Patch occupancy, dispersal, and the rescue effect in fragmented habitats
Occupied patch
Empty patch (recolonizable)
Poor quality patch
Isolated (no dispersal)
Levins (1969) metapopulation model: dp/dt = cp(1−p) − ep
Equilibrium occupancy: p* = 1 − e/c. Patches persist if c > e. Rescue effect: immigration from occupied patches reduces local extinction probability.
Mass effect: high-quality patches act as sources; poor patches are sinks.
Dispersal distance controls which patches are connected — fragmentation reduces effective colonization.