The implications of metalandscape connectivity for population viabilityin migratory songbirds

Landscape connectivity is considered important for species persistence, but linkages among landscape populations (metalandscape connectivity) may be necessary to ensure the long-term viability of some migratory songbirds at a broader regional scale. Because of regional source-sink dynamics, these species can maintain steady populations within extensively fragmented landscapes (landscape sinks) owing to high levels of immigration from source landscapes. We undertook a modeling study to identify the conditions under which immigration, an index of metalandscape connectivity, could rescue declining populations of songbirds in heavily disturbed landscapes. In general, low to moderate levels of immigration (m = 0-20%) were sufficient to rescue species with low edge-sensitivity in landscapes where < 70% habitat had been destroyed. At the other extreme, moderate to high levels of immigration (m = 11-40%) were usually required to rescue highly edge-sensitive species in these same landscapes. Very high levels of immigration (m > 40%) were required to rescue highly edge-sensitive species in extensively fragmented landscapes that had lost > 50% habitat, or when any landscape lost >= 50% habitat gradually over a period of 100 or more years (r = 0.5% habitat lost/year). Paradoxically higher levels of immigration were thus necessary to offset population declines when habitat was lost gradually than when it was lost quickly, where population response lagged behind landscape change. This implies that the importance of metalandscape connectivity for population viability may not be fully appreciated in landscapes undergoing rapid rates of change. Natural immigration rates for migratory songbirds match the very high levels (> 40%) we found necessary to sustain populations in heavily disturbed landscapes, which underscores the importance of metalandscape connectivity for the continued persistence of many migratory songbirds in the face of widespread habitat loss and fragmentation.