Interspecific Dominance Hierarchy Research Articles

Unfamiliarity generates costly aggression in interspecific avian dominance hierarchies

Dominance hierarchies often form between species, especially at common feeding locations. Yet, relative to work focused on the factors that maintain stable dominance hierarchies within species, large-scale analyses of interspecific dominance hierarchies have been comparatively rare. Given that interspecific behavioral interference mediates access to resources, these dominance hierarchies likely play an important and understudied role in community assembly and behavioral evolution. To test alternative hypotheses about the formation and maintenance of interspecific dominance hierarchies, we employ an large, participatory science generated dataset of displacements observed at feeders in North America in the non-breeding season. Consistent with the hypothesis that agonistic interference can be an adaptive response to exploitative competition, we find that species with similar niches are more likely to engage in costly aggression over resources. Among interacting species, we find broad support for the hypothesis that familiarity (measured as fine-scale habitat overlap) predicts adherence to the structure of the dominance hierarchy and reduces aggression between species. Our findings suggest that the previously documented agonistic hierarchy in North American birds emerges from species-level adaptations and learned behaviors that result in the avoidance of costly aggression.

Genetic relatedness and morphology as drivers of interspecific dominance hierarchy in hummingbirds.

A dominance hierarchy is the set of ranks occupied by species within an assemblage. Species with a high position within the dominance hierarchy tend to dominate subordinate species in contests for access to resources. In hummingbirds, greater weight and wing disc loading have been associated with highest ranks within the dominance hierarchy. Nevertheless, the limit to which the difference between the weight of contending species represents a competitive advantage has not yet been determined. Here, we determined the dominance hierarchy of a hummingbird assemblage exploiting the most abundant floral resource (Palicourea padifolia, Rubiaceae) in a cloud forest of central Veracruz, Mexico. Specifically, we tested whether species weight and wing disc loading influence the dominance hierarchy. Additionally, we tested whether the flowers visited per foraging bout increases with species weight and dominance. We further tested whether weight, wing disc loading, and the genetic relatedness between contenders influenced the dominance relationships in species-pair interactions. Our results indicate that the hierarchy is positively influenced by weight. Hummingbirds visited similar number of flowers regardless their weight or their dominance. Nevertheless, the probability that the heaviest contender won contests was positively associated with the differences of weight and genetic relatedness between contenders. Contrarily, the probability that the contender with greatest wing disc loading won contests was positively associated with differences of weight and negatively associated with the relatedness between contenders. However, these models only explained between 22% and 34% of the variation, respectively. Our results demonstrate that the weight was the major contributor to high dominance values. However, future studies should include (1) the temporal variability of the weight and (2) experimental predictor variables such the burst power of the hummingbirds to evaluate its effects on the dynamics of dominance hierarchies in hummingbird assemblages. All the hummingbird species present in the studied assemblage have developed wide behavioral mechanisms that compensate their morphological differences, which allow them to coexist, even when they compete for the access to the same resource.

Sociality and morphology differentiate niches of 13 sympatric Amazonian woodcreepers (Dendrocolaptinae)

Abstract Woodcreepers (Dendrocolaptinae) represent a remarkably uniform group of brownish birds that move by hitching up tree trunks as they forage for arthropod prey. Despite these superficial similarities, we were able to uniquely differentiate the niches of all 13 species north of Manaus by integrating morphological traits (e.g., mass and bill size) with behavioral traits (e.g., sociality, stratum use, and foraging maneuvers). The 5 ant-following (myrmecophilous) species, with their larger bodies and heavier bills, were morphologically distinct from the 7 species that join mixed-species flocks. A combination of vertical stratum, mass, and bill length further distinguished among mixed-flocking species. Two canopy species—the solitary Dendrexetastes rufigula and the mixed-flocking Lepidocolaptes albolineatus—consistently foraged at higher strata than other species. For the remaining mixed-flocking species, the largest 3 species differed significantly by mass, whereas the smallest 3 species, which overlapped broadly in mass, were uniquely distinguished by bill length. The 5 ant-following species differed in their degree of specialization on ant swarms, from facultative (Hylexetastes perrotii) to obligate (Dendrocincla merula). The ant-followers also showed nearly discrete mass distributions that essentially differed by Hutchinsonian 1:1.3 ratios, which likely allows them to maintain interspecific dominance hierarchies at the front of raiding army ant swarms. The behaviors we quantified (sociality, vertical strata, and myrmecophily), together with morphology (mass and bill size), separated all 13 species. We speculate that niche partitioning and competitive exclusion allow each woodcreeper to uniquely access invertebrate prey, permitting coexistence and contributing to high alpha diversity at our study site.

An avian dominance hierarchy at a supplemental water source in the Patagonian steppe.

Birds often compete and engage in interspecific agonistic interactions for access to resources such as food and breeding territories. Based on the observed outcomes from such interactions (i.e., patterns of displacements) dominance hierarchies can be established. Knowing which species can outcompete others for essential resources allows researchers to make predictions about the broader ecological impacts of interspecific interactions. We constructed an interspecific dominance hierarchy of twelve avian species which visited an artificial water source in an arid region of coastal Patagonia, Argentina. Displacements were categorized into four types, based on the behaviors involved in the interaction, and we tested if they could predict the difference in dominance between the interacting species (the difference between calculated dominance coefficients for the two focal species). Indirect displacements, involving only the arrival of the dominant species to the water source without direct aggression toward the subordinate bird, occurred more frequently between species with a large difference in dominance. The most dominant bird observed was the kelp gull (Larus dominicanus), which, due to an increasing population and expanding range, in part due to food supplementation from fisheries waste, is likely to outcompete terrestrial and marine avian species for other scarce resources.

Interspecific social dominance networks reveal mechanisms promoting coexistence in sympatric charr in Hokkaido, Japan.

Coexistence of species requires equalizing mechanisms that minimize fitness differences, which are balanced by stabilizing mechanisms that enhance negative intraspecific interactions versus interspecific ones. Here, we develop a simple theoretical framework that allows measuring the relative strength of intraspecific versus interspecific competition in dominance hierarchies. We use it to evaluate mechanisms promoting coexistence between two congeneric charr that compete for foraging positions, which strongly influence density-dependent growth and survival. Agonistic interactions (n=761) among 71 Dolly Varden Salvelinus malma and whitespotted charr Salvelinus leucomaenis were measured by snorkelling in two pools in the sympatric zone of a Hokkaido stream during two summers. Interspecific dominance hierarchies, analysed using three methods, were closely correlated with fish length but the species treated each other equally. Ranks for the most dominant fish in each pool, determined directly by knockout experiments, were also virtually identical to ranks by length. Similarly, exponential random graph modelling of the social networks provided no evidence that either species was dominant over the other. Instead, larger fish were more likely to win contests, especially over fish of the next lower ranks. These results demonstrated that the two species were nearly ecological equivalents in accessing key resources in this sympatric zone. Nearly identical growth and stable densities over 4years further supported this inference, although Dolly Varden were a minority (29% of the assemblage), a sign of some fitness difference. Detailed foraging observations coupled with two concurrent studies revealed an effective stabilizing mechanism. Dolly Varden shifted to feeding directly from the benthos when drifting invertebrates declined, a behaviour enhanced by morphological character displacement, thereby partitioning food resources and enhancing intraspecific competition while avoiding agonistic encounters with whitespotted charr. The plurality of evidence indicates that fitness differences between these ecologically equivalent species are small in this local assemblage, and balanced by resource partitioning, a modest stabilizing mechanism that promotes coexistence. The theoretical framework presented here is a useful tool to evaluate the strength of interspecific versus intraspecific competition, which combined with information on trade-offs in ecological performance can contribute to a mechanistic understanding of species coexistence.

Egalitarian mixed-species bird groups enhance winter survival of subordinate group members but only in high-quality forests

Only dominant individuals have unrestricted access to contested resources in group-living animals. In birds, subordinates with restricted access to resources may respond to intragroup contests by acquiring extra body reserves to avoid periods of food shortage. In turn, higher body mass reduces agility and increases predation and mortality risk to subordinates. Birds often live in hierarchically organized mixed-species groups, in which heterospecific individuals are considered to substitute for conspecifics as protection against predators at a significantly reduced competition cost. Crested tits (Lophophanes cristatus) and willow tits (Poecile montanus) form mixed-species groups during the non-reproductive season that typically exhibit a nearly linear dominance hierarchy (‘despotic’ social structure) in which the highest ranking male willow tit is fourth in the overall hierarchy after the dominant male, female and subordinate juvenile crested tit, respectively. Much less frequently, ‘egalitarian’ dominance structures occur in which the adult willow tits rank second and the hierarchy is less steep, or linear. We present a rare long-term data set in which egalitarian flocks are common enough to assess the consequences of this simple change in hierarchy structure as well as a potential driver of the pattern. A comparison of individuals in the despotic mixed-species groups revealed a strong negative correlation between subcutaneous fat stores and dominance rank in the interspecific dominance hierarchy, whereas in egalitarian groups, subordinate willow tits had significantly lower fat reserves and they foraged in safer parts of the canopy than willow tits in despotic groups. Moreover, egalitarian groups exhibited markedly less within-group aggression, higher group cohesion and improved winter survival in both tit species. However, winter survival of birds in egalitarian groups was impaired relative to despotic groups in forests recently affected by industrial forestry. This suggests that the more egalitarian bird societies may best be adapted to less-disturbed environments.

Arboreal twig-nesting ants form dominance hierarchies over nesting resources.

Interspecific dominance hierarchies have been widely reported across animal systems. High-ranking species are expected to monopolize more resources than low-ranking species via resource monopolization. In some ant species, dominance hierarchies have been used to explain species coexistence and community structure. However, it remains unclear whether or in what contexts dominance hierarchies occur in tropical ant communities. This study seeks to examine whether arboreal twig-nesting ants competing for nesting resources in a Mexican coffee agricultural ecosystem are arranged in a linear dominance hierarchy. We described the dominance relationships among 10 species of ants and measured the uncertainty and steepness of the inferred dominance hierarchy. We also assessed the orderliness of the hierarchy by considering species interactions at the network level. Based on the randomized Elo-rating method, we found that the twig-nesting ant species Myrmelachista mexicana ranked highest in the ranking, while Pseudomyrmex ejectus was ranked as the lowest in the hierarchy. Our results show that the hierarchy was intermediate in its steepness, suggesting that the probability of higher ranked species winning contests against lower ranked species was fairly high. Motif analysis and significant excess of triads further revealed that the species networks were largely transitive. This study highlights that some tropical arboreal ant communities organize into dominance hierarchies.

Effects of supplementary feeding on interspecific dominance hierarchies in garden birds.

Individuals often differ in competitive ability, which can lead to the formation of a dominance hierarchy that governs differential access to resources. Previous studies of dominance have predominently focussed on within-species interactions, while the drivers of between-species competitive hierarchies are poorly understood. The increasing prevalence of predictable anthropogenic food subsidies, such as that provided by garden bird feeders, is likely to intensify between-species competition. However, the consequences for resource acquisition await detailed study, and in particular, whether competitive interactions are influenced by food quality is not known. Here, we examine competitive interactions amongst ten passerine species of birds utilising supplementary food sources of differing quality. We show that dominance rank is strongly predicted by body mass across species. Socially dominant, heavier species monopolised access to a food that had a relatively short handling time (sunflower hearts), spent longer on supplementary feeders, and pecked at lower rates. In contrast subordinate, lighter species were constrained to feed on a food that had a relatively long handling time (sunflower seeds with the hull intact). Our findings suggest that differences in body mass may result in between-species dominance hierarchies that place the heaviest species in the greatest control of supplementary feeding sites, gaining superior access to higher value foods. This may have important implications for the use of supplementary feeding as a conservation tool.

Fighting over food unites the birds of North America in a continental dominance hierarchy

The study of aggressive interactions between species has, to date, usually been restricted to interactions among small numbers of ecologically close competitors. Nothing is known about interspecific dominance hierarchies that include numerous, ecologically varied species. Such hierarchies are of interest because they could be used to address a variety of research questions, e.g. do similarly ranked species tend to avoid each other in time or space, and what will happen when such species come into contact as climates change? Here, we propose a method for creating a continental-scale hierarchy, and we make initial analyses based on this hierarchy. We quantified the extent to which a dominance hierarchy of feeder birds was linear, as intransitivities can promote local species9 coexistence. Using the existing network of citizen scientists participating in Project FeederWatch, we collected the data with which to create a continent-spanning interspecific dominance hierarchy that included species that do not currently have overlapping geographic distributions. Overall, the hierarchy was nearly linear, and largely predicted by body mass, although there were clade-specific deviations from the average mass-dominance relationship. Most of the small number of intransitive relationships in the hierarchy were based on small samples of observations. Few observations were made of interactions between close relatives and ecological competitors like Melanerpes woodpeckers and chickadees, as such species often have only marginally overlapping geographic distributions. Yet, these species9 ranks--emergent properties of the interaction network--were usually in agreement with published literature on dominance relationships between them.

Trematodes with a reproductive division of labour: heterophyids also have a soldier caste and early infections reveal how colonies become structured

Recent findings have extended the documentation of complex sociality to the Platyhelminthes, describing the existence of a reproductive division of labour involving a soldier caste among the parthenitae of trematode parasites. However, all species examined to date occupy high positions in trematode interspecific dominance hierarchies and belong to two closely related families, the Echinostomatidae and the Philophthalmidae (Superfamily Echinostomatoidea). Further, the two species documented as lacking soldiers also belong to the Echinostomatidae. Here, we examine four species of intermediate dominance, all belonging to the family Heterophyidae (Superfamily Opisthorchioidea): Euhaplorchis californiensis, Phocitremoides ovale, Pygidiopsoides spindalis and Stictodora hancocki, all of which infect the California horn snail, Cerithideopsis californica (=Cerithidea californica). We quantify morphology, distribution and behaviour of rediae from fully developed colonies. We also provide information on colony structure for three developing heterophyid colonies to better understand colony development. We discuss the implications of our findings, particularly with respect to how they suggest alternatives to the conclusions of other researchers concerning the nature of trematode sociality. Our analyses of morphological, distributional and behavioural patterns of developed colonies indicate that these heterophyid trematodes have a non-reproductive caste whose function is defence of the colony from invading trematodes. Hence, a soldier caste occurs for species lower in dominance hierarchies than previously known, and is present in at least two superfamilies of digenean trematodes, suggesting that selection for a soldier caste may be much more common among the Trematoda than previously recognised.

Interspecific variation in competitor avoidance and foraging success in sap-attracted insects

Many insect species attracted to fermenting sap often fight for access to this resource, which results in the establishment of interspecific dominance hierarchies. In one such system, the hornet Vespa mandarinia (Hymenoptera: Vespidae) behaviourally dominates during the daytime and several subordinate species avoid aggressive interactions in various ways. In order to elucidate the interspecific variation in competitor-avoidance behaviour and its subsequent effect on foraging success, the behaviour of species of hornets, beetles and butterflies at patches (exudation spots) in Japan was recorded. The percentage of individuals that succeeded in visiting a patch following departure from one, or an attempted visit, or after waiting near a patch for t 10 s, did not differ greatly among species, despite the distinctive differences in dominance between V. mandarinia and the other species. These results suggest that subordinate species may be equally effective at foraging for sap as the dominant species. The competitor-avoidance behaviour differed among the species. Vespa crabro and satyrine butterflies mainly avoided competition by actively moving away from com- petitors. The beetle Rhomborrhina japonica (Coleoptera: Scarabaeidae) often remained close to an occupied patch and waited for the occupant to leave, whereas V. ducalis and nymphaline butterflies used both tactics. The different costs associated with fighting or flight may have determined the differences in the foraging tactics of the species studied and behavioural switching in those species utilising both tactics.

Social foraging and dominance relationships: the effects of socially mediated interference

In socially foraging animals, it is widely acknowledged that the position of an individual within the dominance hierarchy of the group has a large effect upon its foraging behaviour and energetic intake, where the intake of subordinates can be reduced through socially mediated interference. In this paper, we explore the effects of interference upon group dynamics and individual behaviour, using a spatially explicit individual-based model. Each individual follows a simple behavioural rule based upon its energetic reserves and the actions of its neighbours (where the rule is derived from game theory models). We show that dominant individuals should have larger energetic reserves than their subordinates, and the size of this difference increases when either food is scarce, the intensity of interference suffered by the subordinates increases, or the distance over which dominant individuals affect subordinates increases. Unlike previous models, the results presented in this paper about differences in reserves are not based upon prior assumptions of the effects of social hierarchy and energetic reserves upon predation risk, and emerge through nothing more than a reduction in energetic intake by the subordinates when dominants are present. Furthermore, we show that increasing interference intensity, food availability or the distance over which dominants have an effect also causes the difference in movement between ranks to increase (where subordinates move more than dominants), and the distance over which dominants have an effect changes the size of the groups that the different ranks are found in. These results are discussed in relation to previous studies of intra- and interspecific dominance hierarchies.

Resource partitioning and interspecific interactions among sympatric rain forest arboreal mammals of the Western Ghats, India

Resource partitioning in a community of diurnal arboreal mammals consisting of the lion-tailed macaque Macaca silenus, bonnet macaque (BM) Macaca radiata, Nilgiri langur Semnopithecus johnii, and the Indian giant squirrel Ratufa indica of the Western Ghats, southern India, was studied. Differences in their diet, vertical stratification, food resource niche breadth, niche overlap, and behavioral interactions were examined. Resource partitioning was through differential habitat use, resource use, and vertical stratification. Of the four species, the BM was not a resident species and made frequent forays into evergreen forest from the adjacent deciduous forest during the flowering season of Cullenia exarillata and fruiting season of Ficus microcarpa. The macaques had narrower niches, and the langur and the squirrel had wider niches. Niche overlap was highest between the two macaques. Overlap among the study species was particularly pronounced during the flowering of C. exarillata. There was significant correlation between niche overlap and intolerance among the study species. Certain species pairs showed little or no mutual intolerance despite high overlap. Cooperative interactions such as alarm calls occurred more frequently among the resident species. Interaction matrices revealed an underlying pattern of interspecific dominance hierarchy, with the BM dominating over the other three species. Our study suggests that the BM do not coexist with the other three because of high overlap with its congener and low occurrence of cooperative interactions.

Habitat use under latent predation risk. A case study with wintering forest birds

We test whether the spatial distribution of birds within a habitat is determined by predation risk and also by interspecific competition. The work was carried out in a montane mixed forest of central Spain with four Parus species, the long‐tailed tit (Aegithalos caudatus) and the nuthatch (Sitta europaea). Experimental feeders, that varied in their risk of predation, were used to control the effect of natural variations in food availability and quality on the habitat use patterns of different species. Tree gleaning passerines avoided feeding on dark inner forest places far from edges, distant from protective cover, outside the inner tree canopy and near the ground; they preferred deciduous, relatively clear forest plots. These effects remained invariable across years and weather conditions. There was a common pattern of selection of foraging locations by the four Parus species: proximity to cover and height above ground and over the lowest branches of the tree canopy positively influenced the use of feeding places. According to these patterns, the vigilance proportion of species was significantly higher when feeding far from cover than when birds were feeding near pine foliage. This pattern was also common for the four studied Parus species. Nevertheless, the interspecific dominance hierarchy of the species was positively correlated with the use of the safest feeders (feeders farther the ground and nearer from protective cover within tree canopy), being the converse with the most exposed ones. Therefore, the results of this paper demonstrate that the selection of feeding locations within habitat follows a pattern minimizing predation risk. Interspecific dominance hierarchies can lead to the exploitation of unfavourable risky patches by subordinate species.

Who is the top dog in ant communities? Resources, parasitoids, and multiple competitive hierarchies

A wide variety of animal communities are organized into interspecific dominance hierarchies associated with the control and harvest of food resources. Interspecific dominance relationships are commonly found to be linear. However, dominance relations within communities can form a continuum ranging from intransitive networks to transitive, linear dominance hierarchies. How interference competition affects community structure depends on the configuration of the dominance interactions among the species. This study explores how resource size and the trait-mediated indirect effect (TMIE) specialist phorid fly parasitoids exert on interference competition, affect the transitive nature of competitive interactions in an assemblage of woodland ants. I quantify the linearity of networks of interactions associated with large and small food resources in the presence and absence of phorid parasitoids. Two distinct, significantly linear dominance hierarchies exist within the ant assemblage depending on the size of the disputed resource. However, the presence of phorid fly parasitoids eliminates the linearity of both dominance hierarchies. The host's phorid defense behaviors reduce the competitive asymmetries between the host and its subdominant competitors, increasing the indeterminacy in the outcome of competitive interactions. Thus, both resource size variation and phorid-induced TMIEs appear to facilitate coexistence in assemblages of scavenging ants.

Spacing Patterns and Winter Dominance Relationships among Three Species of Wagtails(Motacilla spp.) in Japan.

Spacing patterns and winter dominance relationships, including tolerance, were studied among three species of wagtails (Motacilla spp.) along the Takano and Kamo rivers in Kyoto City, central Japan from 1981 to 1983. Japanese M. grandis and Grey Wagtails M. cinerea were observed throughout the year throughout the study area, while White (Black-backed) Wagtails M. alba (subspecies M. a. lugens) were observed from October to April in the lower part of the study area, which contained residential areas. The three species occurred in pairs or as solitary birds during the non-breeding season, with the proportion of paired birds (pair ratio) highest in M. grandis, lowest (zero) in M. cinerea, and intermediate in M. a. lugens. All of the social units of M. grandis, 87.1% of M. a. lugens and 70.0% of M. cinerea remained within a particular area for at least one hour (1-hr range) during winter. The 1-hr ranges were separated for conspecific birds, but overlapped among birds of different species. The units holding 1-hr ranges were pairs and solitary birds in both M. grandis (pair ratio = 87.5%) and M. a. lugens (pair ratio = 47.6%), but only solitary birds in M. cinerea. The mean body size of the species concerned was as follows: male M. grandis> male M. a lugens > female M. grandis > female M. a lugens> M. cinerea. A relative dominance relationship prevailed as follows: male M. grandis > female M. grandis > male M. a. lugens > female M. a. lugens > M. cinerea. However, the proportions of non-response of M. grandis toward M. cinerea were higher when M. a. lugens were near M. grandis than when no M. a. lugens were nearby; M. grandis tended to ignore M. cinerea in the presence of M. a. lugens. Primarily the body size, and secondarily the dependent status seem to determine the intersexual and interspecific dominance hierarchy. The difference in microhabitat, and interspecific relationships in dominance and tolerance may enable the three wagtail species to coexist in one area. Territory defendability resulting from such intersexual and interspecific dominance relationships is thought to determine whether a wagtail species occurs in a pair or not during winter.

Competitive interactions for foraging microhabitats in a size-structured interspecific dominance hierarchy of two sympatric stream salmonids in a natural habitat

Aggressive interactions, foraging behaviour, microhabitats, and growth of individuals of two sympatric stream-dwelling salmonids, white-spotted charr (Salvelinus leucomaenis) and masu salmon (Oncorhynchus masou masou), were studied in a mountain stream in central Japan. The fishes within a single pool in the stream formed an interspecific size-structured dominance hierarchy. Dominant individuals of both species maintained foraging territories against both subordinate conspecifics and heterospecifics, whereas subordinate charr adopted nonterritorial tactics. In each species, the most dominant fish usually held the focal point nearest to the pool inlet, which ensured priority of access to drifting food, with subordinates distributed farther downstream. Foraging microhabitats differed vertically in the water column between the two species, charr utilizing a home range near the stream bed and salmon occupying the midlayer. The frequency of foraging attempts by salmon was considerably higher than that by charr. Although daily body mass increments of dominant individuals were significantly larger than those of conspecific subordinates in each species, those of some subordinate salmon exceeded those of dominant charr.

Influence of social status on individual foraging and community structure in a bird guild

We investigated the influence of social interactions on individual foraging behavior and community structure of frugivorous birds in southern Costa Rica. Detailed observations of large, heterospecific feeding assemblages at fruiting trees revealed the existence of an interspecific dominance hierarchy, largely consistent with body and bill size. Social status influenced access to food in several ways. First, subordinate species were interrupted more and tended to have shorter foraging bouts than dominant species (n > 1.000 abouts). Second, analysis of over 7,000 videotaped head movements showed that subordinate species spent a smaller fraction of their foraging bouts actually feeding (as opposed to looking about) than did dominants. Third, when many birds were in a tree simultaneously, the foraging bouts of subordinate species were shortened; this effect was less pronounced or absent for species higher in the dominance hierarchy. Fourth, subordinate species foraged less frequently in mixed-species assemblages than did dominant species. Finally, subordinate species fed disproportionately more in the late afternoon at fruiting trees. The influence of social status appeared to manifest itself at the community level. The species composition of foraging assemblages was compared at isolated fruiting trees situated in an agricultural landscape near to (< 0.5 km) and far from (> 6 km) a large tract of primary forest. Whereas the full complement of avian frugivores foraged at the near trees, visitors to the far trees were predominantly of high social status. We discuss reasons why high social status and associated traits might confer an advantage in exploiting human-dominated habitat.

Distribution of two congeneric charrs in streams of Hokkaido Island, Japan: considering multiple factors across scales.

Salvelinus leucomaenis (white-spotted charr) and S. malma (Dolly Varden) are distributed throughout Hokkaido Island, Japan, but sites where they occur in sympatry are rare. In general, S. malma inhabit upstream reaches and S. leucomaenis extend downstream to the ocean. Factors influencing their distribution were analyzed at four spatial scales ranging from the whole island to individual stream pools. At the island scale, S. leucomaenis were found in the warmer south-west region and at lower altitudes elsewhere, whereas S. malma were found in the colder north-east and at higher altitudes. At a regional scale, the downstream limit of S. malma and upstream limit of S. leucomaenis shifted to lower altitude from south-west to north-east across the island, coincident with the decrease in temperature. Further analysis showed that transition points from S. leucomaenis or sympatry to S. malma in individual watersheds were closely related to an index of cumulative mean monthly temperatures exceeding 5°C. However, at the scale of a single watershed, the transition occurred at different altitudes, gradients, and temperatures in two tributaries, apparently because stream discharge, habitat, and disturbances from floods interacted with these abiotic factors to limit distribution. The two charr species developed interspecific dominance hierarchies in individual pools, and there was strong complementary density compensation among stream pools that could be explained by interspecific competition but not by differences in habitat. However, patterns at watershed and regional scales suggested that interspecific competition interacts with temperature in complex ways. We conclude that the importance of various abiotic and biotic factors in shaping Hokkaido charr distributions depends on the scale at which they are viewed.

Intra‐ and interspecific dominance hierarchies and variation in foraging tactics of two species of stream‐dwelling chars

AbstractAggressive interactions, foraging behavior, habitat use and diet were studied in sympatric populations of white‐sported char,Salvelinus leucomaenis, and Dolly Varden,Salvelinus malma, in a Japanese mountain stream. Underwater observations on individuals of both species revealed two distinct behavioral regimes: aggressive drift foragers and non‐aggressive benthos foragers. Aggressive drift foragers defended partial territories around focal points from which they made forays to capture invertebrates drifting in the water column. Non‐aggressive benthos foragers cruised around and beneath cobble in large foraging ranges that overlapped each other. Intra‐ and interspecific, size‐dependent dominance hierarchies were recognized among aggressive drift foragers, whereas non‐aggressive benthos foragers showed no such relationships. Terrestrial invertebrates were the most abundant prey in the diets of drift foragers, whereas a very small proportion of the diet of benthos foragers was made up of these taxa. Benthos foragers showed more complex diet composition than drift foragers. These results suggest that non‐aggressive benthos foragers may avoid not only interference but also exploitative competition by using alternative foraging tactics. The proportion of drift foragers to benthos foragers among white‐spotted char was more than 35 times that among Dolly Varden. The significant difference in the proportion of each species using the two types of foraging strategy results in interspecific food segregation in sympatric populations.