Not all parts of the earth recieve the same amount of rainfall.  This characteristic is probably the main reason the biomes are so different.  This graph shows the average yearly rainfall.

In the biomes of the Earth the temperature varies a great deal.  This graph shows the drastic change in average yearly temperature between biomes.

Bacteria and Protists
Important bacteria and protists that are needed are Lactid Acid Bacteria (LAB) and other non-pathogic bacterias.  These bacterias help make food stay fresher and not rot.  Other bacteria that is needed helps purify water at sewage plants and helps break down oil after oil spills.  Some bacteria play an important role in the breaking down of organisms.  Another important type helps in digestion and kills harmful organisms.  Only 1% of all bacteria is harmful to humans, so most of them are helpful.