Population Dynamics

Predator-Prey with “fishing”

Prey Starting population
(100 to 1000)

Prey ideal growth rate
(0 to 2)

Prey nutritional value
(0 to 1)

Predator Starting population

Predator death rate
(0 to 2)

Predator effectiveness
(0 to 1)

Fishing effort
(0 to 2)

About This Graph

This graph is the same as the basic predator-prey model except that it introduces a "fishing" term. The word "fishing" is used here because this model was first used to describe the effect of fishing on tuna and shark populations in the Mediterranean Sea during and after World War 1. In general, "fishing" can refer to any situation where both the predator and prey populations are hunted or harvested equally. For example, it might refer to fisherman harvesting both sharks and tuna. It might describe the effects of hunting on wolf and deer populations where both species are hunted equally. It could be used to explore the effect of pesticides that kill both predator and prey populations equally.

The horizontal axis on the graph is the time axis and the vertical axis is the population size. Note that the scale on the vertical axis changes to match the calculated values. Pay close attention to the vertical scale when comparing graphs. The time axis does not represent any particular period of time. It has no units. The point here is to explore patterns of change over time, not to reresent specific time periods. The horizontal axis always represents the same amount of time, though, so you can compare patterns on different graphs against each other. The prey population is shown in purple and the predator population is shown in green.

Fishing effort is a number that indicates the rate at which the populations are harvested. The numbers are arbitrary. Higher numbers mean that a greater proportion of each population is killed or harvested.

Other parameters controlling the initial conditions are the same as for the basic predator-prey model.

Prey starting population and Predator starting population are the number of individuals in each population at the beginning.

Prey ideal growth rate is the rate of prey population increase in the absence of limiting factors, e.g. if there were no predators. The actual growth rate of the prey population will vary with changes in the predator population. The numbers are arbitrary, larger means faster growth rate.

Predator death rate is the rate at which the predator population will die off when there is no prey to feed on. The numbers are arbitrary. Larger numbers mean the predators die off more quickly.

Prey nutritional value indicates the degree to which eaten prey results in new predators. The numbers are arbitrary. Larger numbers mean more new predators result from the same amount of food.

Predator effectiveness represents the degree to which encounters result in the prey being eaten. (A fox doesn't eat a rabbit every time it encounters one.) The numbers are arbitrary.