Figure 2.

Links between specificity and niche space, and the relevance of biotic interactions. (a) While the niche is classically defined as the intersection of tolerances over multiple environmental axes, specificity is the breadth of tolerance on each axis (Futuyma & Moreno 1988). Integrating specificity over all environmental axes defines the niche; thus fundamental and realised niches are, respectively, associated with potential and realised specificity [coloured areas correspond to hypothetical relative frequencies in tolerances or performances (dashed line = fundamental; solid line = realised) for each independent environmental axis]. How specialisation and niche evolution will relate depends on the correlations and pleiotropic effects linking niche axes. (b) The traditional definition of the fundamental niche (F) is that it represents the total multi-dimensional ecological space in which a species could persist. The realised niche (R) is the ecological space in which a species actually persists and is at least partly dependent on biotic interactions with other species. Biotic interactions have the potential to change the presence of a focal species along one or more of the axes that define F. For example, associations with antagonists (predators, pathogens or competitors) may further constrain the realised niche (R_A), while facultative mutualists may increase the potential for a species to expand or shift its realised (and even fundamental) niche along one or more axes depending on the nature of interactions with other species in the community (R_M,A and R_M).