Equilibrium hypotheses for the maintenance of tropical diversity generally invoke some form of niche differentiation. This hypothesis is based upon the idea that ecologically similar species are unable to coexist unless they have developed different patterns of habitat distribution and/or resource use. Within this framework, the more specialized the resource requirements of each species are, the more species can be packed into a given habitat. Niche differences among tropical animals are generally related to the type of food resources utilized, or spatial or temporal differences in habitat use.
For example, otherwise ecologically similar animals can differ in terms of height of activity in the canopy, or the time of day they are active. In contrast, all plant species utilize essentially the same set of basic resources, namely: light, water, carbon dioxide, physical space, and nutrients such as nitrogen, phosphorus and potassium.
However, plant species can differ in terms of more subtle ecological characteristics, such as the efficiency of resource use, tolerance of physiological stress, or dependence on specific pollinators, seed-dispersers or root symbionts. Many studies of tropical forest trees have emphasized differences in the "regeneration niche", or the resources and conditions required by seedlings and saplings to successfully establish in the forest. An important distinction is made between ‘pioneer’ tree species that can grow rapidly in large canopy openings or cleared areas, and "late-successional" or "primary forest" tree species that can establish under low light conditions in the understorey. Other kinds of niche differences among tropical trees include ‘structural niche’ differences related to the size reached by adult trees, and differences in ‘habitat preference’ related to soil characteristics and hydrology.
