Differential survival of macroalgae to digestion by intertidal herbivore molluscs

This study evaluates the capacity of algal propagules to survive digestion by seven intertidal herbivore molluscs. Thirty individuals of Littorina peruviana, 36 of Siphonaria lessoni, 35 of Collisella ceciliana, and 25 each of C. zebrina, Chiton granosus, Fissurella crassa and F. limbata were collected at different dates between October, 1983 and April, 1984. About half of the individuals of each species were examined for gut contents and the other half was used to provide faecal pellets for culture. Diet of these grazers ranged from 8 macroalgal species in Collisella zebrina to 17 in Fissurella limbata. In total, macroalgal propagules of 56% of the 27 algal species found in these gut contents survive digestion. There was no positive correlation between diet and number of algal taxa growing in the respective faecal cultures. Survival through Littorina peruviana and Chiton granosus was low (20–30% of the algal species consumed) while it was high (75–83%) through the two species of Collisella. No significant correlation was found between frequency of algae in the gut contents and its frequency in the faecal cultures. Algal capacity to survive digestion was almost exclusively restricted to opportunistic species, especially in the Chlorophyta and Phaeophyta. It seems that survival of algal propagules through the digestive tract of generalist grazers is a rather random phenomenon.     

Dynamic neuromodulation of aggression by vasotocin: influence of social context and social phenotype in territorial songbirds

The homologous neuropeptides vasotocin (VT) and vasopressin (VP) influence agonistic behaviours across many taxa, but peptide–behaviour relationships are complex and purportedly species-specific. Putative species-specific effects in songbirds are confounded with context, however, such that territorial species have been tested only in resident–intruder paradigms and gregarious species have been tested only in a mate competition paradigm. Using the territorial violet-eared waxbill (Estrildidae: Uraeginthus granatina), we now show that a V_1a receptor antagonist reduces male aggression during mate competition (as in gregarious finches), but does not affect resident–intruder aggression in dominant males. However, the V_1a antagonist disinhibits aggression in less aggressive (typically subordinate) males. These results are consistent with recent data on the activation of different VT cell groups during positive and negative social interactions. Thus, VT influences aggression similarly across territorial and gregarious species, but in context- and phenotype-specific ways that probably reflect the differential activation of discrete VT cell groups.     

The settlement behaviour of planulae larvae of the hermatypic coral Favia fragum (Esper)

Experiments on the settlement behaviour of planulae larvae of the reef coral Favia fragum (Esper) are described. The larvae are positively phototaxic upon release but reverse this and become attracted to dark surfaces, corners, crevices, and the undersides of objects on the bottom. Clean glass surfaces were preferred to surfaces covered with biological slime but there was no preference for rough against smooth surfaces. There was clear evidence of gregarious settlement behaviour, the planulae being able to recognize both adult colonies and previously settled juveniles. A distinction was made between crawling and swimming larvae, and the consequences of their differences in behaviour on the spatial distribution of Favia on reef are discussed. Settlement behaviour of Favia is similar in many respects to that of the Pacific reef coral Pocillopora damicornis (Dana) but is distinguished by gregarious settlement and by a preference for clean surfaces over surfaces covered with biological slime.     

Interspecific competition among urban cockroach species

The aim of this study was to quantify and to compare the effects of intraspecific and interspecific competition among pairs of urban cockroaches [Blatta orientalis L., Periplaneta americana (L.), and Periplaneta australasiae Fabricius (Dictyoptera: Blattodea)] in relation to the limitation of resources such as shelter or food. Our approach was to assess whether the presence of one species affected the resource exploitation of another. A reduction in access to shelters or to food for one species revealed dominant/subordination relationships among species and induced spatial segregation in shelters and temporal segregation during food exploitation. The fragmentation of available resources facilitated spatial segregation and the access of more individuals of the subordinate species. Individuals of each species were aware of the presence of non‐conspecifics. The outcome of interspecific competition under laboratory conditions should help us to understand how segregated spatial distributions occur under natural conditions in urban areas.     

Depth and temperature drive patterns of spatial overlap among fish thermal guilds in lakes across Ontario,Canada

Aim

The spatial distribution of ectotherms is strongly dependent on the temperature of their environments. In temperate lakes, fishes with different thermal optima can become spatially segregated during summer stratification. This habitat partitioning, or niche complementarity, may play a role in the coexistence of trophically similar species; however, the extent of partitioning is dependent on the resources available within each habitat. Although habitat partitioning of fish thermal guilds has been studied in individual lakes, broad-scale patterns of spatial overlap and segregation are not yet understood. In this study, we explore the patterns and drivers of spatial overlap among thermal guilds (cold-, cool-, and warm-water) at a broad scale.

Location

Ontario, Canada.

Methods

We built a multivariate regression tree to explore patterns and environmental drivers of spatial overlap in freshwater fishes across three thermal guilds from 438 lakes.

Results

We identified five clusters of lakes exhibiting different patterns of spatial overlap among the three thermal guilds. Temperature (growing degree days) and maximum lake depth were strong drivers of the spatial overlap patterns.

Main Conclusions

These findings provide a better understanding of broad-scale patterns of spatial overlap and allow us to predict how spatial overlap, and ultimately species interactions and competition, may change under a warming climate.     

Cannibalistic interactions in two co-occurring decapod species: Effects of density, food, alternative prey and habitat

Cannibalism is a potentially important factor in the regulation of populations in a range of habitats. The intensity of this biotic factor may be determined by both intra and interspecific interactions. Cancer pagurus and Porcellana platycheles are two co-occurring decapods on Atlantic rocky shores. In laboratory mesocosms, we investigated intra and intersize class cannibalistic and interspecific predatory behaviours in those species. We addressed the effects of prey and predator densities, food, starvation, alternative prey and habitat type. No agonistic behaviour was noted in P. platycheles, suggesting a non-aggressive co-existence between gregarious individuals. Predation of C. pagurus on P. platycheles was intense, possibly accounting for the spatial segregation observed in the natural environment. Cannibalism among C. pagurus juveniles was low and only on vulnerable prey (i.e. at moulting), suggesting a non-aggressive co-existence among juveniles. However, intersize class cannibalism in C. pagurus was intense (ontogenetic shift), possibly reflecting the juvenile-adult segregation in the natural environment. Prey and predator densities, food and habitat type strongly influenced this behaviour. Possible interference among cannibals was noted, with lower prey consumption at high predator density. Food supply alone had more effect on cannibalistic rate than did alternative prey (P. platycheles) and predator starvation. Structurally complex habitats (small pebble and Fucus serratus habitats) yielded higher prey survival than the sandy habitat, and the behaviours of both prey and cannibals reflected the small-scale spatial distribution of individuals in the wild. Intersize class cannibalism and interspecific agonistic relationships may account for the intertidal distribution of crab species at low tide.     

Niche partitioning of feeding microhabitats produces a unique function for herbivorous rabbitfishes (Perciformes, Siganidae) on coral reefs

Niche theory predicts that coexisting species minimise competition by evolving morphological or behavioural specialisations that allow them to spread out along resource axes such as space, diet and temporal activity. These specialisations define how a species interacts with its environment and, by extension, determine its functional role. Here, we examine the feeding niche of three species of coral reef-dwelling rabbitfishes (Siganidae, Siganus). By comparing aspects of their feeding behaviour (bite location, bite rate, foraging distance) with that of representative species from two other abundant herbivorous fish families, the parrotfishes (Labridae, Scarus) and surgeonfishes (Acanthuridae, Acanthurus), we examine whether rabbitfishes have a feeding niche distinct from other members of the herbivore guild. Measurements of the penetration of the fishes’ snouts and bodies into reef concavities when feeding revealed that rabbitfish fed to a greater degree from reef crevices and interstices than other herbivores. There was just a 40 % overlap in the penetration-depth niche between rabbitfish and surgeonfish and a 45 % overlap between rabbitfish and parrotfish, compared with the almost complete niche overlap (95 %) recorded for parrotfish and surgeonfish along this spatial niche axis. Aspects of the morphology of rabbitfish which may contribute to this niche segregation include a comparatively longer, narrower snout and narrower head. Our results suggest that sympatric coexistence of rabbitfish and other reef herbivores is facilitated by segregation along a spatial (and potentially dietary) axis. This segregation results in a unique functional role for rabbitfishes among roving herbivores that of “crevice-browser”: a group that specifically feeds on crevice-dwelling algal or benthic organisms. This functional trait may have implications for reef ecosystem processes in terms of controlling the successional development of crevice-based algal communities, reducing their potential to trigger macroalgal outbreaks.     

Interactions between root and shoot competition among four plant species in an alpine meadow on the Tibetan Plateau

At low altitudes, the interactions between root and shoot competition on plant growth have been extensively studied, and such interaction effects were found to be positive, negative, or neutral. However, little is known about such effects at high altitudes where the environmental conditions are harsher than those at low altitudes. We carried out a field experiment in an alpine meadow in the northeast Tibetan Plateau to test the hypothesis that a negative interaction between root and shoot competition exists for alpine plant species. Root and shoot competition were experimentally manipulated in the four grass species (Kobresia humilis, Saussurea superba, Stipa aliena and Elymus nutans). We found that K. humilis and S. aliena grew better without competition, whereas S. superba grew better with shoot competition and E. nutans grew better with root competition. The interactions between root and shoot competition were negative in K. humilis, positive in S. superba, but neutral in S. aliena and E. nutans, suggesting that the interaction effects are species-specific. This study also suggested that alpine plants may trade off both plant–plant interactions and competition shift between root and shoot to adapt to stressful environments.     

Refuge utilization and preferences between competing intertidal crab species

Many invertebrates avoid predation risk by seeking and defending refuges that can be in limited supply, producing strong intra- and inter-specific interference competition. Previous experimental studies in central Chile demonstrated that interference competition for refuges is the primary factor driving habitat segregation between the predatory crabs Acanthocyclus gayi and A. hassleri, with the latter species monopolizing galleries inside mussel beds in the mid intertidal zone and limiting A. gayi to rock crevices. Yet, habitat partitioning between rival species can result from differences in habitat preferences and not solely from interference interactions. Moreover, since A. gayi is also known to shelter in turf-forming algae (predominantly Gelidium), which dominates extensive areas in the low intertidal zone, among-sites variation in the turf morphology and abundance could modify habitat preferences and the pattern of inter-specific interactions. We experimentally evaluated refuge habitat preferences of individual, similarly-sized adult A. gayi and A. hassleri in the laboratory, comparing choice patterns across multiple trials with paired combinations of the main refuge types commonly used by crabs in the field: a) mussel galleries, b) rock crevices, c) short algal turf, and d) tall algal turf. Our results showed that both species display a strong ranking of preferences for some refuge habitats over others. In general, mussel galleries were the preferred refuge type for both crab species, but their preference rankings changed depending on turf morphology. When turf was short, A. hassleri and A. gayi made identical refuge choices, strongly preferring mussel galleries over crevices and these over the short turf. In contrast, when the turf was tall A. gayi selected equally the tall turf or mussel galleries, and these were strongly preferred over crevices. A. hassleri, on the other hand, largely ignored tall turf and kept the highest preference for mussel galleries. A field experiment in which crabs were offered to foraging birds demonstrated that A. hassleri is more susceptible than A. gayi to predation by kelp gulls when outside refuges. Differences in patterns of coloration between crabs may underlie between-species differences in predation susceptibility and their habitat choices. These results suggest that the among-site differences in turf morphology (height and shape of fronds), which is largely driven by varying intensity of upwelling, could affect crab preferences for refuge habitats and the relative importance of inter-specific interference competition.     

Species-specific use of allochthonous resources by ground beetles (Carabidae) at a river–land interface

A variety of organisms mediate river–terrestrial linkages through spatial subsidies. However, most empirical studies have classified organisms rather broadly (e.g., by functional group or taxonomic family) and have dismissed species-level linkages at the interface of ecosystems. Here, we show how allochthonous resource use varies among taxonomically similar species of ground beetles (family Carabidae) across seasons (June–September). We investigated seasonal shifts in the distribution of five beetle species and their dietary responses to spatial subsidies (emerging aquatic insects) in a Japanese braided river. Despite their taxonomic closeness, the ground beetles showed species-specific responses to spatial subsidies, and beetle distribution patterns tended to coincide with their diets. Overall, 1–56% of ground beetle diets were derived from aquatic prey. One genus (Bembidion spp.) mainly consumed aquatic prey, while three species fed primarily on terrestrial prey across all seasons. However, one species (Lithochlaenius noguchii) showed shifts in its diet from aquatic to terrestrial prey according to subsidy availability. The observed variation in allochthonous resource use was likely related to species-specific foraging modes, physiological tolerance to dry conditions, and interspecific competition. Our findings suggest that considering species-specific interactions is necessary to fully understand cross-system interactions and recipient food-web dynamics.     

Diel activity and reproductive territory of the Japanese bagrid catfish, <Emphasis Type="Italic">Pseudobagrus ichikawai</Emphasis>

The endangered Japanese bagrid catfish, Pseudobagrus ichikawai, is typically nocturnal, emerging from shelters, such as crevices along the shore, at sunset to actively forage in open areas throughout the night with short-term movements of up to 40 m (except for mature males). Just before sunrise, individuals return to their shelters, from which they rarely emerge in daytime. Mature males (3 years-of-age or older) are found only in the vicinity of their shelters of during both day and night, which they defend with strong territorial behaviour. The restricted reproductive resources (shelters) probably severely limit the population size and reproduction in this species.     

Web-Construction Behavior of Linyphiid Spiders (Araneae,Linyphiidae): Competition and Co-Existence Within a Generalist Predator Guild

The web-construction behavior of three species of linyphiid spider (Erigone autumnalis, Meioneta unimaculata and Bathyphantes pallida) was studied in the laboratory to examine competition and co-existence within predator guilds. Competitive interactions between spiders potentially reduce their role in biological control. We tested the hypothesis that at high densities, intraguild competition for web-sites would occur but spatial separation of microhabitat would reduce interguild competition, thus allowing co-existence. High mortality and reduced web-size were observed at high B. pallida densities but Linyphiinae co-existed with Erigoninae which constructed webs at different strata. Competitive exclusion by larger individuals occurred between species whose microhabitat niche overlapped. The biocontrol potential of spider or arthropod predator guilds could ultimately be enhanced by maximizing the diversity of species whose niche axes vary.     

Spatial associations among algae affect host use in a herbivorous marine amphipod

Host plant choice by herbivores may be constrained by the spatial arrangement of plants with selection of poor quality hosts being more likely if preferred hosts are locally rare. I tested whether the presence of a herbivorous marine amphipod, Peramphithoe parmerong, on a poor quality host could be explained by the relative abundance and spatial arrangement of alternative brown algal hosts. Amphipods strongly preferred the high quality Sargassum linearifolium over the poor quality Padina crassa in laboratory assays with a higher tendency to select Padina crassa when it was relatively more abundant. In the field, however, adults colonised both hosts to equal densities. Rates of colonisation were independent of host quality but strongly affected by the identity of neighbouring hosts with those algae in close proximity to S. linearifolium receiving far greater densities of amphipods than those surrounded by Padina crassa. There was no evidence that the presence of adult Peramphithoe parmerong on Padina crassa was due to local scarcity of S. linearifolium. In contrast, the distribution and behaviour of juveniles were predictable from differences in food quality, with most inhabiting the high quality S. linearifolium and few emigrating from this host. The success of a simple adaptive model based on food quality to predict juvenile, but not adult, behaviour indicates that the relative importance of food quality varies with age, and that other factors are important determinants of adult movements among hosts.     

Indirect Interactions in the High Arctic

Indirect interactions as mediated by higher and lower trophic levels have been advanced as key forces structuring herbivorous arthropod communities around the globe. Here, we present a first quantification of the interaction structure of a herbivore-centered food web from the High Arctic. Targeting the Lepidoptera of Northeast Greenland, we introduce generalized overlap indices as a novel tool for comparing different types of indirect interactions. First, we quantify the scope for top-down-up interactions as the probability that a herbivore attacking plant species i itself fed as a larva on species j. Second, we gauge this herbivore overlap against the potential for bottom-up-down interactions, quantified as the probability that a parasitoid attacking herbivore species i itself developed as a larva on species j. Third, we assess the impact of interactions with other food web modules, by extending the core web around the key herbivore Sympistis nigrita to other predator guilds (birds and spiders). We find the host specificity of both herbivores and parasitoids to be variable, with broad generalists occurring in both trophic layers. Indirect links through shared resources and through shared natural enemies both emerge as forces with a potential for shaping the herbivore community. The structure of the host-parasitoid submodule of the food web suggests scope for classic apparent competition. Yet, based on predation experiments, we estimate that birds kill as many (8%) larvae of S. nigrita as do parasitoids (8%), and that spiders kill many more (38%). Interactions between these predator guilds may result in further complexities. Our results caution against broad generalizations from studies of limited food web modules, and show the potential for interactions within and between guilds of extended webs. They also add a data point from the northernmost insect communities on Earth, and describe the baseline structure of a food web facing imminent climate change.     

Microsatellite evidence for sperm storage and multiple paternity in the marine gastropod Crepidula coquimbensis

In gregarious species with copulation and internal fertilization, male-male competition and female cryptic choice may affect reproductive success of both sexes. We carried out a molecular analysis to study paternity and sperm use by females in the protandrous marine brooding gastropod Crepidula coquimbensis. In the field, a single female inhabits an empty hosting shell with up to six males. This gregarious behavior may promote intra-brood multiple paternity if females can store sperm from several consecutive copulations with the surrounding males. To study female sperm usage, the males sharing shelters with five different adult females were collected and preserved for paternity analysis. Females were transported alive to the laboratory and isolated for six months. After that, an additional male was offered to each of the five study females. Once the females had laid capsules, a total of 528 embryos from the five females were assigned paternity based on five microsatellite loci. Paternity analysis showed that every male sharing the empty hosting shell of a female as well as the additional male were assigned fatherhood of embryos laid by this specific female. Females can thus use sperms from multiple males including sperms stored for at least six months. In addition, in four out of the five offspring arrays, a similar contribution of each male to the brood was observed, a pattern associated with the close relationship between the number of fathers observed and the effective paternity index calculated. These results contrast with those of paternity analyses carried out in another species of the same genus, C. fornicata which is characterized by a stacking behavior in which the closest male to the female achieves the highest reproductive success. Male reproductive success may be largely influenced by the aggregation pattern and male mating opportunities in the Crepidula complex, a hypothesis to be examined further by studying other species exhibiting different grouping behavior.     

Network topology of stable isotope interactions in a sub-arctic raptor guild

Predation is an ecologically important process, and intra-guild interactions may substantially influence the ecological effects of predator species. Despite a rapid expansion in the use of mathematical graph theory to describe trophic relations, network approaches have rarely been used to study interactions within predator assemblages. Assemblages of diurnal raptors are subject to substantial intra- and interspecific competition. Here we used the novel approach of applying analyzes based on network topology to species-specific data on the stable isotopes ¹³C and ¹⁵N in feathers to evaluate patterns of relative resource utilization within a guild of diurnal raptors in northern Sweden. Our guild consisted of the golden eagle (Aquila chrysaetos), the gyrfalcon (Falco rusticolus), the peregrine falcon (Falco peregrinus) and the rough-legged buzzard (Buteo lagopus). We found a modular trophic interaction structure within the guild, but the interactions were less nested than expected by chance. These results suggest low redundancy and hence a strong ecological importance of individual species. Our data also suggested that species were less connected through intra-guild interactions than expected by chance. We interpret our results as a convergence on specific isotope niches, and that body size and different hunting behaviour may mediate competition within these niches. We finally highlight that generalist predators could be ecologically important by linking specialist predator species with disparate dietary niches.     

Where vectors collide: the importance of mechanisms shaping the realized niche for modeling ranges of invasive <Emphasis Type="Italic">Aedes</Emphasis> mosquitoes

The vector mosquitoes Aedes aegypti (L.), native to Africa, and Aedes albopictus (Skuse), native to Asia, are widespread invasives whose spatial distributions frequently overlap. Predictive models of their distributions are typically correlative rather than mechanistic, and based on only abiotic variables describing putative environmental requirements despite extensive evidence of competitive interactions leading to displacements. Here we review putative roles of competition contributing to distribution changes where the two species meet. The strongest evidence for competitive displacements comes from multiple examples of habitat segregation where the two species co-occur and massive reductions in the range and abundance of A. aegypti attributable to A. albopictus invasions in the southeastern U.S.A. and Bermuda (U.K). We summarize evidence to support the primacy of asymmetric reproductive interference, or satyrization, and larval resource competition, both favoring A. albopictus, as displacement mechanisms. Where evidence of satyrization or interspecific resource competition is weak, differences in local environments or alternative ecologies or behaviors of these Aedes spp. may explain local variation in the outcomes of invasions. Predictive distribution modeling for both these major disease vectors needs to incorporate species interactions between them as an important process that is likely to limit their realized niches and future distributions. Experimental tests of satyrization and resource competition are needed across the broad ranges of these species, as are models that incorporate both reproductive interference and resource competition to evaluate interaction strengths and mechanisms. These vectors exemplify how fundamental principles of community ecology may influence distributions of invasive species.     

Competing for crevices: interspecific conflict influences retreat-site selection in montane lizards

Direct interference competition between sympatric taxa affects habitat use and shelter-site selection in species within most major vertebrate lineages. However, studies on interspecific social interference in reptiles largely have been confined to research on interactions between non-native (invasive) species and native fauna. Does interspecific interference also influence habitat use within natural assemblages of reptiles? We studied five broadly sympatric species of viviparous montane skinks within the genera Egernia and Eulamprus in southeastern Australia. Previous work has shown strong interspecific overlap in abiotic attributes of shelter-sites for these taxa, but no joint occupancy of retreats. Laboratory trials in which we manipulated the identity of co-occurring lizards revealed frequent displacement from “preferred” (hotter) shelters, with interspecific interactions more intense than intraspecific conflicts. The five species displayed a linear interspecific dominance hierarchy, with larger species displacing smaller taxa. Field manipulations confirmed that interspecific interactions between these species affect shelter-site use. We conclude that direct agonistic encounters between individuals of different species strongly influence habitat use by lizards within this assemblage.     

Against the odds of the ant competition hierarchy: submissive Myrmica rugulosa block access of the dominant Lasius fuliginosus to its aphids

Many ant assemblages are organised in competitive hierarchies. Myrmica rugulosa, a submissive species at the bottom of the ant competition hierarchy, temporarily took over groups of the myrmecophilic aphid Stomaphis quercus, which is strictly associated with the territorial ant species Lasius fuliginosus. Such previously unknown intervention happened repeatedly on the nest tree of L. fuliginosus. Masses of M. rugulosa intruded into bark crevices harbouring the aphids, and blocked with their own bodies access of L. fuliginosus to the aphids. Lasius fuliginosus showed no aggression towards the intruders, but walked around and on M. rugulosa clusters, palpating foreign ants with their antennae. Their only countermeasure was to briefly drag or carry individual M. rugulosa workers that pressed themselves down and froze for the moment, and then resumed their activities. We interpret the behaviour of M. rugulosa, in the context of its already known interspecific relations, as a specific competitive means of existence with higher-ranked species. The behaviour—by strict definition neither interference nor exploitation competition—effectively combines submissiveness, appeasement and tenacity. We discuss similarities between such behaviour and related behaviour reported earlier in a few other Myrmica species and Manica rubida while confronting physically stronger ants.     

Aggregation behaviour and interspecific relationships in Dermaptera

The rate of gregariousness and intra- and inter-specific attractivity were evaluated under laboratory conditions in five species of Dermaptera. The three species of the genus Forficula (Forficula auricularia L., F. decipiens Gené and F. pubescens Gené) showed distinctly gregarious behaviour in old larvae and adults in the pre-reproductive phase. This gregariousness was seen at the intraspecific level and also among individuals of different species. The other two species, Labidura riparia Pallas and Euborellia moesta Gené, were randomly distributed over the available shelters, with no distinct tendency for aggregation or isolation. In individual tests, insects of all five species were attracted by shelters previously visited by conspecifics in preference to control shelters. For the two non-gregarious species, this phenomenon might function as a burrow marker. The same tests showed an interattractivity for four of the species studied. Only L. riparia did not respond to the trails of the four other species, although its trails were attractive to the other species. The pheromonal nature of the substances involved in the interspecific relationships and the involvement of behaviour in the biology and speciation of Dermaptera are discussed.