Social relations in family-groups of wood miceApodemus sylvaticus under laboratory and enclosure conditions

Two family-groups of wood miceApodemus sylvaticus (Linnaeus, 1758) were observed: one in the laboratory (terrarium 100 × 60 × 60 cm) and the other under enclosure conditions (outdoor enclosure 200 × 100 × 100 cm). Three consecutive periods in the social relations of the family-groups were described: the linear hierarchy, the multidirectional social relations, and the variable dominance-subordination relations. The linear social hierarchy was reflected in the subordination behaviour of family members in relation to the dominant male, the father of the family. During the period of coexistence of a pair of parents with two litters of juveniles, both in the laboratory and under enclosure conditions, amicable relations between individuals occurred. Advanced forms of social care (paternal care, allomaternal lactation, transportation and licking by relatives) were also observed. In the laboratory, agonistic interactions were lacking during the linear hierarchy period, but the attainment of sexual maturity by individuals of the first two litters, accompanied by an increase in the number of mice, resulted in aggression between mature males and competition for dominance. Further, under increasingly crowded conditions, agonistic behaviour prevailed over other types of social interaction, and the survival rate of juveniles decreased. Under enclosure conditions a seasonal variation in agonistic interaction between mature individuals was recorded, similar to that observed in the wild. Despite the systematic increase in population density, no attacks by mature males on juveniles were observed.     

Inter- and intraspecific competition for nest holes in a population of the Collared Flycatcher Ficedula albicollis

Field experiments with nest boxes show that nest holes are a limiting factor on the local population size of the Collared Flycatcher Ficedula albicollis. When this limiting resource occurs with different types that are quality related there is intraspecific competition among males for the best nest boxes, with bigger males being the better competitors. Population dynamics of Great Tit Parus major and Blue Tit P. caeruleus when compared with Collared Flycatchers suggests that there is strong interspecific Competition for available nest holes. This is also shown by removal experiments.     

Facultative nest-parasitism among Iberian shrubsteppe passerines

This paper analyses intra- and interspecific facultative nest parasitism of shrubsteppe passerines on the Iberian peninsula through 4 seasons, from 1991–94. Daily frequency of nest parasitism was very low (f = 3.9 × 10^-3 during laying and f = 2.8 × 10^-4 during incubation). Observation of the 3 examples found in the field of 2 intraspecific and one interspecific instances revealed that the eggs were accepted and, in the latter case, that the parasitic chick was viable. The experimental tests in the field of placing 1 additional egg that was similar to (n = 57) or clearly different from the host (n = 50), in order to simulate both intra and interspecific parasitism revealed: (i) higher rejection rates in areas with higher risk of predation and (ii) similar rejection rates for both types of eggs. The experimental addition of nestlings of other species (n = 6) revealed that in 67% of the cases studied, the parasitic chick had a normal growth rate. It is suggested that nest parasitism among these species is an occasional behaviour pattern, caused by high nest predation rates in Iberian shrubsteppe habitats.     

Nest box contentions: Are nest boxes used by the species they target?

Nest boxes have grown in popularity as a habitat management tool in Australia during the last decade. This management use remains contentious because some studies suggest nest boxes are ineffective. There are three recent contentions: (i) nest boxes mostly benefit common species, (ii) exotic species may be dominant users of nest boxes, and (iii) species of conservation concern use nest boxes infrequently. We address these contentions using data from 1865 nest boxes involving eight nest box designs. These nest boxes were installed predominantly <200 m from a road in association with highway duplication and re‐alignment across 16 projects in New South Wales. The Common Brushtail Possum (Trichosurus vulpecula) is the species of most relevance to contention 1. It used 9% of boxes overall including 26% of ‘possum’ designated boxes. The most frequent nest box users were small petaurid gliders (mostly Sugar Gliders, Petaurus breviceps) which used 63% of ‘small glider’ designated boxes. This nest box and another suited to the Sugar Glider comprised 40% of all boxes installed, so it is not surprising that this species might be a common user. Exotic species were uncommon users of the nest boxes enabling contention 2 to be rejected. Active hives of Feral Honeybees (Apis mellifera) occupied just 1% of boxes, and another 1% of boxes were used by introduced rodents and birds. The Squirrel Glider (Petaurus norfolcensis) is the species most relevant to contention 3. It was seen in 80 boxes across 11 projects, representing 7% of the three types most frequently used. These observations are not consistent with the third contention. Nest boxes can provide many important insights about the requirements and interactions of hollow‐dependent fauna. However, they are not intended as an alternative to retaining hollow‐bearing trees.     

Nest destruction elicits indiscriminate con‐ versus heterospecific brood parasitism in a captive bird

Following nest destruction, the laying of physiologically committed eggs (eggs that are ovulated, yolked, and making their way through the oviduct) in the nests of other birds is considered a viable pathway for the evolution of obligate interspecific brood parasitism. While intraspecific brood parasitism in response to nest predation has been experimentally demonstrated, this pathway has yet to be evaluated in an interspecific context. We studied patterns of egg laying following experimental nest destruction in captive zebra finches, Taeniopygia guttata, a frequent intraspecific brood parasite. We found that zebra finches laid physiologically committed eggs indiscriminately between nests containing conspecific eggs and nests containing heterospecific eggs (of Bengalese finches, Lonchura striata vars. domestica), despite the con‐ and heterospecific eggs differing in both size and coloration. This is the first experimental evidence that nest destruction may provide a pathway for the evolution of interspecific brood parasitism in birds.     

Effects of the risk of competition and predation on large secondary cavity breeders

The effects of competition and risk of predation on secondary cavity breeders were examined between the 2008 and 2009 breeding seasons using an experimental design manipulating two nest entrance sizes (large entrances allowed Barn Owls (Tyto alba) to enter, while the small entrances excluded them). During the 2009 breeding season, the entrance sizes of nest boxes were exchanged, so that if during one year a nest box in a particular location had a small entrance, the second year it had a large entrance and vice versa. Barn Owls and Eurasian Kestrels (Falco tinnunculus) occupied 67.3 and 17.3%, respectively, of large entrance nest boxes. Significantly more Jackdaws (Corvus monedula), House Sparrows (Passer domesticus) and Scops Owls (Otus scops) bred in nest boxes with small than with large entrances. After nest box entrance sizes were exchanged, Barn Owls and smaller species did not breed in the same nest boxes with the new entrance size. Jackdaws probably did not breed in large entrance nest boxes due both to exploitation competition (Barn Owls and Eurasian Kestrels occupied the majority of large entrance nest boxes), and may also have avoided empty nest boxes because of the risk of interference competition; whereas smaller species may have also avoided large entrance nest boxes due to risk of predation.     

Habitat characteristics and species interference influence space use and nest‐site occupancy: implications for social variation in two sister species

Nest‐site selection is an important component of species socio‐ecology, being a crucial factor in establishment of group living. Consequently, nest‐site characteristics together with space‐use proxies may reveal the social organization of species, which is critical when direct observation of social interactions is hindered in nature. Importantly, nest‐site choice is expected to be under strong selective pressures and the object of intra‐ and interspecific competition. Although the bulk of research on sociality focuses on its ecological drivers, our study introduces interspecific competition as a potential factor that could influence social evolution. We investigated the influence of habitat and interspecific competition on the social organization of two sister species of the African four striped mouse, Rhabdomys dilectus dilectus and Rhabdomys bechuanae, in a similar macroenvironment. These species diverged in allopatry and occupy distinct environmental niches. We radiotracked 140 adults to identify their nest‐sites, determine nest characteristics and record groups that shared nest‐sites. Group cohesion was estimated from nest‐site fidelity, group association strength, and home range overlap within versus between group members. We compared the two species in sympatry versus parapatry to determine the impact of species interference on sociality. In parapatry, the two species selected distinct nest‐site types, interpreted as different anti‐predator strategies: R. bechuanae selected fewer, spaced, less concealed nest‐sites whereas R. d. dilectus selected clumped and less visible nest‐sites. Rhabdomys bechuanae also showed more cohesive and stable social groups than R. d. dilectus. In sympatry, compared to R. bechuanae, R. d. dilectus occupied similar nest‐sites, however slightly more exposed and clumped, and displayed similar nest‐site fidelity and group association strength. We conclude that although habitat selection may be an important driver of social divergence in Rhabdomys, species interference, by limiting R. d. dilectus movements and forcing nest‐site sharing may induce new ecological pressures that could influence its social evolution.     

How well do predation rates on artificial nests estimate predation on natural passerine nests?

Daily survival rates (DSR) of three nest types were compared among three passerine species in woodland habitats of the Czech Republic in 1998‐99. The species were: Yellowhammer (YH) Emberiza citrinella, Blackcap (BC) Sylvia atricapilla, Song Thrush (ST) Turdus philomelos. The nest types were: AA, artificial nests with artificial plastic eggs, both mimicking the real models (total n= 432); NA, natural nests left in their original position and baited with artificial plastic eggs (n= 706); and NN, active natural nests observed during the egg stage (n= 596). Rodents were dominant predators of the near‐ground YH nests while predation by corvids slightly prevailed in the shrub nests of BC and ST. There was no consistent relationship between nest type (AA vs. NA) and type of predator. Effects of nest type (AA vs. NA or AA vs. NN, respectively), species, date and study plot on DSR were examined by fitting logistic regression models. Marginally significant (< P < 0.05) differences were found between AA and NA nests in both years. Due to the interaction with date in 1998, the model predicted lower/higher DSR for AA nests before/after about 25 May respectively. In 1999 the DSR of AA nests was consistently higher (difference: 0.011; 95% CI: 0.001 to 0.021). Marginally significant differences between AA and NN nests were found only in 1998: the DSR of AA nests was consistently lower (‐0.014, 95% CI: ‐0.027 to ‐0.001). Comparisons of mean DSR within each species‐year sample showed that the differences between nest types were of both directions and varied from almost zero to 0.02 (ST, BC; ns) or 0.04 (YH; P < 0.05). Although the experimental nests accurately reflected the mean values as well as seasonal trends and interspecific differences in DSR of the real nests in some subsets of data, the pattern was not consistent across species and between years. This study suggests that artificial nests could not be used to assess survival of natural nests without careful validation of the experimental results.     

Use of novel nest boxes by carmine bee‐eaters (Merops nubicus) in captivity

Carmine bee‐eaters make attractive additions to zoo aviaries but breeding programs have had challenges and limited success. The objectives of this study were to document nesting behavior of Carmine bee‐eaters in a captive setting and compare reproductive success between a novel nest box (plastic, 17 × 30 × 22 cm) and a PVC pipe model used previously (30 cm long, 8 cm in diameter). Three bee‐eater pairs were given access to seven nest chambers (six novel boxes, one PVC model). Behavioral observations occurred during a 15‐min period in the morning or afternoon before egg production and continued until chicks fledged for a total of 87 observation periods (21.75 hr). All occurrences by an individual bird entering or exiting a nest tunnel, food provision, and the time (min) spent inside a nest cavity were documented. Additionally, daily temperature within each nest chamber was recorded. Before eggs were produced the average daily temperature (23.02°C) within the nest chambers did not differ, suggesting that nest cavity choice was not influenced by temperature. No differences were detected among pairs in percent of observed time spent inside their nest cavities or number of times a nest tunnel was entered during the incubation or fledging periods. During incubation females spent a greater percent of observed time inside the nest cavity than males (P=0.02). During the fledging period food provision did not differ between the pairs, however males entered their nest tunnels more often per hour than females (P=0.03), and males tended to provide food more often than females (P=0.053). Two pairs nested in novel nest boxes and successfully fledged one chick each. The pair that nested in the PVC model did not fledge a chick. A nest box that aids in keeping eggs intact is essential for breeding bee‐eaters in captivity, and maintaining captive populations will provide opportunities for zoo visitors to enjoy these birds and will reduce the need to remove birds from the wild. Zoo Biol 0:1–13, 2007. © 2007 Wiley‐Liss, Inc.     

The individual nest box as a super-stimulus for domestic hens

Following reports of inconsistent nest choice by hens in groups, choice of nest boxes by solitary hens was investigated. Thirty-two hens of a White Leghorn strain were each isolated with six nest boxes while they laid six eggs. From 16 hens, eggs were collected daily. Only one of these birds laid all her eggs in one nest box, and others used up to four boxes. The other 16 were allowed to accumulate eggs, and these were significantly more consistent in nest choice. Five used only one box. However, the other 11 of this group showed incomplete consistency.An explanation for inconsistent nest choice was suggested by a second experiment. The importance of enclosure in nest-site selection was tested in hens of White Leghorn and Rhode Island Red strains. Individuals about to lay their first egg were isolated with a choice of two types of nest box, selected from five types of different enclosure. Exposed nests, of similar dimensions to natural nests, were rarely used by either strain. By contrast, more enclosed nests were strongly selected. These results indicate that enclosure is an important stimulus in nest-site selection, and that artificial nest boxes, which are more enclosed than natural sites, act as super-normal stimuli (or “super-stimuli”). Choices between super-stimuli are likely to be equivocal, which accounts for inconsistency in the use of nest boxes and other features of the nesting behaviour of domestic hens.     

Provisioning habitat with custom‐designed nest‐boxes increases reproductive success in an endangered finch

Manipulating resource availability to assess the strength and effects of resource limitation on an animal population is relatively straightforward, and thus, common in the scientific literature. Resource quality, however, is rarely manipulated, as this requires a priori knowledge of a relationship between some measurable variation in the resource and individual preference and/or fitness. Recent research on nest‐site selection was used to inform the design of custom‐built nest‐boxes for an endangered hollow‐nester, the Gouldian finch (Erythrura gouldiae). By provisioning breeding populations with these ‘high quality’ artificial nest sites over 3 years, we experimentally investigated the strength and effects of nest‐site limitation in this species. Breeding pairs using nest‐boxes initiated nesting earlier, produced larger clutches and fledged more offspring per season than those in natural hollows. Total and mean reproductive output also increased in nest‐box provisioned sites over the duration of the study. All of these effects were predominantly driven by pairs initiating clutches earlier, potentially because of the reduced intra‐ and interspecific competition for nest sites at optimal breeding times. Our findings suggest that reproduction in wild Gouldian finch populations may be limited by the availability of high quality nest sites in the landscape, and that nest‐boxes could be used as a tool for enhancing reproduction in recovering populations. Furthermore, we conclude that resource quality is equally, if not more important than quantity when supplementing resource‐limited habitats.     

Parent birds assess nest predation risk: influence of cavity condition and avian nest predator activity

Skutch hypothesized that nest predators visually assess parental activities to locate a prey nest, whereas parents modify fitness‐related traits to reduce the probability of nest predation. We examined how cavity condition and parental activity interact with avian nest predators to shape the nest success of two coexisting parid species, marsh tits Poecile palustris and oriental tits Parus minor, breeding in nest‐boxes during the incubation period. Nest‐boxes were manipulated to create a prolonged risk of nest predation (entrance diameter 2.6 cm control vs 5.5 cm treatment) soon after clutch completion. To measure changes in parental behavior, we also simultaneously simulated a pulsed risk of nest predation, using sound playbacks of a coexisting control bird and an avian nest predator. We found that the parent tits merely responded the pulsed risk, presumably due to an environment with high avian nest predator encounters, compared to the prolonged risk. Instead, both species spent more time on vigilance at the nest, only under prolonged risk conditions. The activity of corvids near the nest‐box was higher in the marsh tit than that in oriental tits. This activity was also higher in the treatment nest box than that in the control nest‐box. Nest predation during the incubation period was higher in marsh tits than in oriental tits, presumably due to higher and more plastic vigilance in oriental tits, compared to marsh tits. Our results highlight that the differences in cavity condition and parental activities at the nests of two coexisting non‐excavators may contribute to differential nest predation by attracting avian nest predators.     

A chemically triggered transition from conflict to cooperation in burying beetles

Although interspecific competition has long been recognised as a major driver of trait divergence and adaptive evolution, relatively little effort has focused on how it influences the evolution of intraspecific cooperation. Here we identify the mechanism by which the perceived pressure of interspecific competition influences the transition from intraspecific conflict to cooperation in a facultative cooperatively breeding species, the Asian burying beetle Nicrophorus nepalensis. We not only found that beetles are more cooperative at carcasses when blowfly maggots have begun to digest the tissue, but that this social cooperation appears to be triggered by a single chemical cue – dimethyl disulfide (DMDS) – emitted from carcasses consumed by blowflies, but not from control carcasses lacking blowflies. Our results provide experimental evidence that interspecific competition promotes the transition from intraspecific conflict to cooperation in N. nepalensis via a surprisingly simple social chemical cue that is a reliable indicator of resource competition between species.     

Alternative adaptations, sympatric speciation and the evolution of parasitic, inquiline ants

Inquiline ant species are workerless social parasites whose queens reproduce in colonies of other species alongside the host queens. Inquilines arise either when one non-parasitic species evolves into an inquiline parasite of another non-parasitic species (the interspecific hypothesis), or by the speciation of intraspecific inquilines from their host stock (the intraspecific hypothesis): it is unlikely that inquilines evolve from other forms of social parasite. This paper reviews the evidence for and against the inter-and intraspecific hypotheses. All inquilines are close phylogenetic relatives of their host species (loose ‘Emery's rule’), and some are their host's closest relative (strict ‘Emery's rule’). A problem for the interspecific hypothesis is how to explain the strict Emery's rule, because phylogenetic constraints on host choice are probably quite weak. By contrast, the intraspecific hypothesis has difficulty accounting for the parasites' sympatric reproductive isolation. Facultative polygyny, in which queens may found colonies alone or by adoption into an existing multi-queen colony, should promote the evolution of small intraspecific inquilines. This is because small colony-founding queens should preferentially seek adoption, which provides the opportunity to produce a sexual-only brood. We suggest that microgynes, i.e. miniature queens found in some polygynous ants, represent such parasites. We review the evidence that inquiline species have evolved intraspecifically from microgynes in Myrmica ants. The coexistence within a species of a monogynous (singly-queened) and a polygynous form is probably a phenomenon usually unconnected with inquiline evolution. The reproductive isolation of intraspecific inquilines plausibly arises from divergent breeding behaviour associated with the parasites' small size. Such divergence could involve either a temporal separation in mating episodes, with small parasites maturing early, or a spatial separation, with small males being sexually-selected to mate near the nest with small queens seeking adoption, instead of in mating aggregations. We conclude that inquiline species strictly following Emery's rule could have evolved by the intraspecific route. If so, such species provide evidence for West-Eberhard's “alternative adaptation” hypothesis that between-species diversity frequently stems from diversity within species. They also represent likely cases of sympatric speciation. We suggest work on the parasites' phytogeny, genetics, behaviour and mating biology to test these conclusions further.     

Between-species jamming avoidance in Pipistrelles?

Past studies have reported that bats modify the spectral–temporal features of their vocalizations to avoid interference from the vocalizations of conspecifics (jamming avoidance). We tested for within, and between-species jamming avoidance in two sympatrically occurring species of vespertilionid bats, Pipistrellus pipistrellus and Pipistrellus nathusii. In both species the peak frequencies of the narrowband frequency component from samples of bats flying with conspecifics varied more than those from samples of bats flying alone, indicating intraspecific jamming avoidance. If individuals of the two species were recorded together, we found no reaction of P. nathusii. P. pipistrellus, however, used significantly higher frequencies than when recorded alone. As the narrowband frequency of this species normally overlaps with the frequency modulated (FM) part of P. nathusii calls, this can be interpreted as an interspecific jamming avoidance response.     

Specific nest box designs can improve habitat restoration for cavity‐dependent arboreal mammals

Tree‐cavity‐dependent wildlife faces future shortages of cavities due to a decline in the abundance of large, old trees in many parts of the world. Nest boxes are proposed as a tool to restore habitat value but evidence of their effectiveness for arboreal mammals remains equivocal. This may arise from a poor understanding of design preferences. We conducted investigations in two landscapes in eastern Australia to determine whether species show a preference for specific designs. We observed a preference by some mammal species for particular designs (33–78% occupied/used), suggesting that design refinement can improve the frequency with which nest boxes are used. Although feral species may out‐compete target species for nest boxes, we did not observe this. We recorded feral honeybees (Apis mellifera) in 6–9% of nest boxes but they did not remain, and many occupied boxes were later used by mammals. The introduced common myna bird (Acridotheres tristis) was prevalent in one landscape, but competition for nest boxes was localized. For nest boxes to be an effective habitat restoration tool, they must be able to be occupied over long periods of time. We investigated this for the squirrel glider (Petaurus norfolcensis), an arboreal marsupial threatened through part of its geographic range. Squirrel gliders occupied and bred within nest boxes (100% used) at two locations continuously over a 10‐year period with minimal nest box maintenance. Individuals occupied boxes for up to 7 years. This confirms that targeted nest box programs can be an effective restoration tool for cavity‐dependent arboreal mammals.     

分蘖型克隆植物黍分株和基株对异质养分环境的等级反应

为揭示 1年生分蘖草本黍对异质养分环境的等级反应 ,比较了 4种异质养分环境中黍分株和基株的生长、繁殖特征。在分株水平 ,异质养分环境显著影响源株高度、叶面积、繁殖分配、千粒重和种子数 ,而对生物量无影响 ;异质养分环境显著影响分蘖株的这 6个指标。在基株水平 ,生物量、分蘖数和种子数随环境中养分总量的提高而逐渐增大 ,根重比恰好相反 ;繁殖分配不受异质养分环境的影响 ;除一种处理外 ,其它 3种环境中的千粒重不受养分状况的影响。这些结果指示 :黍分株和基株对异质养分环境的反应具有等级性 ,源株和分蘖株对相同环境的反应存在差异。     

Inter- and Intraspecific Variation in the Acoustic Habitats of Three Marmot Species

Closely related species often have remarkably different vocalizations. Some of the variation in acoustic structure may result from species adapting their calls to maximize transmission through their acoustic environments. We document the relative magnitude of inter- and intraspecific variation in acoustic transmission properties of the habitats of three closely related marmot species to study the relative importance that the acoustic environment may have played in selecting for species-specific marmot alarm calls. We used spectrogram correlation to quantify the degree to which pure tones and alarm calls changed as they were broadcast through marmot home ranges to describe the acoustic habitats of golden (M. candata aurea), yellow-bellied (M. flaviventris), and alpine (M. marmota L.) marmots. Species lived in quantifiably different acoustic habitats. One analysis partitioned variation between species and between marmot social groups (nested within species). We found significant interspecific variation in the acoustic transmission fidelity of the three species' habitats and insignificant intraspecific variation between social groups. Further analysis of a larger sample of alarm calls broadcast through golden marmot social groups found significant intraspecific variation. Interspecific variation greater than intraspecific variation suggests that variable acoustic habitats may be responsible for at least some of the interspecific variation in alarm call structure. This is the first study to use spectrogram correlation to describe habitat acoustics. We discuss aspects of the method that may be useful for others seeking to quantify habitat acoustics.     

PVC nest boxes are less at risk of occupancy by feral honey bees than timber nest boxes and natural hollows

Feral European Honey Bee (Apis mellifera) has been identified as a potential nest competitor for Australian hollow nesting species, but few studies have investigated the impact of feral honey bee competition on Threatened species. Our study used data from Glossy Black‐cockatoo (Calyptorhynchus lathami halmaturinus) nests on Kangaroo Island, monitored and managed over an 11‐year period, and found 12% of nests became occupied by feral honey bees during that period. Our results indicate that feral honey bees were less likely to occupy nest boxes made of PVC (5%) compared with wooden nest boxes (24%) or natural hollows in Eucalyptus trees (14%). The removal of feral honey bee hives from nests is a priority for long‐term conservation of glossy black‐cockatoos on Kangaroo Island. We recommend that PVC nest boxes are chosen for future nesting habitat restoration, due to the more frequent use of wooden nest boxes by feral honey bees.     

Interactions amongst species in a guild of subtidal benthic herbivores

Summary The subtidal coralline flats of northeastern New Zealand support a characteristic guild of grazing herbivores. The most important members of this guild are an echinometrid echinoid, patellid, turbinid and trochid gastropods. Densities of these herbivores fluctuate through time. Interactions within and among the different species of echinoids and gastropods were investigated experimentally. Different combinations of species were caged at densities up to 5 times that of ambient for a 24 week period in an experiment designed to differentiate between intra- and interspecific competition.The echinoidEvechinus chloroticus and the turbinid gastropodCookia sulcata exhibited reduced mean dry weight with increasing intraspecific densities. There was little evidence of density-related mortality in these species. The limpetCellana stellifera showed comparatively large losses of weight and enhanced mortalities in intraspecific experimental treatments but this was not related to density.Investigation of interspecific interactions amongstEvechinus andCookia revealed no evidence of a negative influence of one species on the other. In terms of dry weight,Cookia was indifferent to the presence ofEvechinus, andEvechinus benefited in the reciprocal interaction.Cookia also enjoyed an enhanced mean dry weight when in the presence ofCellana compared to the equivalent intraspecific treatments. There were no coherent trends in proportional mortality in any treatments with enhanced interspecific densities. Cellana, in the presence ofCookia, exhibited a dramatic decrease in mortality rate and increase in mean dry weight. The presence of the turbinid gastropod was clearly beneficial to the limpet when compared to the intraspecific treatments with enhanced intraspecific densities and the control cages containingCellana at ambient density. We suggest that subtidal areas constitute poor habitats for limpets in the absence of agents such asCookia which may provide or maintain suitable sites for attachment and grazing.For the combinations of densities and species investigated there was a consistent trend towards positive interspecific interactions. It seems unlikely that at the sites investigated interspecific competition could act to restrict distributions, or limit abundances of species.