Arrival sequence and diet mediate interspecific competition in an ant community

The arrival sequence of organisms in a habitat and their diet are two factors that are thought to modulate animal performance, affect the outcome of behavioural interactions, and shape communities. In New Zealand, two species that seldom co-occur in field populations are Prolasius advenus and Monomorium antarcticum. Herein we tested the hypotheses that arrival sequence and diet influence the strength of interactions between these two species. These ant species presented asymmetric responses to arrival sequence and diet variations. When arriving first P. advenus displayed increased aggression and M. antarcticum a defensive reaction. Changes in carbohydrate and protein availability modulated colony activity rates of both species. Colonies of M. antarcticum fed on a high carbohydrate and low protein diet displayed higher activity rates than colonies fed on a low carbohydrate and high protein diet. In contrast, control colonies of P. advenus fed on a high carbohydrate and low protein diet displayed lower activity rates than colonies fed on a low carbohydrate and high protein diet. These results indicate that arrival sequence can modulate the agonistic reaction displayed by interacting species in situations of conflict. This work also demonstrates that species adjust activity rates in response to diet, but different species do so differently. Therefore, arrival sequence and diet could explain species mutually exclusive distribution patterns observed in nature.     

Temperature and starvation effects on food exploitation by Argentine ants and native ants in New Zealand

The abundance and distribution of an invasive species is influenced by its relative ability to find resources under a variety of conditions. We examined the exploitative ability of the Argentine ant (Linepithema humile (Mayr)), in comparison with two common New Zealand ant species Monomorium antarcticum (Fr. Smith) and Prolasius advenus (Hymenoptera: Formicidae) (Fr. Smith), using maze trials under different temperature and starvation regimes. Our results showed temperature significantly affected the mean time to discover food resources, but different species responded differently to changes in temperature. A change in temperature from 23°C to 13°C resulted in an approximately 8‐fold increase in the time to discover food for native P. advenus, but discovery times remained relatively similar for invasive Argentine ants. Starvation did not significantly influence the ability of species to find food. Argentine ants consistently located and recruited to food faster than the native species. We examined for variation in walking speed under the experimental conditions as a mechanism for our results. The results revealed Argentine ants and P. advenus to have similar walking speeds at each temperature‐starvation treatment and both were faster than M. antarcticum. However, Argentine ants had rates of turning or returning to the nest that were lower than the native species. This result suggests that Argentine ants show greater ‘exploratory willingness’ or ‘novelty seeking’ behaviour. Our results suggest that Argentine ants are able to discovery and exploit resources more efficiently than these native species under a wide spectrum of environmental and physiological conditions. Such relative efficiencies have likely contributed to the success of this invader.     

Limited niche differentiation within remarkable co‐occurrences of congeneric species: Monomorium ants in the Australian seasonal tropics

Niche theory predicts that few closely related species can co‐occur because such species tend to be ecologically similar and niche differentiation is required to avoid competitive exclusion. We analyse the co‐occurrence of a remarkable 10–15 species of the ant genus Monomorium occurring within single 10 × 10 m plots in a tropical savanna of northern Australia. Most of the species are undescribed, so we use genetic analysis to validate our species demarcations. We document nest dispersion patterns, and investigate differentiation in the three primary niche dimensions: space, time and food. We also examine species differences in competitive abilities, by describing rates of foraging activity, foraging ranges, worker aggression, and levels of behavioural dominance. Analyses of nest and forager distributions showed very limited evidence of spatial segregation within plots. The great majority of species foraged either exclusively or primarily during daylight hours. Body size and isotopic analyses indicated very limited dietary differentiation. Such limited niche partitioning occurred despite the species differing markedly in their competitive abilities as measured by rates of resource discovery, recruitment and monopolization. Our findings defy the traditional assumption that multiple closely related and ecologically similar species of highly interactive taxa cannot co‐occur. It seems very likely that species coexistence in our study system is determined to a very large degree by stochastic processes relating to dispersal and establishment, as predicted by neutral theory. However, neutral theory assumes competitive equivalence, whereas we found very marked differences in the competitive abilities of our co‐occurring species. We suggest that competitive exclusion is prevented by the modular nature of ant colonies, with competition limiting colony performance but not preventing occurrence. We conclude that other factors that allow species persistence, and not just competitive equivalence, can allow dispersal and establishment processes to drive species coexistence.     

Carbohydrate scarcity increases foraging activities and aggressiveness in the ant Prolasius advenus (Hymenoptera: Formicidae)

1. Behavioural responses to varying macronutrient availability are increasingly studied in highly invasive ant species to better understand their ecological success. However, such work is lacking in relation to native ant species confronted with biological invaders. 2. Here the link between diet and behaviour was examined in Prolasius advenus, a native ant from New Zealand facing intense competition for food with invasive insects, including social wasps. A long‐term laboratory experiment was conducted to assess the impact of protein and carbohydrate restriction on several behavioural parameters. 3. Ants shifted their food‐collecting activity towards the least available macronutrient in both protein‐limited and carbohydrate‐limited colonies. But when lacking carbohydrate, they also strongly increased their efforts to discover resources. After 10 weeks, the proportion of the colony patrolling outside of the nest was eight times higher than at the initiation of the experiment. This high patrolling activity was then maintained for several weeks and resulted in a higher efficiency to explore a novel territory. Moreover, ants fed a carbohydrate‐limited diet engaged in longer aggressive acts towards conspecifics. 4. These behavioural responses to carbohydrate scarcity may, in part, enhance the ability of P. advenus to resist a competing invader under natural conditions. While much of the previous research has linked diet and behavioural dominance in invasive ants, the present study shows that conducting similar investigations in the native species to which they are confronted may shed light on the mechanisms behind biotic resistance and the ability of some native species to coexist with highly abundant invaders.     

Diet and trophic niche of Antarctic silverfish Pleuragramma antarcticum in the Ross Sea,Antarctica

The diet of Antarctic silverfish Pleuragramma antarcticum was evaluated by examining stomach contents of specimens collected in the Ross Sea (71°–77° S; 165°–180° E) in January to March 2008. Pleuragramma antarcticum (50–236 mm standard length, L_S) and prey items were analysed for stable‐isotopic composition of carbon and nitrogen. According to index of relative importance (I_RI), which incorporates frequency of occurrence, mass and number of prey items, the most important prey items were copepods (81%I_RI over all specimens), predominantly Metridia gerlachei and Paraeuchaeta sp., with krill and fishes having low I_RI (2·2 and 5·6%I_RI overall). According to mass of prey (M) in stomachs, however, fishes (P. antarcticum and myctophids) and krill dominated overall diet (48 and 22%M, respectively), with copepods being a relatively minor constituent of overall diet by mass (9·9%M). Piscivory by P. antarcticum occurred mainly in the extreme south‐west of the region and near the continental slope. Krill identified to species level in P. antarcticum stomachs were predominantly Euphausia superba (14·1%M) with some Euphausia crystallophorias (4·8%M). Both DistLM modelling (PRIMER‐permanova+) on stomach contents (by I_RI) and stepwise generalized linear modelling on stable isotopes showed that L_S and location were significant predictors of P. antarcticum diet. Postlarval P. antarcticum (50–89 mm L_S) consumed exclusively copepods. Juvenile P. antarcticum (90–151 mm L_S) consumed predominantly krill and copepods by mass (46 and 30%M, respectively). Small adult P. antarcticum (152–178 mm L_S) consumed krill, fishes and copepods (37, 36 and 15%M, respectively). Large adult P. antarcticum (179–236 mm L_S) consumed predominantly fishes and krill (55 and 17%M, respectively), especially in the north (near the Ross Sea slope) and in the SW Ross Sea. Amphipods were occasionally important prey items for P. antarcticum (western Ross Sea, 39%M). General concordance between stomach contents and trophic level of P. antarcticum and prey based on δ¹⁵N was demonstrated. Pleuragramma antarcticum trophic level was estimated as 3·7 (postlarval fish) and 4·1 (fish aged 3+ years).     

The role of resource dispersion in promoting the co‐occurrence of dominant and subordinate ant species

Dominant competitors govern resource use in many communities, leading to predictions of local exclusion and lower species diversity where dominant species are abundant. However, subordinate and dominant species frequently co‐occur. One mechanism that could facilitate resource sharing and co‐occurrence of dominant and subordinate competitors is fine‐scale resource dispersion. Here, we distributed 6 g of a food resource into 1, 2, 8, 32 or 64 units in small 0.40 m² areas centred on nests of the dominant ant Monomorium sydneyense. We tested three hypotheses. First, we hypothesized that the species richness and abundance of foraging ants would increase with increasing resource dispersion. Accordingly, species richness doubled and total ant abundance was two orders of magnitude higher in high resource dispersion treatments. Secondly, we hypothesized that increasing resource dispersion would reduce competitive interactions such as resource turnover events and lower the probability of food resources being occupied. Substantial support for this hypothesis was observed. Finally, we tested the hypothesis that the foraging time of each species would be proportional to the relative abundance of each species solely in high resource dispersion treatments. Expected and observed foraging times were statistically similar for only the dominant ant M. sydneyense. The subdominant Pheidole rugosula increased its foraging time much more than was expected, while two subordinate ants showed no relationship between observed and expected times. Thus, while increasing resource dispersion significantly increased overall species richness, this increase in co‐occurrence did not correlate with a significant increase in foraging time for the two subordinate species. Rather, changes in resource dispersion appeared to benefit only the subdominant species. Inter‐site variation appeared more important for other subordinate species indetermining co‐occurrence and foraging time. Multiple mechanisms facilitate co‐occurrence and resource sharing in this community, and probably in most other communities.     

Relationships between densities of previous and simultaneous foragers and the foraging behaviour of three bumblebee species

1. The local insect composition may be as important as the local floral composition for bumblebees' foraging behaviour. However, little is known about how the local abundance of insects affects the foraging patterns of individuals. 2. Using field observations, we studied the relationships between the local density of previous and simultaneous foragers and the local foraging behaviour of Bombus pascuorum, Bombus lucorum/B. terrestris and Bombus lapidarius while pollinating Centaurea jacea. 3. The number of bumblebees foraging in the plots was positively related to the number of new individuals arriving at these plots. The number of inflorescences contacted and the duration of visits were negatively related to the number of simultaneous foragers, but only in B. lucorum/B. terrestris. The effects of previous foragers on the behaviour of other bumblebees were species‐specific and variable in their direction. Such contrasting effects can be explained in terms of bumblebee species' abundances and functional similarity. In some cases, the effect of previous foragers increased with Centaurea density. 4. The local abundance of previous and simultaneous foragers affected the foraging behaviour of particular bumblebee individuals in complex ways. Future studies on local foraging behaviour might benefit from including the abundance of co‐foragers.     

Reproductive physiology, dominance interactions, and division of labour among bumble bee workers

Abstract. Bumble bee workers (Bombus bifarius, Hymenoptera: Apidae) exhibit aggression toward one another after the colony begins producing female reproductive offspring (the competition phase). Workers in competition phase colonies must continue to perform in‐nest tasks, such as nest thermoregulation, and to forage for food, to rear the reproductives to maturity. Therefore, competition phase workers are faced with potentially conflicting pressures to work for their colonies, or to compete for direct reproduction. The effects of reproductive competition on worker task performance were quantified by measuring relationships of worker body size, reproductive physiology, and aggression with their rates of task performance. If worker division of labour was strongly affected by competition, it was predicted that fecund workers would avoid performing nest maintenance and foraging tasks, focusing instead on reproductive behaviour. Furthermore, it was predicted that fecund workers would dominate their nest mates, and that subordinate workers would perform nonreproductive tasks at higher rates. Worker aggression was associated closely with direct reproductive competition. Both aggression and brood interaction rates were related positively with ooctye development. Furthermore, foraging was associated negatively with ovarian development. However, in‐nest and foraging task performance rates were not associated with social aggression. The results support a partial role for reproductive competition in worker polyethism. Although worker aggression did not directly affect polyethism, reproductively competent workers avoided foraging tasks that would remove them from egg‐laying opportunities. Reproductively competent workers did perform in‐nest tasks, suggesting that these tasks entail little cost in terms of reproductive competition.     

Species interactions and thermal constraints on ant community structure

Patterns of species occurrence and abundance are influenced by abiotic factors and biotic interactions, but these factors are difficult to disentangle without experimental manipulations. In this study, we used observational and experimental approaches to investigate the role of temperature and interspecific competition in controlling the structure of ground‐foraging ant communities in forests of the Siskiyou Mountains of southwestern Oregon. To assess the potential role of competition, we first used null model analyses to ask whether species partition temporal and/or spatial environments. To understand how thermal tolerances influence the structure of communities, we conducted a laboratory experiment to estimate the maximum thermal tolerance of workers and a field experiment in which we added shaded microhabitats and monitored the response of foragers. Finally, to evaluate the roles of temperature and interspecific competition in the field, we simultaneously manipulated shading and the presence of a dominant competitor (Formica moki). The foraging activity of species broadly overlapped during the diurnal range of temperatures. Species co‐occurrence patterns varied across the diurnal temperature range: species were spatially segregated at bait stations at low temperatures, but co‐occurred randomly at high temperatures. The decreased abundance of the co‐occurring thermophilic Temnothorax nevadensis in shaded plots was a direct effect of shading and not an indirect effect of competitive interactions. Thermal tolerance predicted the response of ant species to the shading experiment: species with the lowest tolerances to high temperatures showed the greatest increase in abundance in the shaded plots. Moreover, species with more similar thermal tolerance values segregated more frequently on baits than did species that differed in their thermal tolerances. Collectively, our results suggest that thermal tolerances of ants may mediate competitive effects in habitats that experience strong diurnal temperature fluctuations.     

Manipulation of colony environment modulates honey bee aggression and brain gene expression

The social environment plays an essential role in shaping behavior for most animals. Social effects on behavior are often linked to changes in brain gene expression. In the honey bee (Apis mellifera L.), social modulation of individual aggression allows colonies to adjust the intensity with which they defend their hive in response to predation threat. Previous research has showed social effects on both aggression and aggression‐related brain gene expression in honey bees, caused by alarm pheromone and unknown factors related to colony genotype. For example, some bees from less aggressive genetic stock reared in colonies with genetic predispositions toward increased aggression show both increased aggression and more aggressive‐like brain gene expression profiles. We tested the hypothesis that exposure to a colony environment influenced by high levels of predation threat results in increased aggression and aggressive‐like gene expression patterns in individual bees. We assessed gene expression using four marker genes. Experimentally induced predation threats modified behavior, but the effect was opposite of our predictions: disturbed colonies showed decreased aggression. Disturbed colonies also decreased foraging activity, suggesting that they did not habituate to threats; other explanations for this finding are discussed. Bees in disturbed colonies also showed changes in brain gene expression, some of which paralleled behavioral findings. These results show that bee aggression and associated molecular processes are subject to complex social influences .     

Assessing niche partitioning of co‐occurring sibling bat species by DNA metabarcoding

Niche partitioning through foraging is a mechanism likely involved in facilitating the coexistence of ecologically similar and co‐occurring animal species by separating their use of resources. Yet, this mechanism is not well understood in flying insectivorous animals. This is particularly true of bats, where many ecologically similar or cryptic species coexist. The detailed analysis of the foraging niche in sympatric, cryptic sibling species provides an excellent framework to disentangle the role of specific niche factors likely involved in facilitating coexistence. We used DNA metabarcoding to determine the prey species consumed by a population of sympatric sibling Rhinolophus euryale and Rhinolophus mehelyi whose use of habitat in both sympatric and allopatric ranges has been well established through radio tracking. Although some subtle dietary differences exist in prey species composition, the diet of both bats greatly overlapped (O_jk = 0.83) due to the consumption of the same common and widespread moths. Those dietary differences we did detect might be related to divergences in prey availabilities among foraging habitats, which prior radio tracking on the same population showed are differentially used and selected when both species co‐occur. This minor dietary segregation in sympatry may be the result of foraging on the same prey‐types and could contribute to reduce potential competitive interactions (e.g., for prey, acoustic space). Our results highlight the need to evaluate the spatial niche dimension in mediating the co‐occurrence of similar insectivorous bat species, a niche factor likely involved in processes of bat species coexistence.     

Effects of life phase and schooling patterns on the foraging behaviour of coral‐reef fishes from the genus Haemulon

During this study (December 2009 to December 2010), underwater visual surveys using the focal animal method were performed in the coastal reefs of Tamandaré, north‐eastern Brazil. The aim was to analyse the effects of the life phase (juvenile and adult) and schooling patterns (school and solitary) on the feeding behaviour (foraging rates and substratum preferences) of four species of the genus Haemulon (Haemulon aurolineatum, Haemulon parra, Haemulon plumieri and Haemulon squamipinna). PERMANOVA analysis (P < 0·05) indicated that ontogenetic changes and schooling patterns directly influence foraging behaviour. Schooling individuals had low foraging rates (mean ± s.d . = 2·3 ± 2·1 bites 10 min^?1) and mobility, usually remaining near the bottom; however, solitary fishes had high foraging rates (mean ± s.d . = 12·5 ± 4·6 bites 10 min^?1). Juveniles preferred feeding in the water column (75% of the total number of bites), whereas adults foraged mainly in sand (80%) and bare rock (20%). All four Haemulon species displayed similar patterns of feeding behaviour as well as preferences for foraging sites and display competition for food resources. In contrast, little is known about their habitat use and foraging behaviour over the diel cycle, particularly the newly settled and early juvenile stages.     

Dispersed central-place foraging in the polydomous odorous house ant, Tapinoma sessile as revealed by a protein marker

The odorous house ant, Tapinoma sessile, is a native ant species common throughout North America. In urban areas, this ant is classified a pest species and exhibits several attributes characteristic of invasive “tramp” ants (sensu Passera, 1994). These include: extreme polygyny, colony reproduction by budding, reduced internest aggression, generalist diet, and polydomy. Here we explore the organization of foraging and the pathways of food distribution in polydomous colonies of T. sessile in the laboratory and field using a novel marking technique (rabbit IgG protein) and enzyme linked immunosorbent assay (ELISA). Laboratory assays revealed patterns of food allocation from foragers to other castes and developmental stages. Foragers distributed the IgG- labelled sucrose to the majority of workers within 24 h, and workers retained significantly more sucrose than either queens or larvae. Approximately 50% of queens tested positive for the IgG marker and some queens received significantly more sucrose than others, indicating a possible reproductive dominance hierarchy. Larvae received little sucrose demonstrating their minor reliance on carbohydrates. The results of field experiments showed that odorous house ants are dispersed central-place foragers whereby ants from individual nests exhibit high foraging site fidelity, travel along well-established trails, and forage on a local scale. Dispersed central-place foraging most likely allows the odorous house ant to more efficiently secure both clumped and dispersed food sources and possibly increases its competitive ability. As a result, colonies become numerically large and ecologically dominant. The results of our study contribute to our understanding of the social behavior and colony organization in T. sessile. In addition, they provide a framework for designing more effective ant control programs based on liquid baits. Received 13 December 2005; revised 28 February 2006; accepted 3 March 2006.     

The effect of multi‐species host density on superparasitism and sex ratio in a gregarious parasitoid

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Electrocommunication behaviour during social interactions in two species of pulse‐type weakly electric fishes (Mormyridae)

This study compares electrocommunication behaviour in groups of freely swimming weakly electric fishes of two species, Marcusenius altisambesi and Mormyrus rume. Animals emitted variable temporal sequences of stereotyped electric organ discharges (EOD) that served as communication signals. While the waveform of individual signals remained constant, the inter‐discharge interval (IDI) patterns conveyed situation‐specific information. Both species showed different types of group behaviour, e.g. they engaged in collective (group) foraging. The results show that in each species, during different behavioural conditions (resting, foraging and agonistic encounters), certain situation‐specific IDI patterns occurred. In both species, neighbouring fishes swimming closely together interacted electrically by going in and out of synchronization episodes, i.e. periods of temporally correlated EOD production. These often resulted in echo responses between neighbours. During group foraging, fishes often signalled in a repetitive fixed order (fixed‐order signalling). During foraging, EOD emission rates of M. altisambesi were higher and more regular than those of M. rume. The two species also differed in the quantity of group behaviours with M. altisambesi being more social than M. rume, which was reflected in the lack of specific agonistic IDI patterns, more fixed‐order signalling and more communal resting behaviour in M. altisambesi.     

Diet segregation between two colonies of little penguins Eudyptula minor in southeast Australia

We studied foraging segregation between two different sized colonies of little penguins Eudyptula minor with overlapping foraging areas in pre‐laying and incubation. We used stomach contents and stable isotope measurements of nitrogen (δ¹⁵N) and carbon (δ¹³C) in blood to examine differences in trophic position, prey‐size and nutritional values between the two colonies. Diet of little penguins at St Kilda (small colony) relied heavily on anchovy while at Phillip Island (large colony), the diet was more diverse and anchovies were larger than those consumed by St Kilda penguins. Higher δ¹⁵N values at St Kilda, differences in δ¹³C values and the prey composition provided further evidence of diet segregation between colonies. Penguins from each colony took anchovies from different cohorts and probably different stocks, although these sites are only 70 km apart. Differences in diet were not reflected in protein levels in the blood of penguins, suggesting that variation in prey between colonies was not related to differences in nutritional value of the diet. Anchovy is currently the only available prey to penguins throughout the year and its absence could have a negative impact on penguin food supply, particularly at St Kilda where the diet is dominated by this species. While it is difficult to establish whether diet segregation is caused by inter‐ or intra‐colony competition or spatial differences in foraging areas, we have shown that colonies with broadly overlapping foraging ranges could have significant differences in trophic position, diet composition and prey size while maintaining a diet of similar nutritional value.     

Born among the ice: first morphological observations on two developmental stages of the Antarctic silverfish Pleuragramma antarcticum, a key species of the Southern Ocean

The Antarctic silverfish Pleuragramma antarcticum is a keystone species in the Southern Ocean ecosystem, providing one of the major links between lower and higher trophic levels. Despite the importance of this species, surprisingly little is known of its early development. The first spawning area for the silverfish has been recently identified in the near-shore of Terra Nova Bay (Ross Sea). Evidence indicates that spawning and embryo development occurs in the cryopelagic environment, below the seasonal pack-ice. In order to contribute to the knowledge of the life cycle of this very important Antarctic species, we carried out the first histological characterization on pre-hatching embryos and newly hatched larvae. Embryonated eggs and larvae of P. antarcticum were collected between late October and November 2005 at TNB through holes drilled into the sea ice. Embryonic stage just before hatching and the first post-hatching stage were the most abundant within our samples and thus were analysed using both macroscopic and histological approaches. Early life stages of the Antarctic silverfish revealed interesting features: the sensory system, foraging apparatus and heart appeared well developed, whereas the liver and gills were underdeveloped. Morphological details of the organogenesis were performed, providing the first substantial information on the development of P. antarcticum and representing a further steps towards the knowledge of the life cycle of this important Antarctic key species. An erratum to this article can be found at     

Effects of habitat complexity,dominance and personality on habitat selection: Ideal despotic cichlids

Habitat structure can impede visibility and movement, resulting in lower resource monopolization and aggression. Consequently, dominant individuals may prefer open habitats to maximize resource gain, or complex habitats to minimize predation risk. We explored the role of dominance on foraging, aggression and habitat choice using convict cichlids (Amatitlania nigrofasciata) in a two‐patch ideal free distribution experiment. Groups of six fish of four distinct sizes first competed for shrimp in one‐patch trials in both an open and complex habitat; half the groups experienced each habitat type first. Following these one‐patch trials, each group then chose between habitat types in a two‐patch trial while competing for food. Finally, each fish underwent an individual behavioural assessment using a battery of “personality” tests to determine if behaviour when alone accurately reflected behaviour within a social context. In the one‐patch trials, dominant fish showed similar food consumption between habitats, but chased more in the complex habitat. In the two‐patch choice trials, dominants preferred and defended the complex habitat, forming an ideal despotic distribution with more than half the fish and competitive weight in the open habitat. Within the groups, individual fish differed in foraging and chasing, with repeatabilities of 0.45 and 0.23 across all treatments. Although a higher foraging rate during the individual assessment predicted foraging rate and use of the complex habitat during the group trials, aggression and boldness tests were not reflective of group behaviour. Across groups, heavier dominants and those with higher foraging rate in the open habitat used the open habitat more, suggesting that both risk and energetic state affect habitat preference in dominant convict cichlids.     

Diet and foraging effort of Adélie penguins in relation to pack-ice conditions in the southern Ross Sea

We investigated the diet and aspects of foraging effort among Adélie penguins (Pygoscelis adeliae) breeding at three colonies on Ross Island, in the southwestern Ross Sea – Capes Royds, Bird and Crozier – during the chick-provisioning period of three austral summers, 1994–1995, 1995–1996 and 1996–1997. During the study period, pack-ice cover differed in waters offshore of these colonies, by colony, seasons and year. Diet differed among colonies only slightly. The fish Pleuragramma antarcticum was the most important prey, especially during years or periods within years when little pack ice was present. With respect to krill, which composed the remainder of diet, juvenile Euphausia crystallorophias were consumed predominantly in a year of heavy pack-ice cover; more adult krill were consumed in 2 years when pack ice was sparse. Foraging trip duration differed by colony, season and year and was related directly to distance from the colony to the nearest pack ice. The amount of food brought to chicks increased as trip duration increased, to a point (2 days), but then decreased as duration increased further (up to 4 days). On the basis of data on mass of parents and of meal sizes to chicks, it appeared that on the longest trips more of the food gathered by parents was used for self maintenance; on the longest trips, parents lost body mass. Successful foraging during chick rearing, the period when adult foraging is most intense, appears to depend on the proximity of pack ice to nesting colonies for this penguin species. Received: 1 October 1997 / Accepted: 25 April 1998     

Fungal chemical defence alters density‐dependent foraging behaviour and success in a fungivorous soil arthropod

1. Aggregative behaviour in fungivorous soil arthropods is widespread; its adaptive value, however, is largely unknown. In this study, the spatial foraging behaviour of a collembolan, Folsomia candida, and the fitness consequences of feeding at different densities on the filamentous fungus Aspergillus nidulans were investigated. The effect of two fungal strains were compared; a wild‐type (wt) and a transgenic strain that lacks the ability to express the global secondary metabolite regulator LaeA (ΔlaeA). 2. In laboratory foraging tests, F. candida exhibited aggregated distributions of individuals across four distinct fungal colonies that were arranged in short distances from each other. By quantifying the extent of the feeding damage at each single colony, a more evenly distributed feeding activity was found among wt colonies than among chemical‐deficient colonies. 3. In a fitness experiment, where collembolans at different densities were restricted to feed on single A. nidulans colonies, mean growth rate of F. candida was positively related to density on the wt A. nidulans strain, but negatively related to density on the chemical‐deficient strain. 4. Depending on the fungus' ability to express secondary chemicals and availability of fungal food sources, F. candida may employ different foraging strategies: (i) avoidance of prolonged feeding on single colonies in a rich habitat (travel costs low), and (ii) intensified group feeding on single colonies in a resource‐limited habitat (travel costs high). It was hypothesised that flexibility in fungivore foraging behaviour (clumping vs. spreading feeding activity) is adaptive because it allows avoidance/overcoming induced fungal chemical defence.