Community structure affects trophic ontogeny in a predatory fish

While most studies have focused on the timing and nature of ontogenetic niche shifts, information is scarce about the effects of community structure on trophic ontogeny of top predators. We investigated how community structure affects ontogenetic niche shifts (i.e., relationships between body length, trophic position, and individual dietary specialization) of a predatory fish, brown trout (Salmo trutta). We used stable isotope and stomach content analyses to test how functional characteristics of lake fish community compositions (competition and prey availability) modulate niche shifts in terms of (i) piscivorous behavior, (ii) trophic position, and (iii) individual dietary specialization. Northern Scandinavian freshwater fish communities were used as a study system, including nine subarctic lakes with contrasting fish community configurations: (i) trout‐only systems, (ii) two‐species systems (brown trout and Arctic charr [Salvelinus alpinus] coexisting), and (iii) three‐species systems (brown trout, Arctic charr, and three‐spined sticklebacks [Gasterosteus aculeatus] coexisting). We expected that the presence of profitable small prey (stickleback) and mixed competitor–prey fish species (charr) supports early piscivory and high individual dietary specialization among trout in multispecies communities, whereas minor ontogenetic shifts were expected in trout‐only systems. From logistic regression models, the presence of a suitable prey fish species (stickleback) emerged as the principal variable determining the size at ontogenetic niche shifts. Generalized additive mixed models indicated that fish community structure shaped ontogenetic niche shifts in trout, with the strongest positive relationships between body length, trophic position, and individual dietary specialization being observed in three‐species communities. Our findings revealed that the presence of a small‐sized prey fish species (stickleback) rather than a mixed competitor–prey fish species (charr) was an important factor affecting the ontogenetic niche‐shift processes of trout. The study demonstrates that community structure may modulate the ontogenetic diet trajectories of and individual niche specialization within a top predator.     

Predator foraging strategy mediates the effects of predators on local and emigrating prey

Predation is a dominant structuring force in ecosystems, but its effects are almost always measured in the ecosystem of the predator. However, the effects of predators can potentially extend across ecosystem boundaries during ontogenetic niche shifts in prey. We compared the effects of fish predation on benthic versus emerging aquatic insects, and hypothesized that the relative effects of fish on these two stages of prey are mediated by fish foraging strategy (benthic versus water‐column feeders). Benthic‐feeding smallmouth buffalo reduced benthic insect biomass in the freshwater ecosystem by 89%, and reduced insect emergence to the terrestrial ecosystem by 65%. In contrast, water‐column feeding sunfish had no effect on benthic biomass in the freshwater ecosystem, but reduced emergence to the terrestrial ecosystem by 44% relative to the fishless control. When smallmouth and sunfish were combined in a substitutive design that kept total fish density the same as the single species treatments, their effects on benthic insects (50% reduction) were weaker than expected based on predictions from the single species treatments. In contrast, their combined effects on emergence (46% reduction) were additive. Tetragnathid spider densities increased during peak emergence, but did not respond to changes in emergence among treatments. These results demonstrate that the effects of fish on prey flux to the terrestrial ecosystem are not the same as their effects on benthic prey biomass in the aquatic ecosystem, and that this difference is likely mediated by foraging strategy.     

Landscape context influences the abundance of amphibians and the strength of their food web interactions in small ponds

Many species, including most amphibians, undergo an ontogenetic niche shift (ONS) from an aquatic larval stage to a terrestrial adult stage. We use the ratio of aquatic to terrestrial habitat in a landscape as a tool to understand the influence of landscape context on the population growth of ONS species. The aquatic to terrestrial ratio (ATR) of habitats can be viewed as an analog to the influence of resource ratios on the population growth of consumers and depends on the degree to which each habitat type limits the growth of a given population. Population growth rates of shorter‐lived species tend to be more limited by demographic rates in early (aquatic) life stages. As a result, increasing the ATR should lead to a higher total population size in the landscape (and higher densities in the terrestrial habitat), but have little influence on the density of individuals in any given aquatic habitat. Alternatively, population growth rates of longer‐lived species tend to be more limited by demographic rates in later (terrestrial) life stages and increasing the ATR should have little influence on the total population size in the landscape, but decrease the density of individuals in any given aquatic habitat. We show that among‐landscape variation in the breeding‐pond densities of three widespread amphibians with contrasting life histories is consistent with this framework. Within‐pond densities of Pseudacris crucifer, a species with short‐lived adults, were not influenced by ATR, whereas within‐pond densities of Hyla versicolor, a longer‐lived member of the same family (Hylidae), declined as ATR increased. Ambystoma maculatum, a long‐lived salamander, also had lower densities in ponds with higher ATR. Because A. maculatum larvae are important predators in ponds, we use structural equation modeling to show that landscape context (ATR) can moderate community structure via direct (amphibian abundances) and indirect (prey species richness) effects.     

A hypothesis-testing framework for studies investigating ontogenetic niche shifts using stable isotope ratios

Ontogenetic niche shifts occur across diverse taxonomic groups, and can have critical implications for population dynamics, community structure, and ecosystem function. In this study, we provide a hypothesis-testing framework combining univariate and multivariate analyses to examine ontogenetic niche shifts using stable isotope ratios. This framework is based on three distinct ontogenetic niche shift scenarios, i.e., (1) no niche shift, (2) niche expansion/reduction, and (3) discrete niche shift between size classes. We developed criteria for identifying each scenario, as based on three important resource use characteristics, i.e., niche width, niche position, and niche overlap. We provide an empirical example for each ontogenetic niche shift scenario, illustrating differences in resource use characteristics among different organisms. The present framework provides a foundation for future studies on ontogenetic niche shifts, and also can be applied to examine resource variability among other population sub-groupings (e.g., by sex or phenotype).     

Reliance of brown trout on terrestrial prey varies with season but not fish density

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Adaptive plasticity in ontogenetic niche shifts stabilizes consumer-resource dynamics

Ontogenetic niche shifts, changes in the diet or habitats of organisms during their ontogeny, are widespread among various animal taxa. Ontogenetic niche shifts introduce stage structure in a population with different stages interacting with different communities and can substantially affect their dynamics. In this article, I use mathematical models to test the hypothesis that adaptive plasticity in the timing of ontogenetic niche shifts has a stabilizing effect on consumer-resource dynamics. Adaptive plasticity allows consumers in one ontogenetic niche to perform an early shift to the next ontogenetic niche if the resource density of the first niche is low. The early shift will reduce predation by the consumer on the scarce resource. On the other hand, adaptive plasticity allows consumers to delay their shift to the next niche if the resource density of the first niche is high. The delayed shift will increase the predation on the abundant resource. As a result, the scarce resource will tend to increase, and the abundant resource will tend to decrease. This causes density-dependent negative feedback in the resource dynamics, which stabilizes the consumer-resource dynamics. To test this hypothesis, I compare three consumer-resource models differing in terms of mechanisms controlling the timing of the ontogenetic niche shift: the fixed-age model assumes that the age at which the ontogenetic niche shift occurs is fixed; the fixed-size model assumes that the size at the shift is fixed; and the adaptive plasticity model assumes that the timing of the shift is such that the individual fitness of the consumer is maximized. I show that only the adaptive plasticity model has a locally stable equilibrium and that the stabilizing effect is due to the density-dependent negative feedback in the resource dynamics. I discuss the ontogenetic niche shifts of lake fish in light of the obtained result.     

Emergent impacts of cannibalism and size refuges in prey on intraguild predation systems

Many organisms undergo ontogenetic niche shifts due to considerable changes in size during their development. These ontogenetic shifts can alter the trophic position of individuals, the type and strength of ecological interactions across species, and allow for cannibalism within species. In this study we ask if and how the interaction of a size refuge and cannibalism in the prey alters the dynamics of intraguild predation (IGP) systems. By manipulating the composition of large cannibalistic (Aeshna umbrosa) and predatory (Anax junius) dragonfly larvae in mesocosms we show that the interaction of cannibals and predators was non-linear and increased the survival of prey. The structure of the final resource community shared by prey and predator differed between small and large dragonfly treatments but not within size classes across species. In general, the small prey stage showed similar shifts in microhabitat use and refuge use when exposed to either conspecific cannibals or predators, while large cannibals showed no clear anti-predator response. However, further behavioral experiments revealed that specific behavioral components, such as distances between individuals or number of movements, differed when individuals were exposed to either cannibals or predators. This indicates that individuals discriminated between conspecific or heterospecific predators. Furthermore, in similar experiments large cannibals and predators showed different behaviors when exposed to conspecifics rather than to each other. These changes in behavior are consistent with the observed increase in prey survival. In general, the results indicate that cannibalism and ontogenetic niche shifts can result in behavior-mediated indirect interactions that reduce the impact of the predator on the mortality of its prey and alter the interactions of IGP systems. However, they also indicate that size is not the sole determinant and that we also need to account for the species identity when predicting the dynamics of communities.     

Morphological integration and ontogenetic niche shift: a study of crested newt limbs

This study deals with the ontogenetic and evolutionary aspects of integration patterns in the limbs of crested newt species, which, like most amphibians, have a biphasic life history with two morphologically distinct stages (larval vs. juvenile and adult) that occupy different environments (aquatic vs. terrestrial). We analyzed the structure and pattern of correlation between limb skeletal elements at three ontogenetic stages (larval, juvenile, and adult) of four closely related species that differ in their preferences of aquatic habitats (more terrestrial and more aquatic). We found dynamic changes in the pattern of morphological integration between successive ontogenetic stages, as well as changes over the course of crested newt phylogeny. Generally, equivalent ontogenetic stages of different species of crested newts show higher concordance in the correlation pattern than successive ontogenetic stages within species. Among species, two opposing correlation patterns were observed: in more terrestrial species, homologous limb elements are less correlated and within-limb elements are more correlated; in aquatic species, the reverse pattern occurs. These results indicate that the function seems to be the covariance-generating factor, which has shaped the patterns of morphological integration of crested newt limbs.     

The role of fish life histories in allometrically scaled food‐web dynamics

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The role of refugia for fishes during drought: a review and synthesis

• 1 Drought is a natural disturbance of aquatic ecosystems and can be a major factor in structuring aquatic communities. For individuals, populations and communities to persist in disturbed environments, they must have refuge from disturbance or disturbance must be minimal. Refugia convey spatial and temporal resistance or resilience in the face of disturbance, but the role of refugia in aquatic systems remains poorly understood. 2. We review available literature on aquatic refugia for fishes in order to synthesise current knowledge and provide suggestions for needed research. Our objectives were to clarify definitions of disturbance and refugia in the context of drought in aquatic systems, review how refuge habitats influence fish community structure, and consider the potential impact of refugia on fish population and community dynamics during drought. 3. Droughts cause a decrease in surface area/volume and an increase in extremes of physical and chemical water quality parameters. These conditions are linked with biotic interactions that structure the community of fishes residing in low‐flow or dry season refugia by increasing mortality rates, decreasing birth rates and/or increasing migration rates. Many aquatic organisms seek refuge from disturbance and/or have adaptations (e.g. physiological tolerance) that provide refuge. 4. Drought in aquatic systems leads to shifts in refugia spacing and connectance at multiple spatial and temporal scales. Refuge size, disturbance intensity, and mobility of organisms is predicted to play a large role in population persistence. We expect that refuge habitats will experience net immigration during drying and net emigration after rewetting, with the opposite occurring in surrounding habitat patches. Population dynamics of fishes using refugia during drought are best modelled by modified source‐sink dynamics, but dynamics are likely to change with spatial scale.

     

A multivariate approach reveals diversity of ontogenetic niche shifts across taxonomic and functional groups

1. Shifts in the fundamental and realised niche of individuals during their ontogeny are ubiquitous in nature, but we know little about what aspects of the niche change and how these changes vary across species within communities. However, this knowledge is essential to predict the dynamics of populations and communities and how they respond to environmental change.
2. Here I introduce a range of metrics to describe different aspects of shifts in the realised trophic niche of individuals based on stable isotopes. Applying this multi-variate approach to 2,272 individuals from 13 taxonomic and functional distinct species (Amphibia, Hemiptera, Coleoptera, Odonata) sampled in natural pond communities allowed me to: (1) describe and quantify the diversity of trophic niche shift patterns over ontogeny in multi-dimensional space, and (2) identify what aspects of ontogenetic shifts vary across taxa, and functional groups.
3. Results revealed that species can differ substantially in which aspects of the trophic niche change and how they change over ontogeny. Interestingly, patterns of ontogenetic niche shifts grouped in distinct taxonomic clusters in multi-variate space, including two distinct groups of predators (Hemiptera versus Odonata). Given the differences in traits (especially feeding mode) across groups, this suggests that differences in ontogenetic niche shifts across species could at least partially be explained by variation in traits and functional roles of species.
4. These results emphasise the importance of a multivariate approach to capture the large diversity of trophic niche shifts patterns possible in natural communities and suggest that differences in ontogenetic niche shifts follow general patterns.

     

Trophic ecology of northern Australia's terapontids: ontogenetic dietary shifts and feeding classification

The diets of 21 terapontid species from freshwater environments in northern Australia were investigated to determine the similarity and dissimilarity among species and the extent of any ontogenetic shifts. Distinct ontogenetic dietary shifts occurred in all species for which sufficient data were available, with many species passing through several discrete trophic categories during their life histories. Diets of all juvenile terapontids were similar, mainly comprising aquatic insects and zooplankton. Larger size classes of terapontids diverged into a broad spectrum of feeding groups comprising carnivorous dietary modes (including piscivory and lepidophagy), omnivory (including frugivory and consumption of allochthonous prey), herbivory and detritivory. The results indicate that the terapontids represent Australia's most trophically diverse freshwater fish family.     

Consequences of metamorphosis for the locomotor performance and thermal physiology of the newt Triturus cristatus

During metamorphosis, most amphibians undergo rapid shifts in their morphology that allow them to move from an aquatic to a more terrestrial existence. Two important challenges associated with this shift in habitat are the necessity to switch from an aquatic to terrestrial mode of locomotion and changes in the thermal environment. In this study, I investigated the consequences of metamorphosis to the burst swimming and running performance of the European newt Triturus cristatus to determine the nature and magnitude of any locomotor trade-offs that occur across life-history stages. In addition, I investigated whether there were any shifts in the thermal dependence of performance between life-history stages of T. cristatus to compensate for changes in their thermal environment during metamorphosis. A trade-off between swimming and running performance was detected across life-history stages, with metamorphosis resulting in a simultaneous decrease in swimming and increase in running performance. Although the terrestrial habitat of postmetamorphic stages of the newt T. cristatus experienced greater daily fluctuations in temperature than the aquatic habitat of the larval stage, no differences in thermal sensitivity of locomotor performance were detected between the larval aquatic and postmetamorphic stages. The absence of variation across life-history stages of T. cristatus may indicate that thermal sensitivity may be a conservative trait across ontogenetic stages in amphibians, but further studies are required to investigate this assertion.     

Trophic overlap between fish and riparian spiders: potential impacts of an invasive fish on terrestrial consumers

Studies on resource sharing and partitioning generally consider species that occur in the same habitat. However, subsidies between linked habitats, such as streams and riparian zones, create potential for competition between populations which never directly interact. Evidence suggests that the abundance of riparian consumers declines after fish invasion and a subsequent increase in resource sharing of emerging insects. However, diet overlap has not been investigated. Here, we examine the trophic niche of native fish, invasive fish, and native spiders in South Africa using stable isotope analysis. We compared spider abundance and diet at upstream fishless and downstream fish sites and quantified niche overlap with invasive and native fish. Spider abundance was consistently higher at upstream fishless sites compared with paired downstream fish sites, suggesting that the fish reduced aquatic resource availability to riparian consumers. Spiders incorporated more aquatic than terrestrial insects in their diet, with aquatic insects accounting for 45–90% of spider mass. In three of four invaded trout rivers, we found that the average proportion of aquatic resources in web‐building spider diet was higher at fishless sites compared to fish sites. The probability of web‐building and ground spiders overlapping into the trophic niche of invasive brown and rainbow trout was as high as 26 and 51%, respectively. In contrast, the probability of spiders overlapping into the trophic niche of native fish was always less than 5%. Our results suggest that spiders share resources with invasive fish. In contrast, spiders had a low probability of trophic overlap with native fish indicating that the traits of invaders may be important in determining their influence on ecosystem subsidies. We have added to the growing body of evidence that invaders can have cross‐ecosystem impacts and demonstrated that this can be due to niche overlap.     

Species-specific use of allochthonous resources by ground beetles (Carabidae) at a river–land interface

A variety of organisms mediate river–terrestrial linkages through spatial subsidies. However, most empirical studies have classified organisms rather broadly (e.g., by functional group or taxonomic family) and have dismissed species-level linkages at the interface of ecosystems. Here, we show how allochthonous resource use varies among taxonomically similar species of ground beetles (family Carabidae) across seasons (June–September). We investigated seasonal shifts in the distribution of five beetle species and their dietary responses to spatial subsidies (emerging aquatic insects) in a Japanese braided river. Despite their taxonomic closeness, the ground beetles showed species-specific responses to spatial subsidies, and beetle distribution patterns tended to coincide with their diets. Overall, 1–56% of ground beetle diets were derived from aquatic prey. One genus (Bembidion spp.) mainly consumed aquatic prey, while three species fed primarily on terrestrial prey across all seasons. However, one species (Lithochlaenius noguchii) showed shifts in its diet from aquatic to terrestrial prey according to subsidy availability. The observed variation in allochthonous resource use was likely related to species-specific foraging modes, physiological tolerance to dry conditions, and interspecific competition. Our findings suggest that considering species-specific interactions is necessary to fully understand cross-system interactions and recipient food-web dynamics.     

The status and causes of alien species invasion in China

Data of classification, origin, pathway and environmental impacts of invasive alien micro-organisms, invertebrates, amphibians and reptiles, fish, birds, mammals, weeds, trees, and marine organisms in terrestrial, aquatic and marine ecosystems of China, were analyzed, based on literature retrieval, field survey and consultation. Some 283 invasive alien species were recorded in China, including 19 invasive alien micro-organisms, 18 aquatic plants, 170 terrestrial plants, 25 aquatic invertebrates, 33 terrestrial invertebrates, 3 amphibians and reptiles, 10 fish, and 5 mammals. Of the invasive alien species, 55.1% originated from North and South America, 21.7% from Europe, 9.9% from Asia, 8.1% from Africa and 0.6% from Oceania. Many institutions and individuals in China lack adequate knowledge of ecological and environmental consequences caused by invasive alien species, with some ignorance of the dangerous invasion in the introduction of alien species. For instance, 50.0% of invasive alien plants were intentionally introduced as pasture, feedingstuff, ornamental plants, textile plants, medicinal plants, vegetables, or lawn plants, 25% of alien invasive animals were intentionally introduced for cultivation, ornament, or biological control, In addition, more efforts are being made in the introduction of alien species, and little attention is paid on the management of introduced alien species, which may cause their escape into natural environment and potential threats to the environment. There were also gaps in quarantine system in China. All microorganisms were unintentionally introduced, through timber, seedling, flowerpot, or soil; 76.3% of alien invasive animals invaded through commodity or transportation facility because of the failure of quarantine. Therefore, quarantine measures should be strictly implemented; and meanwhile the intentional introduction of alien species should be strictly managed and a system of risk assessment should be implemented.     

Ontogenetic Diet Shifts of a Neotropical Catfish, Pimelodus maculatus (Siluriformes, Pimelodidae): An Ecomorphological Approach

This study analyses the ontogenetic diet shifts of a Neotropical catfish, Pimelodus maculatus (Siluriformes, Pimelodidae), from an ecomorphological standpoint. We collected 241 individuals in the Piracicaba River (Brazil) and, in the laboratory, seven morphometric variables were recorded from each specimen: standard length (SL), body depth, head length, snout length, eye diameter, mouth height and mouth width. After standardizing these measurements (as a proportion of SL), linear regressions were run to determine whether their growth was isometric or allometric in relation to SL. The diet analysis shows that the main food item for fish in the smaller size classes is aquatic insects, while the largest individuals feed mainly on other fish. As the great majority of the morphometric variables analyzed showed isometric growth as a function of SL – which means that these fish do not change their shape significantly during their lives – the ontogenetic diet shifts may be seen as a consequence of the absolute size increment of their mouth gape, which allows individuals to maximize their energetic gain by ingesting larger prey with a higher caloric content.     

Alternative stable states generated by ontogenetic niche shift in the presence of multiple resource use

It has been suggested that when juveniles and adults use different resources or habitats, alternative stable states (ASS) may exist in systems coupled by an ontogenetic niche shift. However, mainly the simplest system, i.e., the one-consumer-two-resource system, has been studied previously, and little is known about the development of ASS existing in more complex systems. Here, I theoretically investigated the development of ASS caused by an ontogenetic niche shift in the presence of multiple resource use. I considered three independent scenarios; (i) additional resources, (ii) multiple habitats, and (iii) interstage resource sharing. The model analyses illustrate that relative balance between the total resource availability in the juvenile and adult habitats is crucial for the development of ASS. This balance is determined by factors such as local habitat productivity, subsidy inputs, colonization area, and foraging mobility. Furthermore, it is also shown that interstage resource sharing generally suppresses ASS. These results suggest that the anthropogenic impacts of habitat modifications (e.g., fragmentation and destruction) or interaction modifications (e.g., changes in ontogeny and foraging behavior) propagate through space and may cause or prevent regime shifts in the regional community structure.     

Grassland harvesting alters ant community trophic structure: An isotopic study in tallgrass prairies

Disturbances have long been recognized as important forces for structuring natural communities but their effects on trophic structure are not well understood, particularly in terrestrial systems. This is in part because quantifying trophic linkages is a challenge, especially for small organisms with cryptic feeding behaviors such as insects, and often relies on conducting labor‐intensive feeding trials or extensive observations in the field. In this study, we used stable isotopes of carbon and nitrogen to examine how disturbance (annual biomass harvesting) in tallgrass prairies affected the trophic position, trophic range, and niche space of ants, a widespread grassland consumer. We hypothesized that biomass harvest would remove important food and nesting resources of insects thus affecting ant feeding relationships and trophic structure. We found shifts in the feeding relationships inferred by isotopic signatures with harvest. In particular, these shifts suggest that ants within harvest sites utilized resources at lower trophic levels (possibly plant‐based resources or herbivores), expanded trophic breadth, and occupied different niche spaces. Shifts in resource use following harvest could be due to harvest‐mediated changes in both the plant and arthropod communities that might affect the strength of competition or alter plant nitrogen availability. Because shifts in resource use alter the flow of nutrients across the food web, disturbance effects on ants could have ecosystem‐level consequences through nutrient cycling.     

Body shape, diet and ontogenetic diet shifts in young fish of the Sinnamary River, French Guiana, South America

A total of 1468 young fish representing 66 taxa from the Sinnamary River, French Guiana was classified by complete cluster analysis of mean relative body width and mean relative body height into four groups. These had anguilliform, disciform, flat or intermediate body shapes and belonged chiefly to Gymnotiformes, Perciformes, Siluriformes and Characiformes, respectively. Several of the taxa shifted from one to another body shape during ontogenesis. Seven diet groups were defined by complete cluster analysis. Among these, six groups were represented by carnivorous fish. The three most frequent groups had diets of (1) mainly insect larvae and small crustaceans, (2) insect larvae, and (3) predominantly terrestrial insects. The majority of the fish taxa showed ontogenetic diet shifts. Carnivorous fish usually switched from small-size prey, such as small crustaceans, to intermediate-size prey, such as insect larvae and/or to large-size prey, such as insects and/or fish. However, taxa differed in their capacities to switch from small prey to intermediate and/or to large prey. Taxa of different body shapes had significantly different diets. Disciform fish fed mainly on aquatic insect larvae and terrestrial insects but also, in small amounts, on small curstaceans. Most anguilliform taxa ate insect larvae. Individuals belonging to the depressiform or intermediate morphotype had varied diets ranging from plant debris and substratum to fish.