Modeling arbuscular mycorrhizal infection: is % infection an appropriate variable?

Several available models of arbuscular mycorrhizal infection are based on fitting % infection to a logistic curve and then relating the various parameters to biological functions. I suggest here that this direction is misleading. Percent infection is a value derived from the growth of two interdependent but distinct organisms, each of which is seeking to maximize its own growth and survival. I suggest that two-organism models, such as those derived from Lotka-Volterra equations, are more useful for understanding the biology and functioning of mycorrhizae.