Size-dependent Mortality Research Articles

Patterns of forest dynamics in a secondary old-growth beech-dominated forest in the Jizera Mountains Beech Forest Reserve, Czech Republic

Restoring the structural characteristics of secondary old-growth forests that were previously managed is increasingly debated to help increase the area of more complex forests which provide a broader array of forest services and functions. The paucity of long-term data sets in Central Europe has limited our ability to understand the ongoing ecological processes required for effective restoration programs for old-growth forests. To address this, we used repeated census data from eight permanent plots to evaluate forest structural dynamics over a 12-year period in the largest complex of European beech (Fagus sylvatica L.) forests in the Czech Highlands without intensive forestry intervention for almost 50 years. Our results showed that previously managed forests can exhibit structural qualities typically associated with old-growth forests after management has ceased for a period. The stand structural characteristics (e.g., density of large and old trees) is comparable with protected reserves of old-growth European beech-dominated forests. The average stand age was 196 years, but the oldest tree was 289 years old. The annual mortality rate was 0.43% for all species, and the U-shaped distribution indicating size-dependent mortality is likely an important process that is balanced by the turnover of new tree recruitment. During the study period, we detected that the diameter distribution tended towards a rotated sigmoid distribution. The lasting effects of the most recent forest management are evident in the scarcity of dead wood, and a prolonged process of dead wood accumulation has begun. Thus, the abandonment of all management activities in near-natural forest reserves, including dead wood removal, will ensure that the forests will develop characteristics typical of old-growth forests.

Inbreeding depression and differential maladaptation shape the fitness trajectory of two co-occurring Eucalyptus species

The fitness trajectory of long-lived forest species with mixed mating systems is shaped by a dynamic interplay between endogenous (inbreeding depression) and exogenous (environmental maladaptation) factors. Using two eucalypt species, we show that the timing and translation of inbreeding depression from growth to survival through size-dependent mortality may vary between species and may intensify under climate stress. Inbreeding is an important issue in evolutionary biology and breeding, as it can reduce genetic diversity and fitness and ultimately limit the adaptive response of populations to environmental stress. This is particularly relevant to forest tree species, such as eucalypts, which have a mixed mating system and long-generation intervals. Examine the role of inbreeding depression on the fitness trajectory of two eucalypt species, Eucalyptus globulus and E. ovata. Survival, growth, and reproduction of controlled-crossed self and outcross, as well as open-pollinated progeny of each species grown in a common garden field trial were assessed over a 28-year period and analysed using mixed effect models. Inbreeding depression resulted in the purging of inbred progeny through size-dependent mortality with the most death of inbreds occurring between 4 and 13 years. After this period, differential maladaptation of the species was the dominant cause of mortality, associated with a period of drought and high temperatures, and it was evident first in the selfed populations. This study demonstrates the dynamic nature of the selective process in purging inbred progeny from a population, with inbreeding depression the dominant factor early in stand development, leading to older stands being dominated by outcrosses.

Natural selection for body shape in resource polymorphic Icelandic Arctic charr.

Resource polymorphisms exhibit remarkable intraspecific diversity and in many cases are expected to be maintained by diversifying selection. Phenotypic trade-offs can constrain morphologically intermediate individuals from effectively exploiting both alternate resources, resulting in ecological barriers to gene flow. Determining if and how phenotypic trade-offs cause fitness variation in the wild is challenging because of phenotypic and environmental correlations associated with alternative resource strategies. We investigated multiple pathways through which morphology could affect organismal performance, as measured by growth rate, and whether these effects generate diversifying selection in polymorphic Icelandic Arctic charr (Salvelinus alpinus) populations. We considered direct effects of morphology on growth and indirect effects via trophic resource use, estimated by stable isotopic signatures, and via parasitism associated with trophic resources. We sampled over 3years in (lakes) Thingvallavatn and Vatnshlíðarvatn using the extended selection gradient path analytical approach and estimating size-dependent mortality. We found evidence for diversifying selection only in Thingvallavatn: more streamlined and terminally mouthed planktivore charr experienced greater growth, with the opposite pattern in small benthic charr. However, this effect was mediated by parasitism and nontrophic pathways, rather than trophic performance as often expected. Detection of between-morph differences in the presence (Vatnshlíðarvatn) and direction (Thingvallavatn) of size-dependent mortality, together with nontrophic effects of shape, suggests that a morphological trophic performance explanation for polymorphism is insufficient. This rare insight into selection during early diversification suggests that a complex of interacting local factors must be considered to understand how phenotype influences fitness, despite morphological variation reflecting intuitive trade-off explanations.

Life-History Multistability Caused by Size-Dependent Mortality

Body size is a key determinant of mortality risk. In natural populations, a broad range of relationships are observed between body size and mortality, including positive and negative correlations. Previous evolutionary modeling has shown that negatively size-dependent mortality can result in life-history bistability, with early maturation at small size and late maturation at large size representing alternative fitness optima. Here we present a general analysis of conditions under which such life-history bistabilities can occur, reporting the following findings. First, alternative fitness optima can be found for any arbitrarily chosen forms of mortality functions, including functions according to which mortality smoothly declines with size. Second, while bistabilities occur more readily under negatively size-dependent mortality, our analysis reveals that they can also emerge under positively size-dependent mortality, a feature missed in earlier work. Third, any sharp drop of mortality with size facilitates bistability. Fourth, if the mortality regime involves more than one such sharp drop, multistable life histories can occur, with alternative fitness optima straddling each of the drops. Paradoxically, our findings imply that, fifth, a species-poor predator community capable of creating a rugged mortality regime is conducive to evolutionary multistability, which could act as a stepping stone toward prey life-history diversification, whereas a species-rich predator community that results in a smoothly varying mortality regime may prevent diversification through this pathway.

Corrigendum

Corrigendum

Inferring forest fate from demographic data: from vital rates to population dynamic models.

As population-level patterns of interest in forests emerge from individual vital rates, modelling forest dynamics requires making the link between the scales at which data are collected (individual stems) and the scales at which questions are asked (e.g. populations and communities). Structured population models (e.g. integral projection models (IPMs)) are useful tools for linking vital rates to population dynamics. However, the application of such models to forest trees remains challenging owing to features of tree life cycles, such as slow growth, long lifespan and lack of data on crucial ontogenic stages. We developed a survival model that accounts for size-dependent mortality and a growth model that characterizes individual heterogeneity. We integrated vital rate models into two types of population model; an analytically tractable form of IPM and an individual-based model (IBM) that is applied with stochastic simulations. We calculated longevities, passage times to, and occupancy time in, different life cycle stages, important metrics for understanding how demographic rates translate into patterns of forest turnover and carbon residence times. Here, we illustrate the methods for three tropical forest species with varying life-forms. Population dynamics from IPMs and IBMs matched a 34 year time series of data (albeit a snapshot of the life cycle for canopy trees) and highlight differences in life-history strategies between species. Specifically, the greater variation in growth rates within the two canopy species suggests an ability to respond to available resources, which in turn manifests as faster passage times and greater occupancy times in larger size classes. The framework presented here offers a novel and accessible approach to modelling the population dynamics of forest trees.

Spatially dynamic maternal control of migratory fish recruitment pulses triggered by shifting seasonal cues

Environmental regimes set the timing and location of early life-history events of migratory species with synchronised reproduction. However, modified habitats in human-dominated landscapes may amplify uncertainty in predicting recruitment pulses, impeding efforts to restore habitats invaluable to endemic species. The present study assessed how environmental and spawner influences modulate recruitment variability and persistence of the Missouri River shovelnose sturgeon (Scaphirhynchus platorynchus) under modified seasonal spawning and nursery habitat conditions. Using a spatially explicit individual-based biophysical model, spawning cycle, early life-history processes (dispersal, energetics and survival) and prey production were simulated under incrementally perturbed flow (from –10 to –30%) and temperature (+1 and +2°C) regimes over 50 years. Simulated flow reduction and warming synergistically contracted spring spawning habitats (by up to 51%) and periods (by 19%). Under these conditions, fewer mature females entered a reproductive cycle, and more females skipped spawning, reducing spawning biomass by 20–50%. Many spawners migrated further to avoid increasingly unfavourable habitats, intensifying local density dependence in larval stages and, in turn, increasing size-dependent predation mortality. Diminished egg production (by 20–97%) and weakened recruitment pulses (by 46–95%) ultimately reduced population size by 21–74%. These simulations illustrate that environmentally amplified maternal influences on early life histories can lower sturgeon population stability and resilience to ever-increasing perturbations.

Effects of extreme weather on two sympatric Australian passerine bird species.

Despite abundant evidence that natural populations are responding to climate change, there are few demonstrations of how extreme climatic events (ECEs) affect fitness. Climate warming increases adverse effects of exposure to high temperatures, but also reduces exposure to cold ECEs. Here, we investigate variation in survival associated with severity of summer and winter conditions, and whether survival is better predicted by ECEs than mean temperatures using data from two coexisting bird species monitored over 37 years in southwestern Australia, red-winged fairy-wrens, Malurus elegans and white-browed scrubwrens, Sericornis frontalis Changes in survival were associated with temperature extremes more strongly than average temperatures. In scrubwrens, winter ECEs were associated with survival within the same season. In both species, survival was associated with body size, and there was evidence that size-dependent mortality was mediated by carry-over effects of climate in the previous season. For fairy-wrens, mean body size declined over time but this could not be explained by size-dependent mortality as the effects of body size on survival were consistently positive. Our study demonstrates how ECEs can have individual-level effects on survival that are not reflected in long-term morphological change, and the same climatic conditions can affect similar-sized, coexisting species in different ways.This article is part of the themed issue 'Behavioural, ecological and evolutionary responses to extreme climatic events'.

Effects of ambient oxygen and size-selective mortality on growth and maturation in guppies

Growth, onset of maturity and investment in reproduction are key traits for understanding variation in life-history strategies. Many environmental factors affect variation in these traits, but for fish, hypoxia and size-dependent mortality have become increasingly important because of human activities, such as increased nutrient enrichment (eutrophication), climate warming and selective fishing. Here, we study experimentally the effect of oxygen availability on maturation and growth in guppies (Poecilia reticulata) from two different selected lines, one subjected to positive and the other negative size-dependent fishing. This is the first study to assess the effects of both reduced ambient oxygen and size-dependent mortality in fish. We show that reduced ambient oxygen led to stunting, early maturation and high reproductive investment. Likewise, lineages that had been exposed to high mortality of larger-sized individuals displayed earlier maturation at smaller size, greater investment in reproduction and faster growth. These life-history changes were particularly evident for males. The widely reported trends towards earlier maturation in wild fish populations are often interpreted as resulting from size-selective fishing. Our results highlight that reduced ambient oxygen, which has received little experimental investigation to date, can lead to similar phenotypic changes. Thus, changes in ambient oxygen levels can be a confounding factor that occurs in parallel with fishing, complicating the causal interpretation of changes in life-history traits. We believe that better disentangling of the effects of these two extrinsic factors, which increasingly affect many freshwater and marine ecosystems, is important for making more informed management decisions.

Seasonal Growth Fluctuations of Four Species of Neritid Gastropods in an Upper Mangrove Estuary, Ishigaki Island, Japan

Populations of amphidromous neritid gastropods are prone to habitat degradation by anthropogenic disturbances. To accumulate information on the life history of these neritids, seasonal growth patterns of four neritid species, Neritina turrita, N. plumbea, N. auriculata, and Clithon brevispina, were examined in an upper mangrove estuary on Ishigaki Island. Individual neritids in a fixed sampling area were captured and mark-released bimonthly over a 1-yr period. Growth rates of the four species were estimated from linear regression models and Ford-Walford graphs using data from recaptured individuals. A model-selection procedure identified the common L max model as the best model for describing seasonal growth fluctuations of the four species. Growth of the four species was generally higher in the warm season (April–October) and lower in the cool season (October–April). In addition, N. plumbea stopped growing in midsummer (June–August). Temporal changes in the size-frequency histograms showed several recruitment events of the four species. Estimated growth curves, in some cases, did not fit well to temporal changes of the size cohorts derived from the size-frequency histograms, indicating that size-dependent migration or mortality may occur in the sampling area.

Seasonal population dynamics of Sargassum fusiforme (Fucales, Phaeophyta), Suo-Oshima Is., Seto Inland Sea, Japan\u2014development processes of a stand characterized by high density and productivity

Seasonal population dynamics of Sargassum fusiforme, one of the most important edible macroalgae in Japan, were studied. Recruits were mainly generated by vegetative reproduction at the margins of filamentous holdfasts. They first appeared in late spring and peaked in summer as upright thalli of the previous generation withered. After producing recruits, holdfasts withered indicating that holdfasts were also annual, the same as upright thalli. All recruits produced main branches and became new upright thalli in early autumn. During this transitional period, the thallus density decreased due to the crowded conditions induced by simultaneous growth initiation. After this early mortality, however, thallus density remained almost constant over much of the growth season. Thallus growth continued during winter and the stand biomass peaked in spring. During this biomass accumulation, development of a thallus size hierarchy was moderate and no size-dependent mortality was observed. Main branch number per thallus was also constant until spring, indicating the main branches also persisted after being produced in early autumn. This lack of severe intraspecific competition both at a thallus and main branch level is supported by the ambient wave condition of the habitat which gives moderate undulation and enables light and nutrients to be supplied to each thallus, and allowed the S. fusiforme stand to maintain its densely-packed feature with a high productivity.

Adaptation of the Long-Lived Monocarpic Perennial Saxifraga longifolia to High Altitude.

Global change is exerting a major effect on plant communities, altering their potential capacity for adaptation. Here, we aimed at unveiling mechanisms of adaptation to high altitude in an endemic long-lived monocarpic, Saxifraga longifolia, by combining demographic and physiological approaches. Plants from three altitudes (570, 1100, and 2100 m above sea level [a.s.l.]) were investigated in terms of leaf water and pigment contents, and activation of stress defense mechanisms. The influence of plant size on physiological performance and mortality was also investigated. Levels of photoprotective molecules (α-tocopherol, carotenoids, and anthocyanins) increased in response to high altitude (1100 relative to 570 m a.s.l.), which was paralleled by reduced soil and leaf water contents and increased ABA levels. The more demanding effect of high altitude on photoprotection was, however, partly abolished at very high altitudes (2100 m a.s.l.) due to improved soil water contents, with the exception of α-tocopherol accumulation. α-Tocopherol levels increased progressively at increasing altitudes, which paralleled with reductions in lipid peroxidation, thus suggesting plants from the highest altitude effectively withstood high light stress. Furthermore, mortality of juveniles was highest at the intermediate population, suggesting that drought stress was the main environmental driver of mortality of juveniles in this rocky plant species. Population structure and vital rates in the high population evidenced lower recruitment and mortality in juveniles, activation of clonal growth, and absence of plant size-dependent mortality. We conclude that, despite S. longifolia has evolved complex mechanisms of adaptation to altitude at the cellular, whole-plant and population levels, drought events may drive increased mortality in the framework of global change.

How do size distributions relate to concurrently measured demographic rates? Evidence from over 150 tree species in Panama

Abstract:In stable populations with constant demographic rates, size distributions reflect size-dependent patterns of growth and mortality. However, population growth can also affect size distributions, which may not be aligned with current growth and mortality. Using 25 y of demographic data from the 50-ha Barro Colorado Island plot, we examined how interspecific variation in diameter distributions of over 150 tropical trees relates to growth–diameter and mortality–diameter curves and to population growth rates. Diameter distributions were more skewed in species with faster increases/slower decreases in absolute growth and mortality with diameter and higher population growth rates. The strongest predictor of the diameter distribution shape was the exponent governing the scaling of growth with diameter (partial R2 = 0.20–0.34), which differed among growth forms, indicating a role of life history variation. However, interspecific variation in diameter distributions was also significantly related to population growth rates (partial R2 = 0.03–0.23), reinforcing that many populations are not at equilibrium. Consequently, although fitted size distribution parameters were positively related to theoretical predictions based on current size-dependent growth and mortality, there was considerable deviation. These analyses show that temporally variable demographic rates, probably related to cyclic climate variation, are important influences on forest structure.

Spatial patterns of coral survivorship: impacts of adult proximity versus other drivers of localized mortality.

Species-specific enemies may promote prey coexistence through negative distance- and density-dependent survival of juveniles near conspecific adults. We tested this mechanism by transplanting juvenile-sized fragments of the brooding corals Pocillopora damicornis and Seriatopora hystrix 3, 12, 24 and 182 cm up- and down-current of conspecific adults and monitoring their survival and condition over time. We also characterized the spatial distribution of P. damicornis and S. hystrix within replicate plots on three Fijian reef flats and measured the distribution of small colonies within 2 m of larger colonies of each species. Juvenile-sized transplants exhibited no differences in survivorship as a function of distance from adult P. damicornis or S. hystrix. Additionally, both P. damicornis and S. hystrix were aggregated rather than overdispersed on natural reefs. However, a pattern of juveniles being aggregated near adults while larger (and probably older) colonies were not suggests that greater mortality near large adults could occur over longer periods of time or that size-dependent mortality was occurring. While we found minimal evidence of greater mortality of small colonies near adult conspecifics in our transplant experiments, we did document hot-spots of species-specific corallivory. We detected spatially localized and temporally persistent predation on P. damicornis by the territorial triggerfish Balistapus undulatus. This patchy predation did not occur for S. hystrix. This variable selective regime in an otherwise more uniform environment could be one mechanism maintaining diversity of corals on Indo-Pacific reefs.

Comments on the calculation of the specific growth rate in experiments with untagged individuals

The specific growth rate, G , is widely used in articles dealing with the growth of aquatic organisms under experimental conditions. When individuals are untagged, the arithmetic mean of G for a group of animals must be calculated from weight geometric means, not from arithmetic means. The type of weight mean used in articles to calculate the arithmetic mean of G is usually not reported, and an extended use of weight arithmetic means is common. The arithmetic mean of G so calculated is biased according to the increment in the squared coefficient of variation of body weights. Another potential bias in the calculation of the arithmetic mean of G is size-dependent mortality; this bias cannot be avoided when individuals are untagged, but maximal and minimal values can be obtained. In summary, in view of these analytical results, it seems prudent to calculate the arithmetic mean of G for a group of untagged animals from geometric means of weights, and to estimate the maximal error due to the possibility of size-dependent mortality whenever possible.

Death from near and far: alternate perspectives on size-dependent mortality in larval fish

Death from near and far: alternate perspectives on size-dependent mortality in larval fish

Population level effects of multiwalled carbon nanotubes in Daphnia magna exposed to pulses of triclocarban.

Due to the rapid increase of carbon nanotubes (CNT) applications and their inevitable release into the aquatic environment, CNT may interact with and further influence the fate and transport of other pollutants such as triclocarban (TCC). TCC is a high-production-volume chemical that is widely used as an antimicrobial agent, is continually released into the aquatic environment, and is biologically active and persistent. In the present study, the population test with Daphnia magna was performed over 93 days. Different treatments were examined: (a) control, (b) solvent control, (c) 1 mg CNT/L from the beginning, (d) 1 mg CNT/L as of day 14, (e) control with a 2-day pulse of 25 µg TCC/L on day 14, 41 µg TCC/L (day 54), and 61 µg TCC/L (day 68) and (f) same pulses of TCC with co-exposure to 1 mg CNT/L. Significant changes in all three size classes were observed as a result of the long-term exposure to 1 mg CNT/L. Increasing in number of neonates, and decreasing in number of juveniles and adults were observed. Moreover, daphnids were significantly smaller when they were exposed to MWCNT. The exposure with TCC led to size-dependent mortality in Daphnia magna populations and a subsequent recovery. Lower toxicity of TCC was observed, with the presence of MWCNT in the medium. The reported effects of TCC on population level were compared to the output of an individual-based Daphnia magna population model, in order to verify the model predictions with laboratory data.

Trade-offs between growth and maturation: the cost of reproduction for surviving environmental extremes.

Life-history trade-offs and the costs of reproduction are central concepts in evolution and ecology. Episodic climatic events such as drought and extreme temperatures provide strong selective pressures that can change the balance of these costs and trade-offs. We used size-structured matrix models parameterized from field and laboratory studies to examine the effect of periodic drought on two species of aquatic salamanders (greater siren, Siren lacertina; lesser siren, Siren intermedia) that differ in size at reproduction and maximum body size. Post-drought body size distributions of the larger species (S. lacertina) are consistent with size-dependent mortality. Smaller individuals were extirpated from the population during each drought while large animals persisted, a pattern that contrasted with that seen in several ectotherms. This appears to be largely explained by estivation proficiency and a positive relationship between body size and estivation potential. Increased body size, however, may come at the cost of fecundity and maturation rate compared to a closely related congener. The cost of somatic allocation in this case may manifest itself via reduced per-capita competitive ability, which (at least in simulation studies) allows the smaller, fast-maturing species to outcompete the larger, slow-maturing species when drought is minimal or nonexistent.

Why get big in the cold? Size-fecundity relationships explain the temperature-size rule in a pulmonate snail (Physa).

Most ectotherms follow a pattern of size plasticity known as the temperature-size rule where individuals reared in cold environments are larger at maturation than those reared in warm environments. This pattern seems maladaptive because growth is slower in the cold so it takes longer to reach a large size. However, it may be adaptive if reaching a large size has a greater benefit in a cold than in a warm environment such as when size-dependent mortality or size-dependent fecundity depends on temperature. I present a theoretical model showing how a correlation between temperature and the size-fecundity relationship affects optimal size at maturation. I parameterize the model using data from a freshwater pulmonate snail from the genus Physa. Nine families were reared from hatching in one of three temperature regimes (daytime temperature of 22, 25 or 28 °C, night-time temperature of 22 °C, under a 12L:12D light cycle). Eight of the nine families followed the temperature-size rule indicating genetic variation for this plasticity. As predicted, the size-fecundity relationship depended upon temperature; fecundity increases steeply with size in the coldest treatment, less steeply in the intermediate treatment, and shows no relationship with size in the warmest treatment. Thus, following the temperature-size rule is adaptive for this species. Although rarely measured under multiple conditions, size-fecundity relationships seem to be sensitive to a number of environmental conditions in addition to temperature including local productivity, competition and predation. If this form of plasticity is as widespread as it appears to be, this model shows that such plasticity has the potential to greatly modify current life-history theory.

How Body Size and Food Availability Influence First‐Winter Growth and Survival of a Stocked Piscivore

AbstractThe first winter of life can play an important role in the success of age‐0 fishes. First‐winter survival is often size dependent, with larger fish exhibiting higher survival than small fish. Cohorts of age‐0 saugeye female Walleye Sander vitreus × male Sauger S. canadensis stocked into Ohio reservoirs exhibit overwinter shifts toward larger body sizes; however, it is unclear whether growth, size‐dependent mortality, or size‐dependent emigration underlie this phenomenon. Saugeye may experience low prey availability during the overwinter period, making them especially vulnerable to starvation. Furthermore, survivors emerging from winter in poor energetic condition may experience reduced spring foraging success and growth. We used a combination of overwinter PIT tag studies in the field and overwinter outdoor pool experiments to understand these direct and indirect effects of winter on survival and size distributions of cohorts of saugeye. Using PIT tags to allow us to track growth of individuals in reservoirs, we found that saugeye of all sizes increased in length over winter, there was no evidence of size‐dependent overwinter mortality, and rates of emigration out of the reservoir were greater for large saugeye than small saugeye. Thus, only growth rate, and not mortality or emigration biased toward small fish, can explain the observed overwinter shift in size distributions. In pool experiments, we found no direct effects of winter on survival, even in the complete absence of food, and no negative consequences of starvation over the winter on the ability of saugeye to resume feeding in the spring. Our results suggest that shifts in size distributions of first‐year cohorts over winter are driven by growth rather than mortality. Neither direct effects of first winter on survival nor indirect effects mediated through effects of starvation on future foraging ability are important in recruitment success of saugeye in Ohio reservoirs.Received March 18, 2014; accepted June 30, 2014