The primary factor affecting the distribution of organisms is environmental conditions, which include factors like climate, soil type, water availability, and habitat structure. These conditions influence where species can thrive and reproduce. For instance, temperature and rainfall patterns dictate the types of vegetation in an area, which in turn affects the animals that inhabit those regions. Understanding these environmental factors is crucial for ecology and conservation efforts, as they help predict species distributions and potential changes due to climate change and habitat modification, guiding management strategies for biodiversity preservation.