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

Abstract

Summary1. 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. 2. 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. 3. 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. 4. 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. 5. 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.

Sex ratio bias, male aggression, and population collapse in lizards

“…Longer-term monitoring of another set of experimental populations (18) showed similar amplification of sex ratio bias (40% initially, 61% after 4 years, n ϭ 16), parallel with a sustained population decline and high extinction rates (18). Thus, the deleterious demographic effects of male excess on females raise a major threat to population persistence that had not been previously recognized (2,13,14).…”

“…Each disperser was randomly assigned to a new, unfamiliar population of the same sex ratio treatment. The habitat configuration matches the scale and the processes of a natural patchy population, and the enclosures were large enough to support independent demographic units as in a natural metapopulation with frequent emigration and immigration events (18). In our study area, some females start reproduction after their first winter, and small females typically mate later than large females (19).…”

“…After introduction into the patches, individuals were allowed to disperse freely along one-way corridors that offered linear transects of habitat (18). We monitored emigration during the year following introduction.…”

“…Our experimental system comprised enclosed patches of habitat that offered seminatural conditions to lizards (17). Lizards were released in 12 enclosed patches (10 ϫ 10 m) located in a natural meadow and covered by nets to avoid avian predation (18). Each enclosure was extended with a 20-m long one-way corridor ending with a pitfall trap used for daily capture of dispersing lizards.…”

Mother–offspring competition promotes colonization success

“…For each combination of parameters, 100 population trajectories of 1-yr length were drawn by using Monte Carlo simulations. Extinction probabilities were computed as the number of extinct trajectories over the total number of simulated trajectories (59). Simulations were performed with the ULM computer program (60).…”