The component of emigration that is much less easily
understood and has not been widely studied is what
occurs when an individual actually encounters an edge.
There are several factors, both internal and external,
that can influence whether or not an individual crosses
an edge or remains within the habitat patch. Intrinsic
factors may inhibit dispersal (Ehrlich 1961), and these
factors may include genetic dispersal cues and an individual’s
expectation of fitness within the patch (for
reviews of adaptive reasons for dispersal, see Johnson
& Gaines 1990; Stenseth & Lidicker 1992). External
factors, which have received much less study, include
the structure of the edge (which includes abruptness
and degree of contrast), the immediate local environment
at the point of edge encounter, the quality of the
habitat as a whole and the type and quality of the bordering
habitat. Stamps
et al
. (1987) define the probability
that an individual crosses an edge it has
encountered as ‘edge permeability’.