For Nature Photography, you need a tripod, because

good lighting is always at earlier morning or late afternoon when the light is dim. If you shoot birds and animals, they are usually active at morning and evening. BTW, you also need a fast lens to deal with the action of animals. If you are interested in close-up, the tripod is essential.

As what John Shaw (Nature photography Master) said, tripod is not a accessary, it is neccessary.