Occupancy
Occupancy is governed by two primary factors: first, the ability for species to access various sites on the landscape, and second, their ability to persist at these sites. The former varies, in part, as a function of a species’ dispersal capacity, suggesting that movement is important in determining patterns of occupancy. Once a species establishes at a site, its persistence will be driven by the balance between births and deaths, which, in turn, should be governed by the suitability of conditions at the site (i.e., environmental filtering) as well as demographic processes.
Theory supports the above hypotheses. Two concepts from metapopulation theory , which is the principal theory explaining patterns of occupancy, are relevant here. The first is the importance of the balance between colonisation and local extinction in driving occupancy. The second is the distinction between patch and matrix, that is, the presence and spatial arrangement of sites with suitable (patch) or unsuitable (matrix) habitat on the landscape. While the spatial arrangement of patches is a geographic contingency that is beyond the scope of the current discussion, the distinction between patch and matrix acknowledges the role of environmental filtering in driving patterns of occupancy.
The balance between colonisation and local extinction is somewhat more complex, as both processes are intimately tied to abundance: larger local populations produce more propagules, thus increasing the probability of colonising new sites , and are less prone than smaller populations to local extinction resulting from demographic stochasticity. These dynamics have been invoked to help explain the widely observed positive relationship between occupancy and abundance , further underscoring the relationship between these two rarity dimensions. As such, the drivers of abundance indirectly influence occupancy via their effects on colonisation and extinction (Figure 3).
Finally, the importance of colonisation in metapopulation theory underscores the importance of movement as a driver of patterns of occupancy . Based on the discussion in this subsection, we conclude that occupancy is directly driven by environmental filtering and movement, although the indirect effects of demography and interactions, through their influence on local abundance, should not be neglected.