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 (S_J ) and partly from a decline in the survival rate of asexually reproducing females (S_A ). Maturation rate, and with one exception fecundity, also declined but did not contribute consistently to Δλ. λ was most elastic to S_J and S_A , 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 S_J 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.     

Life history buffering in Antarctic mammals and birds against changing patterns of climate and environmental variation

The consequences of warming for Antarctic long‐lived organisms depend on their ability to survive changing patterns of climate and environmental variation. Among birds and mammals of different Antarctic regions, including emperor penguins, snow petrels, southern fulmars, Antarctic fur seals and Weddell seals, we found strong support for selection of life history traits that reduce interannual variation in fitness. These species maximize fitness by keeping a low interannual variance in the survival of adults and in their propensity to breed annually, which are the vital rates that influence most the variability in population growth rate (λ). All these species have been able to buffer these rates against the effects of recent climate‐driven habitat changes except for Antarctic fur seals, in the Southwest Atlantic. In this region of the Southern Ocean, the rapid increase in ecosystem fluctuation, associated with increasing climate variability observed since 1990, has limited and rendered less predictable the main fur seal food supply, Antarctic krill. This has increased the fitness costs of breeding for females, causing significant short‐term changes in population structure through mortality and low breeding output. Changes occur now with a frequency higher than the mean female fur seal generation time, and therefore are likely to limit their adaptive response. Fur seals are more likely to rely on phenotypic plasticity to cope with short‐term changes in order to maximize individual fitness. With more frequent extreme climatic events driving more frequent ecosystem fluctuation, the repercussions for life histories in many Antarctic birds and mammals are likely to increase, particularly at regional scales. In species with less flexible life histories that are more constrained by fluctuation in their critical habitats, like sea‐ice, this may cause demographic changes, population compensation and changes in distribution, as already observed in penguin species living in the Antarctic Peninsula and adjacent islands.     

Life‐history trade‐offs vary with resource availability across the geographic range of a widespread plant

• Trade‐offs between reproduction, growth and survival arise from limited resource availability in plants. Environmental stress is expected to exacerbate these negative correlations, but no studies have evaluated variation in life‐history trade‐offs throughout species geographic ranges. Here we analyse the costs of growth and reproduction across the latitudinal range of the widespread herb Plantago coronopus in Europe.
• We monitored the performance of thousands of individuals in 11 populations of P. coronopus, and tested whether the effects of growth and reproduction on a set of vital rates (growth, probability of survival, probability of reproduction and fecundity) varied with local precipitation and soil fertility. To account for variation in internal resources among individuals, we analysed trade‐offs correcting for differences in size.
• Growth was negatively affected by previous growth and reproduction. We also found costs of growth and reproduction on survival, reproduction probability and fecundity, but only in populations with low soil fertility. Costs also increased with precipitation, possibly due to flooding‐related stress. In contrast, growth was positively correlated with subsequent survival, and there was a positive covariation in reproduction between consecutive years under certain environments, a potential strategy to exploit temporary benign conditions.
• Overall, we found both negative and positive correlations among vital rates across P. coronopus geographic range. Trade‐offs predominated under stressful conditions, and positive correlations arose particularly between related traits like reproduction investment across years. By analysing multiple and diverse fitness components along stress gradients, we can better understand life‐history evolution across species’ ranges, and their responses to environmental change.

     

Assessing the contributions of multiple interacting traits to plant reproductive success: environmental dependence

The reproductive success of sibling cocklebur plants (Compositae: Xanthium strumarium) was monitored after growth at different levels of availability of water and nutrient resources. Variation in reproductive success among individual plants was related to physiological, structural, and phenological characteristics. Reproductive success increased with increased availability of resources, but the relative contribution of particular traits to reproductive success varied with resource availability. Allocation of biomass to different vegetative tissues, time to seedling emergence, degree of branching, transpiration rates, water use efficiency, the rate of decline in height growth after seedling emergence and final plant size all varied significantly with resource availability. However, the changes in each of these phenotypic traits across three garden environments did not always correlate with reproductive success. The shifts across environments in the apparent importance of somatic traits for reproductive success were attributed to plastic changes in the traits but also to changes in the phenotypic correlations of the traits with reproductive success.     

Great tits growing old: selective disappearance and the partitioning of senescence to stages within the breeding cycle

Deterioration of reproductive traits with age is observed in an increasing number of species. Although such deterioration is often attributed to reproductive senescence, a within-individual decline in reproductive success with age, few studies on wild animals have focused on direct fitness measures while accounting for selective disappearance and terminal effects, and to our knowledge none have determined how senescence effects arise from underlying reproductive traits. We show for female great tits that such an approach helps understanding of the onset, impact and architecture of senescence. Cross-sectional analysis of 49 years of breeding data shows annual recruit production to decline from 3.5 years of age, this decline affecting 9 per cent of females each year. Longitudinal analyses, however, show that selective disappearance of poor-quality breeders partly masks senescence, which in fact starts at 2.8 years and affects 21 per cent of females each year. There is no evidence for abrupt terminal effects. Analyses of underlying traits show no deterioration in clutch size, but significant declines in brood size and fledgling number. Furthermore, these traits contribute −9, 12 and 39 per cent to the senescent decline in recruit production, respectively. Besides providing detailed knowledge of the patterns and architecture of senescence in a natural population, these results illustrate the importance of modelling individual variation, and facilitate study of the underlying mechanisms of senescence.     

Contrasting patterns of density‐dependent selection at different life stages can create more than one fast–slow axis of life‐history variation

There has been much recent research interest in the existence of a major axis of life‐history variation along a fast–slow continuum within almost all major taxonomic groups. Eco‐evolutionary models of density‐dependent selection provide a general explanation for such observations of interspecific variation in the "pace of life." One issue, however, is that some large‐bodied long‐lived “slow” species (e.g., trees and large fish) often show an explosive “fast” type of reproduction with many small offspring, and species with “fast” adult life stages can have comparatively “slow” offspring life stages (e.g., mayflies). We attempt to explain such life‐history evolution using the same eco‐evolutionary modeling approach but with two life stages, separating adult reproductive strategies from offspring survival strategies. When the population dynamics in the two life stages are closely linked and affect each other, density‐dependent selection occurs in parallel on both reproduction and survival, producing the usual one‐dimensional fast–slow continuum (e.g., houseflies to blue whales). However, strong density dependence at either the adult reproduction or offspring survival life stage creates quasi‐independent population dynamics, allowing fast‐type reproduction alongside slow‐type survival (e.g., trees and large fish), or the perhaps rarer slow‐type reproduction alongside fast‐type survival (e.g., mayflies—short‐lived adults producing few long‐lived offspring). Therefore, most types of species life histories in nature can potentially be explained via the eco‐evolutionary consequences of density‐dependent selection given the possible separation of demographic effects at different life stages.     

Costs of fear: behavioural and life‐history responses to risk and their demographic consequences vary across species

Behavioural responses to reduce predation risk might cause demographic ‘costs of fear’. Costs differ among species, but a conceptual framework to understand this variation is lacking. We use a life‐history framework to tie together diverse traits and life stages to better understand interspecific variation in responses and costs. We used natural and experimental variation in predation risk to test phenotypic responses and associated demographic costs for 10 songbird species. Responses such as increased parental attentiveness yielded reduced development time and created benefits such as reduced predation probability. Yet, responses to increased risk also created demographic costs by reducing offspring production in the absence of direct predation. This cost of fear varied widely across species, but predictably with the probability of repeat breeding. Use of a life‐history framework can aid our understanding of potential demographic costs from predation, both from responses to perceived risk and from direct predation mortality.     

Paths to selection on life history loci in different natural environments across the native range of Arabidopsis thaliana

Selection on quantitative trait loci (QTL) may vary among natural environments due to differences in the genetic architecture of traits, environment‐specific allelic effects or changes in the direction and magnitude of selection on specific traits. To dissect the environmental differences in selection on life history QTL across climatic regions, we grew a panel of interconnected recombinant inbred lines (RILs) of Arabidopsis thaliana in four field sites across its native European range. For each environment, we mapped QTL for growth, reproductive timing and development. Several QTL were pleiotropic across environments, three colocalizing with known functional polymorphisms in flowering time genes (CRY2, FRI and MAF2‐5), but major QTL differed across field sites, showing conditional neutrality. We used structural equation models to trace selection paths from QTL to lifetime fitness in each environment. Only three QTL directly affected fruit number, measuring fitness. Most QTL had an indirect effect on fitness through their effect on bolting time or leaf length. Influence of life history traits on fitness differed dramatically across sites, resulting in different patterns of selection on reproductive timing and underlying QTL. In two oceanic field sites with high prereproductive mortality, QTL alleles contributing to early reproduction resulted in greater fruit production, conferring selective advantage, whereas alleles contributing to later reproduction resulted in larger size and higher fitness in a continental site. This demonstrates how environmental variation leads to change in both QTL effect sizes and direction of selection on traits, justifying the persistence of allelic polymorphism at life history QTL across the species range.     

Generalists are the most urban‐tolerant of birds: a phylogenetically controlled analysis of ecological and life history traits using a novel continuous measure of bird responses to urbanization

Identifying which ecological and life history traits influence a species’ tolerance to urbanization is critical to understanding the trajectory of biodiversity in an increasingly urbanizing world. There is evidence for a wide array of contrasting patterns for single trait associations with urbanization. In a continental‐scale analysis, incorporating 477 species and >5 000 000 bird observations, we developed a novel and scalable methodology that evaluated the ecological and life history traits which most influence a species’ adaptability to persist in urban environments. Specifically, we assigned species‐specific scores based on continuous measures of response to urbanization, using VIIRS night‐time light values (i.e. radiance) as a proxy for urbanization. We identified generalized, phylogenetically controlled patterns: bird species which are generalists (i.e. large niche breadth), with large clutch size, and large residual brain size are among the most urban‐tolerant bird species. Conversely, specialized feeding strategies (i.e. insectivores and granivores) were negatively associated with urbanization. Enhancement and persistence of avian biodiversity in urban environments probably relies on protecting, maintaining and restoring diverse habitats serving a range of life history strategies.     

Demographic responses to climate‐driven variation in habitat quality across the annual cycle of a migratory bird species

The demography and dynamics of migratory bird populations depend on patterns of movement and habitat quality across the annual cycle. We leveraged archival GPS‐tagging data, climate data, remote‐sensed vegetation data, and bird‐banding data to better understand the dynamics of black‐headed grosbeak (Pheucticus melanocephalus) populations in two breeding regions, the coast and Central Valley of California (Coastal California) and the Sierra Nevada mountain range (Sierra Nevada), over 28 years (1992–2019). Drought conditions across the annual cycle and rainfall timing on the molting grounds influenced seasonal habitat characteristics, including vegetation greenness and phenology (maturity dates). We developed a novel integrated population model with population state informed by adult capture data, recruitment rates informed by age‐specific capture data and climate covariates, and survival rates informed by adult capture–mark–recapture data and climate covariates. Population size was relatively variable among years for Coastal California, where numbers of recruits and survivors were positively correlated, and years of population increase were largely driven by recruitment. In the Sierra Nevada, population size was more consistent and showed stronger evidence of population regulation (numbers of recruits and survivors negatively correlated). Neither region showed evidence of long‐term population trend. We found only weak support for most climate–demographic rate relationships. However, recruitment rates for the Coastal California region were higher when rainfall was relatively early on the molting grounds and when wintering grounds were relatively cool and wet. We suggest that our approach of integrating movement, climate, and demographic data within a novel modeling framework can provide a useful method for better understanding the dynamics of broadly distributed migratory species.     

Life history responses of the cladoceran Ceriodaphnia cf. dubia to variation in food concentration

The effect of five different food concentrations on the life history of Ceriodaphnia cf. dubia was examined by following cohorts of 25 individuals from <24 hours until the death of all individuals. The food concentrations used in the study were chosen to reflect densities found in lentic freshwater systems and those commonly used in toxicity testing, and ranged from 1 × 10⁴ cells mL^-1 to 15 × 10⁴ cells mL^-1. Food concentration was found to have a significant effect (p<0.05) on several life history parameters, with a decrease in food concentration leading to a decrease in brood sizes and population growth rate, and an increase in longevity. Population growth rates varied from approximately 0.39 neonates d^-1 to 0.54 neonates d^-1, while mean lifespan ranged from 16.7 days to 42.9 days. A decrease in food concentration also led to an increase in the mean generation time.     

Effects of variation in resource acquisition during different stages of the life cycle on life‐history traits and trade‐offs in a burying beetle

Individual variation in resource acquisition should have consequences for life‐history traits and trade‐offs between them because such variation determines how many resources can be allocated to different life‐history functions, such as growth, survival and reproduction. Since resource acquisition can vary across an individual's life cycle, the consequences for life‐history traits and trade‐offs may depend on when during the life cycle resources are limited. We tested for differential and/or interactive effects of variation in resource acquisition in the burying beetle Nicrophorus vespilloides. We designed an experiment in which individuals acquired high or low amounts of resources across three stages of the life cycle: larval development, prior to breeding and the onset of breeding in a fully crossed design. Resource acquisition during larval development and prior to breeding affected egg size and offspring survival, respectively. Meanwhile, resource acquisition at the onset of breeding affected size and number of both eggs and offspring. In addition, there were interactive effects between resource acquisition at different stages on egg size and offspring survival. However, only when females acquired few resources at the onset of breeding was there evidence for a trade‐off between offspring size and number. Our results demonstrate that individual variation in resource acquisition during different stages of the life cycle has important consequences for life‐history traits but limited effects on trade‐offs. This suggests that in species that acquire a fixed‐sized resource at the onset of breeding, the size of this resource has larger effects on life‐history trade‐offs than resources acquired at earlier stages.     

Photosynthetic responses to photosynthetically active radiation and temperature including chilling‐light stress on the heteromorphic life history stages of a brown alga,Cladosiphon okamuranus (Chordariaceae) from Ryukyu Islands,Japan

Photosynthetic responses to temperature and photosynthetically active radiation (PAR) were investigated on the heteromorphic life history stages (macroscopic and microscopic stages) of an edible Japanese brown alga, Cladosiphon okamuranus from the Ryukyu Islands. Measurements were carried out by using optical dissolved oxygen sensors and a pulse‐amplitude modulated fluorometer. Maximum net photosynthetic rates and other parameters of the Photosynthesis – PAR curves at 28°C were somewhat similar in both life history stages, without characteristic photoinhibition at 1000 μmol photons m^?2 s^?1. Results of oxygenic gross photosynthesis and dark respiration experiments over a temperature range of 8–40°C revealed similar temperature optima for both stages (29.7°C, macroscopic stage; 30.3°C, microscopic stage), which support their observed occurrences in the habitat during summer. Maximum quantum yields of photosystem II (PSII ) (F _v /F _m ) were relatively stable at low temperatures with the highest at 15.1°C for the macroscopic stage and at 16.5°C for the microscopic stage; but dropped at higher temperatures especially above 28°C. Continuous exposures (6 h) to 200 and 1000 μmol photons m^?2 s^?1 at 8, 16, and 28°C revealed greater depressions in effective quantum yields of PSII (Φ _PSII ) of the microscopic stage at 8°C, as well as its F _v /F _m that barely increased after 6 h of dark acclimation. Whereas post‐dark acclimation F _v /F _m of both stages exposed to low PAR fairly recovered at 28°C, suggesting their photosynthetic tolerance to such high temperature. Under natural conditions, both heteromorphic stages of C. okamuranus may persist throughout the year in this region. Beyond its northern limit of distribution, the microscopic stage of this species may suffer from photodamage, as enhanced by low winter temperatures; hence, its restricted occurrence.