WHY WOULD you bother modelling the number fish in your pond? Well, because you want to understand what the factors are that are responsible for the fish population to boom one year, but crash the next.

Gleick says:

A successful foray into mathematics for an ecologist would let him say something like this: Here's an equation; here's a variable representing reproductive rate; here's a variable representing the natural death rate; here's a variable representing the additional death rate from starvation or predation; and look --- the population will rise at this speed until it reaches that level of equilibrium.
And hey, if you can understand the factors maybe you can predict or at least plan ahead. You could start trading in Fish Food futures.