A guild (or ecological guild) is any group of species that exploit the same resources, often in related ways. It does not follow that the species within a guild occupy the same, or even similar, ecological niches. Guilds are defined according to the locations, the attributes, and the activities of their component species; for example, their mode of acquiring nutrients, their mobility, and the zones of their habitat that they occupy or otherwise exploit. The number of guilds occupying an ecosystem is termed its disparity. Members of a guild within a given ecology could be competing for some resources (such as space or light), while cooperating in resisting wind stresses, attracting pollinators, or detecting predators, such as happens among savannah-dwelling antelope and zebra.
A guild does not typically have strict, or even clearly defined boundaries. A broadly defined guild will practically always have constituent guilds; for example, grazing guilds will have some species that concentrate on coarse, plentiful forage, while others concentrate on low-growing, finer plants. Each of those two sub-guilds may be regarded as guilds in appropriate contexts, and they might in turn have sub-guilds in more closely selective contexts. Some authorities even speak of a fractal resource model. This is a concept that arises in several related contexts, such as the metabolic theory of ecology, the scaling pattern of occupancy, and spatial analysis in ecology, all of which are concepts fundamental in defining guilds.
An ecological guild is not to be confused with a taxocene, a group of phylogenetically related organisms in a community that do not necessarily share the same or similar niches. Nor is a guild the same as a trophic species.