We intend to clarify potential confusions of representation of the Lotka–Volterra model (pp. 53–55) in the article “Mental models of dynamic systems: taking stock and looking ahead” by Stefan Groesser and Martin Schaffernicht (System Dynamics Review, 28(1): 46–68).

  1. The model includes additional structure and extends the conventional Lotka–Volterra model. A maximum-function limits prey deaths to the consumption of prey by predators (Eq. 3, p. 53). In the death rate of prey, a density dependence effect is used which represents that crowding affects the hazard rate of death (Eq. 4, p. 53).
  2. Non-standard notation of the Lotka–Volterra model. In addition, we note that “bPrey” and “dPrey” are the birth rate (inflow) and death rate (outflow) of “PreyP”, the population of prey. The variable “dPrey” does not indicate a derivative of the prey population.
  3. In the paper, the sub-index t is meant to represent continuous and not discrete time. The model was mistakenly documented in discrete time. In the paper, the equations of the Lotka–Volterra model and the references to them in the text used time indices “t” and “t − 1”, which lead to the impression that we propose a discrete time formulation of the model. Following is the complete set of corrected equations. The equation numbers refer to those in the article. Our reasoning assumes continuous time, and we have rewritten the equations. We cannot replace the “t − 1” in the text, but we kindly ask the reader to interpret it as “just before t”.
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