Species Introduction Lab
Gerius Patterson and Betsy Von Holle
Species introductions occur world-wide for a variety of reasons, including agricultural and transportation practices. Even though spider introductions are typically ignored, there are probably many introduced spiders to new environments. There are only two experimentally proven phenomena for species introduction: 1) Propagule pressure increases the chance for successful invasion and persistence, 2) Generalist predators are more likely to have bigger impacts, and thus more likely to be successful.
Outline of this Lab
The purpose of this lab is to discover when these phenomena occur and what factors make an introduction more likely to be successful. There are two separate Ecobeaker Situation files associated with this Lab - one for the Generalist Predation section and one for the Propagule Pressure section. You will have to load each Situation file for the appropriate sections below.
Generalist predation theory predicts that generalist predators are more likely to succeed than herbivores and insectivores. Two spiders are introduced onto an island that already contains two native spiders and an aphid and fly species as prey.
Propagule pressure theory predicts the greater the number and higher the rate of introduction, the better the chance of successful introduction. As most introductions occur randomly across spatial and temporal domains, the settlement procedure for all four species will be a random number. One species will be picked for propagule pressure variation while the others are held at low propagule pressures. Graphical interpretations will be used to discern optimal propagule pressures.
1. Run Ecobeaker (double-click on its icon)
2. Open the Propagule Pressure Situation file
3. The window to the top left is the representation of the island. The square island is 100 miles on each side. The window to the bottom right to the species window where you will see all of the species in the model. Double click on each species to change the parameters of the model. The control panel is where you start and stop the model runs. Always remember to reset the model with the RESET button on the control panel after every parameter change.
4. Run the model once so that you can see how it works. When the model is run, you will see aphids and flies colonizing the island and the four spider species subsequently colonizing the island habitat. To the right of the island is a line graph of the numbers of individuals in each population graphed against the x-axis of time.
5. Watch the population graph for a while. Are all the species surviving? Does one or more spider species survive most of the time? Are all the population numbers of all of the spider species high or low?
6. Now we are going to go over the 'rules' of running a model: there are a set of parameters for the prey species (aphid & fly spp.) and the predators (the 4 spider spp.). The number of the aphids that live on the island is regulated by something Ecobeaker calls Settlement procedure, which is a set of rules governing how many new aphids and flies get placed on the island. Each species has its own settlement procedure. The settlement procedure for the aphids and the flies is density dependence. This represents how aphid and fly species might settle and reproduce in nature. The settlers are a fixed number of creatures per time step and then adds to that a number of additional settlers proportional to the current population size of the species.
7. Double click on the APHID species in the species box and the dialog box will appear where you can set up the rules governing the aphid species. Find the button labeled SETTLEMENT PARAMETERS and click on it. A second dialog box will appear that has the parameter for Density Dependence. You will see that the parameters are Num Immigrants/Turn, Num Settlers/Individual and Possible Uses. The Num Immigrants/Turn is going to decide how many aphids are going to settle there per time step, regardless of how many are there already. Change the 1 Num Immigrants/Turn to 5 Num Immigrants/Turn. Num Settlers/Individual is a way of taking into account the reproduction of the settlers already there. So it is the number of offspring they are allowed to have per time step per individual. Keep the 0.1 Num Settlers/Individual as it is. Ignore Possible Uses, as this is taken care of with the action procedure for animals. Repeat the same procedure for FLIES.
8. Now reset the model (after any time you change the parameter of the model, from now on, remember to push the RESET button on the Control Panel). The run the model again (GO). Again watch the yard and the graph to see how long more than one species of spider survives. Does anything different happen with more aphids/flies?
9. Now lets change the parameters for the spiders. Each species of spider has a settlement procedure similar to that of the aphids and flies, which determines how many spiders from that species are dropped onto the island. The only difference is that we add spiders once, unlike the aphids and the flies, which are added every day. Spiders also have a second set of rules that tell what they do once they land on the island. EcoBeaker calls these rules the Action Procedure, and the action of the spiders is called Predator, as they are eating insects, and in some cases, other spiders.
10. Let us investigate the Generalist Predator theory for invasion biology. Open the Generalist Predator Situation file. Double click on the Native brown spider sp. C in the Species window to bring up the dialog box where you can set the rules for brown spiders. Clicking on the SETTLEMENT PARAMS button will let you change the way the spiders are dropped onto the island. Changing this number works the same way it did for the aphids and flies. In the spiders settlement procedure, change the procedure to HABITAT DENSITY DEPENDENT. Repeat this procedure for the rest of the spiders.
Let us see what happens when one spider is an omnivore and the rest are insectivores. Double click on the action procedure (Predator) of Invasive Spider A and then double click on the prey species. Then click so that this spider eats everything but itself. Now reset the model and run. What happens? Which species has the highest population numbers? Run this model 25 times, recording at approximately the end of 1000 days, which spider 'wins' and the population size. The spider that 'wins' will be the spider with the highest population size. Record the population sizes of the other spider species. How close to zero are the other populations?
11. Now let us see what happens when the invasive species are added randomly, as generally occurs in nature. Change the settling procedures of the invasive spiders to random number. Keep the native spiders at habitat density dependence. Run this model 25 times, recording at approximately the end of 1000 days, which spider 'wins' and the population size. Record the population sizes of the other spider species. How close to zero are the other populations?
12. Now let us investigate the effect of a generalist predator already present among the native species. So we will leave invasive spider species A as an omnivore, but we will add a native omnivore by changing the action procedure for native spider species C. Again, change the action procedure so that spider species C eats everything but itself. Make the settlement procedure for all species HABITAT DENSITY DEPENDENT. Run this model 25 times, recording at approximately the end of 1000 days, which spider 'wins' and the population size. Record the population sizes of the other spider species. How close to zero are the other populations? How does an equivalent functional group (omnivore) mitigate the effect of an introduced omnivore? What predictions would you make about introductions of species into communities with a unique (nonexisting) functional group for that community?
13. Again, let us look at the effect of random settling of invasive spiders by changing the settlement procedures of the invasive spiders (A & B) to random number. Run this model 25 times, recording at approximately the end of 1000 days, which spider 'wins' and the population size. Record the population sizes of the other spider species. How close to zero are the other populations? Does this change the effect of the generalist predator introduced? How might this be reflected in nature?
14. O.K., now that you are aware of the effects of introduced generalists, now let us explore propagule pressure of introduced species. Change the settlement procedure of all of the spider species back to random number. Run the model 25 times, recording population numbers of each spider species. Now change the Num Immigrants/Turn for Invasive spider A from 1 to 5. Run the model. Run this 25 times, recording at approximately the end of 1000 days, which spider 'wins' and the population size. Record the population sizes of the other spider species. How close to zero are the other populations? Repeat the whole procedure for 10, 15, 20, and 25 Num Immigrants/Turn for Invasive spider A, running each model 25 times. What is the optimal or threshold number of immigrants per turn for an invasive speices to add to a community in order for it to be successful? At what point will the other spiders in the community go extinct?
15. Now that you understand the importance of propagule pressure for the successful introduction of species, let us see what happens when this is more reflective of what really happens in nature. You would expect the population growth of native species to be density dependent, while it would be generally expected for invaders to be introduced randomly (the number of spider species that are shipped onto an island via a banana shipment would be random). Change the settlement procedure of the native species back to density dependence and run the model with all of the species arriving at the same Num Immigrants/Turn (1). Run the model 25 times, recording the winner and the population sizes. Now sequentially change the Num Immigrants/Turn for Invasive spider A (5, 10, 15, 20, and 25), running the model 25 times each. What is the optimal or threshold number of immigrants per turn for an invasive speices to add to a community in order for it to be successful? At what point will the other spiders in the community go extinct? Which of the two models (all species settling at a random number, or only the invasive species settling randomly) do you think will more accurately reflect natural conditions?
16. Which of the two phenomena we tested (propagule pressure, generalist) do you think will be more important in determining whether an invasion is successful? under what conditions?
More Things to Experiment With
Now that we have explored the effects of propagule pressure and generalists being introduced into a community, you can start to experiment with the different settling procedures. See how changing the settling procedure of the different spider species (native and nonnative) will change the outcome of coexistance. Try changing the whole community to more of a deterministic model or one that depends on how far away the source population is of spiders (Fixed at X and Interact, respectively) to see how this changes the outcome of the community. Try changing the settling parameter of just the nonnatives or only one invasive species to see if invasion success can depend solely on settling procedure.
Also, you may want to change the functional groups of the natives and the invaders to figure out which functional group will be the most invasive. You can experiment to figure out which functional groups or combination of functional groups in a native community will make the community more resistant to invasion by a particular functional group.
Another fun thing to try is to add disturbance to the model to see how this might change the result of successful invasion with the 1st two exercises. It is possible that a disturbance may preferentially wipe out native species and that this will give the invaders a better foothold for invasion. See the "Hurricane Settlement Procedure" in the manual. Included in this description is how to add species. Have fun experimenting!