Policy and Complex Systems
( ABM ) exhibits the “ stepped pattern of biomass accrual ” found in simple predator – prey dynamics ( Carmichael & Hadzikadic , 2013 ). Briefly , in a three-trophic-level system ( food , prey , predators ), increasing the food available to the prey will cause the predators to increase in population but , surprisingly , does not change the population size of the prey themselves . That is , even though the prey are able to eat more — and thus reproduce faster — with an increase in food , the predators immediately match this change by consuming the extra prey that are now above the equilibrium level . Add food at an even faster rate and the predator population will grow to an even higher population level , and continue to consume all the extra prey . Thus , when resources are increased to the prey , the prey population can eat faster , and reproduce faster , but they are also consumed faster , while the predators alone increase in population size . This non-intuitive result matches the predictions found in Oksanen et al . ( 1981 ).
Another consequence of this dynamic is that the predators do not increase their equilibrium consumption rate at the per capita level . Predator consumption rate does change when they are out-ofequilibrium , but only until reaching the new population level ; at that point the per capita consumption rate has returned to the predators ’ equilibrium level of consumption . Figure 1 illustrates the increased population size for the predators based on an increased resource level for the prey , as well as the concurrent reduction in the prey ’ s average age .
Fig . 1 . Population growth of predators ( purple ) based on increased resources to the prey population ( left ) and the concurrent change in the average age of the prey ( right ). The simulation was run for 6000 time steps ; resource rate was increased after 2000 steps ( a ) from 0.06 to 0.12 , and again at step 4000 ( b ) from 0.12 to 0.18 . Adapted from ( Epstein , 1999 ).
2.1 - Model Description
This generative ABM is purposefully kept as simple as possible , in order to determine baseline properties and consequences of the interacting populations with as few complicating factors as possible .
In the ecological literature , it is noted that the outcomes of this model do not match completely with the real world . This includes Oksanen ’ s results , Gause ’ s Law , the Paradox of Enrichment , and even the Lotka – Volterra model of predator – prey dynamics . This incongruity is acceptable , even expected , as
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