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Individual trophic niche specialization in American beaver (Castor canadensis)
Abstract
The American beaver (Castor canadensis) has been described as a choosy generalist at the species/population scale, yet observational studies have shown little variation in diet among individuals. We compared isotopic values of δ¹³C or δ¹⁵N taken from hair of 32 beaver, representing seven colonies in northern Alabama, USA to determine 1) if colonies of beaver show overlap in isotopic niche width as a result of the similar use of food resources and 2) if individual trophic niche specialization occurs within colonies. Total Trophic Niche Width varied across the wetland with the widest being twice the narrowest. Each of the five niche ellipses overlapped with ≥ two other wetlands studied. The percentages of observed variance attributed to Within Individual Component, Between Individual Component, and Between Location Component for δ¹³C were 37%, 33%, 30%; and those for δ¹⁵N were 16%, 56%, and 28%. Dietary nitrogen differentiated the trophic niches of individual beavers. Our results revealed that colonies across the landscape showed separation in trophic niche, lending support that the choosy generalist classification is correct at the colony scale. Our results also support individual trophic niche specialization within colonies, as seen by the substantial amount variation in both δ¹³C (33%) and δ¹⁵N (56%) between individuals.
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Individual variation is increasingly recognized as a central component of ecological processes, but its role in structuring environmental niche associations remains largely unknown. Species’ responses to environmental conditions are ultimately determined by the niches of single individuals, yet environmental associations are typically captured only at the level of species. Here, we develop scenarios for how individual variation may combine to define the compound environmental niche of populations, use extensive movement data to document individual environmental niche variation, test associated hypotheses of niche configuration, and examine the consistency of individual niches over time. For 45 individual white storks (Ciconia ciconia; 116 individual-year combinations), we uncover high variability in individual environmental associations, consistency of individual niches over time, and moderate to strong niche specialization. Within populations, environmental niches follow a nested pattern, with individuals arranged along a specialist-to-generalist gradient. These results reject common assumptions of individual niche equivalency among conspecifics, as well as the separation of individual niches into disparate parts of environmental space. These findings underscore the need for a more thorough consideration of individualistic environmental responses in global change research. Understanding how individual niches vary can inform ecology and conservation. A study of 45 GPS-tracked white storks across three breeding populations reveals that individual environmental niches are nested, arranged along a specialist-generalist gradient that is highly consistent over time.
Count data can be analyzed using generalized linear mixed models when observations are correlated in ways that require random effects. However, count data are often zero-inflated, containing more zeros than would be expected from the typical error distributions. We present a new package, glmmTMB, and compare it to other R packages that fit zero-inflated mixed models. The glmmTMB package fits many types of GLMMs and extensions, including models with continuously distributed responses, but here we focus on count responses. glmmTMB is faster than glmmADMB, MCMCglmm, and brms, and more flexible than INLA and mgcv for zero-inflated modeling. One unique feature of glmmTMB (among packages that fit zero-inflated mixed models) is its ability to estimate the Conway-Maxwell-Poisson distribution parameterized by the mean. Overall, its most appealing features for new users may be the combination of speed, flexibility, and its interface's similarity to lme4.
American beavers (Castor canadensis) forage on various aquatic and terrestrial plant species. We used stable isotope analysis of carbon (C) and nitrogen (N) to estimate source contributions of seasonal assimilated beaver diets in Voyageurs National Park, Minnesota, from Apr. 2007 to Nov. 2008. Mean (±95% confidence interval) annual beaver diets were estimated as 45.5 ± 11.4% terrestrial and 55.5% aquatic vegetation (22.0 ± 14.5 emergent and 33.5 ± 7.9 floating leaf). Percentages of floating leaf and terrestrial vegetation were similar between winter and summer assimilated diets, but emergent vegetation increased 45% in summer. Although δ15N was 7% greater in summer, δ15N and δ13C were similar by age class and sex, as were assimilated percentages of emergent, floating leaf or terrestrial vegetation. Variation in total assimilated aquatic vegetation did not affect subadult and adult seasonal changes in body mass, tail thickness or tail area, but kit body condition was negatively related to total assimilated aquatic vegetation. Aquatic vegetation accounted for more assimilated diet during winter than previously reported.
Although intra-population variation in niches is a widespread phenomenon with important implications for ecology, evolution and management of a range of animal species, the causes and consequences of this variation remain poorly understood. We used stable isotope analysis to characterise foraging niches and to investigate the causes and consequences of individual niche variation in the European badger, a mustelid mammal that lives in territorial social groups, but forages alone. We found that the degree of individual niche variation within social groups was negatively related to the availability of farmland habitats, which represent an important foraging habitat for badgers; and was positively related to territory size, supporting the idea that resource limitation and ecological opportunity lead to increased individual specialisation. We also found that the degree of individual specialisation related to an individual's body condition and that this effect varied with ecological context; such that specialisation had a stronger positive relationship with body condition in social groups with reduced availability of key farmland habitats. Body condition was also related to the utilisation of specific resources (woodland invertebrates), but again this relationship varied with the availability of farmland foraging habitats. This study supports the idea that resource availability plays an important role in determining patterns of individual niche variation, and identifies the potential adaptive consequences of specialised foraging strategies.
This study integrates laboratory and field results to interpret habitat use and switching by individual bluegill sunfish (Lepomis macrochirus). Prey handling and search times for the bluegill were determined from laboratory feeding experiments at different prey sizes and densities in open water and sediment habitats. Relationships were then constructed to predict the expected return per unit time foraging in these habitats. Experience (learning) was found to increase foraging efficiency up to fourfold over the course of six to eight foraging bouts. A population of 225 bluegills was then introduced into a pond consisting predominantly of open water and sediment habitats. This population exhibited several habitat switches as resources declined during the 3-mo experiment; in addition, the population split over a 5-wk period, with individuals specializing on one habitat or the other. These specialists were considerably more successful than habitat generalists as measured by average amount of food in the stomachs. Return rates estimated from the laboratory experiments indicated that the two habitats were nearly equal in foraging profitability. The individual specialization indicated that learning and mutually interfering foraging mechanisms were involved in the species' use of the two habitats. The results suggest that learning and sampling are important aspects of both habitat use and the specialization of individuals within a population.
Physiological states and foraging behaviors may shape movement patterns of animals. Optimal foraging theory and what we term the deliberate movement hypothesis predict that, to reduce predation risk, central place foragers should move faster with smaller turning angles the further they are from their central place. The complementary bimodal foraging trip hypothesis predicts that the distribution of foraging distances exhibited by central place foragers should be bimodal due to a trade-off between provisioning offspring and self-feeding. We used radio-telemetry to test these hypotheses for American beavers (Castor canadensis) in northern Alabama, United States. American beavers moved faster with increasing distance from lodges in wetland land cover but not in terrestrial land covers, partially supporting the deliberate movement hypothesis. Hourly distances moved from lodges were distributed bimodally during the breeding season, which supports the bimodal foraging trip hypothesis. Therefore, central place foraging may be a determinant of
movement characteristics of American beavers.
Individual specialisation has been identified in an increasing number of animal species and populations. However, in some groups, such as terrestrial mammals, it is difficult to disentangle individual niche variation from spatial variation in resource availability. In the present study, we investigate individual variation in the foraging niche of the European badger (Meles meles), a social carnivore that lives in a shared group territory, but forages predominantly alone. Using stable isotope analysis, we distinguish the extent to which foraging variation in badgers is determined by social and spatial constraints and by individual differences within groups. We found a tendency for individual badgers within groups to differ markedly and consistently in their isotope values, suggesting that individuals living with access to the same resources occupied distinctive foraging niches. Although sex had a significant effect on isotope values, substantial variation within groups occurred independently of age and sex. Individual differences were consistent over a period of several months and in some instances were highly consistent across the two years of the study, suggesting long-term individual foraging specialisations. Individual specialisation in foraging may, therefore, persist in populations of territorial species not solely as a result of spatial variation in resources, but also arising from individuals selecting differently from the same available resources. Although the exact cause of this behaviour is unknown, we suggest that specialisation may occur due to learning trade-offs which may limit individual niche widths. However, ecological factors at the group level, such as competition, may also influence the degree of specialisation.
Home range size of terrestrial animals may be influenced by spatiotemporal dynamics of resources. However, little is known regarding the effects of spatiotemporal resource availability on semi-aquatic central place foragers such as the American beaver Castor canadensis. From January 2011 to April 2012, 26 beavers at 11 wetlands at Redstone Arsenal in north-central Alabama, USA, were captured and radio-tracked using radio telemetry. The objectives of this study were to test the predictions of three hypotheses: (1) the resource dispersion hypothesis: more spatially dispersed resources throughout a landscape increase home range sizes; (2) the temporal resource variability hypothesis: more temporally variable resources result in decreases in home range sizes; (3) the habitat productivity hypothesis: increases in habitat productivity lead to decreases in home range sizes. Twenty-three of 26 radio-tagged beavers had well-bounded home ranges, and their home range sizes were positively related to the diversity of land cover within home ranges as predicted by the resource dispersion hypothesis. Furthermore, home range sizes of 26 beavers, including three seasonally dispersing beavers, decreased with increasing seasonal variability of within-home-range normalized difference vegetation index (NDVI), supporting the prediction of the resource heterogeneity hypothesis. Home range sizes of American beavers increased with increasing total NDVI and proportions of woody plant cover within home ranges probably to avoid overexploitation of woody plants. Our results suggest that the combination of resource quantity, spatial distribution and seasonal variation of resources influences movements and home ranges of central place foragers.
Forage availability can affect body condition and reproduction in wildlife. We used terrestrial and aquatic vegetation sampling, stable isotope analysis, and livetrapping to investigate the influence of estimated forage biomass on diet, body condition, and reproduction in American beavers (Castor canadensis) in the Namakan Reservoir, Voyageurs National Park, Minnesota, USA, May 2008–September 2009. Available terrestrial and emergent aquatic forage varied greatly among territories, but floating leaf aquatic forage was low in abundance in all territories. Variation in estimated biomass of available emergent and terrestrial vegetation did not explain variation in respective assimilated diets, but variation in floating leaf vegetation explained 31% of variation in assimilated floating leaf diets. No models using available vegetation explained variation in body condition. Body condition of individual females in spring did not affect kit catch per unit effort, and overwinter body condition of subadults and adults was similar between territories with and without kits. We found no evidence that available aquatic vegetation affected beaver body condition or fitness. Available forage may be above minimum thresholds to detect differences in diet choice or body condition. Other factors such as water level fluctuations or climatic variables may also explain variation in beaver body condition.
Andersson's (1978; Theor. Popul. Biol. 13: 397-409) optimality model of spatial search allocation was tested for beavers fed aspen saplings in large enclosures. At initial stages of the foraging trials, when resources were uniformly distributed, foraging time per unit area, the number of saplings cut per unit area, and spatial electivity declined linearly with distance from the lodge, as predicted by the model. At subsequent stages of resource depletion there was little change in foraging time per unit area in relation to distance, which is inconsistent with the optimality model. However, the number of saplings cut per unit area and spatial electivity in sapling removal became progressively more homogeneous in relation to distance as resources were depleted, as predicted by the model. These observations suggest that beavers adjust their spatial use of resources in a manner consistent with energy-maximization objectives.
Summary1. Ecological specialization is one of the main concepts in ecology and conservation. However, this concept has become highly context-dependent and is now obscured by the great variability of existing definitions and methods used to characterize ecological specialization.2. In this study, we clarify this concept by reviewing the strengths and limitations of different approaches commonly used to define and measure ecological specialization. We first show that ecological specialization can either be considered as reflecting species’ requirements or species’ impacts. We then explain how specialization depends on species-specific characteristics and on local and contingent environmental constraints. We further show why and how ecological specialization should be scaled across spatial and temporal scales, and from individuals to communities.3. We then illustrate how this review can be used as a practical toolbox to classify widely used metrics of ecological specialization in applied ecology, depending on the question being addressed, the method used, and the data available.4. Synthesis and applications. Clarifying ecological specialization is useful to make explicit connections between several fields of ecology using the niche concept. Defining this concept and its practical metrics is also a crucial step to better formulate predictions of scientific interest in ecology and conservation. Finally, understanding the different facets of ecological specialization should facilitate to investigate the causes and consequences of biotic homogenization and to derive relevant indicators of biodiversity responses to land-use changes.
The objective of this study was to examine the foraging behaviour of the beaver (Castor canadensis Kuhl, 1820) and to explain its selection of terrestrial woody plant species according to central place foraging theory. Limitations in variety of food items in most studies with regard to size and (or) distance from the central place and information on availability of forage choices give a partial view of the subject. In this study, the theory is tested in a natural environment with high variability in food items with regard to these factors. Foraging choices by beavers were inspected by measuring variables on cut and uncut trees of every species encountered within 1 m of trail systems made by 25 beaver colonies in Kouchibouguac National Park in New Brunswick, Canada, thereby quantifying the availability of the different food items. The effect of habitat quality (food availability) on the foraging behaviour of beavers was also tested. The results of this study suggest that with increasing distance from the pond, beavers in high-quality habitats selected fewer, but larger, trees and are more species selective. This selectivity was diminished in habitats of lower quality. The results of this study are consistent with the predictions of the central foraging theory.
Natural populations consist of phenotypically diverse individuals that exhibit variation in their demographic parameters and intra- and inter-specific interactions. Recent experimental work indicates that such variation can have significant ecological effects. However, ecological models typically disregard this variation and focus instead on trait means and total population density. Under what situations is this simplification appropriate? Why might intraspecific variation alter ecological dynamics? In this review we synthesize recent theory and identify six general mechanisms by which trait variation changes the outcome of ecological interactions. These mechanisms include several direct effects of trait variation per se and indirect effects arising from the role of genetic variation in trait evolution.
At many beaver (Castor canadensis) sites at Allegany State Park in New York State, red maple (Acer rubrum) is the only or one of the few tree species left standing at the ponds' edges. The relative palatability of red maple (RM) was studied in three ways. (1) At seven beaver sites, the available and utilized trees were recorded and an electivity index (E) computed. Of 15 tree species, RM ranked second or fourth lowest. (2) In experiment I, RM, sugar maple (A. saccharum, SM), and quaking aspen (Populus tremuloides) logs were presented cafeteria style at 10 colonies. RM was the least preferred. (3) Bark of RM was extracted with solvents. Aspen logs were painted (experiment II) or soaked (experiment III) with this RM extract and presented to beaver cafeteria-style, along with aspen and RM controls. This treatment rendered aspen logs less palatable, indicating that a chemical factor had been transferred.
During the last fifteen years, Akaike's entropy-based Information Criterion (AIC) has had a fundamental impact in statistical model evaluation problems. This paper studies the general theory of the AIC procedure and provides its analytical extensions in two ways without violating Akaike's main principles. These extensions make AIC asymptotically consistent and penalize overparameterization more stringently to pick only the simplest of the true models. These selection criteria are called CAIC and CAICF. Asymptotic properties of AIC and its extensions are investigated, and empirical performances of these criteria are studied in choosing the correct degree of a polynomial model in two different Monte Carlo experiments under different conditions.
Most empirical and theoretical studies of resource use and population dynamics treat conspecific individuals as ecologically equivalent. This simplification is only justified if interindividual niche variation is rare, weak, or has a trivial effect on ecological processes. This article reviews the incidence, degree, causes, and implications of individual-level niche variation to challenge these simplifications. Evidence for individual specialization is available for 93 species distributed across a broad range of taxonomic groups. Although few studies have quantified the degree to which individuals are specialized relative to their population, between-individual variation can sometimes comprise the majority of the population's niche width. The degree of individual specialization varies widely among species and among populations, reflecting a diverse array of physiological, behavioral, and ecological mechanisms that can generate intrapopulation variation. Finally, individual specialization has potentially important ecological, evolutionary, and conservation implications. Theory suggests that niche variation facilitates frequency-dependent interactions that can profoundly affect the population's stability, the amount of intraspecific competition, fitness-function shapes, and the population's capacity to diversify and speciate rapidly. Our collection of case studies suggests that individual specialization is a widespread but underappreciated phenomenon that poses many important but unanswered questions.
Stable isotope mixing models are often used to quantify source contributions to a mixture. Examples include pollution source identification; trophic web studies; analysis of water sources for soils, plants; or water bodies, and many others. A common problem is having too many sources to allow a unique solution. We discuss two alternative procedures for addressing this problem. One option is a priori to combine sources with similar signatures so the number of sources is small enough to provide a unique solution. Aggregation should be considered only when isotopic signatures of clustered sources are not significantly different, and sources are related so the combined source group has some functional significance. For example, in a food web analysis, lumping several species within a trophic guild allows more interpretable results than lumping disparate food sources, even if they have similar isotopic signatures. One result of combining mixing model sources is increased uncertainty of the combined end-member isotopic signatures and consequently the source contribution estimates; this effect can be quantified using the IsoError model (http://www.epa.gov/wed/pages/models/isotopes/isoerror1_04.htm). As an alternative to lumping sources before a mixing analysis, the IsoSource mixing model (http://www.epa.gov/wed/pages/models/isosource/isosource.htm) can be used to find all feasible solutions of source contributions consistent with isotopic mass balance. While ranges of feasible contributions for each individual source can often be quite broad, contributions from functionally related groups of sources can be summed a posteriori, producing a range of solutions for the aggregate source that may be considerably narrower. A paleo-human dietary analysis example illustrates this method, which involves a terrestrial meat food source, a combination of three terrestrial plant foods, and a combination of three marine foods. In this case, a posteriori aggregation of sources allowed strong conclusions about temporal shifts in marine versus terrestrial diets that would not have otherwise been discerned.
Assessments of temporal variation in diets are important for our understanding of the ecology of many vertebrates. Ratios of naturally occurring stable isotopes in animal tissues are a combination of the source elements and tissue specific fractionation processes, and can thus reveal dietary information. We review three different approaches that have been used to resolve temporal diet variation through analysis of stable isotopes. The most straightforward approach is to compare samples from the same type of tissue that has been sampled over time. This approach is suited to address either long or short-term dietary variation, depending on sample regime and which tissue that is sampled. Second, one can compare tissues with different metabolic rates. Since the elements in a given tissue have been assimilating during time spans specific to its metabolic rate, tissues with different metabolic rates will reflect dietary records over different periods. Third, comparisons of sections from tissues with progressive growth, such as hair, feathers, claws and teeth, will reveal temporal variation since these tissues will retain isotopic values in a chronological order. These latter two approaches are mainly suited to address questions regarding intermediate and short-term dietary variation. Knowledge of tissue specific metabolic rates, which determine the molecular turnover for a specific tissue, is of central importance for all these comparisons. Estimates of isotopic fractionation between source and measured target are important if specific hypotheses regarding the source elements are addressed. Estimates of isotopic fractionation, or at least of differences in fractionation between tissues, are necessary if different tissues are compared. We urge for more laboratory experiments aimed at improving our understanding of differential assimilation of dietary components, isotopic fractionation and metabolic routing. We further encourage more studies on reptiles and amphibians, and generally more studies utilizing multiple tissues with different turnover rates.
Dietary diversity often varies inversely with prey resource abundance. This pattern, although typically measured at the population level, is usually assumed to also characterize the behavior of individual animals within the population. However, the pattern might also be produced by changes in the degree of variation among individuals. Here we report on dietary and associated behavioral changes that occurred with the experimental translocation of sea otters from a food-poor to a food-rich environment. Although the diets of all individuals were broadly similar in the food-rich environment, a behaviorally based dietary polymorphism existed in the food-poor environment. Higher dietary diversity under low resource abundance was largely driven by greater variation among individuals. We further show that the dietary polymorphism in the food-poor environment included a broad suite of correlated behavioral variables and that the individuals that comprised specific behavioral clusters benefited from improved foraging efficiency on their individually preferred prey. Our findings add to the growing list of examples of extreme individuality in behavior and prey choice within populations and suggest that this phenomenon can emerge as a behavioral manifestation of increased population density. Individuality in foraging behavior adds complexity to both the fitness consequences of prey selection and food web dynamics, and it may figure prominently as a diversifying process over evolutionary timescales.
• foraging efficiency
• niche width
• polymorphism
• prey handling
Growing evidence that individuals of many generalist animals behave as resource specialists have attracted substantial research interest for its ecological and evolutionary implications. Variation in resource preferences is considered to be critical for developing a general theory of individual specialization. However, it remains to be shown whether diverging preferences can arise among individuals sharing a similar environment, and whether these preferences are sufficiently stable over time to be ecologically relevant. We addressed these issues by means of common garden experiments in feral pigeons (Columba livia), a species known to exhibit among-individual resource specialization in the wild. Food-choice experiments on wild-caught pigeons and their captive-bred cross-fostered descendants showed that short-term variation in food preferences can easily arise within a population, and that this variation may represent a substantial fraction of the population foraging niche. However, the experiments also showed that, rather than being limited by genetic or vertical cultural inheritance, food preferences exhibited high plasticity and tended to converge in the long-term. Although our results challenge the notion that variation in food preferences is a major driver of resource specialization, early differences in preferences could pave the way to specializations when combined with neophobic responses and/or positive feedbacks that reinforce niche conservation.
Foraging herbivores have to trade-off between energy requirements and predator avoidance. We aimed to study the relative roles of these factors in beavers (Castor canadensis) when foraging on land. We hypothesized that beavers were able to assess the risk of predation by using two main cues: the distance from the water and the presence or absence of predator odors. First, we studied the food selection of beavers in relation to distance from the water in natural settings. The transects were made at beaver ponds, and the diameter, species, and distance from the shore of intact and beaver-cut trees were recorded. Secondly, we placed rows of aspen sticks (Populus tremula) perpendicular to the shore around beaver ponds, and treated each row with a neutral, alien, or wolf odor. We found that aspen, downy birch (Betula pubescens), and speckled alder (Alnus incana) were the preferred tree species. More of these species were cut close to the shore, and cut trees were smaller further away from the shore, except in the case of aspen. In the experiment, most of the aspen sticks were taken close to the shore. As predicted, beavers took less aspen sticks in rows treated with wolf odor than water. As the predator odor did not affect the foraging distance from the shore, it is likely that our observation that foraging was the most intense close to shore is due to energetic constraints. However, predation risk probably affects the decision whether to forage on the land in the first place.
Quantified the seasonal food use of Castor canadensis by analysis of stomach contents, and determined if sex or age is related to food selection. -from Authors
The relationship between sizes of trees cut by beavers and distances from the borders of their ponds was examined at three sites in central Massachusetts. For most tree genera, the beavers cut a smaller range of sizes far from shore than close to shore, and relatively more small trees and fewer large trees at greater distances. The second of these results differs from the pattern of preference found in other studies in which predators were much larger than their prey, unlike this case of beavers feeding on trees. Both kinds of results are consistent with an optimal foraging model of size-distance relations in which pursuit or provisioning time depends on size of prey as well as distance (Schoener 1979).
Preference of beavers for tree and shrub species during the autumn food caching period was investigated in western Massachusetts. Cafeteria-style feeding experiments designed to test differences in food selection for immediate consumption vs. longer-term storage were conducted. Beavers removed 80 to 95.9% of all branches of deciduous species offered, while only 65% of the branches of the one evergreen species (Pinus strobus) offered were removed. Witch hazel (Hamamelis virginiana) was consistently cached more often than other species, especially in early autumn. The most abundant species available near the colony area, red maple (Acer rubrum), was removed at a high rate, but used for immediate consumption or construction in early autumn, and not stored at a high rate until November. The differential use of woody species is more evident in early autumn whereas all species, including pine, are stored later in the caching period.
Across their range, a large number of biotic and abiotic factors are known to influence the choice of browse plant and the foraging behaviour of the North American beaver (Castor canadensis). We used generalized linear mixed-effects models to investigate sets of variables that may influence the foraging choices of beaver: forage species, distance of forage from water, forage density, and site. Communities across the study sites in central British Columbia, Canada, were dominated by Salix sitchensis, Salix lucida, and Alnus spp. Density had no impact on forage selection, while site, distance from water, and species identity all influenced the foraging decisions of beaver. We postulated that these factors may be ordered hierarchically: large-scale factors, such as site, followed by the medium-scale distance from water, and species of plant at the finest scale. Forage items in some sites had a higher probability of being browsed than in others, while in all sites the probability of being browsed decreased with increasing distance from water. Beaver appeared to be foraging as “picky” generalists; of the 9 plant species examined, 3 species of Salix (S. scouleriana, S. drummondiana and S. sitchensis) were selected by beaver, Salix bebbiana was avoided, and 5 species were neither selected for nor against. Browse selection within the genus Salix implied that beaver were able to differentiate among closely related species. Detailed information on forage selection is a crucial first step in designing and interpreting models that predict large-scale distributional patterns of beaver.
In six bird species convenient for study, niches are known to be broader on some islands than on the mainland. In every case there is also greater variation in a bill measurement on the islands, except for one species in which the niche on the Canary Islands is narrower than that on the mainland. The adjusted variances average about twice as great in the broader niche. These and other results suggest that continuous variation within local populations is often adaptive in itself and is not part of the genetic or phenotypic load.
One aspect of the niche width of a population refers to the variety of resources used by the entire population. This aspect may be measured by the variance of the population's resource-utilization function. Two components make up a population's niche width. The within-phenotype component measures the spread of resources used by the average individual in the population. The between-phenotype component measures the variety of individual specializations in the population. The sum of these two components yields the total niche width. Calculation of these measures is illustrated with data from Anolis lizards. If the population's niche width is mostly composed of the within-phenotype component, it is virtually a monomorphic population of generalists, whereas if mostly the between-phenotype component, it is virtually a population polymorphic with pure specialists. Anolis populations tend more to be monomorphic with generalists than polymorphic with specialists. Recent ecological theory entails four predictions s...
Relative abundance of woody plants and their utilization by beavers (Castor canadensis) were measured on watersheds in Ashtabula and Columbiana Counties, Ohio, in 1966-67. A food index showed aspen (Populus sp.), alder (Alnus sp.), hop-hornbeam (Ostrya vir- giniana), black cherry (Prunus serotina), willow (Salix sp.)> dogwood (Cornus sp.) and oak (Quercus sp.) to be major foods; 20 different woody species were utilized. The amount of use of a single species seemed to depend largely on the relative availability of all food spe- cies. All topographic features may influence site occupancy by beavers, but in this study, fluctuating water level was the factor most detrimental to sustained habitation. After being exterminated in Ohio by 1830 (Chapman, 1949, p. 174), beavers were reported reappearing in the state in 1936 in counties bordering Pennsylvania (Bednarik, 1965). Since then, they have exerted considerable local influence on streams and watersheds of the area. This study was undertaken to determine utilization of woody vegetation and characteristics of sites occupied by beavers on certain watersheds of northeastern Ohio. Information on the influence of beavers on habitat, in addition to data on beaver productivity (Henry and Book- hout, 1969), is necessary for practical beaver management on a sustained yield basis. THE STUDY AREA
Plusieurs espèces clés d'herbivores de la forêt boréale se nourrissent autant de plantes aquatiques que terrestres. Quantifier la biomasse de plantes aquatiques et terrestres dans les diètes de ces espèces est primordial à la compréhension de leur écologie alimentaire et de leur rôle dans les liens trophiques entre ces écosystèmes. Nous avons examiné le potentiel de l'analyse d'isotopes stables δ13C et δ15N afin de distinguer les sources aquatiques et terrestres des plantes. Nous avons récolté des plantes vasculaires aquatiques et terrestres consommées par les vertébrés herbivores de 2 régions subarctiques canadiennes distinctes et éloignées géographiquement. Nous avons observé de façon systématique des valeurs isotopiques plus positives chez les plantes vasculaires aquatiques que terrestres (+1.8 à 10.4 ‰ pour δ13C et jusqu'à +3.2 ‰ pour δ15N), autant pour les plantes aquatiques de lac ou de rivière. Des analyses groupées des valeurs de δ13C et de δ15N nous ont permis de classer entre 71 et 100 % des valeurs isotopiques des plantes dans les groupes aquatiques et terrestres pour chaque localité. Les plantes aquatiques submergées et terrestres présentaient les plus grandes différences isotopiques alors que les plantes aquatiques émergentes poussant près du rivage contribuaient à un certain chevauchement isotopique entre les catégories aquatique et terrestre. L'ensemble de ces résultats démontre que pour un site spécifique, la distinction entre les plantes vasculaires aquatiques et terrestres grâce à δ13C et δ15N permet d'évaluer quantitativement leur contribution respective aux régimes alimentaires des herbivores et ainsi documenter les liens trophiques entre les interfaces aquatique et terrestre. Nomenclature: ITIS, 2009; USDA NRCS, 2009.
The extent and ecological significance of trophic linkages across ecosystem boundaries have been the subject of considerable recent research attention. North American beavers Castor canadensis engineer terrestrial influences in aquatic ecosystems by constructing terrestrial food caches near their lodges and aquatic influences in terrestrial ecosystems by building dams and flooding low lying areas. However, it is poorly resolved to what extent beavers rely on aquatic food sources and whether this reliance is greater during winter when ice cover physically confines beavers to aquatic habitats or during summer when warm, ice free water promotes the growth and accessibility of aquatic vegetation. Working in a subarctic region, we surveyed the abundance of aquatic and terrestrial food sources in and around lotic and lentic environments and estimated their contributions to beaver diets during open water and ice covered periods using carbon and nitrogen stable isotope analysis of hair samples. Ponds had four times more aquatic vegetation than streams, but terrestrial habitats around ponds had less than half as much shrub cover as habitats adjacent to streams. Beaver diets in this subarctic environment are estimated to be comprised of 60 to 80% aquatic vegetation, with beavers occupying ponds consuming more aquatic vegetation in winter than beavers occupying streams, which rely more on terrestrial shrubs cached near their lodge. Collectively, these results show how the influence of physical barriers on ecosystem linkages can be modified by habitat- and season-specific abundances of preferred resources and the potential for animals to consume food in ecosystems and seasons different from where and when the food was harvested.
In this paper, we tested whether the spatial distribution of a given species in more or less fragmented and disturbed landscapes depends on the species habitat specialization. We studied 891 spatial replicates from the French Breeding Bird Survey (FBBS) monitored at least two years during 2001–2005, and two independent landscape databases measuring respectively landscape fragmentation and recent landscape disturbance on each FBBS replicate. We used a continuous habitat specialization index for the 105 most common bird species monitored by the FBBS. We further modelled the spatial variation in abundance of each species according to fragmentation and disturbance across FBBS replicates, accounting for habitat differences and spatial trends. We then tested whether more or less specialized species responded to landscape fragmentation and disturbance. We found that the more specialist a species, the more negative its spatial response to landscape fragmentation and disturbance. Although there was a very high variation around these tendencies indicating that there are many other drivers of species distribution, our results suggest that measuring specialization may be helpful in predicting which species are likely to thrive in human degraded landscapes. We also emphasize the need to consider both positive and negative species responses when assessing consequences of habitat change in communities.
Multidimensional contingency tables provide a powerful new statistical tool for analyzing interactions among several variables in an ecological study. This technique is applied to a study of selectivity in tree-cutting by two colonies of beavers in central Massachusetts.
At the Blue Heron Cove colony, selection depends jointly on genus and diamter. Beavers cut birch of all diameters available but avoid large diameter maples, pines, and oaks. These beavers are choosy generalists: they show clear preferences in cutting various genera of trees for food yet they cut substantial numbers of trees of non-preferred genera. At the Tamplin Road Pond colony, discrete sites of concentrated cutting activity differ in genera and diameters of trees selected. Trees of all diameters are cut at one site close to water, small diameter trees are selected at two other sites farther from water. Ironwood is preferred at one site but selected against at two other sites. This difference between sites in generic selectivity has two plausible explanations: (1) tree species differ in nutritional and other chemical value between sites, (2) beavers sample trees of the various species present at some sites in order to assess the value of such sites as foraging areas.
The influence of diet on the distribution of nitrogen isotopes in animals was investigated by analyzing animals grown in the laboratory on diets of constant nitrogen isotopic composition.
The isotopic composition of the nitrogen in an animal reflects the nitrogen isotopic composition of its diet. The δ^(15)N values of the whole bodies of animals are usually more positive than those of their diets. Different individuals of a species raised on the same diet can have significantly different δ^(15)N values. The variability of the relationship between the δ^(15)N values of animals and their diets is greater for different species raised on the same diet than for the same species raised on different diets. Different tissues of mice are also enriched in ^(15)N relative to the diet, with the difference between the δ^(15)N values of a tissue and the diet depending on both the kind of tissue and the diet involved. The δ^(15)N values of collagen and chitin, biochemical components that are often preserved in fossil animal remains, are also related to the δ^(15)N value of the diet.
The dependence of the δ^(15)N values of whole animals and their tissues and biochemical components on the δ^(15)N value of diet indicates that the isotopic composition of animal nitrogen can be used to obtain information about an animal's diet if its potential food sources had different δ^(15)N values. The nitrogen isotopic method of dietary analysis probably can be used to estimate the relative use of legumes vs non-legumes or of aquatic vs terrestrial organisms as food sources for extant and fossil animals. However, the method probably will not be applicable in those modern ecosystems in which the use of chemical fertilizers has influenced the distribution of nitrogen isotopes in food sources.
The isotopic method of dietary analysis was used to reconstruct changes in the diet of the human population that occupied the Tehuacan Valley of Mexico over a 7000 yr span. Variations in the δ^(15)C and δ^(15)N values of bone collagen suggest that C_4 and/or CAM plants (presumably mostly corn) and legumes (presumably mostly beans) were introduced into the diet much earlier than suggested by conventional archaeological analysis.
The influence of diet on the distribution of carbon isotopes in animals was investigated by analyzing animals grown in the laboratory on diets of constant carbon isotopic composition.
The isotopic composition of the whole body of an animal reflects the isotopic composition of its diet, but the animal is on average enriched in δ^(13)C by about 1‰ relative to the diet. In three of the four cases examined, the ^(13)C enrichment of the whole body relative to the diet is balanced by a ^(13)C depletion of the respired CO_2. The isotopic relationships between the whole bodies of animals and their diets are similar for different species raised on the same diet and for the same species raised on different diets. However, the δ^(13)C values of whole bodies of individuals of a species raised on the same diet may differ by up to 2‰. The relationship between the ^(13)C/^(12)C ratio of a tissue and the ^(13)C/^(12)C ratio of the diet depends both on the type of tissue and on the nature of the diet. Many of the isotopic relationships among the major biochemical fractions, namely the lipid, carbohydrate and protein fractions, are qualitatively preserved as diet carbon is incorporated into the animal. However, the difference between the δ^(13)C values of a biochemical fraction in an animal and in its diet may be as large as 3‰. The δ^(13)C values of the biochemical components collagen, chitin and the insoluble organic fraction of shells, all of which are often preserved in fossil material, are related to the isotopic composition of the diet.
These results indicate that it will be possible to perform dietary analysis based on the determination of the ^(13)C/^(12)C ratio of animal carbon. Analysis of the total animal carbon will in most cases provide a better measure of diet than the analysis of individual tissues, biochemical fractions, or biochemical components. The limits of accuracy of this method will generally restrict its application to situations in which the diet is derived from sources with relatively large differences in their δ^(13)C values, such as terrestrial vs aquatic organisms or C_3 vs C_4 plants. The method should be applicable to fossil as well as to living material.
A graphical method is discussed which allows a specification of the optimal diet of a predator in terms of the net amount of energy gained from a capture of prey as compared to the energy expended in searching for the prey. The method allows several predictions about changes in the degree of specialization of the diet as the numbers of different prey organisms change. For example, a more productive environment should lead to more restricted diet in numbers of different species eaten. In a patchy environment, however, this will not apply to predators that spend most of their time searching. Moreover, larger patches are used in a more specialized way than smaller patches.
Ecology Letters (2011) 14: 948–958
Many generalist populations are composed of specialised individuals, whose niches are small subsets of the population niche. This ‘individual specialisation’ is a widespread phenomenon in natural populations, but until recently few studies quantified the magnitude of individual specialisation and how this magnitude varies among populations or contexts. Such quantitative approaches are necessary for us to understand how ecological interactions influence the amount of among-individual variation, and how the amount of variation might affect ecological dynamics. Herein, we review recent studies of individual specialisation, emphasising the novel insights arising from quantitative measures of diet variation. Experimental and comparative studies have confirmed long-standing theoretical expectations that the magnitude of among-individual diet variation depends on the level of intra and interspecific competition, ecological opportunity and predation. In contrast, there is little empirical information as to how individual specialisation affects community dynamics. We discuss some emerging methodological issues as guidelines for researchers studying individual specialisation, and make specific recommendations regarding avenues for future research.
1. The use of stable isotope data to infer characteristics of community structure and niche width of community members has become increasingly common. Although these developments have provided ecologists with new perspectives, their full impact has been hampered by an inability to statistically compare individual communities using descriptive metrics. 2. We solve these issues by reformulating the metrics in a Bayesian framework. This reformulation takes account of uncertainty in the sampled data and naturally incorporates error arising from the sampling process, propagating it through to the derived metrics. 3. Furthermore, we develop novel multivariate ellipse-based metrics as an alternative to the currently employed Convex Hull methods when applied to single community members. We show that unlike Convex Hulls, the ellipses are unbiased with respect to sample size, and their estimation via Bayesian inference allows robust comparison to be made among data sets comprising different sample sizes. 4. These new metrics, which we call SIBER (Stable Isotope Bayesian Ellipses in R), open up more avenues for direct comparison of isotopic niches across communities. The computational code to calculate the new metrics is implemented in the free-to-download package Stable Isotope Analysis for the R statistical environment.
Hairs are epidermal derivatives, the type, structure, and arrangement of which are related to the role of the mammalian pelage. Hair follicles probably evolved from amphibian apidermal excrescences which became modified into sensory pits of the reptilelike ancestors of mammals. Pelage affords insulation, is sensitive to external stimuli, may be modified for defense or display, assists in buoyancy and streamlining in water, and aids concealment on land. The basic grouping of hair follicles by threes (trio group) appears almost to have been lost in the Pinnipedia, in which other anatomical adaptations to an aquatic habit include flattened guard hairs, loss of erector (arrector pili) muscles and modified cutaneous glands. Periodic growth and replacement of the pelage is essential to survival. The pelage cycle is closely related to the annual cycle with respect to seasonal requirements dictated by the environment, such as climate and life processes, particularly reproduction. Different pelages occur at differ...
Beavers store and consume tree parts in the bodies of water where they live. We examined whether such soaking renders food more palatable by leaching out undesirable compounds. In experiment 1, saplings of red maple, Acer rubrum (RM), were first soaked in a pond for periods of 2, 18, and 36 days, then offered to free-ranging beavers. Soaking for two days rendered RM slightly more acceptable to beavers. To further examine the time window around two days, RM sticks were soaked in distilled water in the laboratory for 1, 2, 4, and 6 days before presenting them to beavers (experiment 2). In experiment 3, twigs of three species were placed on land. Beavers placed RM in the water for 1 to 3 days before consuming the twigs. In experiment 4, sticks were provided in the water at Cranberry Lake Biological Station (CLBS). Most quaking aspen (QA) was consumed during the first night, and most witch hazel, Hamamelis virginiana (WH), during the third night. At Allegany State Park (ASP), no such difference was found. Twigs were provided in the water in experiment 5. At ASP, WH was taken after three days in the water, and at CLBS little WH was consumed, and only during the third night. A meta-analysis of all experiments shows that relatively more WH is consumed after two days than any other species. Experiment 6 traced the time beavers left their own harvested branches in the water. Unlike other tree species, WH remained in the water for two to four days before being consumed. Experiment 7 measured the phenolics leached into water from RM twigs and small pieces of bark soaked for 10 and 8 days, respectively. Shredded bark lost 50-60% of leachable phenolics into the water, and twigs 70-80%. We conclude that beavers can use water to leach undesirable compounds from their food. Although this effect was not robust, our study is the first of its kind.
Food selection by beavers - a multidimensional contingency table analysis
- Jenkins
Nvnormtest: Normality Test for Multivariate Variables
- S Jarek
Jarek, S., 2012. Nvnormtest: Normality Test for Multivariate Variables. R package
version 0.1-9.
Red maple (Acer rubrum) inhibits feeding by beaver
- D Muller-Schwarze
- B A Schulte
- L X Sun
- A Muller-Schwarze
- C Muller-Schwarze
Muller-Schwarze, D., Schulte, B.A., Sun, L.X., Muller-Schwarze, A., Muller-Schwarze, C.,
1994. Red maple (Acer rubrum) inhibits feeding by beaver (Castor canadensis).