J. Jaime Zúñiga-Vega

Differential mortality drives life-history evolution and population dynamics in the fish Brachyrhaphis rhabdophora

Life-history theory predicts that populations experiencing different levels of extrinsic mortality will evolve divergent reproductive strategies. Previous work in the live-bearing fish Brachyrhaphis rhabdophora shows that individuals from populations that occur with piscivorous fish mature earlier and at smaller sizes and have more and smaller offspring than fish from populations without predators. However, until now, there have been no data to demonstrate that differences in mortality rates actually exist between predator and predator-free sites. Here we present the results of a serial mark-recapture field study designed to estimate mortality rates in natural populations of B. rhabodophora from Costa Rica. We found that fish from predator environments experience higher overall mortality rates and proportionally higher adult mortality rates than fish from predator-free environments. We then ask what impact differences in mortality rates have on B. rhabdophora population dynamics. Using a population matrix modeling approach, we found that B. rhabdophora that co-occur with predators have population growth rates similar to those without predators and both have confidence intervals that span lambda = 1.0. However, elasticity analysis revealed that the most important life-history stages for population growth in predator environments are found early in life and include growth through early ontogenetic stages and survival as small adults; in contrast, the most important life-history stages for population growth in predator-free environments occur late in life, including survival once large juvenile and adult stages are reached. Hence, we demonstrate two important links between predation and population demography, one ecological due to the direct impacts of predator-induced mortality and the other expressed through predator-mediated reproductive adaptation.

A general assessment of the conservation status and decline trends of Mexican amphibians

We present a review on the conservation status and population trends of the 372 amphibian species currently recognized for Mexico. We based our analyses on the information gathered by the International Union for the Conservation of Nature-the Global Amphibian Assessment (IUCN-GAA) as well as on available literature about imminent or potential threats to these organisms in Mexico. This country has the fifth largest amphibian fauna in the world and almost 58% of the species that inhabit this country are considered as threatened. We highlight the proportion of species per order, family, and genus that are currently under severe risk in Mexico. In addition, we prepared a detailed list of the main factors that are threatening amphibians in this country. Evidence is provided that the six main mechanisms that are globally leading amphibians to extinction (alien species, over-exploitation, land use change, global changes, pollution, and infectious diseases) are indeed currently operating in Mexico. We discuss the relative importance of each of these causes. We also highlight the paucity of quantitative studies that support the current conservation status of Mexican amphibian species.

variation in reproductive traits within the lizard genus xenosaurus

Abstract We document variation among 13 populations of distinct species of the lizard genus Xenosaurus in four key reproductive traits: minimum size at maturity, litter size, size at birth, and relative clutch mass (RCM). Despite a common flattened morphology, considerable variation occurs in the examined traits. Minimum size at maturity varied between 92 and 110 mm snout–vent length (SVL), average litter size ranged between 2.1 and 5.7 newborns per female, mean size at birth varied between 40.6 and 51.6 mm SVL, and average RCM ranged between 0.16 and 0.35. We also found considerable interpopulational variation in mean size of reproductive females (from 102.9–119.2 mm SVL) and in the length of the birth season (from about one month to four months distributed between early May and late September). Litter size and RCM were the most variable traits with coefficients of variation above 20, whereas the other traits showed coefficients of variation between 4 and 7.4. Only the minimum size at maturity showed an ...—We document variation among 13 populations of distinct species of the lizard genus Xenosaurus in four key reproductive traits: minimum size at maturity, litter size, size at birth, and relative clutch mass (RCM). Despite a common flattened morphology, considerable variation occurs in the examined traits. Minimum size at maturity varied between 92 and 110 mm snout–vent length (SVL), average litter size ranged between 2.1 and 5.7 newborns per female, mean size at birth varied between 40.6 and 51.6 mm SVL, and average RCM ranged between 0.16 and 0.35. We also found considerable interpopulational variation in mean size of reproductive females (from 102.9–119.2 mm SVL) and in the length of the birth season (from about one month to four months distributed between early May and late September). Litter size and RCM were the most variable traits with coefficients of variation above 20, whereas the other traits showed coefficients of variation between 4 and 7.4. Only the minimum size at maturity showed an allometric effect. Litter size and size at birth were negatively correlated, which we interpret as evidence of a trade-off between these two traits. Cluster analyses revealed the existence of two main life-history strategies: populations with relatively large litters of small young and populations with small litters of large young. We suggest further ecological and phylogenetic analyses to explain the patterns of variation and covariation observed in the

Analysis of the Population Dynamics of an Endangered Lizard (Xenosaurus Grandis) through the Use of Projection Matrices

Abstract We conducted a demographic analysis from 2000 to 2004 of a population of the crevice-dwelling lizard, Xenosaurus grandis, in Veracruz, México. We used population projection matrices to model its population dynamics. Three of the four annual matrices projected an actively growing population, whereas the matrix corresponding to 2003–2004 projected a declining population, apparently associated with a higher-than-average environmental temperature during this period. Observed population structure differed from that expected at equilibrium, according to the four matrices. Highest reproductive values corresponded to adult categories. The transition of newborns and juveniles to larger categories, as well as adult stasis, were the vital rates that made the largest contribution to population growth rate (λ). Growth from one size class to the next was the demographic process that contributed the most to λ, and all size classes had similar elasticities in all years. Stochastic simulations integrating interannual demographic variability projected population growth rates above unity, even under an ecological scenario that consisted of 50% of unfavorable years. Despite this trend toward positive population growth, this X. grandis population faces an imminent threat due to the growing industrial development in the region in which it is endemic. Thus, we suggest this population as a primary target for conservation efforts, as well as a change in the conservation category of this species from “special protection” to “threatened.”

Morphological and reproductive variation among populations of the Pacific molly Poecilia butleri

In viviparous organisms, pregnant females typically experience an increase in body mass and body volume. In this study, the prediction that variation in reproductive traits among populations of viviparous organisms should be related to variation among populations in body shape was tested in the Pacific molly Poecilia butleri, a viviparous fish that inhabits western Mexico and northern Central America. Variation among 10 populations in four reproductive traits was examined: brood size, individual embryo mass, total reproductive allotment and degree of maternal provisioning of nutrients to developing embryos. Variation among these populations in body shape was also examined. Significant variation among populations was observed in both brood size and reproductive allotment but not in embryo mass or degree of maternal provisioning. Significant variation among populations was also observed in body shape. After correcting for female size, however, reproductive traits and body shape were not associated among populations. This suggests that selective pressures acting on reproduction do not necessarily affect morphology and vice versa. Several factors might contribute to this unexpected lack of association between reproductive traits and morphology.

Evolving from static to dynamic signals: evolutionary compensation between two communicative signals.

Signals that convey related information may impose selection on each other, creating evolutionary links between different components of the communicative repertoire. Here, we ask about the consequences of the evolutionary loss of one signal (a colour patch) on another (a motion display) in Sceloporus lizards. We present data on male lizards of four species: two pairs of sister taxa representing two independent evolutionary losses of the static colour patch (Sceloporus cozumelae and Sceloporus parvus; Sceloporus siniferus and Sceloporus merriami). Males of the two species that have undergone an evolutionary loss of blue-belly patches (S. cozumelae, S. siniferus) were less active than their blue-bellied sister taxa (S. parvus, S. merriami), consistent with the suggestion that the belly patches were lost to reduce conspicuousness of species with high predation pressure. In contrast, the headbob display appears to have become more, rather than less, conspicuous over evolutionary time. The colour patch is exhibited primarily during aggressive encounters, whereas headbob displays are multifunction signals used in several different contexts, including aggressive encounters. Males of species that have lost the colour patch produced more motion displays, and the structure of those motion displays were more similar to those produced during combat. In both evolutionary episodes, a static colour signal appears to have been replaced by dynamic motion displays that can be turned off in the presence of predators and other unwanted receivers. The predominant pattern is one of evolutionary compensation and interactions between multiple signals that convey related information.

Reproductive Variation of the Lizard Xenosaurus platyceps: Comparing Two Populations of Contrasting Environments

Abstract We studied litter size, size at birth, and relative litter mass (RLM) in two populations of the crevice-dwelling lizard Xenosaurus platyceps: one located in a low-altitude tropical forest; and the other in a high-altitude temperate oak forest. We found a significant relationship between female size and litter size. Females from the temperate site produced significantly larger litters in comparison with those from the tropical site after adjusting for female size. Neither a relationship between female size and offspring size nor a difference between populations in offspring size was detected, which suggests that this trait could be either constrained or optimized. RLM showed the same pattern as litter size: Larger females exhibited greater RLM, and this trait showed significantly larger values in the temperate locality. This pattern is explained by evidence that females in the temperate population are producing more young of similar size than those produced by their tropical counterparts. We did not find significant interannual variation in any of the reproductive traits studied. We suggest reciprocal transplant or common garden experiments to determine the genetic and proximal causes of the observed intraspecific variation.

Survivorship, Growth, and Detection of a Knob-scaled Lizard in Queretaro, Mexico

Abstract A deep understanding of the processes affecting the population dynamics of living organisms requires fine-scale analyses of basic demographic parameters such as stage-specific survival and growth rates. In this study we estimated survival, detection, and transition rates (growth) for different stages of the life cycle of a Knob-scaled Lizard of the genus Xenosaurus. We also examined potential sources of variation for these parameters by means of a multiple-model inference framework. Our capture–mark–recapture data revealed that survival rates were homogeneous among stage classes but markedly different between the rainy and dry seasons. Contrary to our expectation, survival probability was higher during dry months. Detection probability varied considerably among stage classes and through time. Consistent with theoretical predictions, the rate at which lizards moved from a particular stage class to the following (transition rates) varied among stage classes, with the fastest rates observed in yearlings and the slowest in adults. Also consistent with our predictions, we found a tendency toward faster transition rates during rainy months. We discuss the potential causes and implications of the patterns of variation observed in these key demographic parameters.

Body growth in one montane population of Sceloporus grammicus (Sauria, Phrynosomatidae) in Central México

We conducted an analysis of the body growth pattern of the viviparous lizard Sceloporus grammicus in one of its montane populations in Central Mexico. 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 Mexico 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.

Spatial and temporal variation in superfoetation and related life history traits of two viviparous fishes: Poeciliopsis gracilis and P. infans

Superfoetation is the ability of females to simultaneously bear multiple broods of embryos at different developmental stages. Most studies on the phylogenetic distribution of superfoetation and on the factors that potentially promote superfoetation ignore variation within species. Here, we studied 11 populations of two species of viviparous fishes of the family Poeciliidae (Poeciliopsis gracilis and Poeciliopsis infans) and document wide variation in superfoetation and in three related life history traits: brood size, individual embryo mass and total reproductive allotment. We found significant differences in the average number of simultaneous broods among populations of P. gracilis but not among populations of P. infans. In addition, we found even greater variation between months within populations for both species, although no specific pattern of temporal variation was evident. Instead of the expected consistency of seasonal differences in superfoetation across populations, we found that large variation among months within seasons and the amount and direction of this monthly variation differed widely between populations. Our results emphasize the importance of including intraspecific variation in superfoetation and other life history traits in studies that aimed at finding general explanations of life history trait evolution.