The earth has many ecosystems or communities of living things surviving together within a distinctive collection of geographical features and weather patterns.  These communities can be large and small, simple and complex, populous and sparse.  The living organisms within them interact and form relationships that serve to balance and check populations, are symbiotic, can be predatory and are competitive.  Scientists and other experts break down ecosystems and create a simplistic model to stand for the basic parts of these complex systems.  The field of expertise is called modeling and it is the study of how the pieces fit, whether there are patterns, and whether behavior can be predicted within the ecosystems.  Modeling is especially useful when trying to figure out what effect outside forces, like human activities, have on natural systems.