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.