Giving‐up densities and ideal pre‐emptive patch use in a predatory benthic stream fish

1. We used observational and experimental field studies together with an individual‐based simulation model to demonstrate that behaviours of mottled sculpin (Cottus bairdi) were broadly consistent with the expectations of Giving‐Up Density theory and an Ideal Pre‐emptive Distribution habitat selection model. 2. Specifically we found that: (i) adult mottled sculpin established territories within patches characterised by significantly higher prey densities and prey renewal rates than patches occupied by juveniles or randomly selected patches; (ii) patches abandoned by adult sculpin possessed significantly lower prey densities than newly occupied patches, although this was not true for juveniles; (iii) the observed giving‐up density (GUD) for adult sculpin (i.e. average prey density in patches recently abandoned) increased linearly with increasing fish size up to the average prey density measured in randomly selected patches (i.e. 350 prey items per 0.1 m²) and decreased with increasing sculpin density and (iv) juveniles rapidly shifted their distribution towards the highest quality patches following removal of competitively dominant adult sculpin. 3. These results provide the first evidence of the applicability of GUD theory to a stream‐dwelling organism, and they elucidate the underlying factors influencing juvenile and adult sculpin habitat selection and movement behaviours. Furthermore, optimal patch use, ideal pre‐emptive habitat selection and juvenile ‘floating’ provide behavioural mechanisms linking environmental heterogeneity in the stream benthos to density‐dependent regulation of mottled sculpin populations in this system.     

Environment‐dependent variation in selection on life history across small spatial scales

Variation in life‐history traits is ubiquitous, even though genetic variation is thought to be depleted by selection. One potential mechanism for the maintenance of trait variation is spatially variable selection. We explored spatial variation in selection in the field for a colonial marine invertebrate that shows phenotypic differences across a depth gradient of only 3 m. Our analysis included life‐history traits relating to module size, colony growth, and phenology. Directional selection on colony growth varied in strength across depths, while module size was under directional selection at one depth but not the other. Differences in selection may explain some of the observed phenotypic differentiation among depths for one trait but not another: instead, selection should actually erode the differences observed for this trait. Our results suggest selection is not acting alone to maintain trait variation within and across environments in this system.     

Phenotypic variation and selective mortality as major drivers of recruitment variability in fishes

An individual's phenotype will usually influence its probability of survival. However, when evaluating the dynamics of populations, the role of selective mortality is not always clear. Not all mortality is selective, patterns of selective mortality may vary, and it is often unknown how selective mortality compares or interacts with other sources of mortality. As a result, there is seldom a clear expectation for how changes in the phenotypic composition of populations will translate into differences in average survival. We address these issues by evaluating how selective mortality affects recruitment of fish populations. First, we provide a quantitative review of selective mortality. Our results show that most of the mortality during early life is selective, and that variation in phenotypes can have large effects on survival. Next, we describe an analytical framework that accounts for variation in selection, while also describing the amount of selective mortality experienced by different cohorts recruiting to a single population. This framework is based on reconstructing fitness surfaces from phenotypic selection measurements, and can be employed for either single or multiple traits. Finally, we show how this framework can be integrated with models of density‐dependent survival to improve our understanding of recruitment variability and population dynamics.     

Risk‐taking behaviour may explain high predation mortality of GH‐transgenic common carp Cyprinus carpio

The competitive ability and habitat selection of juvenile all‐fish GH‐transgenic common carp Cyprinus carpio and their size‐matched non‐transgenic conspecifics, in the absence and presence of predation risk, under different food distributions, were compared. Unequal‐competitor ideal‐free‐distribution analysis showed that a larger proportion of transgenic C. carpio fed within the system, although they were not overrepresented at a higher‐quantity food source. Moreover, the analysis showed that transgenic C. carpio maintained a faster growth rate, and were more willing to risk exposure to a predator when foraging, thereby supporting the hypothesis that predation selects against maximal growth rates by removing individuals that display increased foraging effort. Without compensatory behaviours that could mitigate the effects of predation risk, the escaped or released transgenic C. carpio with high‐gain and high‐risk performance would grow well but probably suffer high predation mortality in nature.     

Recruitment variation in a stream galaxiid fish: multiple influences on fry dynamics in a heterogeneous environment

1. The relative importance of density‐dependent and density‐independent processes in explaining fluctuations in natural populations has been widely debated. In particular, the importance of larval supply and whether it may control the type of regulatory processes a population experiences has proved contentious. 2. Using surveys and field experiments conducted in streams in Canterbury, New Zealand, we investigated how variation in the survival of non‐migratory Galaxias vulgaris fry was affected by density‐dependent and density‐independent processes and how this variation influenced recruitment dynamics. 3. Fry populations with high settlement densities experienced a 70–80% reduction in population size from density‐related mortality during the first fourteen days after peak settlement but thereafter the influence of density‐dependent processes on fry was weak. The impact of environmental conditions on fry populations was dependent on fry size and the magnitude of the perturbation, such that flooding effects on fry survival were most severe when fry were small. 4. In streams not affected by flooding, the size and density of introduced trout (Salmo trutta and Oncorhynchus mykiss) were the most significant factors determining the abundance of eventual recruits. A field experiment manipulating brown trout access to fry populations revealed that trout as small as 110 mm may be capable of greatly reducing and possibly preventing galaxiid recruitment. 5. Overall, the results indicated density‐dependent population regulation was only possible at sites with high native fish densities because trout were likely to be suppressing the number of potential recruits at sites with low native fish numbers. Whilst density‐dependent processes had a strong effect on fry survival following the period of peak fry abundance, density‐independent processes associated with flow and predatory trout influences on fry survival largely determined recruitment variability among galaxiid populations. Focusing conservation efforts on improving habitat to increase fry retention and reducing the impacts of trout on galaxiids would ensure more native fish populations reached their potential abundance.     

Predation‐associated divergence of male genital morphology in a livebearing fish

Male genital morphology is remarkably diverse across internally fertilizing animals, a phenomenon largely attributed to sexual selection. Ecological differences across environments can alter the context of sexual selection, yet little research has addressed how this may influence the rapid, divergent evolution of male genitalia. Using the model system of Bahamas mosquitofish (Gambusia hubbsi) undergoing ecological speciation across blue holes, we used geometric morphometric methods to test (i) whether male genital shape (the small, approximately 1 mm long, distal tip of the sperm‐transfer organ, the gonopodium) has diverged between populations with and without predatory fish and (ii) whether any observed divergence has a genetic basis. We additionally examined the effects of genetic relatedness and employed model selection to investigate other environmental factors (i.e. interspecific competition, adult sex ratio and resource availability) that could potentially influence genital shape via changes in sexual selection. Predation regime comprised the most important factor associated with male genital divergence in this system, although sex ratio and some aspects of resource availability had suggestive effects. We found consistent, heritable differences in male genital morphology between predation regimes: Bahamas mosquitofish coexisting with predatory fish possessed more elongate genital tips with reduced soft tissue compared with counterparts inhabiting blue holes without predatory fish. We suggest this may reflect selection for greater efficiency of sperm transfer and fertilization during rapid and often forced copulations in high‐predation populations or differences in sexual conflict between predation regimes. Our study highlights the potential importance of ecological variation, particularly predation risk, in indirectly generating genital diversity.     

Sexual and lifetime selection on body size in a marine fish: the importance of life-history trade-offs

Many field measurements of viability and sexual selection on body size indicate that large size is favoured. However, life-history theory predicts that body size may be optimized and that patterns of selection may often be stabilizing rather than directional. One reason for this discrepancy may be that field estimates of selection tend to focus on limited components of fitness and may not fully measure life-history trade-offs. We use an 8-year, demographic field study to examine both sexual selection and lifetime selection on body size of a coral reef fish (the bicolour damselfish, Stegastes partitus). Selection via reproductive success of adults was very strong (standardized selection differential=1.04). However, this effect was balanced by trade-offs between large adult size and reduced cumulative survival during the juvenile phase. When we measured lifetime fitness (net reproductive rate), selection was strongly stabilizing and only weakly directional, consistent with predictions from life-history theory.     

Climate effects on size‐at‐age: growth in warming waters compensates for earlier maturity in an exploited marine fish

Over the past 3 decades, North Sea Atlantic cod (Gadus morhua) have exhibited variable length‐at‐age along with declines in spawning stock biomass and timing of maturity. Multiple factors affecting growth and development in fish acted on this economically important stock over the same period including warming waters and an intensive fishery. Here, we employ North Sea cod as a model population, exploring how a physiologically relevant temperature metric (the growing degree‐day, GDD; °C day) can be used to compare year‐classes on a physiologically relevant time‐scale, disentangling influences of climate (thermal history) on observed length‐at‐age trends. We conclude that the trends in North Sea cod length‐at‐age observed during the last three decades can be explained by a combination of temperature‐dependent growth increases and a trend toward earlier maturation, the latter likely induced by the intensive fishing pressure, and possibly evidence of fisheries‐induced evolution.     

Density‐dependent and density‐independent growth in a population of juvenile sea‐trout,Salmo trutta,assessed using long‐term data from a small stream in Northwest England

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Diurnal foraging of a wild coral‐reef fish Parapercis australis in relation to late‐summer temperatures

In situ observations of diurnal foraging behaviour of a common site‐attached shallow reef mesopredator Parapercis australis during late summer, revealed that although diet composition was unaffected by seawater temperature (range 28.3–32.4°C), feeding strikes and distance moved increased with temperature up to 30.5°C, beyond which they sharply declined, indicative of currently living beyond their thermal optimum. Diel feeding strikes and distance moved were however, tightly linked to ambient temperature as it related to the population's apparent thermal optimum, peaking at times when it was approached (1230 and 1700 h) and declining up to four fold at times deviating from this. These findings suggest that although this population may be currently living beyond its thermal optimum, it copes by down regulating energetically costly foraging movement and consumption and under future oceanic temperatures, these behavioural modifications are probably insufficient to avoid deleterious effects on population viability without the aid of long‐term acclimation or adaptation.     

Mechanisms of carbohydrate‐fuelled ecological dominance in a tropical rainforest canopy‐foraging ant

1. Canopy‐foraging ants have carbohydrate‐rich diets and the stoichiometric excess of carbon may result in energetic allocation decisions that facilitate ecological dominance. 2. If dietary carbohydrates facilitate ecological dominance in canopy ants, then the mechanism for this relationship is unknown. 3. Four hypotheses were posit that may explain how a carbohydrate‐rich diet might facilitate ecological dominance in canopy ants: Aggressive Defense, Metabolic Fuel, Foraging Success, and Prey Acquisition. 4. To assess these hypotheses, experiments were conducted on the canopy‐foraging bullet ant, Paraponera clavata (Fabricius), an omnivorous species that demonstrates high variability in the relative contribution of carbohydrates to the diets of colonies. 5. No support was found for the Aggressive Defense, Metabolic Fuel and Prey Acquisition hypotheses. 6. The Foraging Success hypothesis was supported, as the proportion of nectar in the diet predicted the overall foraging success. 7. It was argued that there is no explicit advantage in the exploitation of nectar over other food resources, other than the fact that it is the most accessible food resource in the rainforest canopy.     

Body size‐dependent responses of a marine fish assemblage to climate change and fishing over a century‐long scale

Commercial fishing and climate change have influenced the composition of marine fish assemblages worldwide, but we require a better understanding of their relative influence on long‐term changes in species abundance and body‐size distributions. In this study, we investigated long‐term (1911–2007) variability within a demersal fish assemblage in the western English Channel. The region has been subject to commercial fisheries throughout most of the past century, and has undergone interannual changes in sea temperature of over 2.0 °C. We focussed on a core 30 species that comprised 99% of total individuals sampled in the assemblage. Analyses showed that temporal trends in the abundance of smaller multispecies size classes followed thermal regime changes, but that there were persistent declines in abundance of larger size classes. Consistent with these results, larger‐growing individual species had the greatest declines in body size, and the most constant declines in abundance, while abundance changes of smaller‐growing species were more closely linked to preceding sea temperatures. Together these analyses are suggestive of dichotomous size‐dependent responses of species to long‐term climate change and commercial fishing over a century scale. Small species had rapid responses to the prevailing thermal environment, suggesting their life history traits predisposed populations to respond quickly to changing climates. Larger species declined in abundance and size, reflecting expectations from sustained size‐selective overharvesting. These results demonstrate the importance of considering species traits when developing indicators of human and climatic impacts on marine fauna.     

Survival against the odds: ontogenetic changes in selective pressure mediate growth-mortality trade-offs in a marine fish

For organisms with complex life cycles, variation among individuals in traits associated with survival in one life-history stage can strongly affect the performance in subsequent stages with important repercussions on population dynamics. To identify which individual attributes are the most influential in determining patterns of survival in a cohort of reef fish, we compared the characteristics of Pomacentrus amboinensis surviving early juvenile stages on the reef with those of the cohort from which they originated. Individuals were collected at hatching, the end of the planktonic phase, and two, three, four, six and eight weeks post-settlement. Information stored in the otoliths of individual fish revealed strong carry-over effects of larval condition at hatching on juvenile survival, weeks after settlement (i.e. smaller-is-better). Among the traits examined, planktonic growth history was, by far, the most influential and long-lasting trait associated with juvenile persistence in reef habitats. However, otolith increments suggested that larval growth rate may not be maintained during early juvenile life, when selective mortality swiftly reverses its direction. These changes in selective pressure may mediate growth-mortality trade-offs between predation and starvation risks during early juvenile life. Ontogenetic changes in the shape of selectivity may be a mechanism maintaining phenotypic variation in growth rate and size within a population.     

Consistent size‐independent harvest selection on fish body shape in two recreationally exploited marine species

Harvesting wild animals may exert size‐independent selection pressures on a range of morphological, life history, and behavioral traits. Most work so far has focused on selection pressures on life history traits and body size as morphological trait. We studied here how recreational fishing selects for morphological traits related to body shape, which may correlate with underlying swimming behavior. Using landmark‐based geometric morphometrics, we found consistent recreational fishing‐induced selection pressures on body shape in two recreationally exploited marine fish species. We show that individuals with larger‐sized mouths and more streamlined and elongated bodies were more vulnerable to passively operated hook‐and‐line fishing independent of the individual's body size or condition. While the greater vulnerability of individuals with larger mouth gapes can be explained by the direct physical interaction with hooks, selection against streamlined and elongated individuals could either involve a specific foraging mode or relate to underlying elevated swimming behavior. Harvesting using passive gear is common around the globe, and thus, size‐independent selection on body shape is expected to be widespread potentially leaving behind individuals with smaller oral gapes and more compact bodies. This might have repercussions for food webs by altering foraging and predation.     

A trait‐based trade‐off between growth and mortality: evidence from 15 tropical tree species using size‐specific relative growth rates

A life‐history trade‐off between low mortality in the dark and rapid growth in the light is one of the most widely accepted mechanisms underlying plant ecological strategies in tropical forests. Differences in plant functional traits are thought to underlie these distinct ecological strategies; however, very few studies have shown relationships between functional traits and demographic rates within a functional group. We present 8 years of growth and mortality data from saplings of 15 species of Dipterocarpaceae planted into logged‐over forest in Malaysian Borneo, and the relationships between these demographic rates and four key functional traits: wood density, specific leaf area (SLA), seed mass, and leaf C:N ratio. Species‐specific differences in growth rates were separated from seedling size effects by fitting nonlinear mixed‐effects models, to repeated measurements taken on individuals at multiple time points. Mortality data were analyzed using binary logistic regressions in a mixed‐effects models framework. Growth increased and mortality decreased with increasing light availability. Species differed in both their growth and mortality rates, yet there was little evidence for a statistical interaction between species and light for either response. There was a positive relationship between growth rate and the predicted probability of mortality regardless of light environment, suggesting that this relationship may be driven by a general trade‐off between traits that maximize growth and traits that minimize mortality, rather than through differential species responses to light. Our results indicate that wood density is an important trait that indicates both the ability of species to grow and resistance to mortality, but no other trait was correlated with either growth or mortality. Therefore, the growth mortality trade‐off among species of dipterocarp appears to be general in being independent of species crossovers in performance in different light environments.     

Prey experience of predation influences mortality rates at settlement in a coral reef fish, Pomacentrus amboinensis

Previous exposure to predators influenced the survival immediately after settlement in a coral reef damselfish, Pomacentrus amboinensis . Rapid learning of antipredator behaviour may in part have driven the rapid increase in survival in the days following this critical transition.     

Effects of ocean acidification on the early life history of a tropical marine fish

Little is known about how fishes and other non-calcifying marine organisms will respond to the increased levels of dissolved CO₂ and reduced sea water pH that are predicted to occur over the coming century. We reared eggs and larvae of the orange clownfish, Amphiprion percula, in sea water simulating a range of ocean acidification scenarios for the next 50–100 years (current day, 550, 750 and 1030 ppm atmospheric CO₂). CO₂ acidification had no detectable effect on embryonic duration, egg survival and size at hatching. In contrast, CO₂ acidification tended to increase the growth rate of larvae. By the time of settlement (11 days post-hatching), larvae from some parental pairs were 15 to 18 per cent longer and 47 to 52 per cent heavier in acidified water compared with controls. Larvae from other parents were unaffected by CO₂ acidification. Elevated CO₂ and reduced pH had no effect on the maximum swimming speed of settlement-stage larvae. There was, however, a weak positive relationship between length and swimming speed. Large size is usually considered to be advantageous for larvae and newly settled juveniles. Consequently, these results suggest that levels of ocean acidification likely to be experienced in the near future might not, in isolation, significantly disadvantage the growth and performance of larvae from benthic-spawning marine fishes.     

Spatially correlated recruitment of a marine predator and its prey shapes the large-scale pattern of density-dependent prey mortality

Patterns of predator dispersal can be critical to the dynamics of prey metapopulations. In marine systems, oceanic currents may shape the dispersal of planktonic larvae of both predators and prey, producing spatial correlations in the recruitment of both species and distinctive geographic patterns of prey mortality. I examined the potential for this phenomenon in two fishes, a wrasse and its grouper predator, at a Caribbean island where the near-shore oceanographic regime produces a temporally consistent spatial pattern of fish recruitment. I found that recruitment and adult abundance of groupers were spatially correlated with recruitment of wrasse prey. Furthermore, the local abundance of predators strongly affected the nature of density-dependent prey mortality. At sites with few predators, wrasse mortality was inversely density-dependent, while mortality was positively density-dependent at sites with higher predator densities. This phenomenon could be important to the dynamics of any metacommunity in which physical forces produce correlated dispersal.     

A diffusion‐based approach to stochastic individual growth and energy budget,with consequences to life‐history optimization and population dynamics

Using diffusion processes, I model stochastic individual growth, given exogenous hazards and starvation risk. By maximizing survival to final size, optimal life histories (e.g. switching size for habitat/dietary shift) are determined by two ratios: mean growth rate over growth variance (diffusion coefficient) and mortality rate over mean growth rate; all are size dependent. For example, switching size decreases with either ratio, if both are positive. I provide examples and compare with previous work on risk‐sensitive foraging and the energy–predation trade‐off. I then decompose individual size into reversibly and irreversibly growing components, e.g. reserves and structure. I provide a general expression for optimal structural growth, when reserves grow stochastically. I conclude that increased growth variance of reserves delays structural growth (raises threshold size for its commencement) but may eventually lead to larger structures. The effect depends on whether the structural trait is related to foraging or defence. Implications for population dynamics are discussed.