Demography of the lizard Sceloporus grammicus: exploring temporal variation in population dynamics

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]

Relative importance of reproductive life-history paths in one population of the lizard Sceloporus grammicus

“…Similarly, Zúñiga-Vega, Méndez-de la Cruz and Cuellar (2008) found that capture probability (p), which is the other parameter that must necessary be estimated when analyzing mark-recapture data, did not differ among stage classes but differed notably across years. Hence, we constrained p to be similar among stage classes and different through time in all our models.…”

“…A demographic study was previously conducted on this population (Zúñiga-Vega, Méndez-de la Cruz and Cuellar, 2008). In this previous study, mark-recapture data was used to calculate stagespecific survival rates.…”

“…In the present study we use these same demographic data (mark-recapture data for the period comprised between 1991 and 1996) and the same analytical tools (population projection matrices) to estimate the relative contribution of altemative reproductive paths to average fitness by means of loop analysis (van Groenendael et al, 1994). However, we begin by improving the demographic modeling in two ways, compared to the work previously conducted by Zúñiga-Vega, Méndez-de la Cruz and Cuellar (2008). First, we estimate both survival ((^) and growth {f = transition between stage classes) rates using a multi-state framework (Brownie et al, 1993).…”

“…We structured our focal population in three stage classes: juveniles, small adults, and large (asymptotic) adults as per Zúñiga-Vega, Méndez-de la Cruz and Cuellar (2008). Eor these stage classes annual survival {<j>) and transition (l/f) rates were calculated using maximum likelihood procedures implemented in the computer program MARK (White and Burnham, 1999).…”

“…Given that our stage-specific estimates of female (j) varied among years, presumably as a result of environmental variation, the resulting fecundity entries differed among transition matrices. We calculated the total contribution of juveniles to juveniles (the entry a]\ in all matrices) as a combination of the proportion of juveniles staying in this same stage class during an annual transition and the offspring produced by other juveniles that reach sexual maturity and begin reproduction within a single year (as per Zúñiga-Vega, Méndez-de la Cruz and Cuellar, 2008). An average matrix was calculated simply by averaging the entries of all the annual transition matrices (Lemos-Espinal, Rojas-González and Zúñiga-Vega, 2005).…”