The Ecological Footprint Model was constructed in order to calculate the, and define, the amount of biologically productive water and land needed in order to sustain life when following a stated lifestyle. What this means is that the population's use of resources must not exceed the production of resources in order to sustain life. If the population's use of resources exceeds the planet's production of resources, then sustainability of life will lower.
There is, however, one problem with the model. The model does not take into account life outside of human life. While they do not have the same impact upon the resources, animal life is not taken into account, only humans.
Basically, the model was created in order to do two things: show how humanity must change in order to insure that they (mankind) does not use more resources than the earth can create (renewable resources) and provide a time-line, if humanity does not change, which shows how long the planet can maintain life at the rate humanity is using the resources.