Answers to Activity 4

Dispersal and Diversity Simulaiton Model

Students may be expected to make the following observations in the simulations:

1. Many species do not persevere over the course of the simulated time (Species1-3, 6-7, 10-12). Only few proliferate, some faster (Species4, 5, 8) than others (Species9, 13). By Time5, diversity in the inner 9 cells is generally higher compared to the diversity on the fringes, reflecting the maximum interaction between species inside the grid. Note that this particular outcome is the result of the limit of the grid. Unless there are distinct barriers around an area, a natural ecosystem -- however defined -- does interact with ecosystems beyond its defined boundaries.

2. The result of having a high number of unsuccessful species is that species diversity goes down. Individual species display more pointed success or failure patterns, i.e., they quickly are lost or they quickly proliferate. Invasion by the successful species is more pronounced.

3. When all species invade from one side, species diversity develops fairly equally (it is almost symmetrical) on either side of the central column (AG) by Time5. The central column hosts only one species, because of the habitat constraints for this column. Few species are entirely unsuccessful in this run and most others can be found in>50% of the grid cells at Time5.

4. If this simulation is run with the left two columns having only 1's and the right two columns only 0's, diversity will be almost symmetrical around the absolute barrier by Time5. Interestingly in this case, the diversity of the left side of the barrier remains stable from Time1 through Time5. Only on the right hand side does it increase. If, on the other hand, this simulation is run with the original species distribution (copied from the top rows [22-99] into the lower test portion of the model [rows 122-199]), species diversity declines from Time1 through Time5 because of the limited exchange across the barrier. Several species are lost entirely.

5. No matter what the initial species distribution on either side of the high-resistance barrier, diversity can vary quite broadly for this type of model simulation (run several recalculation cycles to see this). The places where species can cross over always host one species at Time5 (according to the habitat constraint) and the cells before and after the cross-over are hardly ever occupied by less than one species. Generally, species diversity in those 4 cells is rather high, and is usually not lower in the inner 9 cells than on the outer ring of the grid.

6. The situation is very similar to that described in point 4. above; however, species diversity is higher overall because there is at least some exchange across the low-resistance barrier.

7. Note the very low number of unsuccessful species in this case! If the no-resistance simulation is run with the left two columns having only 1's and the right two columns with only 0's, diversity will usually be highest in the center of the grid. The inner ring has higher diversity values than the outer ring. If, on the other hand, this simulation is run with the original species distributions (copied from the top rows [22-99] into the lower test portion of the model [rows 122-199]), species diversity is generally lower than in the above-described case and is much more homogenous across the grid. Still the central cell may be the one cell with the highest species diversity.

8. Examples of human interference with species dispersal and diversity include:

Table 3: Computer Simulations and Corresponding Real-World Examples of Human Interference with Ecosystems
Type of Simulation Examples of Human Interference

Change of random numbers to get extreme numbers of unsuccessful species

q repeated destruction of parts of habitats
q total destruction of an entire habitat
q paving an area
q water-logging a previously dry land area
q channeling a stream, eradicating wetland species that grew there before the channeling
q filling in wetlands

Change of beginning maps to the 1-1-0-0-0 pattern (dispersal from one side)
q planting trees in one area of an otherwise bare plot
q planting dune grass to allow dunes to form and persist in the face of erosion

Change of the center column to all 0's (absolute dispersal barrier)
q erecting a wall
q building a canal
q constructing a wide highway

Change of the center column to 0-1-0-1-0 (high-resistance barrier)

q building a road with frog fences; frogs are collected and brought across the road occasionally
q plowing fields; occasionally there is an unplowed edge along which species can disperse

Change of the center column to all 1's (low-resistance barrier)
q allowing the area underneath power lines to grow over, but cutting it occasionally so that some species won't be able to disperse
q installing a gas pipeline

Change of the center column to all 13's (no resistance, corridor)
q intentionally sowing species in a plot
q any freshly opened soil (after a clear-cut; the sides of a road after construction, a plot after burning, etc.)