The largest possible ecosystem is a biosphere.  It includes every organism and species on earth, combining all of the planet's individual ecosystems.

G.E. Hutchinson defined the biosphere in terms of three conditions that allow life in the biosphere.  First, there are large amounts of liquid water.  Second, the biosphere receives a large amount of solar energy from the sun.  Third, matter is able to change from gaseous, liquid, and solid states.

The biosphere is extremely complicated, and has countless diverse features.  It is divided into five kingdoms: monera, prototista, fungi, animala, and plantae. 

Another way to break up the biosphere is into biomes, groups of biotic communities.  There are many different types of these, and they are related through many complex processes such as photosynthesis and respiration.

The most important factor in the biosphere is without doubt humans.  Humans collectively eat massive amounts of food, take over natural systems, consume plants and trees, and create countless problems such as pollution.

Other systems encompassing the entire planet have relationships with the biosphere.  The atmosphere, lithosphere, hydrosphere, and pedosphere all interact with the biosphere in highly complicated and varied ways.

To increase comprehension of the biosphere, scientific research will have to be continued.  Currently, human knowledge is great but highly limited nonetheless.  For example, though humans know of 1.5 million living creatures, some estimates show that there may be as many as 100 million creatures sharing our planet.  Further studies will have to take into account the many levels of the biosphere and all of its unique components.