Can wolves and sheep coexist, or does one always wipe out the other? In the Population Trends graph, sheep-count and wolf-count rise and fall out of step: wolf peaks come roughly 50 ticks after sheep peaks.
Try this Switch model-version to sheep-wolves and press Reset. Without grass, sheep never starve and the swings grow until one side is gone, usually within 200 to 650 ticks: either the wolves eat every sheep and then starve, or the wolves die out and the run stops when the sheep pass 10,000.
About this model Where it ends up
Uri Wilensky's Wolf Sheep Predation (NetLogo Models Library, 1997), an agent-based version of Lotka–Volterra predator-prey dynamics. Instead of solving rate equations, individual sheep and wolves move, eat, reproduce and die, and the population curves come out of those encounters.
Each tick, every sheep in random order turns by up to 50° either way and steps forward 1. In the sheep-wolves-grass version it then pays 1 energy, eats the grass on its patch if there is any (gaining sheep-gain-from-food), dies if its energy is below 0, and otherwise reproduces with probability sheep-reproduce percent, splitting its energy with the lamb. Then every wolf moves the same way, pays 1 energy, eats one sheep on its patch if there is one (gaining wolf-gain-from-food), dies below 0 energy, or reproduces with probability wolf-reproduce percent. Eaten grass grows back after grass-regrowth-time ticks. In the sheep-wolves version there is no grass: sheep never lose energy and only reproduce.
initial-number-sheep and initial-number-wolves are read only at setup, and a model-version change needs Reset to redraw the grass; the gains, reproduction rates and grass-regrowth-time take effect live. The run stops when no animals are left, or when there are no wolves and more than 10,000 sheep. Rules and update order follow the NetLogo code. The show-energy switch has no effect in this port.
