Suboptimal timing of reproduction in Lobelia inflata may be a conservative bet-hedging strategy

Age and size at reproduction are important components of fitness, and are variable both within and among angiosperm species. The fitness consequences of such life-history variation are most readily studied in organisms that reproduce only once in their lifetime. The timing of the onset of reproduction (bolting) in the monocarpic perennial, Lobelia inflata, occurs over a range of dates within a season, and may be postponed to a later year. Empirical relationships among life-history traits, derived from over 950 wild-growing and experimentally manipulated plants in the field, are used to model an optimal changing size threshold (norm of reaction) for bolting over the growing season. Comparisons are made between observed and expected norms of reaction governing bolting. An apparently suboptimal bolting schedule that precludes bolting beyond an early (conservative) date is observed, and is found to be qualitatively consistent with conservative bet hedging under unpredictable season lengths. On this basis we propose the schedule of bolting as a plausible example of a conservative bet-hedging strategy. The results underscore the critical need for long-term studies of fluctuating selection to distinguish suboptimality from bet hedging.     

Time constraints in temperate‐breeding species: influence of growing season length on reproductive strategies

Organisms that reproduce in temperate regions have limited time to produce offspring successfully, and this constraint is expected to be more pronounced in areas with short growing seasons. Information concerning how reproductive ecology of endotherms might be influenced by growing season length (GSL) is rare, and species that breed over a broad geographic range provide an opportunity to study the effects of time constraints on reproductive strategies. We analyzed data from a temperate‐breeding bird, the lesser scaup Aythya affinis; hereafter scaup, collected at eight sites across a broad gradient of GSL to evaluate three hypotheses related to reproductive compensation in response to varying time constraints. Clutch initiation date in scaup was unaffected by GSL and was unrelated to latitude; spring thaw dates had a marginal impact on timing of breeding. Clutch size declined during the nesting season, as is reported frequently in bird species, but was also unaffected by GSL. Scaup do not appear to compensate for shorter growing seasons by more rapidly reducing clutch size. This study demonstrates that this species is remarkably consistent in terms of timing of breeding and clutch size, regardless of growing season characteristics. Such inflexibility could make this species particularly sensitive to environmental changes that affect resource availabilities.     

Divergence among arctic and alpine populations of the annual, Koenigia islandica: morphology, life-history, and phenology

Arctic and alpine habitats occur along complex environmental gradients, and over an extensive geographical range. Despite some selective forces common to these habitats, evolutionary divergence among populations of arctic and alpine plants along this gradient is expected. Of particular significance, both in the context of life-history theory and for implications of climate change, are the few annual species that have adapted to the constraints of an unpredictable, short growing season. In this study, morphological, life-history and phenological characters were found to differ significantly among six widely distributed populations of the arctic-alpine annual Koenigia islandica. On the basis of morphology and life-history traits, populations from high latitudes, with the exception of Svalbard, performed better in simulated arctic conditions, whereas the low latitude alpine plants from Colorado showed enhanced performance under simulated alpine conditions. On the basis of phenology, the six populations can be clearly grouped into arctic, high latitude alpine and alpine populations: arctic plants were found to develop and flower earliest; alpine plants latest. Because these results were obtained using seeds harvested from plants first grown through a complete generation in growth chambers, they indicate strong genetic differentiation. We discuss possible adaptive explanations for observed differences among the six geographically divergent populations.     

Sexual dimorphism in Odonata: age, size, and sex ratio at emergence

Males and females of many organisms differ in important life-history and behavioral characters. Following a recent optimization analysis of sexually dimorphic life histories, we employed an odonate-like parameter set to identify patterns of life history and behavior to be expected in an odonate population. The default parameter magnitudes generated a smaller body size and shorter development time for males than for females, which resulted in a male-biased sex ratio. Whether population growth was density dependent or density independent, and whether development time was fixed or flexible had major impacts on life-history features. The model generated five general predictions for odonate systems. (1) For species with fixed development times, males and females should differ more in activity level, growth and mortality rates than for species with flexible life cycles. (2) In species with fixed development times, populations at high latitude or high altitude should be more active, emerge and reproduce at smaller size and have a more male-biased sex ratio than low latitude and low altitude populations. (3) In density-dependent populations, with density dependence mediated by activity-dependent mortality, higher predation rates should increase activity levels and reduce development time in species with flexible development times. (4) For species with flexible development times, in strongly density-dependent populations with density dependence mediated by mortality, activity levels should decrease and development times should increase at high prey abundance. (5) Males should be larger at emergence relative to females, and the sex ratio at emergence should be more female-biased in territorial than in non-territorial species. Existing empirical evidence concerning these predictions is generally sparse and equivocal; focused tests are clearly needed.     

A change in climate causes rapid evolution of multiple life-history traits and their interactions in an annual plant

Climate change is likely to spur rapid evolution, potentially altering integrated suites of life-history traits. We examined evolutionary change in multiple life-history traits of the annual plant Brassica rapa collected before and after a recent 5-year drought in southern California. We used a direct approach to examining evolutionary change by comparing ancestors and descendants. Collections were made from two populations varying in average soil moisture levels, and lines propagated from the collected seeds were grown in a greenhouse and experimentally subjected to conditions simulating either drought (short growing season) or high precipitation (long growing season) years. Comparing ancestors and descendants, we found that the drought caused many changes in life-history traits, including a shift to earlier flowering, longer duration of flowering, reduced peak flowering and greater skew of the flowering schedule. Descendants had thinner stems and fewer leaf nodes at the time of flowering than ancestors, indicating that the drought selected for plants that flowered at a smaller size and earlier ontogenetic stage rather than selecting for plants to develop more rapidly. Thus, there was not evidence for absolute developmental constraints to flowering time evolution. Common principal component analyses showed substantial differences in the matrix of trait covariances both between short and long growing season treatments and between populations. Although the covariances matrices were generally similar between ancestors and descendants, there was evidence for complex evolutionary changes in the relationships among the traits, and these changes depended on the population and treatment. These results show that a full appreciation of the impacts of global change on phenotypic evolution will entail an understanding of how changes in climatic conditions affect trait values and the structure of relationships among traits.     

Dynamics, diversity, and resource gradient relationships in the herbaceous layer of an old-growth Appalachian forest

The ecological drivers of herbaceous layer composition and diversity in deciduous forests of eastern North America are imperfectly understood. We analyzed the herbaceous layer, across the growing season, in a central Appalachian old-growth forest to examine dynamics, diversity, and relationships to resource gradients. We found clear variation in herb species composition over the growing season. We identified intermingled resource gradients, including soil nutrients, light availability, and topography, that were related to herbaceous composition. We found that herb layer diversity was different among previously identified tree communities, but was not variable over the growing season. We identified a unimodal relationship between diversity and productivity in the herb flora that held throughout the growing season despite changing composition and levels of productivity. Diversity and distributions in the herbaceous community of our study site are linked to a complex of resource gradients.     

Early growth trajectories affect sexual responsiveness

The trajectory of an animal''s growth in early development has been shown to have long-term effects on a range of life-history traits. Although it is known that individual differences in behaviour may also be related to certain life-history traits, the linkage between early growth or development and individual variation in behaviour has received little attention. We used brief temperature manipulations, independent of food availability, to stimulate compensatory growth in juvenile three-spined sticklebacks Gasterosteus aculeatus. Here, we examine how these manipulated growth trajectories affected the sexual responsiveness of the male fish at the time of sexual maturation, explore associations between reproductive behaviour and investment and lifespan and test whether the perceived time stress (until the onset of the breeding season) influenced such trade-offs. We found a negative impact of growth rate on sexual responsiveness: fish induced (by temperature manipulation) to grow slowest prior to the breeding season were consistently quickest to respond to the presence of a gravid female. This speed of sexual responsiveness was also positively correlated with the rate of development of sexual ornaments and time taken to build a nest. However, after controlling for effects of growth rate, those males that had the greatest sexual responsiveness to females had the shortest lifespan. Moreover, the time available to compensate in size before the onset of the breeding season (time stress) affected the magnitude of these effects. Our results demonstrate that developmental perturbations in early life can influence mating behaviour, with long-term effects on longevity.     

Deconstructing latitudinal species richness patterns in the ocean: does larval development hold the clue?

The consistent decrease in species richness with latitude shows several exceptions among marine organisms. We hypothesize that contrasting latitudinal diversity gradients can be explained by differences in critical life-history attributes, such as mode of larval development (MLD). We deconstructed latitudinal species richness patterns of marine benthic invertebrates according to MLD to elucidate differences in patterns of species richness and to reveal underlying processes. The patterns of species richness were remarkably similar across taxa within MLD but differed between MLD. Species richness decreased polewards in planktotrophic species and increased in direct developers. Temperature explained most of the variation in species richness. Low temperature at high latitudes may generally favour direct developing species, but, together with low chlorophyll- a concentration, limit the distribution of planktotrophic species. The contrasting influence of temperature on different MLDs might be explained by its effect on the length of planktonic life and on brooding costs.     

Quantitative genetics of immunity and life history under different photoperiods

Insects with complex life-cycles should optimize age and size at maturity during larval development. When inhabiting seasonal environments, organisms have limited reproductive periods and face fundamental decisions: individuals that reach maturity late in season have to either reproduce at a small size or increase their growth rates. Increasing growth rates is costly in insects because of higher juvenile mortality, decreased adult survival or increased susceptibility to parasitism by bacteria and viruses via compromised immune function. Environmental changes such as seasonality can also alter the quantitative genetic architecture. Here, we explore the quantitative genetics of life history and immunity traits under two experimentally induced seasonal environments in the cricket Gryllus bimaculatus. Seasonality affected the life history but not the immune phenotypes. Individuals under decreasing day length developed slower and grew to a bigger size. We found ample additive genetic variance and heritability for components of immunity (haemocyte densities, proPhenoloxidase activity, resistance against Serratia marcescens), and for the life history traits, age and size at maturity. Despite genetic covariance among traits, the structure of G was inconsistent with genetically based trade-off between life history and immune traits (for example, a strong positive genetic correlation between growth rate and haemocyte density was estimated). However, conditional evolvabilities support the idea that genetic covariance structure limits the capacity of individual traits to evolve independently. We found no evidence for G × E interactions arising from the experimentally induced seasonality.     

Fruit Development in Trillium : Dependence on Stem Carbohydrate Reserves

Leaves are the main source of carbon for fruit maturation in most species. However, in plants seeing contrasting light conditions such as some spring plants, carbon fixed during the spring could be used to support fruit development in the summer, when photosynthetic rates are low. We monitored carbohydrate content in the rhizome (a perennating organ) and the aboveground stem of trillium (Trillium erectum) over the entire growing season (May–November). At the beginning of the fruiting stage, stems carrying a developing fruit were harvested, their leaves were removed, and the leafless stems were maintained in aqueous solution under controlled conditions up to full fruit maturation. These experiments showed that stem carbohydrate content was sufficient to support fruit development in the absence of leaves and rhizome. This is the first reported case, to our knowledge, of complete fruit development sustained only by a temporary carbohydrate reservoir. This carbohydrate accumulation in the stem during the spring enables the plant to make better use of the high irradiances occurring at that time. Many other species might establish short-term carbohydrate reservoirs in response to seasonal changes in growing conditions.     

Life-history variation in relation to time constraints in a damselfly

Although variation within populations in plasticity to time constraints is expected with regard to hatching date, empirical studies are largely lacking. We studied life-history responses to time constraints manipulated by photoperiod and associated with hatching date in larvae of the damselfly Lestes viridis for two populations with a different hydroperiod. In a common garden experiment, early- and late-hatched larvae from both populations were reared at two photoperiods mimicking the start and the end of the egg-hatching season. In a reciprocal transplant experiment, early- and late-hatched larvae from both populations were reared in both ponds. In all these experiments, larvae were reared from egg hatching until adult emergence. Within both populations, larvae reared at the photoperiod indicating a late time point in the growing season, reduced development time to compensate for their perceived shorter development period. Growth rate, however, did not respond to photoperiod, resulting in a lower mass at emergence. As expected, both in the laboratory and in the field, larvae from eggs that hatched later in the season generally had a shorter development time and a faster growth rate, resulting in a higher mass at emergence compared to early-hatched larvae. This may explain the intriguing seasonal increase in mass at emergence in this species, and affect the predictions of optimality models. None of these life-history responses differed between the two populations, despite clear differences in time constraints linked to hydroperiod, suggesting the robustness of the observed patterns. Given the ubiquity of asynchronous hatching in nature, and the adaptive value of the observed differences between early- and late-hatched larvae, we expect the effects of hatching date on life-history plasticity to be widespread.     

Influence of seasonal timing on thermal ecology and thermal reaction norm evolution in Wyeomyia smithii

Evolutionary changes in the seasonal timing of life-history events can alter a population's exposure to seasonally variable environmental factors. We illustrate this principle in Wyeomyia smithii by showing that: (1) geographic divergence in diapause timing reduces differences among populations in the thermal habitat experienced by nondiapause stages; and (2) the thermal habitat of the growing season is more divergent at high compared with low temperatures with respect to daily mean temperatures. Geographic variation in thermal reaction norms for development time was greater in a warm compared with a cool rearing treatment, mirroring the geographic trend in daily mean temperature. Geographic variation in body size was unrelated to geographic temperature variation, but was also unrelated to development time or fecundity. Our results suggest that proper interpretation of geographic trends may often require detailed knowledge of life-history timing.     

Exploring the drivers of variation in trophic mismatches: A systematic review of long‐term avian studies

1. Many organisms reproduce in seasonal environments, where selection on timing of reproduction is particularly strong as consumers need to synchronize reproduction with the peaked occurrence of their food. When a consumer species changes its phenology at a slower rate than its resources, this may induce a trophic mismatch, that is, offspring growing up after the peak in food availability, potentially leading to reductions in growth and survival. However, there is large variation in the degree of trophic mismatches as well as in its effects on reproductive output.
2. Here, we explore the potential causes for variation in the strength of trophic mismatches in published studies of birds. Specifically, we ask whether the changes in the degree of mismatch that have occurred over time can be explained by a bird''s (a) breeding latitude, (b) migration distance, and/or (c) life‐history traits.
3. We found that none of these three factors explain changes in the degree of mismatch over time. Nevertheless, food phenology did advance faster at more northerly latitudes, while shifts in bird phenology did not show a trend with latitude.
4. We argue that the lack of support in our results is attributable to the large variation in the metrics used to describe timing of food availability. We propose a pathway to improve the quantification of trophic mismatches, guided by a more rigorous understanding of links between consumers and their resources.

     

Cascading life-history interactions: alternative density-dependent pathways drive recruitment dynamics in a freshwater fish

Although density-dependent mechanisms in early life-history are important regulators of recruitment in many taxa, consequences of such mechanisms on other life-history stages are poorly understood. To examine interacting and cascading effects of mechanisms acting on different life-history stages, we stocked experimental ponds with fathead minnow (Pimephales promelas) at two different densities. We quantified growth and survival of the stocked fish, the eggs they produced, and the resulting offspring during their first season of life. Per-capita production and survival of eggs were inversely related to density of stocked fish; significant egg cannibalism by stocked minnows resulted in initial young-of-the-year (YOY) densities that were inversely related to adult densities. Subsequent growth and survival of YOY were then inversely related to these initial YOY densities, and survival of YOY was selective for larger fish. Because of these compensatory processes in the egg and YOY stages, treatments did not differ in YOY abundance and mean size at the end of the growing season. Because of differences in the intensity of size-selective mortality, however, variation in end-of season sizes of YOY was strongly (and inversely) related to densities of stocked fish. When mortality was severe in the egg stage (high densities of stocked fish), final YOY size distributions were more variable than when the dominant mortality was size-selective in the YOY stage (low stocked fish densities). These differences in size variation could have subsequent recruitment consequences, as overwinter survival is typically selective for YOY fish larger than a critical threshold size. Density-dependent effects on a given life stage are not independent, but will be influenced by earlier stages; alternative recruitment pathways can result when processes at earlier stages differ in magnitude or selectivity. Appreciation of these cascading effects should enhance our overall understanding of the dynamics of stage-structured populations.     

Testing the effect of early‐life reproductive effort on age‐related decline in a wild insect

The disposable soma theory of ageing predicts that when organisms invest in reproduction they do so by reducing their investment in body maintenance, inducing a trade‐off between reproduction and survival. Experiments on invertebrates in the lab provide support for the theory by demonstrating the predicted responses to manipulation of reproductive effort or lifespan. However, experimental studies in birds and evidence from observational (nonmanipulative) studies in nature do not consistently reveal trade‐offs. Most species studied previously in the wild are mammals and birds that reproduce over multiple discrete seasons. This contrasts with temperate invertebrates, which typically have annual generations and reproduce over a single season. We expand the taxonomic range of senescence study systems to include life histories typical of most temperate invertebrates. We monitored reproductive effort, ageing, and survival in a natural field cricket population over ten years to test the prediction that individuals investing more in early‐reproduction senesce faster and die younger. We found no evidence of a trade‐off between early‐life reproductive effort and survival, and only weak evidence for a trade‐off with phenotypic senescence. We discuss the possibility that organisms with multiple discrete breeding seasons may have greater opportunities to express trade‐offs between reproduction and senescence.     

Effects of an autocorrelated stochastic environment and fisheries on the age at maturity of Chinook salmon

Chinook salmon (Oncorhynchus tshawytscha) reproduce only once in their lifetime, and their age at reproduction varies among individuals (indeterminate semelparous). However, the factors that determine their spawning age still remain uncertain. Evidence from recent studies suggests that individual growth and reproduction of Chinook salmon are affected by the rate of coastal upwelling, which is shown to be positively autocorrelated between years. Therefore, the serially autocorrelated environmental is expected to play an important role in determining their spawning age. In the present study, I demonstrate the advantage of an indeterminate maturation strategy under a stochastic environment. I then present theoretical evidence for the advantage of adjusting the maturation probability based on the environment they experienced and demonstrate that fisheries reduce the fitness of the strategy to delay maturation. The results presented herein emphasize the importance of incorporating detailed life-history strategies of organisms when undertaking population management.     

Seasonal plasticity in life history traits: growth and development in Polygonia c-album (Lepidoptera: Nymphalidae)

The potentially multivoltine comma butterfly, Polygonia c-album L., hibernates in the adult stage. The adult seasonal morph is demonstrated to be a good indicator of whether an individual has entered reproductive diapause or is developing directly to sexual maturation. This fact, and the assumption that a short development time is not equally important to all categories of individuals, was used to test predictions on variation in life-history traits among categories (morphs and sexes) and environments (temperature and photoperiod) at the level of individuals and to some extent families and populations (the univoltine Stockholm population and the partially bivoltine Oxford population). Individuals developing to adults in a short time were expected to be smaller and lighter as a result of a basic trade-off between the two traits. Development times varied in accordance with predictions, but in most cases this was due to plastic growth and development in both the larval and pupal stages rather than through variation in size or weight, i.e. size was a highly canalized trait. This suggests a relationship between plasticity and canalization and a strong potential for plasticity to shield life-history traits from selection. Individuals regulated development times also within developmental pathways, in response to photoperiods indicating the progression of the season. These and other results suggest that development times are not normally minimized in temperate butterflies unless this is enforced by direct development and protandry. There is thus scope for a high degree of adaptive plasticity in growth- and developmental rates which may devalue the basic trade-offs assumed by life-history theory and account for inconsistencies with its predictions.     

Countergradient variation in growth rate: compensation for length of the growing season among Atlantic silversides from different latitudes

Summary How do organisms adapt to the differences in temperature and length of the growing season that occur with latitude? Among Atlantic silversides (Menidia menidia) along the east coast of North America, the length of the first growing season declines by a factor of about 2.5 with increasing latitude. Yet body size at the end of the first growing season does not decline. High-latitude fish must, therefore, grow faster within the growing season than do low-latitude fish. This geographical pattern has a genetic basis. Laboratory experiments on fish from six different locations revealed a latitudinal gradient in the capacity for growth (i.e., maximum growth potential). In two subsequent experiments using fish from Nova Scotia (NS), New York (NY) and South Carolina (SC) that had been separately reared in a common environment for several generations, differences in growth rate among populations were highly significant. The rank order was NS>NY>SC, but the difference among populations depended on temperature. High-latitude fish outperformed those from low latitudes primarily at the high temperatures that low-latitude fish would be expected to experience most often in nature. These results suggest that instead of being adapted for growth at low temperatures, fish from high latitudes are adapted for rapid elevation of growth rate during the brief interval of the year when high temperatures occur. Selection on growth rate results from sizedependent winter mortality: the importance to winter survival of being large increases with latitude but the length of the growing season simultaneously decreases. The end result is countergradient variation in growth rate, a phenomenon that may be much more widespread than currently recognized.     

Terre et Vie : des histoires imbriquées

The Earth's history consists in recurrent flashbacks of similar events and scenarios: redistribution of continents, orogenic cycles, glaciations, marine transgressions and regressions, etc. In contrast, Life's history evolves according to a succession of stages: prokaryotic stage, eukaryotic cell stage, pluricellular organism stage, terrestrialization, development of animal societies, hominization. With each successive stage the biosphere rises to a higher level of organization and complexity. This evolution results from the natural trend of living organisms to extend their control over the entire planet while they progressively escape the constraints of the aquatic environments and climates. During the last four billion years close and complex interactions prevailed between the history of both the Earth and Life. Living organisms have a profound effect on their environment and on the processes of the Earth dynamics, while the planetary environment controls the evolution of living species. Nevertheless, from time to time, the fragile equilibrium established between Earth's and Life's dynamics breaks down and triggers mass extinctions. It is presently the case of the increasing impact of human activities on the integrity of our planet, a major challenge for humankind during the 21st century.