Variation In Juvenile Survival Research Articles

Survival of juvenile greater sage‐grouse in Wyoming

An understanding of vital rate contributions to population growth is necessary for species of conservation concern, such as greater sage‐grouse Centrocercus urophasianus. Sage‐grouse demographic rates are generally well described; however, a notable exception is juvenile survival during the post‐fledging period. We evaluated juvenile survival at two study areas in central and south–central Wyoming. We captured and monitored 124 juvenile sage‐grouse (77 females and 47 males) in 2017–2019 in the central Wyoming study area and 68 (29 females and 39 males) in 2020–2021 in the south–central Wyoming study area. Monthly survival generally increased from September to March in each year and study area. In both study areas, we found no evidence that monthly mortality risk differed between male and female juvenile sage‐grouse. In central Wyoming, seven‐month survival estimates from September to March were 0.28 (85% CI: 0.18–0.44) from 2017–2018, 0.28 (85% CI: 0.20–0.39) from 2018–2019, and 0.43 (85% CI: 0.34–0.55) from 2019–2020. In south‐central Wyoming, survival estimates were 0.34 (85% CI: 0.25–0.47) from 2020–2021 and 0.78 (85% CI: 0.68–0.90) from 2021–2022. Overall, we found evidence that body condition at time of capture and weather (temperature and precipitation) during the pre‐fledging period influenced juvenile mortality risk, but the most supported intrinsic and extrinsic factors varied between study areas. Our results provide additional estimates of juvenile survival that will be useful for understanding sage‐grouse demography. However, the spatial and temporal variation in juvenile survival that we documented should be accounted for when evaluating how management actions may influence sage‐grouse populations.

Noninvasive camera data and spatial capture–recapture models reveal strong temporal variation in fawn survival

AbstractIn south Florida, white‐tailed deer (Odocoileus virginianus) are the primary prey of the endangered Florida panther (Puma concolor coryi). Deer populations in some regions of south Florida have declined in recent years, and the role of fawn survival and recruitment in these declines is unknown. Determining known‐fate survival of fawns is challenging, requires invasive and costly methods, and often has a limited geographic scope. We deployed 180 cameras throughout the Florida Panther National Wildlife Refuge and the Big Cypress National Preserve to understand how environmental variables influence fawn survival. We identified 271 fawns from 12,715 photographs in 2015 and 2016. We utilized a noninvasive sampling method coupled with a spatial capture–recapture model to estimate the number of fawns born, the spatial distribution of birth locations, and the number of fawns that survived to recruitment (180 days old) during two fawning seasons. We found strong evidence of temporal variation in survival, but little evidence of spatial variation. Within the 10,941‐ha study area, we estimated that 305 (95% CI: 245–385) fawns were born in 2015 and 278 (212–381) fawns were born in 2016. In 2015, 36% (110) of the estimated 305 fawns survived to 180 days. However, in 2016, only 13% (36) of the estimated 278 fawns survived to 180 days. The large difference in recruitment between years was likely driven by record flooding in 2016. Our data suggest that extreme weather events, coupled with high adult mortality, likely contributed to recent deer population decline in south Florida through reduced fawn recruitment. Unlike studies of known‐fate fawn survival that require labor‐intensive and invasive capture of both adults and neonates, our approach relies exclusively on camera data, which makes it possible to conduct studies over broad spatiotemporal scales in challenging environments to illuminate the drivers of variation in juvenile survival.

Life-history responses of a freshwater rotifer to copper pollution.

In organisms with dormant stages, life‐history responses to past pollution can be studied retrospectively. Here, we study such responses in a rotifer (Brachionus calyciflorus) from the once heavily copper‐polluted Lake Orta (Italy). We extracted resting eggs from sediments, established clonal lineages from hatchlings, and exposed newborns of these lineages to one of three copper concentrations that each mimicked a specific period in the lake's pollution history. For each rotifer, we daily collected life‐table data. We then estimated treatment‐specific vital rates and used a stage‐structured population model to project population growth rate λ. We also estimated elasticities of λ to vital rates and contributions of vital rates to observed Δλ between copper treatments. As expected, λ decreased with increasing copper concentration. This decrease resulted mostly from a decline in juvenile survival rate (SJ ) and partly from a decline in the survival rate of asexually reproducing females (SA ). Maturation rate, and with one exception fecundity, also declined but did not contribute consistently to Δλ. λ was most elastic to SJ and SA , indicating that survival rates were under stronger selection than maturation rate and fecundity. Together, our results indicate that variation in juvenile survival is a key component in the rotifers’ copper response. The consistent decrease in SJ with increasing copper stress and the sensitivity of λ to that decrease also suggest that juvenile survival is a useful indicator of population performance under environmental pollution.

Dietary-based developmental plasticity affects juvenile survival in an aquatic detritivore.

Developmental plasticity is ubiquitous in natural populations, but the underlying causes and fitness consequences are poorly understood. For consumers, nutritional variation of juvenile diets is probably associated with plasticity in developmental rates, but little is known about how diet quality can affect phenotypic trajectories in ways that might influence survival to maturity and lifetime reproductive output. Here, we tested how the diet quality of a freshwater detritivorous isopod (Asellus aquaticus), in terms of elemental ratios of diet (i.e. carbon : nitrogen : phosphorus; C : N : P), can affect (i) developmental rates of body size and pigmentation and (ii) variation in juvenile survival. We reared 1047 individuals, in a full-sib split-family design (29 families), on either a high- (low C : P, C : N) or low-quality (high C : P, C : N) diet, and quantified developmental trajectories of body size and pigmentation for every individual over 12 weeks. Our diet contrast caused strong divergence in the developmental rates of pigmentation but not growth, culminating in a distribution of adult pigmentation spanning the broad range of phenotypes observed both within and among natural populations. Under low-quality diet, we found highest survival at intermediate growth and pigmentation rates. By contrast, survival under high-quality diet survival increased continuously with pigmentation rate, with longest lifespans at intermediate growth rates and high pigmentation rates. Building on previous work which suggests that visual predation mediates the evolution of cryptic pigmentation in A. aquaticus, our study shows how diet quality and composition can generate substantial phenotypic variation by affecting rates of growth and pigmentation during development in the absence of predation.

Survival during the pre-fledging period rather than during post-fledging drives variation in local recruitment of an endangered migratory shorebird, the Southern Dunlin Calidris alpina schinzii

The declines in wet-grassland breeding shorebird populations are considered to mainly result from changes in reproduction. While there is plenty of information on nest survival, little reliable information exists on local recruitment due to confounding effects of permanent emigration. Furthermore, few studies have been able to study the roles of pre- and post-fledging survival on local recruitment. Therefore, it is unclear whether local recruitment of young reflects conditions at the breeding sites or at non-breeding sites. We studied an isolated population of the endangered Southern Dunlin (Calidris alpina schinzii) breeding on the west coast of Sweden to examine (1) brood survival (probability of at least one chick fledging) by following broods fates and (2) local recruitment (survival from hatching to 1 year old) using capture-recapture data. We then examined how much of the annual variation in juvenile survival was explained by variation in brood survival. Brood survival was on average 0.58 (annual range 0.08–1.00) and explained 64% of variation in annual local recruitment. Still local recruitment was rather high for a shorebird (0.17, SE = 0.023), which reflects the isolated nature of the study population. Our results suggest that local recruitment seems to be mainly constrained by chick survival during the pre-fledging period. Therefore, management of breeding sites leading to increased brood survival, e.g., reducing predation on chicks, should have strong impacts on local recruitment and local population growth.

Seeing spots: quantifying mother-offspring similarity and assessing fitness consequences of coat pattern traits in a wild population of giraffes (Giraffa camelopardalis).

Polymorphic phenotypes of mammalian coat coloration have been important to the study of genetics and evolution, but less is known about the inheritance and fitness consequences of individual variation in complex coat pattern traits such as spots and stripes. Giraffe coat markings are highly complex and variable and it has been hypothesized that variation in coat patterns most likely affects fitness by camouflaging neonates against visually hunting predators. We quantified complex coat pattern traits of wild Masai giraffes using image analysis software, determined the similarity of spot pattern traits between mother and offspring, and assessed whether variation in spot pattern traits was related to fitness as measured by juvenile survival. The methods we described could comprise a framework for objective quantification of complex mammal coat pattern traits based on photographic coat pattern data. We demonstrated that some characteristics of giraffe coat spot shape were likely to be heritable, as measured by mother-offspring regression. We found significant variation in juvenile survival among phenotypic groups of neonates defined by multivariate clustering based on spot trait measurement variables. We also found significant variation in neonatal survival associated with spot size and shape covariates. Larger spots (smaller number of spots) and irregularly shaped or rounder spots (smaller aspect ratio) were correlated with increased survival. These findings will inform investigations into developmental and genetic architecture of complex mammal coat patterns and their adaptive value.

Contrasting effects of climate and population density over time and life stages in a long-lived seabird.

Summary Although population responses to environmental variability have been extensively studied for many organisms, few studies have considered early‐life stages owing to the inherent difficulties in tracking the fate of young individuals. However, young individuals are expected to be more sensitive to environmental stochasticity owing to their inexperience and lower competitive abilities. Thus, they are keys to understand demographic responses of an age‐structured population to environmental variability.In this study, we used capture–recapture modelling, based on a 49 year‐long individual‐based longitudinal monitoring dataset, to investigate climatic and population density effects on immature demographic parameters in a long‐lived seabird, the wandering albatross.We provide evidence that climate and population size affected both survival and recruitment age of young individuals although in different ways according to the trait. We found that early‐life survival was mainly affected by population density, whereas recruitment age variation appeared to be better explained by climatic conditions, with a surprising long‐term effect of climate. While population size explained 60% of the variation in juvenile survival, the average Southern Annular Mode over the five previous years explained 52% of variation in recruitment age.In addition, although early‐life survival was consistently negatively affected by population size, the relationship between recruitment age and population size shifted from negative to positive over time from the 1970s to 2000s, showing that density dependence mechanisms can temporarily disappear.Finally, we found that similar climatic conditions may affect individual performances in opposite ways according to the life stage of individuals. This result underlines the critical need to assess age‐specific functional responses to environmental variability to allow accurate demographic predictions. By revealing the poorly known demographic process of younger age classes, the results of this study improve our understanding of population dynamics of long‐lived marine species. A lay summary is available for this article.

Season of birth affects juvenile survival of giraffe

AbstractVariation in timing of reproduction and subsequent juvenile survival often plays an important role in population dynamics of temperate and boreal ungulates. Tropical ungulates often give birth year round, but survival effects of birth season for tropical ungulate species are unknown. We used a population of giraffe in the Tarangire Ecosystem of northern Tanzania, East Africa to determine whether calf survival varied by season of birth. Variation in juvenile survival according to season of birth was significant, with calves born during the dry season experiencing the highest survival probability. Phenological match may confer a juvenile survival advantage to offspring born during the dry season from greater accumulated maternal energy reserves in mothers who conceive in the long rainy season, high‐protein browse in the late dry‐early short rainy seasons supplementing maternal and calf resources, reduced predation due to decreased stalking cover, or some combination of these. Asynchrony is believed to be the ancestral state of all ungulates, and this investigation has illustrated how seasonal variation in vegetation can affect juvenile survival and may play a role in the evolution of synchronous births.

Maternal effects and Symbiodinium community composition drive differential patterns in juvenile survival in the coral Acropora tenuis.

Coral endosymbionts in the dinoflagellate genus Symbiodinium are known to impact host physiology and have led to the evolution of reef-building, but less is known about how symbiotic communities in early life-history stages and their interactions with host parental identity shape the structure of coral communities on reefs. Differentiating the roles of environmental and biological factors driving variation in population demographic processes, particularly larval settlement, early juvenile survival and the onset of symbiosis is key to understanding how coral communities are structured and to predicting how they are likely to respond to climate change. We show that maternal effects (that here include genetic and/or effects related to the maternal environment) can explain nearly 24% of variation in larval settlement success and 5–17% of variation in juvenile survival in an experimental study of the reef-building scleractinian coral, Acropora tenuis. After 25 days on the reef, Symbiodinium communities associated with juvenile corals differed significantly between high mortality and low mortality families based on estimates of taxonomic richness, composition and relative abundance of taxa. Our results highlight that maternal and familial effects significantly explain variation in juvenile survival and symbiont communities in a broadcast-spawning coral, with Symbiodinium type A3 possibly a critical symbiotic partner during this early life stage.

Dependent vs. independent juvenile survival: contrasting drivers of variation and the buffering effect of parental care

Juvenile survival is often found to be more sensitive than adult survival to variation in environmental conditions, and variation in juvenile survival can have significant impacts on population growth rates and viability. Therefore, understanding the population-level effects of environmental changes requires understanding the effects on juvenile survival. We hypothesized that parental care will buffer the survival of dependent juveniles from variation in environmental conditions, while the survival of independent juveniles will respond more strongly to environmental variation and, in turn, drive the overall variation in annual juvenile survival. We tested this parental-care hypothesis using a 30-year mark-recapture data set to model the survival of juvenile Song Sparrows (Melospiza melodia) during the dependent and independent stages. We examined the effects of weather, density, and cohort mean fledge date and body mass on annual variation in survival during the first 12 weeks after fledging, as well as effects of individual fledge date and body mass on individual variation in survival. The primary driver of annual variation in juvenile survival was precipitation during the previous rainy season, consistent with an effect on food availability, which had a strong positive effect on the survival of independent juveniles, but no effect on dependent juveniles. We also found strong support for effects of body mass and fledge date on individual survival probability, including striking differences in the effect of fledge date by stage. Our results provided evidence that different mechanisms influence juvenile survival during each stage of fledgling development, and that parental care buffers the survival of dependent juveniles from variation in environmental conditions. Consequently, variation in juvenile survival was driven by independent juveniles, not dependent juveniles, and studies focused only on survival during the dependent stage may not be able to detect the major drivers of variation in juvenile survival. We recommend that future efforts to understand or project the population-level effects of environmental change not only examine the effects on juvenile survival, but specifically consider the survival of independent juveniles, as well as how the drivers of variation in juvenile survival may vary by stage.

Temporal variation of juvenile survival in a long-lived species: the role of parasites and body condition

Studies of population dynamics of long-lived species have generally focused on adult survival because population growth should be most sensitive to this parameter. However, actual variations in population size can often be driven by other demographic parameters, such as juvenile survival, when they show high temporal variability. We used capture-recapture data from a long-term study of a hunted, migratory species, the greater snow goose (Chen caerulescens atlantica), to assess temporal variability in first-year survival and the relative importance of natural and hunting mortality. We also conducted a parasite-removal experiment to determine the effect of internal parasites and body condition on temporal variation in juvenile survival. We found that juvenile survival showed a higher temporal variability than adult survival and that natural mortality was more important than hunting mortality, unlike in adults. Parasite removal increased first-year survival and reduced its annual variability in females only. Body condition at fledging was also positively correlated with first-year survival in treated females. With reduced parasite load, females, which are thought to invest more in their immune system than males according to Bateman's principle, could probably reallocate more energy to growth than males, leading to a higher survival. Treated birds also had a higher survival than control ones during their second year, suggesting a developmental effect that manifested later in life. Our study shows that natural factors such as internal parasites may be a major source of variation in juvenile survival of a long-lived, migratory bird, which has implications for its population dynamics.

Seasonal variation in foraging conditions for Ring Ouzels Turdus torquatus in upland habitats and their effects on juvenile habitat selection

Recent studies indicate that variation in juvenile survival may be particularly important in driving avian population dynamics. The quality of habitats available to inexperienced juveniles of migrant species is critical to their survival because they must obtain enough food to build up fat reserves for migration, while avoiding predation. We radiotracked 110 juvenile Ring Ouzels Turdus torquatus, a species of high conservation concern in the UK, to quantify for the first time seasonal patterns in foraging habitat and food abundance during this potentially key life‐history period. Key attributes of foraging plots were compared with those on control plots (representing the broad habitat types selected by foraging juveniles) during 2007–08. Birds foraged on invertebrates in grass‐rich plots during June to mid‐July and then shifted to foraging mainly on moorland berries in higher‐altitude, heather‐rich plots during mid‐July to early‐September. Juveniles selected invertebrate foraging plots with low soil acidity, and increasingly selected plots with high earthworm (an important food) biomass and grass cover, but low grass and all vegetation height as the season progressed. In contrast, earthworm biomass and grass cover remained constant, and grass and all vegetation height increased, on control plots. Juveniles selected berry foraging plots with higher abundance of ripe Bilberries Vaccinium myrtillus and Crowberries Empetrum nigrum than found on control plots. Juvenile Ring Ouzels thus appear to require access to short, grass‐ and invertebrate‐rich habitat during early summer, and taller, heather‐dominated and berry‐rich areas in late summer. The use of two distinct habitat types during the pre‐migratory period illustrates the need for a detailed understanding of the requirements of juvenile birds.

Pre‐reproductive survival in a tropical bird and its implications for avian life histories

The factors that affect survival until reproduction are essential to understanding the organization of life histories within and among species. Theory predicts, for example, that survival until reproduction influences the optimum level of reproductive investment by parents, which might partly explain prolonged parental care in species with high first-year survival. Tests and refinements of life-history theory have been hampered, however, by a lack of field-based estimates of pre-reproductive survival, especially for tropical species, which have been the subject of many comparative analyses. Tropical species are predicted to have higher first-year survival and delayed reproduction compared to Northern Hemisphere species. We estimated survival until reproduction, age at first reproduction, and sources of variation in juvenile survival in a Neotropical passerine, the Western Slaty-Antshrike (Thamnophilus atrinucha), in central Panama. We observed that fledged antshrikes had 76% survival through the dependent period and 48% survival to the age of 1 year; survival rate was lowest during the first week after leaving the nest. Timing of fledging within the breeding season, fledgling mass, and age at dispersal influenced survival, while sex of offspring and year did not. Individuals did not breed until two years of age, and post-fledging pre-reproductive survival was 41% of annual adult survival. High survival until reproduction in antshrikes balanced their low annual productivity, resulting in a stable population. Survival during the post-fledging period of dependence and the first year of independence in the Western Slaty-Antshrike exceeded estimates for Northern Hemisphere species. This difference appears to be associated with the extended post-fledging parental care, delayed dispersal, low costs of dispersal, and the less seasonal environment of antshrikes.

Long-term trends in survival of a declining population: the case of the little owl (Athene noctua) in the Netherlands

The little owl (Athene noctua) has declined significantly in many parts of Europe, including the Netherlands. To understand the demographic mechanisms underlying their decline, we analysed all available Dutch little owl ringing data. The data set spanned 35 years, and included more than 24,000 ringed owls, allowing detailed estimation of survival rates through multi-state capture–recapture modelling taking dispersal into account. We investigated geographical and temporal variation in age-specific survival rates and linked annual survival estimates to population growth rate in corresponding years, as well as to environmental covariates. The best model for estimating survival assumed time effects on both juvenile and adult survival rates, with average annual survival estimated at 0.258 (SE = 0.047) and 0.753 (SE = 0.019), respectively. Juvenile survival rates decreased with time whereas adult survival rates fluctuated regularly among years, low survival occurring about every 4 years. Years when the population declined were associated with low juvenile survival. More than 60% of the variation in juvenile survival was explained by the increase in road traffic intensity or in average temperature in spring, but these correlations rather reflect a gradual decrease in juvenile survival coinciding with long-term global change than direct causal effects. Surprisingly, vole dynamics did not explain the cyclic dynamics of adult survival rate. Instead, dry and cold years led to low adult survival rates. Low juvenile survival rates, that limit recruitment of first-year breeders, and the regular occurrence of years with poor adult survival, were the most important determinants of the population decline of the little owl.Electronic supplementary materialThe online version of this article (doi:10.1007/s00442-010-1868-x) contains supplementary material, which is available to authorized users.

Avian cholera, post‐hatching survival and selection on hatch characteristics in a long‐lived bird, the common eider Somateria mollisima

Infectious diseases can have dramatic impacts on animal population dynamics, but how they influence vital rates remains understudied. We took advantage of the appearance of an avian cholera epizootic in an arctic colony of common eiders Somateria mollissima to study variation in juvenile survival and selection on hatch characteristics in relation to this highly infectious disease. Avian cholera is one of the most important infectious diseases affecting wild birds and is thought to primarily affect adult survival. Here, we show that avian cholera was associated with a 90% decline in duckling survival, leading to almost zero recruitment. Before the cholera outbreak, there was significant stabilizing selection on hatching date and significant positive directional selection on hatching mass. During cholera outbreaks, selection on hatch characteristics was no longer significant. These results were based on a low sample of surviving ducklings in cholera years, but suggested that date and mass at hatching did no longer affect duckling survival in the presence of cholera. These effects of avian cholera on post‐hatching survival were likely not only the consequence of the disease per se, but also a consequence of an increase in predation rates that followed the emergence of avian cholera. Our results emphasize the dramatic direct and indirect impacts that infectious disease can have on vital rates, and thus population dynamics.

Is lack of evidence of inbreeding depression in a threatened New Zealand robin indicative of reduced genetic load?

Continuous inbreeding exposes deleterious recessive alleles to selection and can thereby lead to partial purging of the genetic load and reduced inbreeding depression. Purging has been well documented in experimental laboratory populations, but evidence of reduced inbreeding depression due to purging in wild populations is largely lacking. This study examines the inbreeding load associated with juvenile survival at a protected island site of a bottlenecked population of the Stewart Island robin Petroica australis rakiura. Based on a complete pedigree of the island population, we found little evidence that inbreeding coefficients explained any additional variation in juvenile survival, once demographic factors such as the effects of density, timing of fledging and age of mother were taken into account. Lethal equivalents, a standardized measure of the strength of inbreeding depression, were close to zero (B=0.24, 95% CI=−1.92-1.04, n=326) and lower than that documented for an island population of a widespread congener (B=6.71, 95% CI=−0.66-14.08, n=238) and for several other species for which significant inbreeding depression was detected (B=1.30-7.47). There are several reasons as to why studies can fail to detect pedigree-based inbreeding depression in wild populations, but evidence presented here of low lethal equivalents and relatively high survival of both inbred and non-inbred juveniles are consistent with a population that has undergone partial purging of its genetic load during historical population bottlenecks. Although our study does not imply a complete absence of inbreeding depression, it is one of the first studies of a wild population where weak inbreeding depression for juvenile survival appears to be associated with a prolonged bottleneck.

Within‐ and between‐year variation in the juvenile survival of Common GuillemotsUria aalge

We studied juvenile survival of 20 cohorts of Common GuillemotUria aalgechicks colour‐ringed on the Isle of May, Scotland, using both live observations at the colony and dead recoveries, allowing estimation of fidelity to the colony as well as survival. In this seabird, chicks leave the colony when only partly grown and are cared for by the male parent for several weeks afterwards. First‐year survival varied strongly between cohorts, with a mean of 56% (range 30–91%). We did not identify any covariates which could explain this variation, whether relating to climate, population size or prey density. Survival was low during two regime shift episodes in the North Sea (1987–90 and 2000 onwards). Early hatched chicks were substantially more likely to survive than those hatching later in most years, whereas body condition at ringing had no detectable effect. Ringing recoveries indicated that mortality was highest in mid‐winter, i.e. well after the cessation of paternal care. These results do not support the hypothesis that variation in prey quantity or energy content before fledging is a primary driver of variation in juvenile survival. Rather, it seems that chicks of high‐quality parents are more likely to survive, as high‐quality females tend to lay earlier in the season, and high‐quality males presumably are better able to prepare their chicks to survive their first winter at sea. Very few (4%) Guillemots emigrated permanently before age 3 years, but from age 5 onwards 25–30% of birds annually left the colony or otherwise became unobservable.

Variation in juvenile survival and related physiological traits among dipterocarp species co‐existing in a Bornean forest

AbstractQuestion: What environmental factors and physiological traits determine the juvenile survival rates of 11 dipterocarp species (Dipterocarpaceae) co‐occurring in a Bornean tropical rain forest?Location: Tropical northwestern Borneo.Methods: In 248 quadrats, distributed over 12 ha, the juvenile (height< 2.0 m) survival of 11 shade‐tolerant dipterocarp species was monitored for one year. The probability of survival for each species was expressed as a logistic function of plant height and three environmental factors: canopy openness, soil water potential and conspecific individual density. The correlation between survival under shaded conditions and several physiological traits of juveniles was tested by normal Pearson correlation and a phylogeny based manner using independent contrasts.Results: Seven species had increased survival under more open canopies, one species had increased and two species had decreased survival at drier sites. Six species had higher survival as the density of conspecifics increased, whereas two species had lower survival. The magnitudes of the effects for the three environmental factors were similar. However, the interspecific difference in survival was largely determined by a dependence of small seedlings on the light environment. Survival under low light was correlated positively with root dry mass and negatively with total leaf area of juveniles.Conclusions: Responses of seedlings to the fine‐scale gradient of light availability were considerably different among the 11 species. Species that suffered increased mortality under low light had a set of structural traits that would potentially allow rapid growth under bright conditions. Differential performance along the light gradient may contribute to the stable coexistence of these species.

Variation in juvenile survival and related physiological traits among dipterocarp species co-existing in a Bornean forest

Abstract Question: What environmental factors and physiological traits determine the juvenile survival rates of 11 dipterocarp species (Dipterocarpaceae) co-occurring in a Bornean tropical rain forest? Location: Tropical northwestern Borneo. Methods: In 248 quadrats, distributed over 12 ha, the juvenile (height< 2.0 m) survival of 11 shade-tolerant dipterocarp species was monitored for one year. The probability of survival for each species was expressed as a logistic function of plant height and three environmental factors: canopy openness, soil water potential and conspecific individual density. The correlation between survival under shaded conditions and several physiological traits of juveniles was tested by normal Pearson correlation and a phylogeny based manner using independent contrasts. Results: Seven species had increased survival under more open canopies, one species had increased and two species had decreased survival at drier sites. Six species had higher survival as the density of conspecific...

Winter weather affects asp viper Vipera aspis population dynamics through susceptible juveniles

Detailed studies on mammals and birds have shown that the effects of climate variation on population dynamics often depend on population composition, because weather affects different subsets of a population differently. It is presently unknown whether this is also true for ectothermic animals such as reptiles. Here we show such an interaction between weather and demography for an ectothermic vertebrate by examining patterns of survival and reproduction in six populations of a threatened European snake, the asp viper (Vipera aspis), over six to 17 years. Survival was lowest among juvenile and highest among adult snakes. The estimated annual probability for females to become gravid ranged from 26% to 60%, and was independent of whether females had reproduced in the year before or not. Variation in juvenile survival was strongly affected by winter temperature, whereas adult survival was unaffected by winter harshness. A matrix population model showed that winter weather affected population dynamics predominantly through variation in juvenile survival, although the sensitivity of the population growth rate to juvenile survival was lower than to adult survival. This study on ectothermic vipers revealed very similar patterns to those found in long-lived endothermic birds and mammals. Our results thus show that climate and life history can interact in similar ways across biologically very different vertebrate species, and suggest that these patterns may be very general.