Interspecific territorialism and competition in Acrocephalus warblers as revealed by playback experiments in areas of sympathy and allopatry

Playback experiments with Acrocephalus warblers in areas of sympatry and allopatry reveal that significant interspecific responses only occurred under certain conditions of competition in sympatry. These were when a later arriving species overlapped and invaded the habitat already occupied by the earlier arriving species. The latter were dominant in interspecific interactions. Earlier models which suggested that interspecific territorialism results largely through failure to discriminate between two species are critically discussed, and found inadequate to explain the present and similar cases. It is suggested that learning may also play a role, and that certain individuals may learn to recognize and respond to the songs of congenerics who are regularly heard and encountered in competitive situations.     

THE ORIGINS OF ADAPTIVE INTERSPECIFIC TERRITORIALISM

1. In order to understand fully the evolution of a behavioural trait one must not only consider whether it is adaptive in its present environment but also whether it originated as an adaptation to existing selective forces or as a fortuitous consequence of selection for a different role in other environments (i.e., as a pre-adaptation) or of selection for different traits (e.g., as a pleiotropic effect). In this paper interspecific territorialism is examined in species of humming-birds, sun-birds, tropical reef fishes, stingless bees, stomatopods, crayfish, and limpets as a means of determining its adaptiveness and its origins. 2. Humming-birds form complex assemblages with species sorted out among the available resources. Dominant species establish feeding territories where flowers provide sufficient nectar. A few large, dominant species, usually uncommon, are marauders on others' territories. Subordinate species establish territories where flowers are more dispersed or produce less nectar, or they fly a circuit from nectar source to nectar source when flowers are even more dispersed, a foraging pattern called ‘traplining’, or they steal nectar from the territorial species by being inconspicuous while foraging. Two species, Amazilia saucerottei and Selasphorus sasin, subordinate in one-to-one encounters, are able to take over rich resources by establishing several small territories within a territory of a dominant and forcing it to forage elsewhere. 3. Among humming-birds, territorial individuals attacked not only subordinate competitors but marauding humming-birds and some insects, which stayed in the territory and foraged at will, and seemingly inappropriate targets, such as non-competitors. This suggests that the stimulus for aggression is ‘any flying organism near the food resources’, regardless of its appearance. The behaviour rather than the identity of the intruder is the stimulus. 4. Sun-birds resemble humming-birds to the extent that dominants establish territories on rich nectar sources and subordinates establish territories on less rich nectar sources or steal from the territories of dominants. The diversity of foraging patterns is not so great as in humming-birds, perhaps because so few species of sun-birds have been studied. However, the advantage of territorialism has been measured in the sun-bird Nectarinia reichenowi. Individuals with territories lose much less nectar to competitors than do those without territories. 5. Field work on three species of tropical reef fishes involved a single aggressive species whose individuals attacked a wide range of species intruding on their territories. The stimulus for aggression in Pomacentrus jenkinsi seemed to be an “object moving through [its] territory”. As suggested for humming-birds, the stimulus is the behaviour rather than the identity of the intruder. 6. The relationships found in stingless bees, stomatopods, crayfish, and limpets are simpler. The dominant and subordinate species divide the resources in their habitat, the dominants' aggression preventing the subordinates from using resources that were otherwise available to them. 7. A general pattern emerges. Mutual interspecific territorialism occurs between species that (i) have different geographic ranges, (ii) occupy different habitats, or (iii) use different resources within the same habitat. Examples of two species holding separate territories on the same resources within the same habitat are rare and occur when the dominant species is rare relative to the available resources. These observations are contrary to the usual view that interspecific territorialism is an adaptation that permits co-existence of potential competitors within the same habitat. 8. Interspecific territorialism is sometimes adaptive and sometimes maladaptive, depending upon the species and the situation. 9. The general pattern of occurrence of the behaviour and the general nature of the stimulus for aggression, i.e., the behaviour rather than the identity of the intruder, suggest that interspecific territoriality is a fortuitous consequence of selection for intraspecific territorialism, the latter being not only an adaptation to the presence of conspecific competitors but a pre-adaptation to the presence of competitors of other species, should they occur.     

Compensatory and additive helper effects in the cooperatively breeding Seychelles warbler (Acrocephalus sechellensis)

In cooperatively breeding species, care provided by helpers may affect the dominant breeders’ investment trade‐offs between current and future reproduction. By negatively compensating for such additional care, breeders can reduce costs of reproduction and improve their own chances of survival. Alternatively, helper care can be additive to that of dominants, increasing the fledging fitness of the current brood. However, the influence helpers have on brood care may be affected by group size and territory quality. Therefore, the impact of helping needs to be disentangled from other factors determining offspring investment before conclusive inferences about the effect of help on additive and compensatory care can be made. We used 20 years of provisioning data to investigate the effect of helping on provisioning rates in the facultative cooperatively breeding Seychelles warbler Acrocephalus sechellensis. Our extensive dataset allowed us to statistically disentangle the effects of helper presence, living in larger groups and different food availability. We show compensatory and additive care (i.e., partial compensation) in response to helper provisioning. Helpers lightened the provisioning load of the dominant male and female and increased total provisioning to nestlings. This was irrespective of group size or territory quality (food availability). Moreover, our results illustrate sex‐specific variation in parental care over the course of the breeding event. We discriminate between temporal variation, group size, and territory quality processes affecting cooperative care and as such, gain further insight into the importance of these factors to the evolutionary maintenance of helping behavior.     

Investigations of density interactions among breeding birds in ponderosa pine forests: correlative and experimental evidence

Summary We assessed the importance of interspecific competition among insectivorous birds breeding in northcentral Arizona's ponderosa pine forests. We examined density interactions among species using two analytic approaches; correlative and experimental. The correlative approach examined patterns of change in breeding densities over four years at the community level and within two foraging guilds; picker-gleaners and aerial feeders. The relationships between morphological and behavioral similarity with pairwise density interactions were also assessed. Our experimental approach involved placement of nest boxes on two treatment plots to increase breeding densities of secondary cavity nesting birds that were in foraging guilds with open nesting insectivores.We found little evidence of interspecific competition. Patterns of density fluctuations indicated large positive covariances among species at both the community level and within guilds. Pairwise density interactions were independent of morphological or behavioral similarity. Nest boxes significantly increased breeding densities of the secondary cavity nesters. However, these increases did not induce reprocal density changes in the open nesting species. Interspecific competition for food during the breeding season appears to be unimportant in ponderosa pine bird communities.     

High resource overlap and small dietary differences are widespread in food-limited warbler (Parulidae) communities

Although both interspecific competition and coexistence mechanisms are central to ecological and evolutionary theory, past empirical studies have generally focused on simple (two-species) communities over short time periods. Experimental tests of these species interactions are challenging in complex study systems. Moreover, several studies of ‘imperfect generalists’, consistent with Liem's Paradox, raise questions about the ability of evolved species differences to partition niche space effectively when resources vary considerably across the annual cycle. Here we used a recently developed theoretical framework to combine past research on population-level processes with observational data on resource use to test for ongoing interspecific competition and understand the nature of resource overlap. We compared species diet overlaps and differences in several distinctive communities centred on a focal species, the American Redstart Setophaga ruticilla replicated both spatially and seasonally, in combination with documentation of population regulation to assess the ability of similar species to partition dietary niche space and limit interspecific competition. Our results document high dietary overlap in most of the communities studied, with only subtle differentiation consistent with known species differences in foraging behaviour and morphology. These findings are largely consistent with species foraging as imperfect generalists. However, in contrast to past studies, the high diet overlaps observed here during times of inferred resource scarcity were driven by low-value prey taxa (e.g. small ants) and did not involve truly ‘private’ resources. All of these factors increase the potential negative impacts of interspecific competition, and limit the ability of these birds to avoid competition if food availability deteriorates further than observed in our study, either seasonally or at longer intervals.     

Direct interference or indirect exploitation? An experimental study of fitness costs of interspecific competition in voles

Studies on competing mammalian species in the past have focused mainly on the competitive exclusion of one species from the preferred habitat of the other. Investigations on effects of competition and coexistence on individual fitness are rare . In this study we were able to measure effects of interspecific competition on major fitness components, using a system with two vole species in asymmetric competition. Survival, reproduction and space use of bank vole Clethrionomys glareolus females were monitored in 32 enclosed populations over four replicates of eight parallel run enclosures. Into half of the enclosures we introduced an additional number of field voles Microtus agrestis , a dominant competitor.
Survival of bank vole females was lower under competitive conditions. Total number of breeding females was lower in populations coexisting with competitors. Territory size of bank vole females decreased. Females body weight and litter size bank vole litters conceived during the experiment were not affected by interspecific competition. These characteristics should respond to differences in food resources, and territory size should increase if food was scarce, thus we found no indication of direct exploitation competition between the two species. Space use was overlapping between the species, but individuals of both species were never caught together in the same trap, indicating avoidance behaviour.
We conclude that adult bank vole females do suffer fitness consequences through interference competition with field voles, probably basing on increased number of aggressive encounters in the presence of the dominant species. Our results suggest, that direct interference rather than indirect exploitation competition may be the cause for observed fitness decrease in bank vole females.     

Changes in sexual selection resulting from novel habitat use in the sand goby

Resource availability may determine local breeding systems and may also vary locally between different habitats, affecting the way individuals distribute themselves between these habitats. For nest-breeding fishes, nest site availability is a crucial resource that has been found to affect local sexual selection regimes and thus breeding systems. In this study, we compared the availability and size distribution of nest sites and their consequences for habitat preference, fish distributions and the breeding system in sand gobies ( Pomatoschistus minutus ) breeding in two different habitats. The usual breeding sites of sand gobies are shallow sandy beaches, but here we report their breeding in a novel environment, on rocky bottoms. We found obvious differences between the two habitats. The density of occupied nests was nearly 50 times higher in rock habitat than on sand bottoms. However, competition for nest sites was stronger on sand; nearly all natural nest sites found were in use and empty nest sites were occupied at a much higher rate on sand than in rock habitat. In addition to the numerical differences, nests were larger and contained more eggs in rock habitat than in sand habitat. Moreover, we found significant habitat-specific differences in fish size distribution. The intensity of intrasexual competition as a result of the degree of nest site availability explains the observed patterns within the habitats, but not between them. These habitat-related differences are better explained by the larger average size of nests on rocky bottoms. The effect of nest size on habitat preference was confirmed experimentally by laboratory experiments. When nest size did not differ between the habitats, gobies originating in the two environments showed an equal preference for the sand habitat.     

Interspecific territoriality in gibbons (Hylobates lar and H. pileatus) and its effects on the dynamics of interspecies contact zones

We investigated the ecology and interspecific interactions of the two gibbon species (Hylobates lar and H. pileatus) that overlap in distribution within a narrow zone of contact in the headwaters of the Takhong River at Khao Yai National Park in central Thailand. The zone is about 10-km wide, with phenotypic hybrids comprising 6.5% of the adult population. We compared species with respect to diet, territory size, intra- and interspecific group encounters, and territory quality. The two gibbon species exploited the same types of resources within their territories despite variation in the relative abundance of food-plant species between territories. The gibbons were interspecifically territorial, and males of both species displayed aggressive behaviors at both intraspecific and interspecific territorial boundaries. There was no marked difference in the amount of overlap between territories of conspecific and heterospecific pairs of groups. Although the habitat was not homogeneous, territory quality did not vary significantly between species. The species have not diverged in habitat preference or in diet. Neither species dominated in interspecific encounters, and both were reproducing well in the contact zone. We analyzed the potential consequences of several types of interspecific interactions on individual dispersal options and on the structure of the contact zone. Interference competition through interspecific territoriality affects the dispersal of individuals into the range of the other species. In general, territorial competition coupled with limited hybridization leads to predictions of a narrow contact zone or parapatry between species; thus, behavioral and ecological interactions between species need to be considered as potential factors in explaining range borders of primate species.     

Transcontinental latitudinal variation in song performance and complexity in house wrens (Troglodytes aedon)

There is growing interest in latitudinal effects on animal behaviour and life history. One recent focus is on birdsong, which is hypothesized to be more elaborated or complex in the north temperate zone compared with the tropics. Current evidence is mixed and based on cross-species comparisons, or single species with restricted distributions. We circumvent these limitations using a transcontinental sample of 358 songs from house wrens (Troglodytes aedon) at 281 locations spanning more than 100° of latitude (52° N–55° S) across the Americas. We found a significant latitudinal gradient in several basic elements of song performance and complexity between north temperate and tropical populations. Furthermore, we document convergence in song patterns between populations at higher latitudes in the Northern and Southern Hemispheres. Effects were strongest for the number of elements in a song, and the rate of element production, both increasing towards the poles, with similar but weaker effects for other song dimensions (e.g. number of unique elements, trills and trill rate). We consider possible causes related to variable habitats and morphology, concluding that the shorter breeding seasons at higher latitudes in both hemispheres may favour greater song elaboration to mediate territory competition and mate choice.     

Evolution of leaf warbler songs (Aves: Phylloscopidae)

Songs in passerine birds are important for territory defense and mating. Speciation rates in oscine passerines are so high, due to cultural evolution, that this bird lineage makes up half of the extant bird species. Leaf warblers are a speciose Old‐World passerine family of limited morphological differentiation, so that songs are even more important for species delimitation. We took 16 sonographic traits from song recordings of 80 leaf warbler taxa and correlated them with 15 potentially explanatory variables, pairwise, and in linear models. Based on a well‐resolved molecular phylogeny of the same taxa, all pairwise correlations were corrected for relatedness with phylogenetically independent contrasts and phylogenetic generalized linear models were used. We found a phylogenetic signal for most song traits, but a strong one only for the duration of the longest and of the shortest element, which are presumably inherited instead of learned. Body size of a leaf warbler species is a constraint on song frequencies independent of phylogeny. At least in this study, habitat density had only marginal impact on song features, which even disappeared through phylogenetic correction. Maybe most leaf warblers avoid the deterioration through sound propagation in dense vegetation by singing from exposed perches. Latitudinal (and longitudinal) extension of the breeding ranges was correlated with most song features, especially verse duration (longer polewards and westwards) and complexity (lower polewards). Climate niche or expansion history might explain these correlations. The number of different element types per verse decreases with elevation, possibly due to fewer resources and congeneric species at higher elevations.     

Community convergence in bird song

Species in similar habitats are often similar in morphology or behaviour, attributed to adaptation to similar environmental selection pressures, sometimes mediated by competitive interactions. For passerine songs, similarity of phenotype in identical habitats and character displacement have been documented, the former due to adaptation to the acoustics of the habitat, and the latter due to competition for acoustic space among species. If these phenomena are widespread, they should lead to community convergence of bird songs. Here, we test if passerine communities in similar habitats converge in song attributes or in acoustic differentiation among species. We compared the songs of European and North American Mediterranean climate passerine communities in open and closed habitats. Song frequency varied across different habitats but not continents. This was independent of both phylogeny and body size, indicating community convergence due to acoustic adaptation, rather than species sorting or similarity as a by-product of another type of ecological convergence. We found little evidence for regular spacing in song features among species, as would be expected if acoustic competition shapes within-community structure. However, for one of five song components, the open habitat communities showed a similar distribution of phenotypes on each continent. The proportion of interspecific variation in song explained by these effects was small. The fact that songs are complex signals that vary in many dimensions may explain why competition for acoustic space seems to be of small importance in structuring songs in these passerine communities.     

Elevational replacement of two Himalayan titmice: interspecific competition or habitat preference?

Elevational species replacement is a widely documented pattern in montane species. Although interspecific competition has been shown to be important in setting species elevational limits in tropical habitats, its effect in species of temperate regions is poorly studied. We tested the role of interspecific competition for space in the breeding season and for food in the non‐breeding season in mediating the distribution of two resident titmice species in the Himalayas. We show that high elevation green‐backed tits Parus monticolus are behaviourally dominant over low elevation cinereous tits Parus cinereus in both song playback and feeder trials. Despite being subordinate, at their elevational upper limit, cinereous tits occur in sympatry in human modified habitats. Our study suggests that the loss of natural habitats in the sympatric zone, not interspecific competition, might be limiting the distribution of the high‐elevation green‐backed tits and facilitating an upward range shift through human association in cinereous tits.     

Habitat shift and geographical variation in North American tanagers (Thraupinae: Piranga)

Summary Relationships of habitat structure among North American tanagers were studied in large areas of their breeding ranges in the USA. In the large areas sampled, the four species differ significantly in multivariate scores of habitat structure. In Scarlet and Summer Tanagers (for which the most data was collected) there is intraspecific geographical variation in habitat features. Variation in habitats of Summer Tanagers is more pronounced than that of Scarlet Tanagers, and is related to changes of macrohabitats in low elevations from oak-pine and mixed woods in the southeast to riparian vegetation in the dry southwest. In the southeast these two species often breed in the same localities, and establish territories next to each other. In six localities in which I found both of them, the difference between the two species in habitat structure is significantly greater than in allopatric areas in the southeast. These results, and those of another study on song variation and behavioral responses to song suggest that coexistence of Scarlet and Summer Tanagers in sympatry can be explained both by partial habitat shift, and by interspecific aggressive division of space. It is concluded that quantitative sampling of habitats in large areas of distributional ranges of species is needed before any conclusions on species-specific preference and interspecific competition can be made.     

Structure in temperate predaceous diving beetle communities (Coleoptera: Dytiscidae)

Collection records of adult dytiscid beetles (Coleoptera, Dytiscidae) from 312 sites within the province of Alberta, Canada, were analyzed for patterns of similarity in species distribution. Sites compared on the basis of species occurrence (Jaccard coefficient of similarity and group average cluster analysis) separated into 12 principal clusters, interpreted as communities. However, cluster inclusion occurred at very low levels of similarity and 30% of sites were unclustered outliers. Ecological charade us tics of sites that were recognized as varying between dusters were salinity, productivity, stability, water temperature, substrate type, flow and vegetation. An index relating these qualitatively defined variables to species numbers is presented. An analysis of co-occurrence patterns of species was conducted as an alternative approach to definition of multispecies associations. Each possible combination of co-occurrence between pairs of species occurring in more than 5 collections each was tested for significance (p = 0.005) by means of Yates corrected χ² tests. Results of tests were expressed in binary terms, blank – no significance, x - significant. Species were then clustered on the basis of patterns of significant co-occurrences as above. This analysis was used as the basis for ordination of species in a symmetrical matrix with species ordered so as to maximize density of points of significant co-occurrence (x) around the principal diagonal. Areas in the matrix showing high densities of co-occurrence were interpreted as representing communities. The cluster analysis of species produced a better cluster pattern than obtained for sites but while generally similar, the species clustering pattern was less fine. The principal species groups consisted of those of saline water, alpine/subalpine lotic sites, other lotic habitats, and lentic habitats with two subgroups, species in forest areas and grassland species. The ordination, while reflecting this grouping of species, demonstrated that each group graded into others through loss of certain species and acquisition of others and thus showed the continuum nature of these communities. Restricting ordination to congeneric species indicated that these do not usually show identical patterns of co-occurrence but neither are patterns coincident with mutual exclusion. Within genera, considerable overlap in occurrence between species is the norm but species tend to occupy slightly different positions relative to one another along qualitatively defined ecological gradients. Relative position of species in these ordinations shows good correlation with patterns of geographical distribution lending support to the interpretation of ordination patterns reflecting relative ecological segregation. The dytiscid beetle fauna of north temperate regions is especially rich. Also, the large number of significant co-occurrences observed between species shows that species packing in many habitats of this region is dense. It is speculated that this species richness is possible because of the seasonality of habitats at these latitudes which reduces competition and predation from other groups and produces seasonal pulses of high productivity permitting a species rich fauna to develop because of abundant resources. It is likely that factors other than interspecific competition for resources are important in shaping dytiscid communities in habitats with strong nutrient pulses.     

Timing of resource input and larval competition between invasive and native container-inhabiting mosquitoes (Diptera: Culicidae).

Container-inhabiting mosquito species are subject to both intraspecific and interspecific competition during larval development in resource-limited habitats. The arrival of an invasive species, Aedes albopictus, in the U.S. has altered competitive interactions among container-inhabiting mosquito species and, in some cases, has led to displacement of these species. Resource enrichment of container habitats has been shown to alleviate competitive interactions and to promote species co-existence; however, the importance of the timing of enrichment has yet to be explored. Larval competition between Ae. albopictus and a native species, Ochlerotatus triseriatus, was explored when resources were added either gradually or in a single pulse. Replacement series experiments revealed that Ae. albopictus was able to outcompete and displace Oc. triseriatus via resource monopolization when all resources were made available simultaneously; however, when the same resource amount was added over time, survival was high for both species, leading to co-existence. Timing of resource input also had an effect in monospecific treatments, indicating that intraspecific competition impacts survival as well. Duration of larval development was influenced by both species presence and by timing of resource input for Oc. triseriatus. These results indicate competitive outcome is condition-specific and that timing of resource input can determine whether a dominant invasive competitor displaces a native species, or if the two species are able to co-exist. Both intraspecific and interspecific competition occur at different temporal scales due to species-specific differences in larval developmental time. Timing of resource availability in container habitats can impact mosquito survival via competitive interactions, which can ultimately influence vector population size and behavior, possibly influencing vector-borne disease transmission.     

Costs of coexistence along a gradient of competitor densities: an experiment with arvicoline rodents

1. Costs of coexistence for species with indirect resource competition usually increase monotonically with competitor numbers. Very little is known though about the shape of the cost function for species with direct interference competition. 2. Here we report the results of an experiment with two vole species in artificial coexistence in large enclosures, where density of the dominant competitor species (Microtus agrestis) was manipulated. Experimental populations of the subordinate vole species (Clethrionomys glareolus) were composed of same aged individuals to study distribution of costs of coexistence with a dominant species within an age-cohort. 3. Survival and space use decreased gradually with increasing field vole numbers. Thus, responses to interference competition in our system appeared to be similar as expected from resource competition. The total number of breeders was stable. Reproductive characteristics such as the timing of breeding, and the litter size were not affected. In the single species enclosures a proportion of surviving individuals were not able to establish a breeding territory against stronger conspecifics. Under competition with heterospecifics such nonbreeders suffered high mortality, whereas the breeders survived. 4. Combined interference of dominant conspecifics and heterospecifics probably increased the frequency of aggressive interactions, social stress and mortality for the weaker individuals within a homogeneous age cohort of the subordinate competitor population. 5. Our results suggest, that in open systems where bank voles are outcompeted over the breeding season by faster reproducing field voles, animals able to establish a territory may be able to withstand competitor pressure, while nonbreeding bank vole individuals are forced to emigrate to suboptimal forest habitats.     

Competitive interactions among raptors in boreal forests

We examined inter-specific interactions among goshawks (Accipiter gentilis), common buzzards (Buteo buteo) and honey buzzards (Pernis apivorus) in western Finland in 1983–1996. Because goshawks are among the largest birds of prey species in boreal forests they may take over the nest of smaller and less-competitive forest-dwelling raptors when searching for suitable places for breeding. Accordingly, more than half of newly established goshawk territories were found on the territories previously occupied by the common buzzard and the honey buzzard. Otherwise, territory sharing between these species was rare. Fledgling production of honey buzzards was not associated with the presence of goshawks, probably owing to the almost 2 months later onset of breeding. This probably decreases competitive interactions between these two species. An intensive interference competition, instead, seemed to be evident between common buzzards and goshawks, because the fledgling production of common buzzards was decreased by 20% as a result of failures during incubation and nestling period in the vicinity (<1 km) of occupied goshawk nests. Similarly, territory occupancy of common buzzards till the next breeding season was significantly reduced in the presence of goshawks. Relatively high proportions of occupied buzzard territories (17%) in the study area were shared by breeding goshawks on the same territory. This suggests that although their diets are dissimilar they inhabit similar habitats and might compete for the available prime nesting habitats within forest landscapes. In addition, goshawks benefit from taking over the complete nests of other raptors, imposing upon the original owners of the nest, because building a large stick nest is probably energetically costly. As a large raptor, the goshawk apparently has a competitive advantage over smaller ones, and may have an ever-increasing impact on smaller birds of prey, if there is a lack of sheltered forests inducing competition for the available nest sites.     

Manifestation d'un comportement territorial chez les crevettes pontoniinae associées aux mollusques pinnidae a tuléar (Madagascar)

Shrimps associated with molluscs are generally found in pairs in their host and observations show that this disposition leads to active competition, both intra- (between individuals of same sex) and interspecific.The territory in dispute is the entire palliai ensemble of the mollusc. The weapons are the second pair of pereiopods and their size is of prime importance in determining the outcome of any encounter.In interspecific confrontations, Paranchistus ornatus Holthuis is regularly expelled by Anchistus custos (Forskål), and Conchodytes biunguiculatus (Paulson) will displace both of these species.     

Co-existence of nine gill ectoparasites (Dactylogyrus: monogenea) parasitising the roach (Rutilus rutilus l.): history and present ecology

Co-existence among potentially competing species can be favoured by niche specialisation and/or by reducing the overall intensity of competition via aggregated utilisation of fragmented resources. We investigated the respective roles of niche specialisation and aggregation in the case of nine congeneric monogenean parasites on the gills of Roach (Rutilus rutilus L.) belonging to the genus Dactylogyrus. The position of each individual parasite of the nine Dactylogyrus species was recorded. Niche breadth and niche overlap of parasite species were estimated. Comparative methods, which take into account phylogenetic information of the analysed species, were used. We reconstructed a phylogeny of the nine Dactylogyrus species based on morphological characters. We used the 'aggregation model of co-existence' in the model to test if species co-existence is facilitated when intraspecific aggregation exceeds interspecific aggregation. We observed a lack of negative correlation in abundance between pairs of parasites, and a negative correlation between niche size and parasite aggregation, for both intraspecific and interspecific aggregation. Our comparative analysis showed that parasite abundance is positively correlated with niche breadth. Then parasite abundance, and not interactions between Dactylogyrus species, seems to be the most important factor determining niche size This result gives some support to niche segregation by specialisation. Niche size was negatively correlated with both intraspecific and interspecific aggregation. No relationship was found between an increase of interspecific aggregation with an increase of niche overlapping, which suggests that competition may play little role. A lack of competition could be also confirmed by the lack of negative correlation in abundance between species pairs. A parsimony analysis of the evolution of gill distribution indicates a change in one parameter of the niche (arch, segment and/or area) at each branching event.