What are seasonal variations and how do they effect ecosystems

Seasonal variations are your basic seasons and the act of them naturally changing- spring, summer, fall and winter. This affects ecosystems by limiting or producing abundant food supplies, water access, and other factors that would otherwise allow ecosystems- particularly the animals- to thrive in the area. The biggest changes are in spring and fall, when you have major animal migrations. Ecosytems naturally change because when the seasons change and the base of the food chain (plants) is limited, prey animals usually either migrate or hibernate, and predators have to search elsewhere for food. Reversed, in times when plants are abundant, prey animals are everywhere and predators generally eat well where they hunt (depending on location). 
Basically, seasons affect the general makeup of an ecosystem temporarily.