Resource partitioning and competition for shells in a subtidal hermit crab species assemblage

Summary Shell and habitat utilization are quantified for 12 hermit crab species occurring subtidally in the San Juan Archipelago, Washington. The mechanism of competition for shells between these species is investigated using laboratory experiments to determine shell preferences, shell acquisition rates, and rates of exchange of shells via shell fighting. This information is used to estimate relative intensities of inter- and intra-specific competition for shells between the species in this assemblage. In contrast to earlier findings on intertidal hermit crab assemblages, a significant number (5) of the species in this assemblage appear to experience a greater reduction in their shell supply due to members of other species than due to other members of their own species. The relative amounts of inter- and intra-specific competition differ greatly for different species in the community. The high average figures for interspecific/intraspecific competition are largely a result of the presence of three abundant and very generalized species. In spite of the large number of species and relatively high ratios of interspecific to intraspecific competition, the species in this group are not close to a limiting similarity in resource use. There is suggestive evidence that greater selection pressures for divergence in habitat use may have resulted in the lower amounts of overlap observed in intertidal hermit crab assemblages in previous studies.     

Competition in an Indo-Pacific hermit crab community

Summary This article uses a method developed previously (Abrams 1980) to estimate the relative intensities of inter- and intra-specific competition between pairs of species in a highdiversity intertidal hermit crab community in the tropical Indo-Pacific. The community could be separated into two sub-communities, with relatively weak interactions between members of different sub-communities. The average amount of inter-specific competition experienced by the members of this community was greater than that for the species in a less diverse community in Panama (1980). Average competition ratios for the species in the Indo-Pacific community were still less than competition coefficients calculated from overlap data for other taxa of animal.     

Resource partitioning between sexes in the "unconventional" hermit crab, Calcinus tubularis

Together with Calcinus verrilli from Bermuda, the Mediterraneanhermit crab C. tubularis is unique in that it exhibits a sharpsexual dimorphism in resource use in which one sex, the male,occupies loose shells and the other, the female, occupies attachedtubes. Field surveys and laboratory experiments addressed twoissues that help understand male-female resource partitioningin this species. First, the value of shells as shelters differsbetween sexes. Shells furnish mobility to their inhabitants,but also greater opportunities to grow because they can providelarger size than can the tubes, whereas both mobility and largedimensions are crucial selective factors for the reproductiveoutcomes of males only. In fact, egg production seemed not tobe affected by females' sedentary life and the number of eggswas not related to the female size. Second, previous shelterexperience plays a role in reducing male-female competition.I found that individual crabs, once presented simultaneouslywith a shell and a tube, more likely selected the type of shelterthat they were collected in. Then, in male-female competitionexperiments, pairs composed of one crab found in a shell andone crab found in a tube and offered a shell and a tube in conjunctionoccupied the housing in accordance to the shelter previouslyoccupied without any influence of their sex or size. The questionremains as to how, why, and when such a sexual dimorphism inthe distribution of C. tubularis between microhabitats did occurduring a hermit crab's life.     

Resource partitioning and competition for shells between intertidal hermit crabs on the outer coast of Washington

Summary Resource partitioning was quantified for 6 species of intertidal hermit crabs in the genus Pagurus, that occur on the outer coast of Washington. This, together with field evidence of shell shortage and with laboratory experiments to quantify the mechanism of interactions for shells, allowed estimation of the relative intensities of inter-and intraspecific competition between these species. The findings were that: (1) the magnitude of intraspecific competition was greater than any single interspecific competitive effect for all of the species; and (2) the relative proportion of intraspecific competition was greater for the middle and upper intertidal species than for the lower intertidal species. Studies at several outer coast sites supported these generalizations. Both of these findings are consistent with the hypothesis that competitive divergence has occurred in the past. The structure of the outer coast hermit crab assemblage is compared with that of the San Juan Archipelago hermit crab assemblage. Differences between the two do not seem to be the result of adaptive responses to the presence of more competing species in the former group.     

Shell fighting and competition between two hermit crab species in Panama

Summary This article is a study of shell fighting between two intertidal hermit crab species in Panama. Laboratory results showed some cases of high exchange frequencies between Calcinus obscurus and Clibanarius albidigitus when the former occupied poor-quality shells. Exchange frequencies varied considerably between collecting sites, and were always low when the defending Clibanarius came from Venado Beach. Shell exchange frequencies estimated from field experiments were similar to those obtained in the laboratory. Observations on relative shell sizes occupied by both species in areas of sympatry and allopatry failed to provide clear evidence that Calcinus reduced the shell size of Clibanarius or that Clibanarius increased the shell size occupied by Calcinus. Results obtained here differ from those obtained in previous studies (Abrams 1980; Bertness 1981a, b), and these differences are discussed. Although shell fighting may be an important component of the interaction of these species, it is likely that roughly 90% of the competition experienced by each species is intraspecific.     

Resource partitioning and interspecific competition in snakes: the search for general geographical and guild patterns

The role of interspecific competition as a key factor in the ecology of natural communities where species exploit limited resources is well established, and the study of competition dynamics in snake communities has received much attention in recent years. Twenty years ago, an acclaimed review ( Toft 1985 ) suggested that snakes were atypical among vertebrates because sympatric species usually partition the food niche. Here, I review the articles published in the last two decades with the aim of finding any general geographical or guild patterns and assessing if Toft's main conclusion is still supported by new evidence. Where appropriate, I use Monte Carlo simulations to establish whether observed patterns of niche overlap are real, or if they have occurred by chance. My study shows clear congruence in the patterns of coexistence exhibited by snake communities in different regions of the world, i.e.: (1) cold regions of the northern hemisphere (high latitudes and altitudes) exhibit low species richness and a very low, or even absent, potential for interspecific competition; (2) aquatic snakes that form communities in temperate regions generally partition the food type available and exhibit a broad similarity in habitat use with subtle differences in microhabitat use; (3) terrestrial snake communities in temperate regions are very variable in terms of their coexistence dynamics and show no evidence of generalised patterns; (4) sympatric viperids in Europe, North America and, most interestingly, tropical Asia partition the available habitat but not the prey resource; (5) competition is much stronger in tropical snake communities, and the intensity of this process fluctuates throughout the year being most intense during periods of low food availability; (6) in general, tropical snakes partition the food resource (prey type and/or prey size), but when this resource is not partitioned competitive exclusion can occur. Prey resource availability is a fundamental variable for all snake communities; this is clearly documented by studies on terrestrial snakes in Australia where, due to a relative scarcity of prey availability in the field, sympatry among species is much rarer than in other continents. I conclude that, although there are several notable exceptions, Toft's main conclusion is still supported by empirical evidence. However, I disagree with Toft's conclusion that most snakes are food specialists, and I contend that interspecific competition is important in structuring many (if not most) of the snake communities around the world.     

Resource partitioning in a tropical stream fish assemblage

An assemblage of freshwater fishes inhabiting a medium-sized stream in tropical central Vietnam was investigated with respect to spatial and trophic distribution. Point-abundance sampling, gut content analysis and relative intestine length yielded interspecific differences in niche expression. Conspecific juvenile and adult habitat and feeding niches were also distinct. Niche differences arose mainly from differences in current velocity, substrata and foraging preferences. Extreme specializations, such as selective feeding on hard incustrations of cyanobacteria by adult Annamia normani , were present. The low niche overlap suggests that this tropical fish assemblage is structured by competitive interactions.     

Intraspecific relationships of the hermit crab Diogenes pugilator: predation and competition

The population of the hermit crab Diogenes pugilator is numerically quite substantial in the examined area, therefore it is analysed if the population was subject to any pressure by predators and by other species of hermit crabs living in that zone. It was found there is a potential pressure by predators which might influence it. Only Liocarcinus depurator is able to keep the population trend of D. pugilator under control. With regard to the relationships with the other species of hermit crab, D. pugilator dominates only over Pagurus anachoretus, while in the other cases, there was just competition for food.     

Intraspecific competition and aggression for shells in the hermit crab Pagurus samuel1s

Many studies have investigated shell‐related behaviour in hermit crabs. Few studies, however, have focused specifically on the intraspecies aggression associated with shell competition. We examined intraspecies aggression in hermit crab (Pagurus samuelis) pairs as it relates to competition for a limiting resource, gastropod shells. Pairs of hermit crabs were observed in the laboratory in four different treatments that varied the presence or absence of shells for one or both of the crabs. Measurements of the latency to respond, the number of bouts, and the fight durations were recorded. There was a significant difference among treatments for all three measurements, and naked hermit crabs were much more aggressive than housed hermit crabs. There was no significant difference in aggression between males and females in any of the three treatments. The heightened aggression observed in naked P. samuelis is likely in service of acquiring a protective shell.     

Resource partitioning in a cetacean community from Uruguayan waters

The study of resource partitioning within a community is central to understanding the processes that enable interspecific competition and coexistence. This study aimed to understand the resource partitioning and habitat preferences among odontocetes in Uruguay. We analyzed stable carbon and nitrogen isotopes data from the bone tissue of common dolphins (Delphinus delphis), Fraser's dolphins (Lagenodelphis hosei), false killer whales (Pseudorca crassidens), Lahille's bottlenose dolphins (Tursiops truncatus gephyreus), Burmeister's porpoises (Phocoena spinipinnis), and franciscana dolphins (Pontoporia blainvillei). Based on the isotopic values, false killer whale individuals were divided into two different ecological groups (ecotype 1 and 2). Isotopic values also suggest that franciscana dolphins, Burmeister's porpoises, Lahille's bottlenose dolphins, and false killer whales ecotype 1 feed on the continental shelf and/or high trophic level preys. At the other end of the gradient, values suggest Fraser's dolphins and false killer whales ecotype 2 use resources from the oceanic environment and/or lower trophic preys. Isotopic niche overlap was found between the common dolphin and Lahille's bottlenose dolphin and between Lahille's bottlenose dolphin and false killer whale ecotype 1. This work provides novel basic information about the trophic habit and feeding habitat of the odontocetes inhabiting one of the most important estuaries of South America.     

Pair formation in a coral inhabiting hermit crab

Summary The aqueous extract of Euphorbia granulata Forssk., prepared in Hoagland's nutrient solution; and underlying soil significantly inhibited germination and radicle growth of Dichanthium annulatum, Cynodon dactylon, Setaria italica, Pennisetum americanum, Euphorbia pilulifera, Oxalis corniculata and Lactuca sativa invariably in laboratory bioassays. Artificially decomposed E. granulata litter in a nutrient medium significantly reduced germination and fresh and dry weight of the aforementioned test species. The toxins affected germination and growth independently. The local dominance of this weed over its counterparts was primarily due to allelopathy, and it is therefore suggested that its litter must be removed from the fields during weeding.     

元宝山冷杉群落种内与种间竞争的数量关系

元宝山冷杉 (AbiesyuanbaoshanensisY .J .LuetL .K .Fu)特产广西 ,是种群数量极少的濒危物种。根据调查资料 ,采用Hegyi提出的单木竞争指数模型CI=∑Nj=1 (Dj/Di)·1Lij对元宝山冷杉群落的种间、种内竞争强度进行定量分析。结果表明 :元宝山冷杉种内竞争较之与其伴生树种间的竞争剧烈 ;竞争木对对象木的竞争强度与对象木的胸高直径服从幂函数关系CI=ADB,竞争强度随对象木个体的增大而减小。当元宝山冷杉胸高直径达到 35～ 40cm后 ,竞争强度变化不明显。利用模型预测了元宝山冷杉种内和种间的竞争强度     

Resource partitioning and competition in honeyeaters of the genus Meliphaga

Six species of Meliphaga studied in detail in South Australia all differ in some important aspect of their ecology consistent with the concept of interspecific competition. Four species are very similar in their feeding ecology but show distinct habitat preferences. The two remaining species are rather different in their feeding ecology and frequently overlap with their congeners in habitat. Two other species, not studied in detail, also appear to have distinct habitat preferences. One species has increased its range of habitat on Kangaroo Island, in the absence of potential competitors. The case for interspecific competition playing a part in the moulding of the niches of species in this genus is thus strengthened.     

Temporal resource partitioning mitigates interspecific competition and promotes coexistence among insect parasites

A key to understanding life's great diversity is discerning how competing organisms divide limiting resources to coexist in diverse communities. While temporal resource partitioning has long been hypothesized to reduce the negative effects of interspecific competition, empirical evidence suggests that time may not often be an axis along which animal species routinely subdivide resources. Here, we present evidence to the contrary in the world's most biodiverse group of animals: insect parasites (parasitoids). Specifically, we conducted a meta-analysis of 64 studies from 41 publications to determine if temporal resource partitioning via variation in the timing of a key life-history trait, egg deposition (oviposition), mitigates interspecific competition between species pairs sharing the same insect host. When competing species were manipulated to oviposit at (or near) the same time in or on a single host in the laboratory, competition was common, and one species was typically inherently superior (i.e. survived to adulthood a greater proportion of the time). In most cases, however, the inferior competitor could gain a survivorship advantage by ovipositing earlier (or in a smaller number of cases later) into shared hosts. Moreover, this positive (or in a few cases negative) priority advantage gained by the inferior competitor increased as the interval between oviposition times became greater. The results from manipulative experiments were also correlated with patterns of life-history timing and demography in nature: the more inherently competitively inferior a species was in the laboratory, the greater the interval between oviposition times of taxa in co-occurring populations. Additionally, the larger the interval between oviposition times of competing taxa, the more abundant the inferior species was in populations where competitors were known to coexist. Overall, our findings suggest that temporal resource partitioning via variation in oviposition timing may help to facilitate species coexistence and structures diverse insect communities by altering demographic measures of species success. We argue that the lack of evidence for a more prominent role of temporal resource partitioning in promoting species coexistence may reflect taxonomic differences, with a bias towards larger-sized animals. For smaller species like parasitic insects that are specialized to attack one or a group of closely related hosts, have short adult lifespans and discrete generation times, compete directly for limited resources in small, closed arenas and have life histories constrained by host phenology, temporal resource subdivision via variation in life history may play a critical role in allowing species to coexist by alleviating the negative effects of interspecific competition.     

Filter feeding in the hermit crab

Summary The hermit crab, Pagurus bernhardus is able to remove both Artemia nauplii and unicellular algae from suspension. Crabs with wet weights of 1.1–9.2 g consumed all of the 300 Artemia nauplii contained in 200 ml of sea water within 1 h. Crabs weighing 0.7–1.1 g wet weight filtered suspended Dunaliella algae (8 m) from concentrations of 10–350 million cells per liter at a rate of 26% and 47% within 2 and 5 h, respectively. A similar result was obtained with an 11 g crab. During filter feeding activity a water current is generated by the flagella of the exopods of the second and third maxillipeds. Artemia nauplii are caught by grasping movements of the endopods of the third maxillipeds, whereas filtering of unicellular algae is probably achieved by the two maxillae. It is assumed that filter feeding activity depends on the same structures and behavior as in deposit feeding. P. bernhardus is one more example of a benthic marine animal which may use any food source which becomes available in the course of the seasons.     

Species partitioning in a temperate mountain chain: Segregation by habitat vs. interspecific competition

Disentangling the relative influence of the environment and biotic interactions in determining species coexistence patterns is a major challenge in ecology. The zonation occurring along elevation gradients, or at bioclimatic contact zones, offers a good opportunity to improve such understanding because the small scale at which the partitioning occurs facilitates inference based on experiments and ecological modelling. We studied the influence of abiotic gradients, habitat types, and interspecific competition in determining the spatial turnover between two pipit and two bunting species in NW Spain. We explored two independent lines of evidence to draw inference about the relative importance of environment and biotic interactions in driving range partitioning along elevation, latitude, and longitude. We combined occurrence data with environmental data to develop joint species distribution models (JSDM), in order to attribute co‐occurrence (or exclusion) to shared (or divergent) environmental responses and to interactions (attraction or exclusion). In the same region, we tested for interference competition by means of playback experiments in the contact zone. The JSDMs highlighted different responses for the two species pairs, although we did not find direct evidence of interspecific aggressiveness in our playback experiments. In pipits, partitioning was explained by divergent climate and habitat requirements and also by the negative correlations between species not explained by the environment. This significant residual correlation may reflect forms of competition others than direct interference, although we could not completely exclude the influence of unmeasured environmental predictors. When bunting species co‐occurred, it was because of shared habitat preferences, and a possible limitation to dispersal might cause their partitioning. Our results indicate that no single mechanism dominates in driving the distribution of our study species, but rather distributions are determined by the combination of many small forces including biotic and abiotic determinants of niche, whose relative strengths varied among species.     

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.     

Levels of understanding in ecology: interspecific competition and community ecology

Abstract Ecological interpretation has been subject to several divisive controversies, involving, for example, the significance of density dependence and interspecific competition as ecological processes. Generally, resolution has been obtained through compromise and concensus or calls for yet more data. Essentially, both sides in the discussion are seen to have been correct to some extent. As a consequence the debates have been portrayed widely as having been sterile. We agree, but only because they have been conducted at a level so superficial that the relevance of the original criticisms to the theoretical structure of ecology has not been widely appreciated, nor resolved. Debate that deals with ecological generalizations must be conducted at a level appropriate to such aims.     

Movement,activity pattern,and rôle of a hermit crab population in a sublittoral epifaunal community

The movements and activity of hermit crabs in a sublittoral soft-bottom community in the North Adriatic Sea (Gulf of Trieste) were monitored by means of observations by a SCUBA diver and time-lapse photography. Tagging experiments showed that the hermit crabs exhibit a pattern of movement which causes them to remain in a defined area for extended periods. During a two-year period, the radius of activity of a crab is estimated to be under 15 m. This leads to a high recovery rate of tagged shells. The average speed of a hermit crab was 2.1 m/h and the average distance travelled per day was 21.6 m as calculated from the analysis of the time-lapse films. This distance is an order of magnitude larger than that estimated from the tagging experiments (1.1 m/day). The activity of the crabs was restricted to the sediment surface between multi-species clumps. The approximate area of the sediment surface traversed by a crab is 0.4 m²/day. Many inter- and intraspecific encounters were observed, particularly among hermit crabs and the brittle stars Ophiothrix quinquemaculata (D.Ch.) and Ophiura lacertosa (Pennant). The daily activity pattern of the crabs is related to the light and dark phases on the sediment surface. The high density and conspicuous activity of the hermit crabs point to the important rôle of the pagurids in the investigated community.