Summary1. Habitat fragmentation is involved in the present extinction crisis and is known to in¯uence many aspects of population dynamics. The level of connectivity between populations is one of its components. In an experimental study on the common lizard (Lacerta vivipara Jacquin), we analysed in two contrasting habitats the in¯uence of connection on dispersal patterns, on dierences between dispersing and philopatric individuals, on population size, survival rates, and reproduction. 2. The experimental design consisted of eight two-patch units. Half of them were connected by dispersal. We performed this treatment in a grassland and a wood clearance, two habitats with dierent resource availability (respectively called`rich' and`poor' habitat). 3. We expected that the loss of connection would modify juvenile dispersal patterns, have a dierent eect on dispersing and philopatric individuals, negatively aect survival rates and fecundity by modifying social interactions and would have dierent consequences in`rich' habitats compared to`poor' ones. 4. Connection modi®ed dispersal patterns. Two dispersal periods occurred in unconnected units, but only one occurred in connected ones. Moreover, dispersers of the second dispersal period were morphologically similar to philopatric individuals. The loss of connection probably caused the late dispersal of individuals which would have remained philopatric in a connected context. Dispersers of the second period seemed to have lower winter survival rates than philopatric individuals, as expected if these individuals had been`forced' to disperse. 5. We found that connection modi®ed demography through juvenile winter survival rates and female reproduction depending on habitat type. Connection led to higher juvenile survival rates in`rich' habitats, whereas it was not the case in`poor' habitats. The loss of connection decreased female reproductive rate whatever the type of habitat. 6. This study suggests that connection can directly modify demographic parameters depending on habitat quality and involving behavioural mechanisms. It con®rms that conservation management, like installation of dispersal corridors, must take into account habitat characteristics and behavioural features.

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.

Parasites affect the life‐histories and fitness of their hosts. It has been demonstrated that the ability of the immune system to cope with parasites partly depends on environmental conditions. In particular, stressful conditions have an immunosuppressive effect and may affect disease resistance. The relationship between environmental stress and parasitism was investigated using a blood parasite of the common lizard Lacerta vivipara. In laboratory cages, density and additional stressors had a significant effect on the intensity of both natural parasitaemia and parasitaemia induced by experimental infection. Four weeks after infection, crowded lizards had three times more parasites than noncrowded lizards. After 1 month of stress treatment, naturally infected lizards had a significantly higher level of plasma corticosterone and a higher parasite load than nonstressed individuals. In seminatural enclosures, stress induced by the habitat quality affected both the natural blood parasite prevalence and the intensity of parasitaemia of the host.