No abstract

We conducted a 5 year demographic study in one population of the viviparous lizard Sceloporus grammicus Wiegmann, 1828 in central México. The population was structured in three size classes (juveniles, small adults, and asymptotic adults) for which we estimated annual survival and fecundity rates. A population projection matrix was constructed for each annual transition. All of them resulted in finite rates of population growth (l) that, although variable from year to year (from 0.808 to 1.065), were not significantly different than unity, indicating population stability. Elasticity analysis revealed that survival staying in the same size class was the demographic process that made the greatest contribution to l values in most years. Similarly, the stasis of large adults was the vital rate with the highest relative importance for population persistence. To incorporate the observed yearly variation in long-term population projections, we used a mean matrix, a stochastic simulation, and a resampling procedure. All these resulted in long-term population growth rates that were not significantly different than unity. Our results indicate overall demographic stability for the studied population of S. grammicus.Résumé : Nous avons mené une étude démographique de 5 ans sur une population du lézard vivipare Sceloporus grammicus Wiegmann, 1828 dans le centre du Mexique. La population est structurée en trois classes de taille (jeunes, petits adultes, adultes de taille asymptotique) pour lesquelles nous avons estimé les taux annuels de survie et de fécondité. Nous avons construit une matrice démographique de projection pour chaque transition annuelle. Toutes les matrices donnent des taux réels de croissance de la population (l) qui, bien que variables d'une année à l'autre (de 0,808 à 1,065), ne diffèrent pas significativement de 1, ce qui indique une population stable. Une analyse d'élasticité montre que la survie des individus qui restent dans la même classe de taille est le processus démographique qui contribue le plus aux valeurs de l la plupart des années. De même, la stase des grands adultes est le taux vital qui présente l'importance relative la plus grande pour la persistance de la population. Afin d'incorporer les variations annuelles observées dans les projections démographiques à long terme, nous avons utilisé une matrice moyenne, une simulation stochastique et une procédure de ré-échantillonnage. Toutes ces méthodes ont fourni des taux de croissance de population qui ne diffèrent pas significativement de l'unité. Nos résultats indiquent globalement la stabilité démographique de la population de S. grammicus que nous avons étudiée.[Traduit par la Rédaction]

We conducted an analysis of the body growth pattern of the viviparous lizard Sceloporus grammicus in one of its montane populations in Central México. We calculated 177 individual growth rates for the period comprised between June 1991 and June 1997. Von Bertalanffy growth model provided the best fit to our data in comparison with logistic models. No intersexual differences were found in the growth pattern. The constructed growth curve estimated an age at maturity of 12 and a half months for females and 14 months for males. In spite of marked seasonality in the region, growth rates were not significantly different between seasons. Our results, together with previous studies on the growth ecology of other populations of S. grammicus, suggest that in the volcanic mountains of Central México body growth, and therefore age and size at maturity, are relatively more limited during the whole year in comparison with populations inhabiting northern or lower-altitude localities.