5/10/89 S H A R K S ! A Predator-Prey Simulator for EGA Thomas R. Wilson 4407 1/2 Stanford

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5/10/89 S H A R K S ! A Predator-Prey Simulator for EGA Thomas R. Wilson 4407 1/2 Stanford St. Houston, Texas 77006 SHARKS! is a graphic population model. Predators in the form of sharks hunt their prey, fish, in a struggle for sur- vival. Several independent variables, or "life parameters" can be controlled while the program is running. By altering their values, the interdependency of the two populations can be studied. More importantly, it is FUN TO WATCH! PROGRAM MECHANICS: The struggle is played out on an ocean of variable size. There are two kinds of creatures in the sea - sharks and fish. During a time cycle each creature has the opportunity to make one move in any of four possible directions. By repeating this procedure over and over, the passing of time is simulated. Each creature's life is governed by a set of rules that it must follow. Fish seek only open spaces, and if the amount of time allotted by the fish breed rate has passed, a new fish appears in that space. If the fish is surrounded by other fish, or it does not find an adjacent space empty, it will wait to breed. It is assumed that the fish have an endless food supply in the form of lower sea animals. Sharks seek out fish. Eating a fish delays starvation. If, after a search, no fish is found, a shark will move into an adjacent blank space if one exists. If the starve time is reached and no fish has been found, the shark will starve and disappear from the screen. Like the fish, if a shark is able to survive to reach breed time, a new shark will be born in an adjacent blank space. The sea is shaped like a torroid, or doughnut. The area on the screen is actually a two dimensional map of the volume's surface. Imagine that the top of this flat map can be curved back to meet the bottom, forming a cylinder. The ends are then brought together in a circle, completing the torroid. Thus, the map edges are connected and the creatures are permitted to scroll off the boundries and reappear on the opposite side. LIFE PARAMETERS: FISH BREED rate - How fast the fish breed. If this number is small, the fish breed quickly. Correspondingly, more food becomes available for the sharks to eat. Must be less than 64. SHARK BREED rate - How fast the sharks breed. This increases competition for the current food supply. Must be less than 64. SHARK STARVE rate - How long the sharks can go without eating. Increasing this makes the sharks less vulnerable to local food shortages. Must be less than the shark breed rate. SHARK SMARTS - How many directions the sharks will look in the search for a fish. Increasing this makes the fish more vulnerable to capture by the more cunning sharks. Vary from 1 to 4. POPULATION THEORY: A graph on the lower half of the screen keeps track of up- dated shark and fish totals. With time, trends in the two curves become apparent. Because the sharks are dependent on the fish population, there is a tendency for the shark population to "follow" that of the fish. As the fish breed and their numbers rise, more food becomes available for the sharks to eat. Food stimulates the shark population, causing it to increase as long as the feeding frenzy continues. With more predators hunting them, the fish population reaches some maximum, then begans to drop off. With less prey to eat, the shark population will also level off at some max- imum, then fall as more and more sharks starve in response to the food shortage. The cycle is completed when the shark population falls low enough that the fish start to multiply in response to the added safety of a sea with less predators. The curves approach two sine waves with the predator (shark) population lagging the prey (fish) population by some constant phase. In other words, both the populations tend to rise and fall cyclicly. The shark curve follows the fish curve because predator populations are a function of how much food there is available. The exact shape of these waves is determined by the input parameters. Varying the input parameters will vary the ouput waves' periods, amplitudes, and deviations as well as causing obvious changes in the distribution of the creatures on the screen. FAILURE CRITERIA: The system fails if conditions cause the loss of a species. SHARKS! detects failure and responds by restocking the sea. If the random attribute is set, new input parameters will be gen- erated, otherwise the old ones will be run again. Two possible events constitute failure: Predator extinction - Conditions were not favorable enough to ensure the survival of the shark species. Perhaps the area food supply was in- sufficient, or the sharks breed rate made competition for food too tough. Perhaps the sharks were not smart enough to consistantly find food, or they had to eat too often to make it through food shortages. Both species extinct - The struggle was too difficult for the fish to survive. With the sea clear of food, the sharks quickly follow into ex- tinction. If the fish did not breed fast enough, they may have been overwhelmed by the sharks. Another possibility is that they bred too fast, making a feast for the sharks. The sharks respond to food by proliferating larger, hungrier generations. The system collapses when the food supply cannot support the new predators' needs. It is important to realize the significance of the inter- dependancy of the populations on each other. Changing one parameter may affect a species in a predictable way, but the effects on the other population are less apparent. Real ecosystems have many links in the chain, increasing the un- predicatability of population manipulation. Especially curious is the paradox that a possible cause of failure is lack of predator intelligence, yet a more probable one is that the sharks are too smart for their own good. Their cleverness at hunting can destroy the fish, their only source of food. LIMITATIONS: SHARKS! is a simplified model of a complex system. Natural systems, of course, have many more variables that determine the two populations. The programming involved in setting up more realistic models would be tremendous, assuming that all of the independent variables could even be quantitized. These other variables tend to steady things out a bit. Real predator/prey populations do not have the dynamic range that is so evident in SHARKS! This is fortunate indeed, because the sea on the screen is quite volatile - it can turn from a harmonious struggle to a void wasteland of extinction in a flash. The situation and units described in SHARKS! should be viewed as being arbitrary. That is, any predator/prey combin- ation could be substituted for sharks and fish. Likewise, the individual areas on the grid could represent square miles or square feet. Each population unit could be thousands of crea- tures, and the time units could represent hours, or even years. COMMAND SUMMARY: F1 - decrease fish breed F5 - increase fish breed F2 - decrease shark breed F6 - increase shark breed F3 - decrease shark starve F7 - increase shark starve F4 - decrease shark smarts F8 - increase shark smarts 'h'- Help. Halts and displays command information on the screen. Hit any key to continue. 'c'- Clear sea. The sea is cleared, then restarted with new populations. If the random attribute is set, new parameters are chosen, otherwise the old ones remain in effect. 's'- Sea size. Changes the area of the sea under study. Four sizes are available. Smaller sea areas run faster but are statistically more vulnerable to extinctions. 'f'- Fast. Toggles fast/slow. When in slow mode, there is a delay each time all the creatures in the sea have had an oppurtunity to make one move. 'p'- Plot. Toggles on/off. If on, population data is logged to the graph in the lower half of the screen. Also pop- ulation totals are updated to the right of the sea. If plot is set to off, no data are written. This makes creature movement faster. 'r'- Random. Toggles on/off. If on, random breed parameters will be chosen each time the sea is cleared, or one of the failure criteria is met. 'd'- Dots. Toggles on/off. If on, single pixels will be used to represent creatures rather than graphic symbols. Dots run much faster than symbols because of the decreased need for memory access. If dots are selected, the dimensions of the sea are doubled to take advantage of this speed increase. 'x'- Scatter. Toggles on/off. If on, fish and sharks will be dis- tributed to their respective grid locations with small variations in position. Scattering the creatures eliminates the ordered "row-column" look that normally accompanies such a model. 'space'- Run/Stop. Movement may be frozen anytime, then restarted with the space bar. 'ESC'- Quit. Returns to DOS. REFERENCE: - "Computer Recreations", A. K. Dewdney, Scientific American, 12/84, pp. 14-22. - SHARKS! was created using Turbo-C 2.0 from Borland Internat'l. IMPORTANT: SHARKS! may be copied and distributed freely about the planet. SHARKS! is one of thousands of programs created by thousands of programmers that is available to you, the user, free of charge. We create these programs out of some specific need, or out of sheer love for programming, then pass them along to the rest of the world. However, if you appreciate excellence in software, and the fantastic network we have for its distribution, you should support those involved. If you enjoyed SHARKS! please send $10 to the above address. It won't break you and can only help your karma. ALSO by Thomas R. Wilson... PETE the Amazing Negro - this colorful character comes to your EGA screen to dazzle you and your friends for hours (or days!) on end. His talent for exciting the senses is sure to please. EGA_LSD - Take a "trip" into the psychedelic world of the infamous hallucinogen LSD...without leaving your seat or even taking the drug! PRISM - An animation from Pink Floyd's Dark Side of the Moon. Spinning rainbows and sparkling light for the easily amused. S O F T W A R E F O R T H E P E O P L E O F T H E U N I V E R S E !


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