on a smaller scale, the two sides of a valley, or of a tree, may be home to very different kinds of organisms if the two sides receive unequal amounts of sunlight or rain. when ecologists study the distribution of organisms, they try to discover the physical and biological factors that influence the presence or absence of particular species. they also look for any historical factors that may have affected where species are found, and for patterns that might indicate how the distribution of populations could change in the future. this is especially important in the case of rare or endangered species.