ISLAND BIOGEOGRAPHY Lab 7

Transcription

1 Reminders! Bring memory stick Read papers for Discussion Key Concepts Biogeography/Island biogeography Convergent evolution Dynamic equilibrium Student Learning Outcomes After Lab 7 students will be able to: 1. explain the principles of biogeography and relate them to terrestrial habitats. 2. demonstrate the complex interactions of longterm ecological processes in biogeography including immigration and extinction using graphs and apply these ideas to broader evolutionary concepts. Book Chapters Campbell: Chp ISLAND BIOGEOGRAPHY Lab 7 I. BIOGEOGRAPHY Biogeography is the study of the spatial and geographical distribution of organisms. The central question of biogeography is, Why do we find these organisms at this location? The answer to this question includes both large scale answers (the community of species found at a given location is a function of the species pool from the larger surrounding region) and small scale answers (local competition and habitat conditions determine community composition), as well as, both historical (the community composition is a function of species and conditions that were there previously) and modern answers (the community composition is strictly a function of recent processes). When discussing the composition of a community, we can analyze it at many taxonomic levels (e.g. genus, family, etc.), not just at the species level. Biogeography also deals with functional questions of why certain areas tend to have similar or different communities. For instance, Figure 1 Figure 1. Desert grasslands from the Gobi Desert in Asia and the Chihuahuan desert in New Mexico. Can you tell which one is which? shows desert habitats from very distant locations in Asia and North America. Despite very different geographical locations, they support very similar communities of organisms. Similarly, the organisms within those communities may be very distantly related, but show convergent evolution toward a similar ecological role. For example, Figure 2. The Tasmanian wolf is more closely related to a kangaroo than a wolf. Fall

2 the recently extinct Tasmanian wolf (Figure 2) was a predatory animal very similar to North American and Eurasian wolves. However, it is a marsupial and is more closely related to kangaroos than wolves (yep, it s got a pouch and everything). II. ISLAND BIOGEOGRAPHY The theory of biogeography was developed by Robert MacArthur and Edward O. Wilson and published in their book on in Many early naturalists, including from Captain Cook s voyage, noticed a relationship between the size of an and the number of species it supports (species richness), with larger s supporting more species. In the Galapagos Islands, MacArthur and Wilson documented a similar pattern (Figure 3). This trend is true not just for plants in the Galapagos, but also for bird species in Hawaii, bat species in the Caribbean, amphibians and reptiles in the West Indies, and others. dynamic equilibrium because the composition of species on the might change over time, but the total number of species generally remains about the same. Imagine a new that appears off the coast of the mainland. Species that colonize this will come from the pool of species found on the mainland (or whatever is the source of new migrants). As the number of new immigrants increases, the rate of immigration will slow down because there will be fewer species remaining in the pool of potential immigrants that could migrate to the. Immigration reaches zero when all the species from the mainland pool have reached the. Initially, extinction rates on this new will be random. As more and more immigrants arrive, however, extinction rates will increase. This is because there are a larger number of species on the that could potentially go extinct, as well as, because habitat space is being filled up and more and more species on the are competing for fewer and fewer resources. If we plot immigration rates and extinction rates on the same graph, we can find the equilibrium number of species at the point where the two curves intersect (Figure 4). Figure 3. The relationship between size and the number of plant species. (Image from Campbell and Reece 2005). In MacArthur and Wilson s view, the number of species on an is a dynamic equilibrium between immigration and extinction rates. The balance between the number of new species arriving and the number of species going extinct on the will determine the equilibrium number of species on the. This is called a Figure 4. The relationship between immigration and extinction rates on the equilibrium number of species on an. (Image from Campbell and Reece 2005). We can look at the effect of immigration on the equilibrium number of species in more depth by including the effect of distance from the pool of Fall

3 potential colonizers (Figure 5). Islands that are close to the mainland will attract immigrants at a higher rate for the simple reason that it is easier for organisms to travel that shorter distance. Immigration rate Far Near Immigration or extinction rate à Far Near Small Large resident species Figure 5. The relationship between immigration rates and the number of resident species. We can also look at the effect of extinction on the equilibrium number of species in more depth by considering the size of the. Larger s will have lower extinction rates because they presumably have more space and a wider variety of habitats so could support a larger number of species (Figure 6). Extinction rate Small Large Equilibrium number of species Figure 7. Equilibrium species numbers with different extinction and immigration rates. Immigration curves in red and extinction curves in black. See also Figures 5 & 6 for more on immigration and extinction curves. (Image from Campbell and Reece 2005). Large s that are close to the mainland will support the largest number of species, while small s that are far from the mainland, will support the fewest number of species. Distant large s and close small s will support a similar number of species. The theory of biogeography was originally developed by researchers working on oceanic s. However, the concepts apply to any isolated patch of habitat. This could include cold mountain tops separated by warm valleys, ponds separated by dry land, or separate individuals of a host species for a parasite. A notable example in Hawaii is a kipuka. A kipuka is a patch of forest in which flowing lava has destroyed surrounding areas making it an of forest in a sea of lava rock (Figure 8). resident species Figure 6. The relationship between extinction rates and the number of resident species. By combining these two graphs, we can simultaneously examine the effects of both size and distance from the mainland on the equilibrium number of species (Figure 7). Figure 8. Kipukas on the of Hawaii. Fall

4 III. LABORATORY EXERCISES We have so far examined population and community ecology from a variety of perspectives. Today you will be examining the effects of the immigration and extinction of species on the make-up (species richness) of an community. General Procedures For this simulation in biogeography, the class will be divided into four teams. Collections of small cups will represent s, ping pong balls will represent species, bouncing them into the cups will represent immigration events, and dice rolls will represent extinction events. You will keep a running tally of immigration, extinction, and the species richness of your s, and then generate a graph as in Figure 7. Each group will be assigned one of the following scenarios by your TA. At the end you will combine the data for your homework within your class and with some other sections to have a big enough sample size. Near Large Islands The large will consist of two sets of 6 numbered, plastic cups one blue set and one red set placed next to each other. 1. Place a mark one meter away. 2. Take 5 ping pong balls and from one meter away, try bouncing them into the cups that make up the (they have to bounce at least once). Try a few practice rounds before recording data. 3. A ball bouncing into a cup is a successful colonization of a species onto that. A ball that lands on top of the cups is not successful, nor is a ball that lands in the same cup as another ball (think competitive exclusion principle). 4. Everyone on the team should take turns at colonization attempts to control for any differences in colonization ability (i.e. bouncing skills) amongst group members. Rotate jobs after each round (1 round= 5 balls) 5. On Data Sheet 1, record the starting number of species in that round, the number of successful colonizations, and the new total number of species (i.e. # of balls in cups). 6. Randomly choose either the red or blue die and roll to determine if extinction occurs. If there is a ball in the cup with the number matching the die roll, remove that ball an extinction has occurred. If there is no ball in that cup, no extinctions have occurred. (A piece of tape wrapped sticky side out can help you remove the balls). 7. Record the new number of species on the for the beginning of the next round. 8. Repeat steps 2-7 for a total of 30 rounds. Each round, attempt colonization with 5 ping pong balls, regardless of events in previous rounds. Near Small Islands The small will consist of one set of 6 numbered, plastic cups either blue or red. 1. Follow directions as for the near large, but with only one set of cups and one die for extinctions. 2. Record data on Data Sheet 1. Note that the max number of resident species you can have in Datasheets 1A and 1B is 6 (not 12, so ignore rows for species 7-12). Far Large Islands The large will consist of two sets of 6 numbered, plastic cups one blue set and one red set placed next to each other. 1. Follow directions as for the near large, but place the mark two meters away. 2. Record data on Data Sheet 1. Far Small Islands The small will consist of one set of 6 numbered, plastic cups either blue or red. 1. Follow directions as for the near small, but place the mark two meters away. 2. Record data on Data Sheet 1. Note that the max number of resident species you can have in Datasheets 1A and 1B is 6 (not 12, so ignore rows for species 7-12). Fall

5 Data Compilation For each of the four possible scenarios above, you will be generating two graphs (eight graphs total). One graph of immigration and extinction rates vs. number of resident species (as in Figure 4) and another graph of resident species number vs. time (or numbers of rounds). Remember to number the figure, label the axes, and your curves, and add a caption for each. To help you with the data for these graphs, there are data tables for compiling your section s data. Fill in the datasheet that corresponds to the scenario you collected data for. On the immigration data compilation sheet 1A, the initial number of resident species is from column 1 of Data Sheet 1. The number of colonization attempts is the number of rounds in which you started with X number of resident species multiplied by five attempts (ping pong balls) per round. (For example, if you had 8 of the 30 rounds start with no resident species in the cups, the number of colonization attempts for zero number of initial resident species is 8 * 5 = 40). For column 2 of Data Sheet 1A, to get the number of successful colonization events, sum up the number of successful colonization events from column 2 of Data Sheet 1 for those rounds in which you started with X number of resident species. The extinction rate compilation sheet 1B is similar to the immigration rate compilation sheet, however, the number of resident species after colonization is from column 3 of Data Sheet 1. In 1B, the number of extinction trials is the number of rounds in which you started with X number of species from column 3. To get the number of extinction events, sum up the number of extinction events from column 4 of Data Sheet 1 for those rounds in which you started with X number of resident species. IV. ASSIGNMENT (35 pts.) Do not leave lab early. If there is time, complete at least the graphs for Question 1 before leaving. Make sure you label all axes and add captions. The datasheets for question 3 will be a compilation of several sections and will be provided to you by your TA within 48hrs of your lab. 1. Use your section data to plot the number of resident species vs. time (rounds) for the simulations of all four groups within your section. You should end up with a total of 4 graphs, one per scenario (Near-Large, Near- Small, Far-Large, Far-Small). (4 pts.) 2. Describe each of your four graphs from Question #1 (e.g. - does the number of resident species fluctuate, does it level off quickly, etc.) in a cohesive paragraph including the following questions: For each of the scenarios, did the number of resident species on the reach equilibrium? (i.e. did it achieve a stable number of resident species?) If it did reach equilibrium, how many rounds did it take to reach that number? If it did not reach equilibrium, why do you think it didn t? How do the plots for small s compare to large s (think about the shapes of the plots, the variability, and the final equilibrium point)? How do near s compare to far s? Is this what you expected according to what you know about the theory of Island Biogeography? (8 pts.) 3. For the Near-Large Island scenario, using the compiled data from several sections (provided by your TA within 24hrs), plot the mean immigration and extinction rates vs. number of resident species on a single graph (see Figure 4). Do a XY scatter plot. Add polynomial trendlines for both data sets!! Repeat this for each of the other scenarios (Near-Small, Far-Large, Far-Small) so you get a total of 4 graphs. (4 pts.) Fall

6 4. a. What do your 4 graphs from Question #3 predict to be the equilibrium number of species for each of the four scenarios? (2 pts.) b. Do these predictions match the number of species you found at the end of 30 rounds of your sections simulations (see graphs of Question #1)? (1 pt.) Why do you think that they do or do not match? Explain. (1 pt.) c. How do your sectionʻs simulations rank in terms of the number of resident species (use number of resident species at end of the 30 rounds for each scenario from graphs in Question #1; e.g., large near : 10 species, rank 1; large far : 8 species, rank 2; etc.)? (1 pt.) Do these ranks (Question #1; number of resident species over time) match the predicted ranks (number of species at equilibrium from graphs in Question #3 )? (1 pt.) V. REFERENCES Blackburn, T.M., P. Cassey, R.P. Duncan, K.L. Evans, and K.J. Gaston Avian Extinction and Mammalian Introductions on Oceanic Islands. Science 305: MacArthur, R.H. and Wilson, E.O An equilibrium theory of insular zoogeography. Evolution 17(4): Let s say hypothetically, that one group in the class was much better than others at bouncing ping pong balls into cups. In our simulation, this would symbolize a higher immigration rate. Name two ecological or environmental conditions in which you might find variation in colonization rates to s that are otherwise matched for size and distance from the mainland. (5 pts.) 6. Write a cohesive paragraph on the material you read for your class discussion today according to the specifications of your TA. (8 pts.) Fall

7 Data Sheet 1: Island Scenario (circle one for both) Near / Far Small / Large Round # Column 1 Initial Resident Species (# of balls in cups) Column 2 Successful Colonization Events Column 3 Species after Colonization (Columns 1 + 2) Column 4 Extinction Events Fall

8 Data Sheet 1A: Immigration rate compilation table Island Scenario (circle one for both) Near / Far Small* / Large Initial number of resident species Column 1 Colonization Attempts Column 2 Successful Colonization Events *If you have the small then your max of resident species is 6 Data Sheet 1B: Extinction rate compilation table Island Scenario (circle one for both) Near / Far Small* / Large resident species after colonization Column 1 Extinction trials Column 2 Extinction Events *If you have the small then your max of resident species is 6 Fall