s work on niches led to many important advances in our understanding of the factors that limit species distributions and species diversity within communities.

Commanding The Niche Wars In this lab you will experiment with the relationship between the ecological niche and competition. Your model organisms will be rabbits raised in a pen and provisioned with food. Every day, food will be added to the rabbit pen. Individual rabbits will hop around looking for and eating food. Rabbits gain energy from eating and use up energy staying alive. If a rabbit’s energy runs out, it dies. If, on the other hand, it acquires more than it needs to survive, it will reproduce. The key niche dimensions in this simulation are habitat (the pen) and food. As you work through the exercises, you will have the opportunity to modify your rabbits’ niches. The first exercise allows you make some basic observations and to perform a “bunny” version of one of Gause’s original experiments (he used Paramecium in test tubes, which aren’t quite as cute as rabbits). After you’ve experimented with the basic features of competitive exclusion, you will move on to the second and third exercises, in which you will explore mechanisms that allow species with similar niches to coexist within the same habitat. In addition to examining how coexistence can be achieved, you will also observe some of the patterns that can arise as a result of competition.

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In the final exercise, you will use the power of computer modeling to tackle the challenge of modifying the niches of your rabbits such that more than one species can coexist, even though their resources are extremely limited. When you have completed the exercises, you should be able to: –

Define the term ‘ecological niche’.

–

Compare and contrast fundamental and realized niches.

–

Explain the competitive exclusion principle in terms of ecological niches.

–

Compare and contrast the two scenarios you investigated under which species coexistence is impossible.

–

Describe the conditions under which multiple species can coexist in the same community, including conditions in which competition is present.

–

Explain the relationship between competition, the fundamental niche, and the realized niche using specific examples from the lab as well as hypothetical examples.

The Niche Wars Simulation Model in SimBio Virtual Labs This lab uses a simulation model of a rabbit pen containing four different “species” of rabbits. The model establishes rules for each species that are based on a number of important characteristics reflecting their ecological niches and their competitive abilities. These characteristics include how far rabbits can see (to find food), how fast they can hop (to acquire food), how much energy they use each day just to stay alive, how much energy they must accumulate before reproducing, and how much energy they absorb from each type of food they eat. As you will see, the simulation model is “parameterized” by assigning values to the variables for the rules. You will have the power to change each of the rules separately for each of the rabbit species, and you will also be able to change the amount of lettuce (and later on carrots) that the rabbits are fed every day.

Part 1: Competitive Exclusion Exercise 1. Rabbits and Their Habits: Competitive Exclusion After reading the introduction, start the program by double-clicking the SIMBIO VIRTUAL LABS icon or by selecting it from the Start Menu on your computer. [1]

When SIMBIO VIRTUAL LABS opens, select NICHE WARS from the EcoBeaker suite. You will see several panels on the screen: – The large panel on the left shows the rabbit pen. It is initially provisioned with lettuce; as the simulation runs, you will also see four different “species” (colors) of rabbits moving around the pen. The rabbits have no predators, and their color differences do not provide them any advantages or any disadvantages. – The bar graphs on the right show the population sizes of all species in the pen. – A drop-down menu at the top of the screen lets you select each of the exercises in this lab. Be sure the exercise Rabbits and Their Habits is selected. – A Control Panel in the bottom left corner on the screen lets you start and stop the simulation.

[ 2 ] Advance the simulation one week by clicking the STEP 1 button in the Control Panel. Then click the STEP 1 button three or four more times, watching the rabbits hopping around eating the lettuce that is tossed into the pen. Notice that the total time elapsed is tracked at the bottom of the screen. [ 2.1 ]

Do the graphs indicate that any species (=colors) are at a greater risk of going extinct than any others? If so, which one(s) and why?

Of the bunny populations, the black bunnies are the most at risk of extinction because there is only one black bunny currently alive in the pen.

[3]

Click the RESET button, then click the STEP 100 button. The simulation will stop automatically after running for 100 virtual weeks. Note: speed things up by moving the speed slider to the right. It’s a bit like feeding your rabbits caffeine! [ 3.1 ]

Did all four species survive? Briefly describe what happened in the space below.

Only the white bunnies survived after 100 weeks. One by one each type of bunny started to die off leaving more food for the other bunnies, the white bunnies acquired most of the food and were able to out reproduce the others not leaving any food for the rest so they were the only ones to survive.

[ 4 ]

Click the RESET and then the GO button. This time run the simulation until only one species survives, recording the order of extinctions and surviving “color” in the Run 1 column below. Then run the simulation 2 more times, similarly recording your results in the Run 2 and Run 3 columns. [ 4.1 ]

Order of extinctions: RUN 1

[ 4.2 ]

1

White

2

Red

3

Black

Survivor

Brown

RUN 2

RUN 3

Brown

Red

Red

White

White

Brown

Black

Black

What evidence do you have that random chance determines which species will survive?

The order in which the bunny populations die off is different everytime. Although black survived twice, it still shows random chance because before brown survived and in the trial for number 3, white survived. It ultimately just depends on the location of the bunnies when the food is placed which is random.

[5]

The competitive exclusion principle (described in the Introduction) applies when two or more species occupy the same niche. To determine whether the rabbits in the simulation occupy the same niche, click on the MODEL SETUP button at the right on the Control Panel. A floating window will appear that lets you see the model settings (parameters) for each of the four “species” of rabbits. Select the tab for the Red rabbit and look at the settings. Then select the tabs for each of the other 3 species and look at their settings. [ 5.1 ]

Based on the model parameters, are there any differences in the niches of the 4 rabbit species? If so, what are they?

All the bunnies have the exact same values granting none of them a competitive advantage over the others.

[ 5.2 ] Given what you’ve discovered about each species’ niche and your results from running the simulation, explain how the competitive exclusion principle applies to these species. The competitive exclusion principle describes how species with the same niche can’t coexist forever. We see this with the bunnies who all have the same niche, yet one species still out lasts the others due to random chance.

[6]

Close the MODEL SETUP window.

[ 7 ] Competitive exclusion occurs when a critical resource is limited in the environment. You will now modify the simulation to increase the amount of lettuce (the limiting resource) available each day. To the right of the rabbit pen, find the box labeled Lettuce/day. The number is currently set at 4 heads of lettuce. Click inside the box and change the value to 8. [ 7.1 ] With twice as much lettuce available, will multiple species be able to coexist? Explain your reasoning.

No the lettuce is still a limiting resource.

[ 8 ] Now test your prediction. Click the RESET and GO buttons to run the simulation. As before, run the simulation several times. [ 8.1 ]

Did you predict correctly? Explain your results.

Yes my prediction was correct. All that changed was the amount of bunnies present and the time it took for extinction of the other bunnies.

[ 9 ] As you read in the Introduction, Gause concluded that two species requiring the same limiting resource can’t coexist, because one species will inevitably be better adapted to its environment —specifically, better adapted to finding and using that limiting resource—than the other. It will, therefore, outcompete the other species. In your previous experiments, the four species were identical (i.e., they had the same model settings). In your final experiment of this exercise, you will test Gause’s hypothesis by modifying one species to make it a better competitor than the others. Two of the most important characteristics determining how well rabbits can acquire and use food are found in the MODEL SETUP: sight distance (how far away a rabbit can see, and thus detect food) and speed (how fast the rabbit can get to the food once it’s been seen). [ 9.1 ]

[ 10 ]

If one species has an advantage in either sight distance or speed, what outcome do you expect? Explain your reasoning.

Now you get to test your prediction. Click the MODEL SETUP button and select the tab for one of the rabbit species. Increase the value for either speed or sight distance (up to a maximum value of 10). If you make a mistake, click the RESET DEFAULTS button at the bottom of the window. RESET the simulation, and click GO to run. As before, repeat the simulation several times. [ 10.1 ] Did you predict correctly? Describe your results and explain them in terms of competitive exclusion.

The bunny with the advantage will survive in the trial.

[ 11 ]

Click the RESET DEFAULTS button in the MODEL SETUP window to restore the original settings. Now give your species a smaller advantage than you did previously and repeat the experiment. [ 11.1 ]

Did you predict correctly? Describe your results, explaining them in terms of competitive exclusion.

Yes, my prediction was correct. I gave the black bunny a speed value of 5 and it survived in all 3 of the trials I ran. The black bunny no longer occupied the same niche and thus was able to survive.

[ 12 ] In this exercise, you observed that only one species of rabbit can survive in the pen when all rabbits require the same food source. This was true when all species were identical in every characteristic, and when one species was better adapted to find and acquire food. [ 12.1 ] When all four species were biologically identical, were you able to correctly predict which species would survive competition?

No it was purely random.

[ 12.2 ]

What about when one was given an advantage?

Yes, the bunny with the advantage would survive.

[ 13 ]

Click the TEST YOUR UNDERSTANDING button in the bottom right corner of the screen and answer the questio nin the window that pops up.

Part 2: When Can Species Coexist? Exercise 2. Shoots and Roots: Niche Divergence and Overlap As Hutchinson noted, thinking about competitive exclusion becomes particularly interesting when species have niches that do not overlap completely and/or when species reduce their fundamental niches as a result of competition. In this exercise, you will explore how species can coexist within the same community. To do this, you will focus on the extent to which your rabbits require the same food resource. [ 1 ] What would happen if your rabbits were provided with two food resources; lettuce and carrots? To explore this question, select the Shoots and Roots exercise from the drop-down menu on the top toolbar. Note that you now have two boxes at the bottom of the pen for controlling food: one for Lettuce/day and one for Carrots/day. The default value for each is 2, for a total of 4 food items per day. [ 1.1 ]

Will adding carrots as a food source allow more than one species to persist in the pen? Explain your reasoning.

I don’t think a new food source will change the species of rabbits that can survive.

[2]

Test your prediction by running the simulation several times (clicking the RESET and GO buttons). Remember to increase the speed slider if you find yourself wishing the rabbits would hop faster. [ 2.1 ]

Did you predict correctly? Describe your results below.

Yes my prediction was correct. Since all the bunnies eat both lettuce and carrots, it’s the same scenario as just increasing the amount of leccuce in the pen. Yet again only one type of rabbit remained.

[ 3 ] You can change what a rabbit species eats...