Connectivity and homogenisation of population sizes: an experimental approach in Lacerta vivipara

Abstract

Summary At regional scales, dispersal is known to prevent metapopulation extinction by buffering stochastic processes. Theory predicts that connectivity, through density‐dependent dispersal rates, should spatially homogenize population density and synchronize local population dynamics in the long term. However, empirical evidence for the effect of connectivity on synchrony and local population dynamics remains scarce. We experimentally manipulated connectivity in order to investigate the homogenisation effect on population size. The experimental design consisted of 16 patches of common lizard populations (Lacerta vivipara), half of which were connected by dispersal. The design allowed us to identify candidates for dispersal in unconnected patches. We found that population sizes became spatially more and more homogeneous with time in connected patches, whereas extinctions or demographic explosions were observed in unconnected patches. Juvenile dispersal was density‐dependent in connected patches but not in unconnected ones. These results suggest that the loss of connection modifies population functioning by influencing how dispersal is determined by local conditions. Finally, population explosions in unconnected patches were followed by a sharp decrease in population size. So non‐extinct, unconnected populations did not stabilize. This could be due to over‐compensatory density dependence. Population viability analysis models suggest that environmental stochasticity and catastrophic events, in addition to the density‐dependent process, are required to explain population size variation and extinction.