Too close for comfort: spatial patterns in acorn barnacle populations

Spatial patterns in aggregations form as a result of the interplay between costs and benefits experienced by individuals. Such self-organisation of aggregations can be explained using a zonal model in which a short-range zone of repulsion and longer-range zone of attraction surrounding individuals leads to emergent pattern properties. The signal of these processes can be detected using spatial pattern analyses. Furthermore, in sessile organisms, post-settlement mortality reveals the relative costs and benefits of positions within the aggregation. Acorn barnacles are known to require contact with conspecifics for reproduction and are therefore believed to aggregate for this purpose; isolated individuals may also be more susceptible to abiotic stress and predation. At short distances, however, competition for space and resources is likely to occur. In this study spatial patterns of barnacles (Semibalanus balanoides L.) were analysed using pair-correlation functions. Individuals were dispersed at distances below 0.30 cm, but peak relative density occurred at a distance of 0.36 cm from conspecifics. This is much closer than required for reproductive access, implying a strong aggregative drive, up to the point of physical contact with neighbours. Nevertheless, analysis of dead barnacles illustrated that such proximity carries a cost as barnacles with many neighbours were more likely to have died. The inferences obtained from these patterns are that barnacles aggregate as closely as they can, and that local neighbourhood competition is a powerful determinant of mortality. These processes give rise to the observed pattern properties.     

Social effects on foraging behavior and success depend on local environmental conditions

In social groups, individuals' dominance rank, social bonds, and kinship with other group members have been shown to influence their foraging behavior. However, there is growing evidence that the particular effects of these social traits may also depend on local environmental conditions. We investigated this by comparing the foraging behavior of wild chacma baboons, Papio ursinus, under natural conditions and in a field experiment where food was spatially clumped. Data were collected from 55 animals across two troops over a 5‐month period, including over 900 agonistic foraging interactions and over 600 food patch visits in each condition. In both conditions, low‐ranked individuals received more agonism, but this only translated into reduced foraging performances for low‐ranked individuals in the high‐competition experimental conditions. Our results suggest one possible reason for this pattern may be low‐ranked individuals strategically investing social effort to negotiate foraging tolerance, but the rank‐offsetting effect of this investment being overwhelmed in the higher‐competition experimental environment. Our results also suggest that individuals may use imbalances in their social bonds to negotiate tolerance from others under a wider range of environmental conditions, but utilize the overall strength of their social bonds in more extreme environments where feeding competition is more intense. These findings highlight that behavioral tactics such as the strategic investment of social effort may allow foragers to mitigate the costs of low rank, but that the effectiveness of these tactics is likely to be limited in certain environments.     

Agonistic behaviour over food and perch space in male Quelea quelea

A measure commonly used in previous studies of fighting in Quelea quelea (the Dominance Index) was found to be unsuitable for the ranking of animals in small groups. It is argued as preferable to study interactions between pairs of individuals within groups. Rank relationships between individuals were found to be similar in competition over food and perch space in two series of observations 8 weeks apart. Fights within the groups did not show an overall decrease with time since group formation but were related to flight activity (between perches) and time spent at the food hopper. Success in competition at the food hopper gave priority of access to the food, but success in competition over perching space did not lead to those birds winning more encounters sitting in a central position within the resting groups. These results are discussed in relation to possible advantages to individuals and to the ecology of Quelea.     

A minimalist approach to the effects of density-dependent competition on insect life-history traits

Abstract  1. Due to its effects on the phenotypic and genotypic expression of life-history traits, density-dependent competition is an important factor regulating the growth of populations. Specifically for insects, density-dependent competition among juveniles is often associated with increased juvenile mortality, delayed maturity, and reduced adult size.
2. The aim of the work reported here was to test whether the established phenotypic effects of density-dependent competition on life-history traits could be reproduced in an experimental design requiring a minimal number of individuals. Larvae of the mosquito Aedes aegypti were reared at densities of one, two, or three individuals per standard Drosophila vial and in six different conditions of larval food availability. This design required relatively few individuals per independent replicate and included a control treatment where individuals reared at a density of one larva per vial experienced no density-dependent interactions with other larvae.
3. Increased larval densities or reduced food availability led to increased larval mortality, delayed pupation, and the emergence of smaller adults that starved to death in a shorter time (indicating emergence with fewer nutritional reserves).
4. Female mosquitoes were relatively larger than males (as measured by wing length) but males tended to survive for longer. These differences increased as larval food availability increased, indicating the relative importance of these two traits for the fitness of each sex. The role of nutritional reserves for the reproductive success of males was highlighted in particular.
5. This minimalist approach may provide a useful model for investigating the effects of density-dependent competition on insect life-history traits.     

A test of the Menge-Sutherland model of community organization in a tropical rocky intertidal food web

Summary Menge and Sutherland (1976) predicted that in physically benign habitats: (1) community structure will be most strongly affected be predation, (2) the effect of predation will increase with a decrease in trophic position in the food web, (3) trophically intermediate species will be influenced by both predation and competition, and (4) competition will occur among prey species which successfully escape consumers. These predictions were tested in a tropical rocky intertidal community on the Pacific coast of Panama. The most abundant mobile species included fishes and crabs, which occupied the top trophic level, and predaceous gastropods and herbivorous molluscs, which occupied intermediate trophic levels. The most abundant sessile organisms were encrusting algae, foliose algae, barnacles, and bivalves. Diets were broad and overlapping, and 30.3% of the consumers were omnivorous. Each consumer group had strong effects on prey occurring at lower trophic levels: (1) Fishes and crabs reduced the abundance of predaceous snails, herbivorous molluscs, foliose algae, and sessile invertebrates. (2) Predaceous gastropods reduced the abundance of herbivorous molluscs and sessile invertebrates. (3) Herbivorous molluscs reduced the abundance of foliose algae and young stages of sessile invertebrates, and altered relative abundances of the encrusting algae. The encrusting algae, although normally the dominant space occupiers, proved to be inferior competitors for space with other sessile organisms when consumers were experimentally excluded. However, the crusts escaped consumers by virtue of superior anti-herbivore defenses and competed for space despite intense grazing. Observations do not support the hypothesis that the trophically intermediate species compete. Hence, with the exception of this last observation, the predictions of the Menge and Sutherland model were supported. Although further work is needed to evaluate other predictions of the model in this community, evidence from this study joins an increasing body of knowledge supporting the model. Contradictory evidence also exists, however, indicating that aspects of the model require revision.     

Effects of early recruits on temperate sessile marine community composition depend on other species recruiting at the time

In many environments recruitment of dispersive propagules (e.g. seeds, spores and larvae) can vary from situations when particular taxa recruit in relative isolation to times when they recruit simultaneously with other, functionally quite different taxa. Differences in the identity and density of recruiting taxa can have important consequences on community structure, but it is still not clear how the effects of individual taxa on communities are modified when they recruit together with other species. Using an experimental approach we compared early development of a temperate marine sessile community after the recruitment of mixtures of botryllid ascidians and barnacles to that when barnacles or botryllid ascidians recruited alone. Communities exposed to recruitment of botryllid ascidians in isolation differed from those that received barnacles, a mixture of botryllids and barnacles or no recruitment in 2-week-old communities. These early differences were driven by higher abundances of the species that were present as initial recruits in experimental treatments. After 2 months communities also differed between barnacle and mixed recruitment treatments but not mixed and botryllid or botryllid and barnacle treatments. These differences were not directly due to differences in the abundances of our manipulated taxa but occurred because of two abundant arborescent bryozoans, Bugula dentata, which occupied more space in communities that initially received mixed recruitment than in those that received barnacle or no recruitment, and Zoobotryon verticillatum, which occupied more space in communities that initially received only barnacle recruitment than those that initially received botryllid or mixed recruitment. These effects did not persist, and communities did not differ after 6 months. These results suggest that, more generally, species may influence community dynamics differently when they recruit alongside other species than when they recruit in relative isolation.     

The influence of resident adults on recruitment: a comparison to settlement

For species recruiting into established sessile communities, the adult colonies and individuals already present form a significant part of the environment and have the potential to alter both larval settlement rates and post-settlement mortality. Settlement rates can be reduced by predation on larvae, by the removal or addition of substratum space, or by stimulation or prohibition of larvae from settling on adjacent substratum. Once attached, the recruiting individual can still be influenced by predation or overgrowth by residents, by the added physical structure for firmer attachment, or by being camouflaged from motile predators. To examine those processes by which residents affect recruitment we exposed experimental substrata with three densities of adults of a single species at a site in eastern Long Island Sound, USA for a 1-wk period. Seven different species of common invertebrates were used in nine separate experiments. The major effect of most resident species was the usurpation of space and the restricting of recruitment to adjacent unoccupied areas. This was particularly true for resident ascidians and bryozoans, but less so for barnacles and oysters. In fact several species recruited in higher densities on or next to oysters and barnacles. Comparison to 1-day settlement experiments indicated that the encrusting ascidian species Diplosoma and possibly Botryllus reduced recruitment relative to settlement, probably by overgrowing newly-settled individuals. However, in the presence of most resident species, recruitment patterns were not greatly different from settlement patterns, indicating that the effects of the attached community on recruitment may result from influences on settlement.     

The Effect of Hunger on Shoal Choice in Golden Shiners (Pisces: Cyprinidae,Notemigonus crysoleucas)

When an animal has a choice of joining one group over another, its decision may depend on its relative vulnerabilities to predation and starvation. For example, a well-fed animal may choose a large group of individuals with body size matching its own because this gives good protection against predators, but a hungry animal may prefer smaller groups made up of smaller individuals because this decreases food competition. To test this idea, a choice between various shoals was given to golden shiners, Notemigonus crysoleucas, that were either well fed or deprived of food for 48 h. In a choice of 10 vs. 3 shoalmates, both well-fed and hungry shiners spent more time near the shoal of 10. In a choice of 20 vs. 3 shoalmates, both well-fed and hungry shiners again preferred the larger shoal, but in one replicate this preference was significantly weaker in the hungry fish. This reduced preference did not appear to be an artefact of increased mobility by hungry fish searching for food. In a choice between shoals of small vs. large conspecifics, small well-fed shiners, small hungry shiners, and large well-fed shiners preferred shoalmates with body size matching their own, but large hungry shiners preferred smaller individuals. These results are consistent with the hypothesis that hungry fish sacrifice safety from predation in their shoaling behaviour (by avoiding larger groups to a certain extent and by risking the oddity effect) so as to decrease food competition.     

Resource partitioning between unionid mussels in a Swedish lake outlet

Partitioning of food and space resources of two dominating unionid mussel species, Anodonta piscinalis and Unio pictorum , was investigated in a South Swedish lake outlet. Differences in diurnal activities were not considered to be a mechanism for niche segregation. The two species ingested particles of almost identical size distribution indicating a high overlap in food. Mussel distribution was correlated to environmental variables including substrate properties, water depth, current velocity, and distance from the lake, the latter being a relative measure of food quality and quantity, U. pictorum dominated near the lake and in the centre of the river while A. piscinalis had its peak density ca. 500 m below the lake and was relatively more abundant along the river margins. At maximum mussel densities, which were found near the lake, a separation in space was more obvious than downstream, implying possible interspecific interactions.     

Multiple foundation species shape benthic habitat islands

Pattern generation by foundation species (FS) is a primary structuring agent in marine and terrestrial communities. Prior research, focused on single-species or guild-dominated habitats, stressed the role of facilitation in maintaining community structure. However, many habitats are developed by multiple FS from different guilds. Competition between these FS may provide an additional agent potentially responsible for spatial and temporal patterns. In the White Sea, epibenthic patches formed by barnacles (Balanus crenatus) and solitary ascidians (mainly Styela spp. and Molgula spp.) on small stones and empty bivalve shells (mainly Serripes groenlandicus) produce microhabitats for different sessile taxa. We hypothesized that: (1) several FS would provide habitats for most of other species in the community; (2) different FS promote different assemblages of sessile organisms; (3) the interplay of facilitation and competition best explains observed patterns of abundance and demography in FS; and (4) these interactions shape the whole community, increasing the diversity compared to less heterogeneous patches constituted by single FS. We examined 459 patches and the results generally supported this hypothesis. The number of FS in a patch positively affected species diversity. Most sessile species (72% of individuals) resided on barnacles, ascidians and red algae, except barnacles that dominated the primary substrate. The size structure of barnacles (live individuals and empty shells) and ascidians were interrelated, suggesting long-term patch dynamics whereby ascidians regularly replace barnacles. Following this replacement, we expect consequent changes to the entire dependent assemblage. Evidence for these changes exists in the spatial pattern: most sessile and motile taxa demonstrated significant associations with either FS. Our results indicate that the small-scale patterns observed in patches formed by multiple FS are primarily generated by facilitation of dependent taxa by FS, and facilitation and competition between different FS. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Dedicated to E. A. Ninbourg, our late tutor.     

Mating group size and evolutionarily stable pattern of sexuality in barnacles

Barnacles, marine crustaceans, have various patterns of sexuality depending on species including simultaneous hermaphroditism, androdioecy (hermaphrodites and dwarf males), and dioecy (females and dwarf males). We develop a model that predicts the pattern of sexuality in barnacles by two key environmental factors: (i) food availability and (ii) the fraction of larvae that settle on the sea floor. Populations in the model consist of small individuals and large ones. We calculate the optimal resource allocation toward male function, female function and growth for small and large barnacles that maximizes each barnacle's lifetime reproductive success using dynamic programming. The pattern of sexuality is defined by the combination of the optimal resource allocations. In our model, the mating group size is a dependent variable and we found that sexuality pattern changes with the food availability through the mating group size: simultaneous hermaphroditism appears in food-rich environments, where the mating group size is large, protandric simultaneous hermaphroditism appears in intermediate food environments, where the mating group size also takes intermediate value, the other sexuality patterns, androdioecy, dioecy, and sex change are observed in food-poor environments, where the mating group size is small. Our model is the first one where small males can control their growth to large individuals, and hence has ability to explain a rich spectrum of sexual patterns found in barnacles.     

Does Feeding Competition Influence Tammar Wallaby Time Allocation?

Animals may aggregate to reduce predation risk, but this potentially incurs the cost of increased competition. We studied the degree to which competition for food influenced the time tammar wallabies (Macropus eugenii) allocate to foraging and vigilance by experimentally manipulating access to food, while holding other factors constant. Groups of six wallabies were observed when they had access to either one or six non‐depleting bins of supplemental food. Food availability had no effect on the time allocated to foraging, looking or affiliative interactions, and this was true whether individuals or groups were treated as the unit of analysis. However, wallabies engaged in substantially more aggressive acts in the high‐competition treatment. These results, when combined with other findings, suggest that the moderately social tammar wallaby receives an antipredator benefit by aggregating with conspecifics which is not reduced significantly by foraging competition.     

Modeling the succession of barnacles and mussels on a sandstone reef in Pernambuco State, Brazil

Settlement and recruitment rates, biological interactions, environmental parameters, and larval supply are factors that influence community structure in areas where barnacles and mussels are the dominant fauna. This study modeled the succession populations of two barnacle species, Tetraclita stalactifera (Lamarck, 1818) and Chthamalus bisinuatus (Pilsbry, 1916), and a bivalve, Brachidontes solisianus (d’Orbigny, 1846), on a sandstone reef at Piedade Beach, Pernambuco State, Brazil. The different stages of the life cycles of these invertebrates are represented by adult and immature forms in this model and are simulated as numbers of individuals per 100 cm². Recruitment rates are represented as forcing functions based on field recruitment experiments with time-steps of 1 day. In addition to intraspecific competition, the model takes into account interspecific competition based on reciprocal effects and carrying capacities. Field data from field succession experiments were used to calibrate the model. Recruitment and succession data were collected from random 10 × 10-cm plots within a 1-m transect on a Brachidontes bed from April to December 2007. Correlation coefficients between field data and simulation results were statistically significant for both populations; sensitivity analysis showed that increases and decreases in invertebrate population densities were always smaller than 10% compared to baseline model values. Patch closure by mussels population growth was one of the main succession processes observed during the rainy season. However, barnacles were the dominant species at the end of both field and simulation succession experiments, with an average density of close to 100 individuals per 100 cm². Cirripedia individuals may inhibit bivalve re-colonization by settling over shells and more efficiently occupying free space in addition to having higher recruitment rates than bivalves during the dry season.     

Intra-sexual competition alters the relationship between testosterone and ornament expression in a wild territorial bird

In a reliable signalling system, individual quality is expected to mediate the costs associated with ornamental displays, with relatively lower costs being paid by individuals of higher quality. These relative costs should depend not only on individual quality, but also on levels of intra-sexual competition. We explored the current and delayed effects that testosterone implants have on bird ornamentation in populations with contrasted population densities, as a proxy for intra-sexual competition. In a replicated experiment, we manipulated testosterone in 196 yearling male red grouse Lagopus lagopus scoticus in autumn in populations of high and low levels of intra-sexual competition. Males were assigned to one of three exogenous testosterone (T) treatments: empty implants (T0), small T implants (T1) or larger T implants (T2). We monitored subsequent changes in testosterone levels, ornament size and carotenoid-based colouration, carotenoid levels and body condition from autumn to spring. Testosterone implants increased testosterone levels, comb redness and comb size, and decreased body condition but these effects depended on levels of intra-sexual competition. Specifically, T2-implanted birds increased testosterone levels and comb size more, and reduced body condition more, in populations where intra-sexual competition was low. In the following spring, testosterone levels of T2-treated birds kept increasing in populations where intra-sexual competition was high but not in populations where intra-sexual competition was low. Our results highlight that levels of intra-sexual competition alter the relationship between testosterone levels and ornament expression, influencing their condition-dependence; they also indicate that the outcome of standard hormone manipulation conducted in free-living animals vary depending on the population context.     

Resource defense and monopolization in a marked population of ruby‐throated hummingbirds (Archilochus colubris)

Resource defense behavior is often explained by the spatial and temporal distribution of resources. However, factors such as competition, habitat complexity, and individual space use may also affect the capacity of individuals to defend and monopolize resources. Yet, studies frequently focus on one or two factors, overlooking the complexity found in natural settings. Here, we addressed defense and monopolization of nectar feeders in a population of free‐ranging ruby‐throated hummingbirds marked with passive integrated transponder (PIT tags). Our study system consisted of a 44 ha systematic grid of 45 feeders equipped with PIT tag detectors recording every visit made at feeders. We modeled the number of visits by competitors (NVC) at feeders in response to space use by a focal individual potentially defending a feeder, number of competitors, nectar sucrose concentration, and habitat visibility. Individuals who were more concentrated at certain feeders on a given day and who were more stable in their use of the grid throughout the season gained higher exclusivity in the use of those feeders on that day, especially for males competing against males. The level of spatial concentration at feeders and its negative effect on NVC was, however, highly variable among individuals, suggesting a continuum in resource defense strategies. Although the apparent capacity to defend feeders was not affected by competition or nectar sucrose concentration, the level of monopolization decreased with increasing number of competitors and higher nectar quality. Defense was enhanced by visibility near feeders, but only in forested habitats. The reverse effect of visibility in open habitats was more difficult to interpret as it was probably confounded by perch availability, from which a bird can defend its feeder. Our study is among the first to quantify the joint use of food resource by overlapping individuals unconstrained in their use of space. Our results show the importance of accounting for variation in space use among individuals as it translated into varying levels of defense and monopolization of feeders regardless of food resource distribution.     

Feeding changes for three Sphoeroides species (Tetraodontiformes: Tetraodontidae) after Isidore hurricane impact in Carbonera Inlet, Southeastern Gulf of Mexico

The coexistence of ecologically similar species may occur because of resources distribution, such as prey and habitat type and segregation time, that minimizes the interspecific competition. The changes brought about by Hurricane Isidore in the distribution of food resources by three coexisting fish species of the family Tetraodontidae (Sphoeroides nephelus, S. spengleri and S testudineus), were analyzed at the Carbonera Inlet. Sphoeroides spp. based their food on benthic organisms; principally, they consume mussels (Brachidontes sp.), barnacles (Balanus sp.) and gastropods (Crepidula sp). Before hurricane impact, the three species share the available food resources in different proportions (bivalves, gastropods, barnacles and decapods), according to different strategies that enabled them to coexist and reduce interspecific competition. After the impact, the abundance of available prey decreased and the interespecific competition for food increased, leading to S. testudines and S. nephelus change their trophic spectrum (xiphosurans, amphipods, isopods and detritus) and displacing S. splengleri of the inlet. The distribution of food resources was conditioned by the abundance and diversity of prey, as well as the adaptive response of each species.     

Severe food stress has no detectable impact on developmental instability in Daphnia magna

Groups of individuals (three replicates of four treatment groups of 10 individuals) of Daphnia magna were exposed to nutritional stress in the forms of reduced food quantity and/or quality (phosphorus-limitation). Both types of stress led to reduced growth and reproduction, producing a gradient from surplus of high quality food to severe nutritional stress. There was no effect of nutritional stress on developmental instability measured either as fluctuating asymmetry or as phenotypic variability in body shape. Computer simulations confirmed that the lack of effect was not simply due to low statistical power. Low food quantity led to larger variability among individuals in overall body size, but this was most likely due to exploitative competition and competitive dominance of larger individuals. Low food quantity also led to increased occurrence of abnormal postabdominal spines (two spines appearing from one basis), but other analyses suggested that abnormal spines may actually represent an adaptive phenotypic response rather than a deviant phenotype caused by developmental instability. In conclusion, the results suggest that Daphnia magna are able to maintain high developmental stability even under extreme nutritional stress. This casts doubt on the utility of using any of the investigated measures of developmental instability as general indicators of environmental stress.     

Effect of feeding frequency on food intake and growth of Arctic charr, Salvelinus alpinus L.

Social interactions and dominance hierarchy effects are important factors governing rates of growth of Arctic charr, Salvelinus alpinus L. The effects of hierarchy were increased as access to food became more restricted, i.e. feeding frequency was reduced, but these effects could not be attributed to direct competition for food since fish were fed to satiation at each feeding period. The results suggest that, whilst some fish on the restricted feeding regime were able to maintain good rates of growth, feeding by the majority of the fish was inhibited by the presence of larger individuals. Due to the importance of these hierarchy effects it was not possible to demonstrate physiological adaptations in fish allowed infrequent access to food.     

Lack of disturbance as an explanation for the additive basal area phenomenon in a stratified Afrotemperate forest

Abstract Explanations for the additive basal area (BA) phenomenon in forests are frequently given in the context of stratification and the avoidance of competition for space or resources (e.g. light, nutrients, moisture). Thus, large individuals avoid competition for light by emerging above the canopy and, in so doing, the BA of emergent individuals is often ‘additive’ to that of the rest of the individuals in a stand. The additive BA phenomenon was evident in a stratified Afrotemperate forest and was not confined to the emergent stratum but occurred also within the canopy stratum where gymnosperm BA appeared to be additive. However, there was no evidence that stand BA was at carrying capacity (i.e. presumably neither space nor nutrient resources were limiting) and there was no statistical evidence of competitive effects. We argue that avoiding competition is an insufficient explanation for how biomass accumulates in stratified forests and suggest that local variation in disturbance regime and tree life‐history provide a more plausible and general explanation for current forest stand structure and dynamics. We suggest that the additive effect is a result of long‐lived individuals persisting in the prolonged absence of disturbance to canopy and emergent strata.