Between‐individual variation drives the seasonal dynamics in the trophic niche of a Neotropical marsupial

The dynamics of population niches result from the variation in resource use within individuals and also from the variation between individuals. The prevalence of one mechanism or the other leads to competing hypotheses about the major mechanisms underlying the empirical observations of the contraction/expansion dynamics of the trophic niche in natural populations. In this study, we investigated how within‐ and between‐individual variation in resource use shapes the food niche dynamics of the woolly mouse opossum, Marmosa paraguayana (Didelphimorphia: Didelphidae), in a remnant of the highly seasonal Cerrado in south‐eastern Brazil. To do so, we analysed the faecal samples of live‐trapped individuals to determine their diets within the wet and dry seasons. In addition to a seasonal shift in the composition of the diet, the population trophic niche was significantly wider during the dry season than the wet season. This expansion resulted from larger between‐individual variation in the dry season that was not related to sex preferences, whereas the individual niche widths did not significantly increase from the wet to the dry seasons. Our findings add to the growing list of animal populations that show individual‐level variation in resource use. Furthermore, these results represent a pattern of individual‐level response to seasonal changes that is different from patterns reported for other organisms. We suggest that a pathway to build more realistic foraging models and produce more accurate predictions on population and community dynamics is to consider between‐individual variation and short‐term niche dynamics.     

Availability of food resources and habitat structure shape the individual‐resource network of a Neotropical marsupial

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Beyond ecological opportunity: Prey diversity rather than abundance shapes predator niche variation

1. Ecological opportunity (i.e. the diversity of available resources) has a pivotal role in shaping niche variation and trophic specialisation of animals. However, ecological opportunity can be described with regard to both diversity and abundance of resources. The degree to which these two components contribute to niche variation remains unexplored.
2. To address this, we used an extensive dataset on fish diet and benthic invertebrate diversity and density from 73 sampling events in three Norwegian rivers in order to explore realised trophic niches and the response of dietary niche variation along gradients of resource diversity (potential trophic niches), resource density (as a proxy of resource abundance) and fish density (as a proxy of inter‐ and intra‐specific competition) in a freshwater top predator (the brown trout, Salmo trutta L.).
3. Linear models indicated that individual and population niche variation increased with increasing ecological opportunity in terms of prey diversity. However, no simple cause‐and‐effect associations between niche indices and prey abundance were found. Our multiple regression analyses indicated that the abundance of certain resources (e.g. Chironomidae) can interact with prey diversity to determine individual and population realised trophic niches. Niche variation (within‐individual component and inter‐individual diet variation) decreased with increasing inter‐ and intra‐specific competition.
4. This study extends prevailing trophic ecology theory by identifying diversity, rather than density, of available prey resources as a primary driver of niche variation in fish of temperate riverine systems with no extensive resource limitation. The study also shows that ecological opportunity may mask the direction of the effect (compression or expansion) of competition on niche variation when food resources are diverse.
5. Our study provides novel empirical insight to the driving forces behind niche variation and reveals that diversity, rather than density, of available prey resources may be a primary driver of niche variation in freshwater fish. Our study supports the view that a broader potential trophic niche promotes broader realised trophic niche variation by individuals, which leads to individual niche diversification by opening access to alternatives resources, resulting in a concomitant rise in the realised trophic niche width of the population.

     

Seasonality and interspecific competition shape individual niche variation in co‐occurring tetra fish in Neotropical streams

The drivers of intraspecific niche variation and its effects on species interactions are still unclear, especially in species‐rich Neotropical environments. Here, we investigated how ecological opportunity and interspecific competition affect the degree of individual trophic specialization and the population niche breadth in tetra fish. We studied the four ecologically similar species (Psalidodon aff. gymnodontus, P. aff. paranae, P. bifasciatus, and Bryconamericus ikaa) in subtropical headwater streams (three sites with two co‐occurring species and three sites with only one species). We sampled fish in two contrasting seasons (winter/dry and summer/wet) and quantified their trophic niches using gut content analysis. Psalidodon bifasciatus was the only species distributed over all the sampled streams. We observed seasonal differences in population trophic niche breadth of P. bifasciatus just when this species co‐occurred with P. aff. gymnodontus. These findings confirm the complex nature of the effects of interspecific competition, depending, for instance, on the identity of the competitor. The degree of individual specialization of P. bifasciatus was higher in the winter, and it was not influenced by the presence of another species. Conversely, the other two Psalidodon species studied presented greater individual specialization in the summer, when fish consumed a higher proportion of allochthonous items (terrestrial insects and seeds), and there were no effects only for B. ikaa. Herein, our results suggest that seasonality in food‐resource availability is a major driver of niche variation and it has the potential to play an important role in how these similar tetra species interact and coexist. Abstract in Portuguese is available with online material.     

Lifetime individual plasticity in body condition of a migratory bird

Body condition is an important trait that reflects the capacity of individuals to acquire food or resist disease, eventually allowing successful reproduction and survival. We first quantified the effects of condition on life history and a secondary sexual trait using long‐term lifetime data on condition in a migratory bird species, the barn swallow, Hirundo rustica. Second, we quantified sources of individual variation in condition by separating the within‐individual from the between‐individual components of variation of environmental condition and age on body condition. Therefore, we were able to partition variation as a result of selection and phenotypic plasticity. Within‐individual variation in body condition increased in early life until middle age (i.e. 3–4 years of age) in the two sexes followed by only a slight decrease in body condition during senescence in males but not in females. After accounting for age‐dependent variation, condition could be partitioned into a within‐individual plastic response to environmental conditions during migration and a nonplastic response (i.e. a between‐individual difference) to environmental conditions experienced in the African winter quarters. Specifically, there was a within‐individual increase in body condition as environmental conditions during migration improved in both males and females, independent of age. There was a between‐individual effect of condition found in the winter quarter in body condition of females, but not in males, which was attributed to the disappearance of females in poor body condition from the study population as a result of the higher natal dispersal of low‐quality females compared to high‐quality ones during years with favourable environmental conditions in the African winter quarters. Males and females also tended to be in better body condition during the warmer springs upon arrival at the breeding grounds. There was a temporal decline in female body condition during 1991–2007, whereas no significant trend was detected in males. Therefore, both intrinsic (e.g. age and sex) and extrinsic factors (e.g. climate) affected body condition. © 2011 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 420–434.     

Age‐related change in melanin‐based coloration of Barn owls (Tyto alba): females that become more female‐like and males that become more male‐like perform better

Ornament expression fluctuates with age in many organisms. Whether these changes are adaptively plastic is poorly known. In order to understand the ultimate function of melanin‐based ornaments, we studied their within‐individual fluctuations and their covariation with fitness‐related traits. In barn owls (Tyto alba), individuals vary from reddish‐brown pheomelanic to white, and from immaculate to marked with black eumelanic spots, with males being less reddish and less spotted than females. During the first molt, both sexes became less pheomelanic, females displayed larger spots and males fewer spots, but the extent of these changes was not associated with reproduction. At subsequent molts, intra‐individual changes in melanin‐based traits covaried with simultaneous reproduction changes. Adult females bred earlier in the season and laid larger eggs when they became scattered with larger spots, whereas adults of both sexes produced larger broods when they became whiter. These results suggest that the production of melanin pigments and fitness‐related life‐history traits are concomitantly regulated in a sex‐specific way. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 101 , 689–704.     

Seasonal population and individual niche dynamics in a tetra fish in the Pantanal wetlands

In seasonal tropical regions, rainfall and/or temporary floods during the wet season generally increase the abundance and diversity of food resources to many consumers as compared to the dry season. Therefore, seasonality can affect intraspecific competition and ecological opportunity, which are two important ecological mechanisms underlying population and individual niche variations. Here, we took advantage of the strong seasonality in the Pantanal wetlands to investigate how within‐ and between‐individual diet variations relate to seasonal population niche dynamics of the tetra fish Astyanax lacustris. We quantified dietary niche using gut contents and stable isotopes. Tetras had higher gut fullness and better body condition in the wet season, suggesting that competition is more intense in the dry season. The population niche was broader in the wet season due to an increase in diet divergence between individuals, in spite of potential stronger competition in the dry season. We posit that low ecological opportunity in the dry season limits the diversifying effect of intraspecific competition, constraining population niche expansion. Our results add new insights on how seasonality affects population and individual diets, indicating that intraspecific competition and ecological opportunity interact to determine temporal niche variations in seasonal environments.     

Examining the link between dietary specialization and foraging modes of stream‐dwelling brown trout Salmo trutta

The aim of this study was to explore differences in dietary specialization across two foraging modes (benthic v. surface‐drift foraging) of stream‐dwelling brown trout Salmo trutta. The degree of inter‐individual niche variation within each foraging mode was high, but the dietary specialization was maintained between foraging modes. This study supports the view that if aquatic invertebrates are more abundant and accessible than surface prey, the individuals will not specialize on surface prey (surface‐drift foraging).     

Sexual dimorphism of head morphology in three‐spined stickleback Gasterosteus aculeatus

This study examined sexual dimorphism of head morphology in the ecologically diverse three‐spined stickleback Gasterosteus aculeatus. Male G. aculeatus had longer heads than female G. aculeatus in all 10 anadromous, stream and lake populations examined, and head length growth rates were significantly higher in males in half of the populations sampled, indicating that differences in head size increased with body size in many populations. Despite consistently larger heads in males, there was significant variation in size‐adjusted head length among populations, suggesting that the relationship between head length and body length was flexible. Inter‐population differences in head length were correlated between sexes, thus population‐level factors influenced head length in both sexes despite the sexual dimorphism present. Head shape variation between lake and anadromous populations was greater than that between sexes. The common divergence in head shape between sexes across populations was about twice as important as the sexual dimorphism unique to each population. Finally, much of the sexual dimorphism in head length was due to divergence in the anterior region of the head, where the primary trophic structures were found. It is unclear whether the sexual dimorphism was due to natural selection for niche divergence between sexes or sexual selection. This study improves knowledge of the magnitude, growth rate divergence, inter‐population variation and location of sexual dimorphism in G. aculeatus head morphology.     

Ovarian self‐incompatibility in Narcissus papyraceus (Amaryllidaceae) is the result of a pre‐zygotic response

Ovarian self‐incompatibility, including pre‐ and post‐zygotic reactions, is a complex mechanism for which we still lack many details relating to its function and significance. The joint presence of ovarian self‐incompatibility with style polymorphism is a rare combination that is found in the genus Narcissus. Usually, style polymorphic species have heteromorphic (diallelic and linked to style length locus) incompatibility, which prevents fertilization between individuals of the same morph, thereby helping to maintain equal proportions of floral morphs in populations. However, when present, self‐incompatibility in Narcissus is not linked to style polymorphism and cross‐fertilization within each morph is possible. Hence, self‐incompatibility in Narcissus is of particular interest when attempting to unravel the nature of the rejection reaction and aiming to assess possible cryptic differences in the fertilization process in intra‐ and inter‐morph crosses, which might ultimately explain the wide variation of morph‐ratio in the field. We examined the breeding system of Narcissus papyraceus, a style‐dimorphic species that has biased morph ratios in most of its populations. We studied pollen‐tube growth in the pistil and ovule fate after experimentally controlled hand pollinations. The growth of pollen tubes in self‐ and intra‐ and inter‐morph crosses was similar up to the point of micropyle penetration in both morphs but, subsequently, a pre‐zygotic failure appeared to affect male and female gametophytes in selfed pistils. A high proportion of ovules from self‐pollinated flowers showed signs of collapse and self‐pollen tubes were blocked or behaved abnormally before entering the embryo sac. Self‐incompatibility was stronger in the long‐styled morph than in the short‐styled morph. We did not find any conclusive sign of differential functioning between intra‐ and inter‐morph cross‐pollinations in any morph. These results enable us to rule out the possible effects of pollen–pistil interactions in N. papyraceus as a cause of morph‐ratio biases and confirm the exceptional nature of the self‐incompatibility mechanism in this polymorphic species. © 2015 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 177 , 629–643.     

Scramble or contest competition over food in solitarily foraging mouse lemurs (Microcebus spp.): New insights from stable isotopes

The relationships between resource distribution, type of competition, and consequences for social organization have been formalized in the socioecological model (SEM) which predicts that ecological factors are the main determinants of female distribution. We tested this basic prediction in two solitary primates (Microcebus berthae and M. murinus) which differ in female association patterns. Using stable nitrogen and carbon isotope data of hair samples and food sources we quantified inter‐specific differences in diet. δ¹³C in M. berthae reflected a diet composed mainly of insect secretions. Higher within‐species as well as seasonal variation in δ¹³C of M. murinus indicated a wider trophic niche including plant and animal source food. Constantly elevated δ¹⁵N in M. murinus most likely reflected extended torpor during the lean season. This energy‐saving strategy together with a wider, more opportunistic feeding niche might reduce female competition in this species, facilitating smaller female ranges, and a higher association potential. In contrast, δ¹⁵N fluctuated seasonally in M. berthae, most likely indicating varying amounts of arthropod food in the diet. Intense scramble competition over small and seasonally limited resources might lead to female spatial avoidance and a reduced association potential in M. berthae. Thus, differences in female association patterns between these two solitary foragers are due to different types of competition and overall intensities of intra‐specific competition. Am J Phys Anthropol, 2010. © 2009 Wiley‐Liss, Inc.     

Polymorphism in lake trout in Great Bear Lake: intra‐lake morphological diversification at two spatial scales

Great Bear Lake is the most northerly lake of its size and provides unique opportunities for intraspecific diversification. Despite increasing attention to intraspecific polymorphism, several knowledge gaps remain (e.g. determining the extent of intraspecific diversification in large relatively pristine lakes and at which spatial scale it can occur). We focused on geographical patterns of morphological differentiation within lake trout (Salvelinus namaycush) to describe two levels of intralake diversification in Great Bear Lake. We used a combination of geometric and traditional linear measurements to quantify differences in body shape, head shape, and fin and body lengths among 910 adult lake trout from the five distinct arms of Great Bear Lake. Although head and fin linear measurements discriminated the three common morphotypes at the whole‐lake level, inter‐arm variation in body shape was observed within each morphotype. A comparison of genetic and morphological distance matrices revealed the lack of an association between the two sets of data, although both comparisons revealed an association in the inter‐arm variation patterns among morphotypes, suggesting a phenotypically plastic response to distinct environments. The whole‐lake and inter‐arm morphological variation observed within lake trout demonstrates the importance of considering scale, especially across large lakes that exhibit marked complexity and a variety of freshwater habitats. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 114 , 109–125.     

Variance in developmental event timing is greatest at low biological levels: implications for heterochrony

Intraspecific variation in developmental event timing is common and may be the raw material from which heterochronies (altered timing of developmental events between ancestors and descendants) arise. However, our understanding of how variance in intraspecific developmental event timing is distributed across different hierarchical, biological levels is poor, despite recent evidence suggesting a genetic basis for such inter‐individual differences. In the present study, we used high (temporal and spatial) resolution bio‐imaging of the entire embryonic development of the pond snail, Radix balthica, to investigate how variance was partitioned between the biological levels of interpopulation, inter‐egg mass and individual, with respect to the relative timing of four key developmental events (four‐cell division, a discrete heart beat, capsule rupture, and hatching), as well as egg volume (an index of maternal investment), hatchling size and shape (to compare embryonic with post‐hatch variance). We found that the timing of all but one (four cell division) of the developmental events, together with all measures of hatchling size and shape, had most variance partitioned at the individual level; variance at the interpopulation level was surprisingly low despite sampling populations from across the geographical range of R. balthica. These patterns highlight the importance of studying the distribution of variance in developmental event timing with the aim of understanding how it might drive organismal ecology and evolution. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110 , 581–590.     

Landscape heterogeneity drives intra‐population niche variation and reproduction in an arctic top predator

While intra‐population variability in resource use is ubiquitous, little is known of how this measure of niche diversity varies in space and its role in population dynamics. Here we examined how heterogeneous breeding environments can structure intra‐population niche variation in both resource use and reproductive output. We investigated intra‐population niche variation in the Arctic tundra ecosystem, studying peregrine falcon (Falco peregrinus tundrius, White) breeding within a terrestrial‐marine gradient near Rankin Inlet, Nunavut, Canada. Using stable isotope analysis, we found that intra‐population niches varied at the individual level; we examined within‐nest and among‐nest variation, though only the latter varied along the terrestrial‐marine gradient (i.e., increased among‐nest variability among birds nesting within the marine environment, indicating higher degree of specialization). Terrestrial prey species (small herbivores and insectivores) were consumed by virtually all falcons. Falcons nesting within the marine environment made use of marine prey (sea birds), but depended heavily on terrestrial prey (up to 90% of the diet). Using 28‐years of peregrine falcon nesting data, we found a positive relationship between the proportion of terrestrial habitat surrounding nest sites and annual nestling production, but no relationship with the likelihood of successfully rearing at least one nestling reaching 25 days old. Annually, successful inland breeders raised 0.47 more young on average compared to offshore breeders, which yields potential fitness consequences for this long‐living species. The analyses of niche and reproductive success suggest a potential breeding cost for accessing distant terrestrial prey, perhaps due to additional traveling costs, for those individuals with marine nest site locations. Our study indicates how landscape heterogeneity can generate proximate (niche variation) and ultimate (reproduction) consequences on a population of generalist predator. We also show that within‐individual and among‐individual variation are not mutually exclusive, but can simultaneously arise and structure intra‐population niche variation.     

Individual–resource networks reveal distinct fruit preferences of selective individuals from a generalist population of the Helmeted Manakin

Ecological studies traditionally assume that generalist populations are homogeneous in the use of food resources, but empirical evidence supports that intraspecific differences in morphology, physiology and behaviour affect foraging decisions and promote diet variation among individuals. Furthermore, the temporal availability of resources may shape the dynamics of population trophic niche, which ultimately depends on individual niches. In this study, we investigated the seasonal changes in individual-based networks between the Helmeted Manakin Antilophia galeata, a generalist frugivorous bird, and fruiting plants, following theoretical models of interindividual diet variation based on the Optimal Diet Theory. Selective individuals were the majority of the generalist population of the Helmeted Manakin. Our results suggest that the structure of the individual-resource networks varied seasonally. We found that modularity was higher than expected by chance in the wet season, when fruit availability was also higher. In the dry season, modules disappeared and the network became more nested. These findings are consistent with the Distinct Preference Model of diet variation. We suggest that downscaling ecological networks to the individual level may reveal emergent properties that, albeit existent, are not evident in species–resources networks.     

Scales of spatiotemporal variability in macroinvertebrate abundance and diversity in monsoonal streams: detecting environmental change

1. We quantified spatial and temporal variability in benthic macroinvertebrate species richness, diversity and abundance in six unpolluted streams in monsoonal Hong Kong at different scales using a nested sampling design. The spatial scales were regions, stream sites and stream sections within sites; temporal scales were years (1997–99), seasons (dry versus wet seasons) and days within seasons. 2. Spatiotemporal variability in total abundance and species richness was greater during the wet season, especially at small scales, and tended to obscure site‐ and region‐scale differences, which were more conspicuous during the dry season. Total abundance and richness were greater in the dry season, reflecting the effects of spate‐induced disturbance during the wet season. Species diversity showed little variation at the seasonal scale, but variability at the site scale was apparent during both seasons. 3. Despite marked variations in monsoonal rainfall, inter‐year differences in macroinvertebrate richness and abundance at the site scale during the wet season were minor. Inter‐year differences were only evident during the dry season when streams were at base flow and biotic interactions may structure assemblages. 4. Small‐scale patchiness within riffles was the dominant spatial scale of variation in macroinvertebrate richness, total abundance and densities of common species, although site or region was important for some species. The proportion of total variance contributed by small‐scale spatial variability increased during the dry season, whereas temporal variability associated with days was greater during the wet season. 5. The observed patterns of spatiotemporal variation have implications for detection of environmental change or biomonitoring using macroinvertebrate indicators in streams in monsoonal regions. Sampling should be confined to the dry season or, in cases where more resources are available, make use of data from both dry and wet seasons. Sampling in more than one dry season is required to avoid the potentially confounding effects of inter‐year variation, although variability at that scale was relatively small.     

Evidence for a multi‐peak adaptive landscape in the evolution of trophic morphology in terapontid fishes

Using the Australian marine‐freshwater terapontid fishes as a model system, we examined the role of dietary phenotypic optima in an adaptive macro‐evolutionary landscape. Comparative modelling relying on both a priori and data‐driven identification of selective regimes suggested multi‐peak models as best describing much of the dietary phenotypic landscape of terapontids. Both approaches identified common phenotypic optima for different lineages of marine and freshwater herbivores, and minimal differentiation between carnivores and omnivores, irrespective of their phylogenetic relationships, as the model best describing morphological evolution. Significant correlations also existed between these phenotypic axes and proportions of non‐animal dietary items in species’ diets. While simulation results provided evidence for a multi‐peak adaptive landscape in the evolution of trophic morphology in terapontids, they could not rule out chance convergence in these adaptive peaks. However, they do provide scope for identifying areas for more detailed, functionally specific study of phenotypic convergence in herbivorous terapontid trophic habits. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 623–634.     

Size‐dependent growth of individual Atlantic salmon Salmo salar alevins from hatch to first feeding

Variation in egg size, hatch timing and size at hatch, and their influence on individual growth rates of Atlantic salmon Salmo salar alevins up to first feeding were examined in pure strain and hybrid crosses of fish from Scotland and Canada. At the intra‐female, intra‐cross type and inter‐cross type levels, specific growth rates prior to first feed were strongly size dependent, with smaller and later hatching alevins growing significantly faster. The magnitude of this size‐dependent growth was greatest in the hybrid crosses. This resulted in a 40% reduction in the coefficient of variation (c.v. ) in alevin size from post‐hatch to first feeding at the intra‐female level, and a reduction of both intra‐ and inter‐cross differences in alevin sizes in the same period.     

Sex,androgens, and whole‐organism performance in an Australian lizard

Understanding underlying physiological differences between the sexes in circulating androgens and how hormonal variation affects morphology–performance relationships may help clarify the evolution of sexual dimorphism in diverse taxa. Using a widely distributed Australian lizard (Eulamprus quoyii) with weak sexual dimorphism and no dichromatism, we tested whether circulating androgens differed between the sexes and whether they covaried with morphological and performance traits (bite force, sprint speed, endurance). Males had larger head dimensions, stronger bite force, faster sprint speed, and longer endurance compared to females. We found that the sexes did not differ in androgen concentrations and that androgens were weakly associated with both morphological and performance traits. Interestingly, high circulating androgens showed a nonlinear relationship with bite force in males and not females, with this relationship possibly being related to alternative male reproductive tactics. Our results suggest that androgens are not strongly correlated with most performance and morphological traits, although they may play an important organizational role during the development of morphological traits, which could explain the differences in morphology and thus performance between the sexes. Differences in performance between the sexes suggest differential selection on these functional traits between males and females. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 111, 834–849.     

Sex‐specific phenotypic plasticity in response to the trade‐off between developmental time and body size supports the dimorphic niche hypothesis

Female‐biased sexual size dimorphism (SSD) is widespread in many invertebrate taxa. One hypothesis for the evolution of SSD is the dimorphic niche hypothesis, which states that SSD evolved in response to the different sexual reproductive roles. While females benefit from a larger body size by producing more or larger eggs, males benefit from a faster development, which allows them to fertilize virgin females (protandry). To test this hypothesis, we studied the influence of temperature and intraspecific density on the development of Chorthippus montanus. We reared them in monosexual groups under different conditions and measured adult body size, wing length, nymphal mortality, and development time. The present study revealed an inverse temperature–size relationship: body size increased with increasing temperature in both sexes. Furthermore, we found intersexual differences in the phenotypic response to population density, supporting the dimorphic niches hypothesis. At a lower temperature, female development time increased and male body size decreased with increasing density. Because there was no food limitation, we conclude that interference competition hampered development. By contrast to expectations, mortality decreased with increasing density, suggesting that interference did not negatively affect survival. The present study shows that sex‐specific niche optima may be a major trigger of sexual dimorphisms. © 2015 The Linnean Society of London, Biological Journal of the Linnean Society, 2015, 115 , 48–57.