Metapopulation Stability in Fragmented Forests

Abstract

Metapopulation theory predicts that a network of habitat patches connected by dispersal can maintain a species regionally even when individual local populations are extinction-prone, provided that the colonisation rate from occupied to empty patches exceeds the local extinction rate and that connectivity among patches is sufficient for rescue effects to stabilise populations approaching extinction. This study empirically tested metapopulation stability predictions from the Levins and Hanski incidence function model frameworks against 18-year occupancy and genetic connectivity data from 284 forest vertebrate populations across 48 fragmented forest landscapes in Italy, spanning a gradient from highly fragmented (mean patch area 4.2 ha; inter-patch gap 1.8 km) to moderately connected (mean patch area 48.4 ha; inter-patch gap 0.4 km). Twelve species representing diverse forest vertebrate guilds (woodland birds, small mammals, reptiles, amphibians) were monitored annually. Metapopulation occupancy turnover (annual local extinction and colonisation rates) was significantly predicted by patch area (area coefficient b = +0.48; p < 0.001) and isolation (distance coefficient b = -0.42; p < 0.001) across all 12 species, consistent with incidence function model predictions. Population genetic structure (RADseq; n = 28,484 SNPs per species) confirmed that effective gene flow between patches (migrants per generation Nm > 1) was limited to inter-patch gaps < 0.8 km for all amphibian and reptile species, < 1.4 km for small mammals, and < 2.8 km for woodland birds. The regional metapopulation viability threshold -- the minimum patch-network configuration sustaining regional persistence -- was estimated at mean patch area >= 12.4 ha combined with inter-patch gap <= 0.8 km for low-dispersal species, and >= 4.4 ha with <= 2.4 km gap for high-dispersal birds. Landscape genetics confirmed that current Italian forest fragmentation patterns render 64.4% of amphibian and reptile metapopulations below the estimated viability threshold, identifying patch-network enhancement through corridor planting and forest patch enlargement as the priority interventions under the EU Nature Restoration Law