They developed a model that could be used to predict at what body size an organism, given a choice of alternate habitats, would optimally switch from one habitat to another. The model casts the question of evolutionary fitness as a trade-off of the benefits vs costs of living in one place or moving to an alternative area. The insights of this model are easily shown with graphical comparisons. As the fish grows, however, the food in H1 is less well suited, and eventually H2 presents a better environment in terms of growth. Growth rate is a good parameter to track, because it integrates all of these various biotic and abiotic factors.
Growth rate represents the benefits accrued by the fish from living in one or another habitat.
Certainly, the risk of dying represents the ultimate cost. To make the Werner-Gilliam model work, m must be greater in H2 than H1. Note that m varies between 0 and 1, because it is a risk, and a risk is nothing more than a probability. In this case, G is the benefit, and m is the cost. Werner, Gilliam, and their colleagues tested this model in lakes and experimental ponds.