Abstract

The paper by Don Hugie and Tamara Grand (2003) confirms the validity of their previous
results that had been called into question by us. This new paper reassures us that both
parties are still of one mind regarding what we feel is the key message from the original
paper of Hugie and Grand (1998): that non-IFD movements are likely to be crucial to our
understanding of animal distributions.
The new results presented by Don and Tamara bear strong similarities to those presented
previously in both Hugie and Grand (1998) and Ruxton and Humphries (1999). Using an
analytic model, Hugie and Grand (1998) predicted a ‘single stable distribution’ for
each competitor type, while Ruxton and Humphries’ (admittedly crude) individual-based
simulation model predicted a distribution of potential equilibria resulting from shifts from
one equilibrium to another over time. Don and Tamara now show that, when their original
model is translated more carefully into an individual-based simulation, ‘variation in the
competitor distribution appears to be centred on the equilibrium distribution of the large
population scenario’ (Hugie and Grand, 2003: 141). Thus, although a dynamic equilibrium
results in the large population model, when a finite population is considered, ‘the probabilistic
nature of competitor movements causes the flow of individuals moving in either direction
to vary, preventing any competitor distribution from persisting indefinitely’ (p. 141).
The main difference from our results is that we predicted temporal variation with the system
moving between a number of possible IFD distributions, whereas Hugie and Grand (2003)
predict similar variation centred on a single distribution. Hugie and Grand’s (1998) analytic treatment of an infinite population size does not predict temporal variation at all.