Effect of competitor abundance on feeding territoriality in a grazing fish,the ayuPlecoglossus altivelis

The grazing fish, ayu,Plecoglossus altivelis Temminck & Schlegel, establishes feeding territorialiry during the young stage. The population density fluctuates from year to year by more than a hundredfold, but the determinant of territory size is less well known. The feeding territoriality of ayu was examined under simulated habitat conditions where fish density was manipulated and food resources were renewable. Fish competed for algae attached to the substrata and were divided into residents with territories, and floaters without territories. By experimental alteration of fish density the number of residents increased with density and rerritory size decreased with density. Floaters intruded into territories in a school to feed on algae, which induced overt aggression of the resident and reduced the productivity of algae growing there. Both the intruding frequency of floaters over territorial areas and their feeding pressure on algae increased at higher floater density. Floaters functioned to shift cost-benefit relationships for various territory sizes. They acted as food competitors to restrict territory size below a maximum through competitive interference. Although the growth rate of residents was inversely related to fish density, residents grew faster than floaters in each group. Under a given set of competitor abundances, economic defensibility determined territory size.     

Depth-related differences in territory size and defense in the herbivorous cichlid,neolamprologus moorii, in lake tanganyika

Neolamprologus moorii, a small herbivorous cichlid of Lake Tanganyika, defends its territory against food competitors. Territory size and territorial defense were compared between two sites at different depths. In the shallow site, more fishes intruded into the territories than in the deep site, owing to the higher density fish population in the former. The territory holders in the shallow site attacked intruders more often, although the ratio of attacks to number of intruders was similar to that of the deep site. The territories in the deep site were larger and contained a greater amount of algae than the shallow site. The larger territory size in the deep site may have resulted from the lower cost of territory defense due to the lower competitor density and from the need to maintain a greater amount of food resources to compensate for lower algal productivity.     

Allometric influence on phenotypic variation in the Song Sparrow (Melospiza melodia)

Song Sparrow ( Melospiza melodia ) populations found along the Pacific Coast of North America, from Baja California to the islands off the coast of Alaska, exhibit extensive morphological variation. With a multivariate analysis of size and shape, I describe a portion of this pattern and examine how it could be maintained despite gene flow among the populations. Because shape differences fall along geographic barriers, I suggest that similarities among Song Sparrow populations in multivariate shape reflect their pattern of genetic relatedness. A general pattern of Song Sparrow post-nestling growth allometry has been discovered: bill characteristics are positively allometric and all other characteristics are negatively allometric. In contrast to shape, multivariate patterns of body size variation do not correspond to geographic relationships. In combination with evidence of Song Sparrow phenotypic plasticity, it is proposed that multivariate body size is an environmentally plastic trait and that specific traits exhibit levels of phenotypic plasticity in proportion to their rate of growth with respect to body size. In this way local environmental factors which alter body size may change an entire suite of allometrically related traits and thus create striking patterns of morphological variation.     

Intraspecific and interspecific competition induces density‐dependent habitat niche shifts in an endangered steppe bird

Interspecific competition is a dominant force in animal communities that induces niche shifts in ecological and evolutionary time. If competition occurs, niche expansion can be expected when the competitor disappears because resources previously inaccessible due to competitive constraints can then be exploited (i.e., ecological release). Here, we aimed to determine the potential effects of interspecific competition between the little bustard (Tetrax tetrax) and the great bustard (Otis tarda) using a multidimensional niche approach with habitat distribution data. We explored whether the degree of niche overlap between the species was a density‐dependent function of interspecific competition. We then looked for evidences of ecological release by comparing measures of niche breadth and position of the little bustard between allopatric and sympatric situations. Furthermore, we evaluated whether niche shifts could depend not only on the presence of great bustard but also on the density of little and great bustards. The habitat niches of these bustard species partially overlapped when co‐occurring, but we found no relationship between degree of overlap and great bustard density. In the presence of the competitor, little bustard's niche was displaced toward increased use of the species' primary habitat. Little bustard's niche breadth decreased proportionally with great bustard density in sympatric sites, in consistence with theory. Overall, our results suggest that density‐dependent variation in little bustard's niche is the outcome of interspecific competition with the great bustard. The use of computational tools like kernel density estimators to obtain multidimensional niches should bring novel insights on how species' ecological niches behave under the effects of interspecific competition in ecological communities.     

Territory tenure in song sparrows is related to song sharing with neighbours, but not to repertoire size

Song repertoires may be a product of sexual selection and several studies have reported correlations of repertoire size and reproductive success in male songbirds. This hypothesis and the reported correlations, however, are not sufficient to explain the observation that most species have small song repertoire sizes (usually fewer than 10, often fewer than five song types). We examined a second important aspect of a male's song repertoire, the extent to which he shares songs with his neighbours. Song sharing has not been measured in previous studies and it may be partially confounded with repertoire size. We hypothesized that in song sparrows, Melospiza melodia, song sharing rather than repertoire size per se is crucial for male territorial success. Our longitudinal study of 45 song sparrows followed from their first year on territory showed that the number of songs a bird shares with his neighbourhood group is a better predictor of lifetime territory tenure than is his repertoire size. We also found that song sharing increases with repertoire size up to but not beyond eight to nine song types, which are the most common repertoire sizes in the population (range in our sample 5-13). This partial confound of song sharing and repertoire size may account for some earlier findings of territory tenure-repertoire size correlations in this species and other species having small- or medium-sized repertoires. Copyright 2000 The Association for the Study of Animal Behaviour.     

An energy-based model of optimal feeding-territory size

An energy-based model of feeding territoriality is described. The model predicts an optimal territory size where the territory holder's net energy intake is maximized, on a daily or seasonal time scale. Simulated effects on optimal territory size of animal size, food availability, and competitor density are in general agreement with observed relationships in a wide variety of animals. When salmonid data are used to estimate the model parameter values, predicted territory sizes are similar to those actually observed. When emigration is allowed, the model predicts that territoriality, through individual selection alone, can regulate population size, but that this regulation breaks down when initial densities exceed some threshold value. Sensitivity analysis shows optimal territory size to be most affected by those parameters influencing food intake and energy expenditure. Some alternative criteria for optimization are also discussed. An animal maximizing net foraging efficiency has a smaller territory than one maximizing net energy, but the effects of animal size, food availability, and competitor density on territory size are the same in either case.     

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.     

Diet niche relationships among North American grassland and shrubsteppe birds

Summary We consider the dietary relationships of the numerically dominant breeding bird species in four North American grassland/shrubsteppe habitats, sampled over 2–3 consecutive years. Overall, the diets of these species contained primarily insects: orthopterans comprised 29% of the diet biomass, coleopterans 24%, and lepidopteran larvae 23%, while seeds contributed 15% of the average diet. These diets varied substantially, however, and we evaluated several aspects of this variation. Intersexual differences in diets within a species were few, despite the occurrence of significant sexual size dimorphism in several species. For many species, however, there were substantial shifts in dietary composition between years at a given location; overall, the average between-year similarity of species' dietary composition was 70%. Different species exhibited rather different diet patterns. Horned Larks were relatively omnivorous, had broad diet composition niches, and varied considerably in diets between different locations. Meadowlarks were also broad-niched and geographically variable in their diets, but were the most highly carnivorous of the species we considered. Dietary niche breadths of Grasshopper Sparrows were intermediate, but diet composition was rather stable, both between years and between locations. Chestnut-collared Longspurs exhibited narrow diet niches, but substantial annual variation: each year this species apparently exploited a different but limited set of prey types rather heavily. Larger avian predators generally consumed a broader array of functional groups of prey, but did not differ in the taxonomic variety of their diets from small birds. Variation in diet composition between individuals within local populations was considerable; in most species, an individual contained on the average 30–40% of the prey taxa represented in entire population smaples.Patterns of dietary overlap among species were quite inconsistent from year to year at most locations, although at the shrubsteppe site overlap among all species present was consistently quite high. Relatively few cooccurring species pairs exhibited low diet overlap. The degree of diet niche overlap was unrelated to body size differences of the birds, despite as much as six-fold differences in weight among some coexisting species. Relationships of the bird species on another dimension of the trophic niche, prey size, also differed substantially between sites and years. The ranking of co-occurring species by the mean sizes of the prey they consumed generally did not parallel their rankings by body sizes, and in some cases the smallest and the largest species present ate prey of similar sizes. At the shrubsteppe site, all the breeding species exhibited quite similar frequency distributions of prey sizes in their diets.As species number and diversity increased in the breeding avifaunas, diet niche breadths generally decreased, species packing by prey size decreased, and diet composition niche overlap remained relatively unchanged. These trends are in at least partial agreement with predictions of diffuse competition theory, but the patterns were derived from broad inter-site comparisons of overall site averages, and the relationships generally did not hold within local assemblages of species. In general, our attempts to match values of dietary niche features with site characteristics failed to demonstrate close agreement with the predictions of prevailing ecological theory based upon assumptions of resource limitation and competition. Instead, our findings seem generally most consistent with the suggestion that food is not normally limiting to bird populations in these systems, and individuals and populations are exploiting the food resources in an opportunistic fashion, which leads to considerable individual, between-year, and between-location variation in diet compositions and interspecific overlaps.Our attempts to discern clear relationships that accord with theoretical expectations in these avian assemblages are thwarted by our lack of detailed information on the resource base and by the lack of clear tests that will separate alternative hypotheses of community organization and structuring. We suggest that these complications may compromise the findings of many community studies.     

Intraspecific competition drives multiple species resource polymorphism in fish communities

It has been hypothesized that inter-specific competition will reduce species niche utilization and drive morphological evolution in character displacement. In the absence of a competitor, intra-specific competition may favor an expansion of the species niche and drive morphological evolution in character release. Despite of this theoretical framework, we sometimes find potential competitor species using the same niche range without any partitioning in niche. We used a database on test fishing in Sweden to evaluate the factors (inter- and intraspecific competition, predation, and abiotic factors) that could influence habitat choice of two competitor species. The pattern from the database shows that the occurrence of perch and roach occupying both littoral and pelagic habitats of lakes in Sweden is a general phenomenon. Furthermore, the results from the database suggest that this pattern is due to intra-specific competition rather than inter-specific competition or predation. In a field study, we estimated the morphological variation in perch and roach and found that, individuals of both species caught in the littoral zone were more deeper bodied compared to individuals caught in the pelagic zone. Pelagic perch fed more on zooplankton compared to littoral perch, independent of size, whereas the littoral perch had more macroinvertebrates and fish in their diet. Pelagic roach fed more on zooplankton compared to littoral roach, whereas littoral individuals fed more on plant material. Furthermore, we sampled littoral and pelagic fish from another lake to evaluate the generality of our first results and found the same habitat associated morphology in both perch and roach. The results show a consistent multi-species morphological separation in the littoral and pelagic habitats. This study suggests that intra-specific competition is possibly more important than inter-specific competition for the morphological pattern in the perch-roach system.     

An experimental analysis of territory size in juvenile steelhead trout

I experimentally manipulated levels of food abundance and density of competitors to determine how these factors influence the territory size of juvenile steelhead trout, Oncorhynchus mykiss. Steelhead trout were held in artificial stream channels and I followed cohorts that were fed at one of three levels of food abundance and stocked at one of three levels of fish density. By measuring territory size over a 2-month period, while the fish were growing, I was also able to assess the effects of body size in determining the size of a territory. Defended and foraging areas were similar in absolute size, but the frequency of space use was different for defence than for foraging. As predicted, territory size decreased with increasing levels of food abundance and increased with decreasing levels of fish density. In addition, territory size increased with increasing body size even after controlling the effects of food abundance and competitor density. In comparison to previous studies, territory size of steelhead trout changed more dramatically in response to changing levels of food and competitors. For territorial animals with indeterminate growth, territory size is not only adjusted as a trade-off between the costs and benefits of defence, but also with respect to body size due to increasing metabolic demands as individuals grow. Copyright 2000 The Association for the Study of Animal Behaviour.     

Body size, competitive interactions, and the local distribution of Triturus newts

1. Pairs of European Triturus newt species of similar size tend not to co-occur syntopically, suggesting that similarity in body size is associated with competitive interactions that prevent coexistence. I tested this hypothesis with an experiment involving larvae of four species in 675-L artificial ponds. 2. There were strong interactions between most species pairs. Even the small T. helveticus had a clear impact on the larger T. alpestris. Pairs of species with different body sizes did not interact less strongly. 3. A standard increase in competitor biomass (c. 2 g mass at metamorphosis) caused 42% lower expected survival from hatching to 1 year of age, regardless of whether the species were of similar or different size. In most cases this resulted from delayed metamorphosis, reduced size at emergence, and slightly lower larval survival. 4. A standard increase in competitor density (0.74 individuals m(-2)) caused a greater reduction in expected 1-year survival when the competitor was larger (18% decline) than when both species were of similar size (6% decline), primarily because the very large T. cristatus consumed the smallest species. 5. These findings suggest that species interactions during the larval stage cannot explain distribution patterns of same- and different-sized Triturus.     

An investigation of dominance in phytoplankton using the PROTECH model

1. Using a model (PROTECH-C) that simulates the simultaneous daily growth of eight phytoplankton species, the following hypotheses were tested: (i) for each given set of simulated conditions, the species with the most appropriate trait, as predicted by a functional group classification, should dominate the community; (ii) with removal of this dominant species, the next best-adapted species should dominate and should be from the same, or a close, functional group where available; (iii) a reduction in the inoculum size of the initially dominant species will not prevent its eventual dominance of the community.
2. For clearer insight into the mechanisms underlying these community processes, a functional group classification based upon species morphology has been used to produce a matrix analogous to Grime's CSR (C, competitor; S, stress tolerator; R, ruderal) paradigm. The effects upon this phytoplankton community of temperature, grazing, limiting light and nutrients over a simulated year were recorded.
3. The results supported all three hypotheses. It was found that, for a given selective constraint, functional traits provided excellent predictors of the dominant types. Also, under conditions of resource competition, the number of functional groups represented decreased. Competition was greatest within functional groups where niche overlap was high, but one species was always clearly the strongest competitor, i.e. its superiority over its nearest functional competitor was regularly expressed even when the difference in inoculum size was great (1000-fold). These conclusions emphasized the power that trait selection can have in the shaping of communities.     

The sterile release method for population control with interspecific competition

A differential equations model of competing species with the release of sterile individuals of one of the species is examined. The system is found to have two positive steady states for certain parameter values; one of these is stable and the other is unstable. The system is quite resilient around the stable steady state. The release of steriles causes the nontarget species to increase in numbers. There exists a value of the release rate above which the pest species collapses to extinction. The existence of the competitor species assists the sterile release program since the pest equilibrium at any release rate is lower with the competitor species present than without it; in addition the release rate required to cause collapse of the pest species is lower with the competitor species present than without it. The effects of the parameters on the ease of eradication were examined. It was found that the ideal competitor species should have a high rate of increase, a large carrying capacity and exert strong competitive depression on the pests. The ideal pest would have a low rate of increase, a low carrying capacity and be a poor competitor.     

Population size dependence, competitive coexistence and habitat destruction

1. Spatial dynamics can lead to coexistence of competing species even with strong asymmetric competition under the assumption that the inferior competitor is a better colonizer given equal rates of extinction. Patterns of habitat fragmentation may alter competitive coexistence under this assumption.
2. Numerical models were developed to test for the previously ignored effect of population size on competitive exclusion and on extinction rates for coexistence of competing species. These models neglect spatial arrangement.
3. Cellular automata were developed to test the effect of population size on competitive coexistence of two species, given that the inferior competitor is a better colonizer. The cellular automata in the present study were stochastic in that they were based upon colonization and extinction probabilities rather than deterministic rules.
4. The effect of population size on competitive exclusion at the local scale was found to have little consequence for the coexistence of competitors at the metapopulation (or landscape) scale. In contrast, population size effects on extinction at the local scale led to much reduced landscape scale coexistence compared to simulations not including localized population size effects on extinction, especially in the cellular automata models. Spatially explicit dynamics of the cellular automata vs. deterministic rates of the numerical model resulted in decreased survival of both species. One important finding is that superior competitors that are widespread can become extinct before less common inferior competitors because of limited colonization.
5. These results suggest that population size–extinction relationships may play a large role in competitive coexistence. These results and differences are used in a model structure to help reconcile previous spatially explicit studies which provided apparently different results concerning coexistence of competing species.     

Green woodhoopoe Phoeniculus purpureus territories remain stable despite group-size fluctuations

Cooperatively breeding groups may be constrained in size by the territory available to them, or territories may be expanded to accommodate extra group members. Here, we show that there was no relationship between the number of adult green woodhoopoes Phoeniculus purpureus in a group and the size of its territory. Furthermore, territories were remarkably stable between seasons, with no significant changes in area, despite fluctuating group sizes. These results suggest that food was not limiting at the group sizes found in this study: sufficient resources were available within existing territories for groups that were expanding in size. Following an increase in group membership, a larger proportion of the available area was utilised. Groups also used a larger area in the non-breeding season compared to when breeding: in the latter instance, foraging was concentrated in the vicinity of the nest.     

Trophic ecology of adult male Odonata. I. Dietary niche metrics by foraging guild,species, body size,and location

1. Information on the dietary niches of adult odonates is sparse, as they are highly mobile and evasive animals, which makes them difficult to observe in their natural habitat. Moreover, there is a lack of knowledge on how varying behavioural traits of odonates relate to phenomena like niche partitioning. 2. This study investigated niche partitioning amongst odonate species, foraging guilds and size classes in a riverine system in the Eastern Cape province of South Africa. A combination of stable isotope and fatty acid‐based niches was used to infer odonate feeding. 3. Both fatty acid and stable isotope‐based niches showed that there was niche separation amongst odonates that forage in flight (fliers) and those that forage from a perch (perchers), amongst odonates of different size classes (damselflies, medium‐ and large‐sized dragonflies), and amongst species, although varying levels of niche overlap were observed in each case. 4. Niche sizes of odonates varied between an upstream and a downstream site. Generally greater niche overlap was recorded at the narrow upstream site (associated with low insect emergence rates) than the wider downstream site (associated with high insect emergence rates), indicating that a greater degree of resource sharing occurred at the upstream site where aquatic food was less abundant. 5. The findings of this study suggest that dietary niches of odonates can be influenced by foraging guild, body size, and/or environmental conditions, and additional study in a variety of regions is recommended to determine the greater applicability of these findings.     

Ecological Similarity and Coexistence of Epiphytic Ice-Nucleating (Ice) Pseudomonas syringae Strains and a Non-Ice-Nucleating (Ice) Biological Control Agent

De Wit replacement series were used to study competitive interactions between epiphytic IcePseudomonas syringae strains and the biological frost control agents IceP. syringae TLP2del1 and Pseudomonas fluorescens A506. Mixtures containing two strains in different proportions but at a constant total population size were inoculated onto potato leaves. The population sizes of each strain and the total population size were determined when the community had reached equilibrium. A near-isogenic P. syringae strain pair exhibited an interaction similar to that expected for strains competing equally for limiting environmental resources. Replacement series with nonisogenic Ice and IceP. syringae strain pairs suggested that these strains competed for limiting resources according to their relative competitive abilities. There was no evidence of any niche differentiation between the IceP. syringae strains and the IceP. syringae strain. The growth responses of epiphytes following addition of nutrients to the phyllosphere indicated that the epiphytic P. syringae populations were nutrient limited and that, under growth chamber conditions, the populations were more limited by the availability of carbon than by the availability of nitrogen. Determination of in vitro carbon source utilization profiles provided further evidence for the lack of niche differentiation between the Ice and the IceP. syringae strains. Niche overlap indices calculated for the IceP. syringae strains with respect to IceP. syringae TLP2del1 were uniformly high, indicating ecological similarity, and were consistent with the observed low level of coexistence. The biological frost control agent P. fluorescens A506 replaced P. syringae. This was correlated with a high degree of niche overlap between these species.     

Song‐type sharing in a population of Song Sparrows in the eastern United States

Male Song Sparrows (Melospiza melodia) sometimes interact with neighboring territory owners by song‐type matching or repertoire matching. In some Song Sparrow populations, levels of song sharing are high and most neighbors can interact by matching, but levels of song sharing are much lower in other populations, limiting the degree to which males can match their neighbors. One explanation for variation in sharing levels is that the importance of song‐type and repertoire matching, and therefore the extent of song sharing, varies geographically in North America, being greater in western populations than eastern populations. However, to date, two studies of eastern populations have provided conflicting evidence concerning levels of song sharing by Song Sparrows. Thus, we measured sharing of whole songs and introductory phrases of songs between males with adjacent territories in another population of Song Sparrows in the eastern United States (North Carolina), near the eastern and southern limits of the species’ breeding range. Males (N = 17) in our study shared an average of only 8.7% of their song types with neighbors, and more than half of neighbor pairs shared no whole songs. Sharing of introductory phrases was more common (mean = 22.8%). The level of whole song sharing in our study is the second lowest yet reported for any Song Sparrow population, supporting the hypothesis that sharing is generally lower in eastern than in western populations.     

Determining habitat quality for the cooperatively breeding Rufous Treecreeper,Climacteris rufa

Abstract The theory of habitat selection predicts that organisms should use habitat that maximizes their fitness. The cooperatively breeding Rufous Treecreeper, Climacteris rufa, exhibits non‐random habitat use at a number of spatial scales. By assessing correlative relationships between nest‐site use and nest success, and territory use and reproductive success and survival, it was determined whether non‐random use of habitat yields fitness benefits. It was also determined whether breeding group size contributed significantly to fitness once differences in territory quality had been considered. Structural characteristics of nest sites that were positively correlated with the probability of a site being used had no relationship with nest success. This result probably reflects the relatively unrestricted access to an abundance of suitable nest sites in the study area. Habitat traits that predicted territory use by treecreepers were positively correlated with a number of fitness measures. They were also positively correlated with breeding group size and provisioning rate to nestlings, which in turn were correlated with fitness. However, group size was not significantly related to any measure of fitness, except primary male survival, once territory quality had been considered. The quality of territories occupied by Rufous Treecreepers appeared to be a significant factor for breeding group fitness.     

Cooperative nest‐defence behaviour and territory quality in a resident and socially monogamous passerine

Nest defence is a fundamental aspect of parental care in secondary cavity‐nesting birds, and predation or competition for nesting sites can involve different defensive behaviours. Because habitat quality determines breeding success, we were interested in whether breeding pairs of the Eurasian nuthatch, Sitta europaea, established in more favourable environment also manifest higher probability of cooperative behaviour during their nest‐site defence. To explore this relationship, we quantified behavioural displays of both parents and analysed activity budget ethogram data from simulated territorial intrusions performed in the chick‐feeding phase with one conspecific and two different heterospecific stimuli (dummies of nuthatch, starling and woodpecker). We found that paired individuals shared their roles during nest‐site defence to a considerable extent. Males had a significantly higher number of attacks on intruders than females, and females performed more threat displays and controls of the brood than males. Multinomial analysis of the cooperative behaviour suggested that pairs in a high‐quality territory had higher probability of reciprocal substitution of different roles towards a balance between attacks, threat displays and nest controls. Contrary to this, pairs in a low‐quality territory had less likely pairwise combinations of simultaneous behavioural states that are associated with effective nest‐site defence. The difference in response probability according to territory quality was, however, highly variable in view of the stimulus that was used in simulated territorial intrusion. Because individual roles and the complex behavioural repertoire of pairs altered in response to territory quality and potential nest‐site competitor or brood predator, our results suggest that the cooperative nest‐defence behaviour could be linked to the breeding success of this year‐round territorial species living in a heterogeneous forest habitat.