Few topics have attracted the attention of ecologists more than fluctuations in the numbers of plants and animals through time and their variation in abundance through space. Understanding population fluctuations, and thus population conservation, requires understanding the links between demographic processes--birth, death, immigration and emigration--and the environments in which populations exist. It is only by viewing populations from these several perspectives at once that one can both appreciate the enormous complexities of population abundance and distribution and begin to organize and understand that complexity. This understanding is an essential prerequisite to developing a practical theory of population ecology that can aid in conserving biological diversity.