Density dependence plays an important role in the regulation of most populations. Descriptive studies provide only limited evidence, while density manipulations are thought to be a more powerful tool. Here, we describe such a manipulation. We experimentally analyzed demographic responses to changes in density in common lizard (Lacerta vivipara) populations. Two neighboring sites were studied from 1986 to 1988. At the end of the first year, density was artificially decreased at one site and increased at the other. The evolution of demographic parameters over time (before and after manipulation) was compared between the two sites. We found that: (1) density in each age and sex class quickly converged to pre—experimental values at both sites; (2) survival rates (introduced individuals excluded) were unchanged, except that juvenile mortality was positively related to density; (3) emigration rates remained low and did not seem to be influenced by density, while immigration rates were negatively influenced by density; (4) the proportion of young reproductive females was negatively influenced by density; (5) body length and growth rate were negatively influenced by sensitivity only in yearlings, and at least in females, no catchup on growth is apparently possible; no difference in reserve storage was found; and (6) female reproduction was modified by the manipulation through clutch size and hatching success, which were negatively influenced by density; however, no difference was detected for hatching date, number of abortive (clear) eggs, prenatal mortality, reproductive investment, and body mass of live neonates. Rapid density readjustments were mostly explained by immigration into the site where density was decreased, and by high mortality of introduced individuals and of native juveniles where density was increased. Other parameters that were influenced by the manipulation could only have induced delayed effects on population density. Different age and sex classes showed different responses to density. These responses indicate that density dependence plays a key role in shaping the demography of this lizard species.

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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.

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We experimentally investigated the relative role of kinship and density on juvenile dispersal in the common lizard. A few days after birth, juveniles were introduced into seminatural enclosures, where they experienced different social environments: in the first experiment we varied the density of unrelated adults (males or females) within the enclosure (0, 1, or 2 adults), and in the second experiment, we varied the level of kinship and familiarity between juveniles and adults. Each enclosure was connected to a second enclosure by small holes which allowed only juveniles to move between enclosures. Juvenile movements were monitored during 14 days after birth, as juvenile dispersal is mainly completed within 10 days after birth under natural conditions. Most juveniles did not return to the first enclosure. Sex had no effect on juvenile dispersal. Adult density and kinship with adults both affected dispersal. Adult female density increased juvenile dispersal whatever the level of kinship and familiarity with the females. Dispersers had better body condition than nondispersers at high female density, and this difference was significantly greater when the mother and the familiar female were present in the enclosure. Furthermore, body condition of mothers and familiar females was positively correlated with juvenile dispersal, whereas there was no such correlation in the case of unfamiliar and unrelated females. These results strongly suggest that adult female density is a major factor promoting dispersal in this species and that both intraspecific and kin competition motivate dispersal.

While males gain obvious direct advantages from multiple mating, the reproductive capacity of females is more constrained. The reason why polyandry evolved in females is therefore open to many conjectures. One hypothesis postulates that females gain indirect benefits by increasing the probability of siring young from high quality males. To explore this hypothesis, we used the natural variation of the reproductive value that males and females undergo through age. The age-related variation of phenotypic performance might then induce variations in mating strategies in males and females. Using the common lizard (Lacerta vivipara) as our model system, we showed that reproductive immaturity and senescence created variability in both male and female reproductive success (including survival of offspring). Consistent with theory, males at their best-performing phenotype adopted a polygynous strategy. These males were of an intermediate age and they produced offspring of higher viability than younger and older males. In contrast, females at their best performing phenotype, also of an intermediate age, were less polyandrous than other less-performing females. Middle-aged females tended to mate with males of an intermediate age and produced litters with higher viability independently from their reproductive strategy. Males of an intermediate age enhanced their fitness by additional matings with young or old females. Young and old females increased their fitness by being more polyandrous. Polyandry therefore appears as means to seek for good males. A positive correlation between males and their partners' fitness disagree with the idea that polyandry is the result of a sexual conflict in this species.

The common lizard (Lacerta vivipara) is a small live-bearing lacertid that reproduces once a year. In order to document the poorly known mating system of this species, we present here an assessment of multiple paternity using microsatellite markers. Paternities were established within 122 clutches belonging to two wild populations from contrasted areas and to four seminatural enclosed populations. The proportion of multiply sired clutches was found to be very high (between 50.0% and 68.2%) and similar among populations, which suggests that the mating system of this species may be insensitive to environmental and population conditions.

Dispersal of organisms generates gene flow between populations. Identifying factors that influence dispersal will help predict how species will cope with rapid environmental change. We developed an innovative infrastructure, the Metatron, composed of 48 interconnected patches, designed for the study of terrestrial organism movement as a model for dispersal. Corridors between patches can be flexibly open or closed. Temperature, humidity and illuminance can be independently controlled within each patch. The modularity and adaptability of the Metatron provide the opportunity for robust experimental design for the study of 'meta-systems'. We describe a pilot experiment on populations of the butterfly Pieris brassicae and the lizard Zootoca vivipara in the Metatron. Both species survived and showed both disperser and resident phenotypes. The Metatron offers the opportunity to test theoretical models in spatial ecology.