The HANDY Model is Based On Predator-Prey Cycles

As indicated above, Human And Nature DYnamics (HANDY) was originally built based on the predator-prey model. We can think of the human population as the predator", while nature (the natural resources of the surrounding environment) can be taken as the prey", depleted by humans. In animal models, carrying capacity is an upper ceiling on long-term population. When the population surpasses the carrying capacity, mechanisms such as starvation or migration bring the population back down. However, in the context of human societies, the population does not necessarily begin to decline upon passing the threshold of carrying capacity, because, unlike animals, humans can accumulate large surpluses (i.e., wealth) and then draw down those resources when production can no longer meet the needs of consumption. This introduces a dierent kind of delay that allows for much more complex dynamics, fundamentally altering the behavior and output of the model. Thus, our model adds the element of accumulated surplus not required in animal models, but which we feel is necessary for human models. We call this accumulated surplus "wealth".

Empirically, however, this accumulated surplus is not evenly distributed throughout society, but rather has been controlled by an elite. The mass of the population, while producing the wealth, is only allocated a small portion of it by elites, usually at or just above subsistence levels. Based on this, and on the historical cases discussed in the introduction, we separated the population into Elites" and Commoners", and introduced a variable for accumulated wealth. For an analysis of this two-class structure of modern society, see Dragulescu and Yakovenko [2001]; Banerjee and Yakovenko [2010]. This adds a a dierent dimension of predation whereby Elites prey" on the production of wealth by Commoners. As a result, HANDY consists of four prediction equations: two for the two classes of population, Elites and Commoners, denoted by xE and xC, respectively; one for the natural resources or Nature, y; and one for the accumulated Wealth, w, referred to hereafter as Wealth". This minimal set of four equations seems to capture essential features of the human-nature interaction and is capable of producing major potential scenarios of collapse or transition to steady state.