9 research outputs found
Harvest-induced disruptive selection increases variance in fitness-related traits
The form of Darwinian selection has important ecological and management implications. Negative effects of harvesting are often ascribed to size truncation (i.e. strictly directional selection against large individuals) and resultant decrease in trait variability, which depresses capacity to buffer environmental change, hinders evolutionary rebound and ultimately impairs population recovery. However, the exact form of harvest-induced selection is generally unknown and the effects of harvest on trait variability remain unexplored. Here we use unique data from the Windermere (UK) long-term ecological experiment to show in a top predator (pike, Esox lucius) that the fishery does not induce size truncation but disruptive (diversifying) selection, and does not decrease but rather increases variability in pike somatic growth rate and size at age. This result is supported by complementary modelling approaches removing the effects of catch selectivity, selection prior to the catch and environmental variation. Therefore, fishing most likely increased genetic variability for somatic growth in pike and presumably favoured an observed rapid evolutionary rebound after fishery relaxation. Inference about the mechanisms through which harvesting negatively affects population numbers and recovery should systematically be based on a measure of the exact form of selection. From a management perspective, disruptive harvesting necessitates combining a preservation of large individuals with moderate exploitation rates, and thus provides a comprehensive tool for sustainable exploitation of natural resources
Cytosine methylation patterns suggest a role of methylation in plastic and adaptive responses to temperature in European grayling (Thymallus thymallus) populations
Temperature is a key environmental parameter affecting both the phenotypes and distributions of organisms, particularly ectotherms. Rapid organismal responses to thermal environmental changes have been described for several ectotherms; however, the underlying molecular mechanisms often remain unclear. Here, we studied whole genome cytosine methylation patterns of European grayling (Thymallus thymallus) embryos from five populations with contemporary adaptations of early life history traits at either 'colder' or 'warmer' spawning grounds. We reared fish embryos in a common garden experiment using two temperatures that resembled the 'colder' and 'warmer' conditions of the natal natural environments. Genome-wide methylation patterns were similar in populations originating from colder thermal origin subpopulations, whereas single nucleotide polymorphisms uncovered from the same data identified strong population structure among isolated populations, but limited structure among interconnected populations. This was surprising because the previously studied gene expression response among populations was mostly plastic, and mainly influenced by the developmental temperature. These findings support the hypothesis of the magnified role of epigenetic mechanisms in modulating plasticity. The abundance of consistently changing methylation loci between two warmer-to-colder thermal origin population pairs suggests that local adaptation has shaped the observed methylation patterns. The dynamic nature of the methylomes was further highlighted by genome-wide and site-specific plastic responses. Our findings support both the presence of a plastic response in a subset of CpG loci, and the evolutionary role of methylation divergence between populations adapting to contrasting thermal environments.Peer reviewe
Pathogens trigger top-down climate forcing on ecosystem dynamics
Evaluating the effects of climate variation on ecosystems is of paramount importance for our ability to forecast and mitigate the consequences of global change. However, the ways in which complex food webs respond to climate variations remain poorly understood. Here, we use long-term time series to investigate the effects of temperature variation on the intraguild-predation (IGP) system of Windermere (UK), a lake where pike (Esox lucius, top predator) feed on small-sized perch (Perca fluviatilis) but compete with large-sized perch for the same food sources. Spectral analyses of time series reveal that pike recruitment dynamics are temperature controlled. In 1976, expansion of a size-truncating perch pathogen into the lake severely impacted large perch and favoured pike as the IGP-dominant species. This pathogen-induced regime shift to a pike-dominated IGP apparently triggered a temperature-controlled trophic cascade passing through pike down to dissolved nutrients. In simple food chains, warming is predicted to strengthen top–down control by accelerating metabolic rates in ectothermic consumers, while pathogens of top consumers are predicted to dampen this top–down control. In contrast, the local IGP structure in Windermere made warming and pathogens synergistic in their top–down effects on ecosystem functioning. More generally, our results point to top predators as major mediators of community response to global change, and show that size-selective agents (e.g. pathogens, fishers or hunters) may change the topological architecture of food webs and alter whole ecosystem sensitivity to climate variation
Body downsizing caused by non-consumptive social stress severely depresses population growth rate
Chronic social stress diverts energy away from growth, reproduction and immunity, and is thus a potential driver of population dynamics. However, the effects of social stress on demographic density dependence remain largely overlooked in ecological theory. Here we combine behavioural experiments, physiology and population modelling to show in a top predator (pike Esox lucius) that social stress alone may be a primary driver of demographic density dependence. Doubling pike density in experimental ponds under controlled prey availability did not significantly change prey intake by pike (i.e. did not significantly change interference or exploitative competition), but induced a neuroendocrine stress response reflecting a size-dependent dominance hierarchy, depressed pike energetic status and lowered pike body growth rate by 23 per cent. Assuming fixed size-dependent survival and fecundity functions parameterized for the Windermere (UK) pike population, stress-induced smaller body size shifts age-specific survival rates and lowers age-specific fecundity, which in Leslie matrices projects into reduced population rate of increase (λ) by 37–56%. Our models also predict that social stress flattens elasticity profiles of λ to age-specific survival and fecundity, thus making population persistence more dependent on old individuals. Our results suggest that accounting for non-consumptive social stress from competitors and predators is necessary to accurately understand, predict and manage food-web dynamics
Biotic and abiotic effects on cohort size distributions in fish
Intraspecific variation in body size is common in animals and plants. Body size affects trophic interactions like foraging ability and vulnerability to predation, which in turn affect individual fitness as well as population stability and extinction risk. Experimental and theoretical work has shown that the size distribution of individuals within cohorts is strongly influenced by intraspecific competition for resources, often leading to skewed frequency distributions. However, little is known about the effects of environmental factors such as climate and eutrophication on the cohort size-structure of natural populations. We use a long-term time series of scientific monitoring of a freshwater fish (European perch Perca fluviatilis) to investigate the effects of density dependence, predation, nutrient availability, climate and the timing of spawning on the cohort size distributions. We find that the mean length of the fish is best predicted by the extrinsic factors phosphorus concentration and summer temperature, and the densities of the different age-classes, whereas the skewness of the length distribution is best predicted by phosphorus concentration, summer temperature, abundance of small fish, and the timing of spawning. Higher nutrient levels, temperatures and densities of small fish increase food availability and thus reduce competition, which is reflected in increased mean length and decreased skewness. The timing of spawning affects skewness presumably through changes in the initial size variation of the cohort and the length of the first growth season. Our results indicate that higher temperatures increase the mean length and decrease skewness due to the concurrent eutrophication of the lake. The study thereby highlights the potential impact of human-induced environmental change on the size structure of fish populations. More studies are needed to understand better the complex mechanisms through which these factors alter the intensity of intraspecific competition in fish communities
The ideal free pike: 50 years of fitness-maximising dispersal in Windermere
The ideal free distribution (IFD) theory is one of the most influential theories in evolutionary ecology. It predicts how animals ought to distribute themselves within a heterogeneous habitat in order to maximize lifetime fitness. We test the population level consequence of the IFD theory using 40-year worth data on pike (Esox lucius) living in a natural lake divided into two basins. We do so by employing empirically derived density-dependent survival, dispersal and fecundity functions in the estimation of basin-specific density-dependent fitness surfaces. The intersection of the fitness surfaces for the two basins is used for deriving expected spatial distributions of pike. Comparing the derived expected spatial distributions with 50 years data of the actual spatial distribution demonstrated that pike is ideal free distributed within the lake. In general, there was a net migration from the less productive north basin to the more productive south basin. However, a pike density-manipulation experiment imposing shifting pike density gradients between the two basins managed to switch the net migration direction and hence clearly demonstrated that the Windermere pike choose their habitat in an ideal free manner. Demonstration of ideal free habitat selection on an operational field scale like this has never been undertaken before
Stage-specific biomass overcompensation by juveniles in response to increased adult mortality in a wild fish population
Recently developed theoretical models of stage-structured consumer–resource
systems have shown that stage-specific biomass overcompensation can arise in response to
increased mortality rates. We parameterized a stage-structured population model to simulate
the effects of increased adult mortality caused by a pathogen outbreak in the perch (Perca
fluviatilis) population of Windermere (UK) in 1976. The model predicts biomass
overcompensation by juveniles in response to increased adult mortality due to a shift in
food-dependent growth and reproduction rates. Considering cannibalism between life stages
in the model reinforces this compensatory response due to the release from predation on
juveniles at high mortality rates. These model predictions are matched by our analysis of a 60-
year time series of scientific monitoring of Windermere perch, which shows that the pathogen
outbreak induced a strong decrease in adult biomass and a corresponding increase in juvenile
biomass. Age-specific adult fecundity and size at age were higher after than before the disease
outbreak, suggesting that the pathogen-induced mortality released adult perch from
competition, thereby increasing somatic and reproductive growth. Higher juvenile survival
after the pathogen outbreak due to a release from cannibalism likely contributed to the
observed biomass overcompensation. Our findings have general implications for predicting
population- and community-level responses to increased size-selective mortality caused by
exploitation or disease outbreaks
Density dependence and density independence in the demography and dispersal of pike over four decades
Quantifying the effects of density-dependent and density-independent factors in
demographic and dispersal processes remains a major challenge in population ecology. Based
on unique long-term capture–mark–recapture (CMR) data (1949–2000) on pike (Esox lucius)
from Windermere, United Kingdom, we provide estimates of density-dependent and densityindependent
effects, under the influence of individual size and sex, on natural survival, fishing
mortality, and dispersal. Because survival is expected to be related to the individual growth
process, we also explore the degree of parallelism between the two processes by applying the
best-supported survival model structure to individual growth data. The CMR data were
analyzed using sex- and age-structured multistate models (two lake basins: north and south)
assuming no seasonal variation in survival and dispersal. Total survival and dispersal
probabilities were insensitive to this assumption, and capture probability was shown to be
robust to assumptions about intra-annual variation in survival and dispersal. The analyses
revealed that large pike (.55 cm) displayed marked basin-specific differences in survival and
dispersal responses to the abundance of conspecifics in which pike from the south basin show
high density dependence in survival and a low degree of density dependence in dispersal,
whereas the opposite was found for those of the north basin. Both large- and small-pike
dispersals were found to be dependent upon the between-basin gradient in perch (Perca
fluviatilis, the main prey) abundance, but most so for north-to-south dispersal. The strength
and pattern of density-dependent mortality in small pike was influenced by temperature in a
peculiar way: at low summer temperatures survival was lowest at high small-pike abundances
and low perch abundances; at low temperatures, survival was lowest at high small-pike
abundances and high perch abundances. Analyses of individual small-pike growth trajectories
showed a similar modulation of the positive perch abundance effect on growth rate when
temperature increases. The growth analysis also indicated that cannibalism may be biased in
favor of rapidly growing individuals or at least increased growth rate for the surviving
individuals. Altogether, this study provides evidence of a complex interplay between densitydependent
and density-independent factors affecting survival, dispersal, and individual growth
of an aquatic top predator
Six decades of pike and perch population dynamics in Windermere
We use six decades of catch-at-age data for perch (Perca fluviatilis) and pike (Esox lucius) in Windermere (UK) to estimate age- and sex-specific population sizes, natural mortalities and catchabilities in both species. Population sizes are estimated by fitting age structured population models to the catch-at-age data using standard maximum likelihood methods. We validate our methods using data simulations, and use our estimates of vital rates (natural mortality, recruitment and catahability) to address important aspects of fisheries biology. Our model indicates that strong fishery selection against male perch apparently triggered a population collapse, highlighting that sex-selective fisheries can be harmful even at a reasonable exploitation rate (here ≤30%). Recruitment (R) increased with the abundance of spawners (S) in both species, but it also responded to both abiotic and other biotic factors. In particular, increased predator (pike) abundance induced a change from compensation to depensation in the prey (perch) SR relationship, thus favouring the occurrence of an Allee effect. Our study provides reference points for the effective exploitation of pike and perch populations, and underscores the need for ecosystem-based harvesting management