Effects Of Intraspecific Competition Research Articles

Prey Abundance, Intraguild Predators, Ants and the Optimal Egg-laying Strategy of a Furtive Predator

Larval performance can have a great influence on female oviposition choice, especially in insects where the newly hatched offspring are unable to move any great distance to find an appropriate food source. For furtive predators, like the predatory midge Aphidoletes aphidimyza which preys on aphids while simultaneously residing and remaining undetected within their colonies, oviposition behaviour is crucial because these slow moving offspring are restrained to their natal colony. Here we develop a new model for predicting the optimum number of eggs that a furtive predator should lay in an aphid colony, based on: (1) the number of available prey (aphids); (2) the protection from predation conferred by “hiding” in the colony and (3) the effects of interspecific and intraspecific competition. We also explore the effect of aphid attendance by ants on oviposition behavior. We compare model predictions with empirical field observations of the clutch sizes of A. aphidimyza in apple orchards. The simplest of the four models best fits the observed data and provides the first field evidence that a furtive predator adjusts its clutch size as a function of prey density. The slope of the relationship between clutch size and aphid number is quite close to that predicted by our models suggesting that intra-clutch competition is the main factor governing furtive aphid midge oviposition choice.

Density-dependent effects on morphological plasticity of Mytilus gallloprovincialis in suspended culture

Bivalve molluscs are characterized by high morphological plasticity in response to variations in local environmental conditions. In the present study, we evaluate this capacity in the mussel Mytilus galloprovincialis with regard to intra-specific competition caused by cultivation density. Suspended cultivation ropes at different initial densities (220, 370, 500, 570, 800 and 1150 individuals per meter of rope) were placed on a raft in the Ría Ares-Betanzos, following standard cultivation techniques. From May to October, covering the period from thinning out to harvest, various morphological indicators (length/width, length/height and height/width ratios) and allometric relationships (Volume-Length and Projected area-Length) in addition to energy distribution (dry tissue weight/dry valve weight) were analyzed. Differences in morphological indicators due to cultivation density were observed while no influence on the energy distribution was detected. From the second cultivation month (June), a decrease was recorded in the length/height ratio of individuals due to cultivation density. Similarly, there was also a decrease in the length/width ratio, although this decrease only became statistically significant from September. In addition, the allometric relationships studied (Volume-Length and Projected area-Length) suggested asymmetric competition processes. Differences in volume or projected area between densities were only detected at the end of the experimental culture (October) and only in the smaller individuals. These morphological adaptations can be understood as a strategy to mitigate the effects of intra-specific competition, palliating the consequences of physical interference at high densities.

Two sympatric root hemiparasitic Pedicularis species differ in host dependency and selectivity under phosphorus limitation

Parasitic biology of Pedicularis L. (Orobanchaceae) has been underinvestigated despite its wide distribution and potential ecological significance. To better understand the parasitic aspects of the root hemiparasites, host-parasite interactions were investigated with two sympatric Pedicularis species, Pedicularis rex C. B. Clarke and Pedicularis tricolor Hand.-Mazz., at two developmental stages. Plant DW, shoot phosphorus (P) content, root:shoot ratio and number of haustoria were measured in Pedicularis grown with either a host plant or a plant of its own species in pot experiments. In addition, effects of parasitism and intraspecific competition on growth and biomass allocation in four host species belonging to three major functional groups (grasses, legumes and forbs) were investigated. The two Pedicularis species showed obvious host preference, but preferred different host species. Interactions between Pedicularis and their hosts depended on both species identity and developmental stages of the partners. Overall, P. rex showed much weaker host dependency and less damage to hosts than P. tricolor. Interspecific variations were observed among different host species in their responses to intraspecific competition and parasitism. We concluded that different Pedicularis-host pairs showed different interaction patterns. Sympatric Pedicularis may have differential influence on plant community structure and productivity.

Variation in breeding parameters of Eleonora's falcon (Falco eleonorae) and factors affecting its reproductive performance

AbstractWe gathered data for a four‐year period (2004–2007) on the nesting ecology and reproductive performance of Eleonora's falcon in the Aegean Sea. We investigated in an indirect way the relation between clutch size and pre‐laying food availability, the significance of site and pair quality on productivity, and the effects of habitat and intraspecific competition on breeding success. Overall, the species selected nest sites sheltered from sun exposure but not from the prevailing wind. Hatching, fledging, and breeding success rates were estimated at 64, 72%, and 60%, respectively. Fledglings per breeding pair ranged from 1.19 to 1.75, and fledglings per successful pair from 1.84 to 2.0, between the years. Productivity depended on parental care rather than nest‐site quality. Breeding parameters varied significantly between the years, exhibiting a strong spatial yet local effect. Low success rates were recorded in specific colonies, which were attributed to adverse weather conditions and habitat degradation. Insect food availability prior to egg‐laying, estimated via plant biomass, was positively correlated to clutch size. Meanwhile, low wind strengths in August, large distances from the mainland, and population size in the vicinity of the colonies had negative effects on breeding success. Considering data at the Mediterranean scale, a longitudinal trend is observed across the breeding range of this species, with breeding parameters slightly decreasing from west to east.

Inter- and intraspecific interactions among green crabs ( Carcinus maenas) and whelks ( Nucella lapillus) foraging on blue mussels ( Mytilus edulis)

Blue mussels ( Mytilus edulis) play an important role in many coastal marine communities, regulating biodiversity and serving as food for many predators. Interactions between predators mediate their impact on mussel prey, and thus have indirect effects on the mussel bed community. We studied the effects of predator density and species composition on mussel predation by green crabs ( Carcinus maenas) and dogwhelks ( Nucella lapillus). We conducted an experiment in which predators at high and low natural intertidal densities were enclosed with mussels, in either single- or multiple-predator species treatments. Biomass consumed by each species in each treatment was determined and effects of multiple predators on consumption rates were estimated. Mean size of mussels eaten by each predator species was also measured and compared between treatments. Feeding by whelks and crabs was inhibited by conspecifics, and whelk foraging was also depressed in the presence of crabs. The presence of whelks had a positive effect on crab foraging, which may have moderated the negative effects of intraspecific competition between crabs. Sizes of mussels consumed did not differ statistically between treatments. In mixed-species treatments, crabs appear to have kleptoparasitized mussels being eaten by whelks, possibly facilitating consumption of larger mussels by crabs. Results indicate that interactions between invertebrate predators of blue mussels cause shifts in the feeding patterns of these predators, and are therefore an important part of mussel bed community dynamics.

Competition among loblolly pine trees: Does genetic variability of the trees in a stand matter?

The intensity of intraspecific competition among trees in a stand was compared for seed origin versus clonal origin stands of loblolly pine using tree size-based competition indices. A method that uses maximum likelihood with simulated annealing (MLSA) and one that uses discrete radii with simple correlation coefficients (DRSC) were applied and their results compared to stand level observable effects of competition. DRSC was found to give estimates of competitor search radii that were reasonable and in line with observable stand level effects of intraspecific competition. MLSA appeared to overestimate the optimum competitor search radius, had the potential to give estimates of the competitor search radius that were not unique, and was generally not conclusive on which genetic variety had the most intense level of competition among trees in its stand. The DRSC approach provides a simple alternative to MLSA where the assumptions behind MLSA do not hold or where it may not be easy for them to be checked. A comparison of competitor search radii in the different genetic variety stands and the observable stand level effects of competition did not find evidence to support the hypothesis that greater genetic uniformity among trees in loblolly pine stands would result in more intense intraspecific competition among the trees in the stand.

Effects of inter- and intra-specific competition on the growth rates of juvenile European barbel Barbus barbus used in the stock enhancement of UK fisheries

Fish stocking can result in increased resource competition for coexisting species in receiving fisheries, with potential implications for the subsequent growth of individuals. The effects of increased exploitative competition for limited food resources were tested on the somatic growth of European barbel Barbus barbus (Linnaeus 1758). A substitutive–additive design was used, utilising common carp ( Cyprinus carpio Linnaeus, 1758), tench ( Tinca tinca Linnaeus, 1758) and B. barbus conspecifics. Additive treatments revealed that the growth of B. barbus was significantly suppressed following the introduction of C. carpio, with the magnitude of growth suppression directly proportional to increased density and biomass. The strength of this competition was, however, similar to that elicited by T. tinca at the same density, but at lower biomass, suggesting density was the important determinant of inter-specific competition strength and was independent of species. A substitutive treatment testing the effect of intraspecific competition revealed that when B. barbus was introduced at a similar density and biomass to C. carpio, there was no significant difference in the extent of the suppressed growth. There was no effect of the treatments on fish condition and coefficients of variation of the treatments suggested there was no establishment of hierarchies in any treatment, with growth rates consistent between individuals.

The effects of intraspecific competition on the prey capture behavior and kinematics of the bluegill sunfish, Lepomis macrochirus

Competition has broad effects on fish and specifically the effects of competition on the prey capture kinematics and behavior are important for the assessment of future prey capture studies in bony fishes. Prey capture kinematics and behavior in bony fishes have been shown to be affected by temperature and satiation. The densities at which bony fish are kept have also been shown to affect their growth, behavior, prey selection, feeding and physiology. We investigated how density induced intraspecific competition for food affects the prey capture kinematics of juvenile bluegill sunfish, Lepomis macrochirus. High speed video was utilized to film five bold individuals feeding at three different densities representing different levels of intraspecific competition. We hypothesized that: (1) the feeding kinematics will be faster at higher levels of competition compared to lower levels of competition, and (2) bluegill should shift from more suction-based feeding towards more ram-based feeding with increasing levels of competition in order to outcompete conspecifics for a prey item. We found that, with increased intraspecific competition, prey capture became faster, involving more rapid jaw opening and therefore greater inertial suction, shorter mouth closing times, and shorter gape cycles. Furthermore, the attack velocity of the fish increased with increasing competition, however a shift towards primarily ram based feeding was not confirmed. Our study demonstrates that prey capture kinematics are affected by the presence of conspecifics and future studies need to consider the effects of competition on prey capture kinematics.

House sparrows selectively eject parasitic conspecific eggs and incur very low rejection costs

Most host species of obligate interspecific brood parasites are under strong selection because such parasitism, e.g., that involving evictor nestmates, is highly costly. Egg rejection is one of the most efficient host defences against avian brood parasites. Many hosts have thus evolved egg-recognition ability and rejection behaviour. However, this defensive mechanism has not evolved in most species where only intraspecific brood parasitism occurs, probably because (1) the eggs of conspecific females are very similar in appearance, making egg rejection less likely to emerge, and (2) such parasitism is frequently less costly than interspecific parasitism. Using a captive population of house sparrows (Passer domesticus) with a low breeding density, we here provide new evidence showing that this species actually has a fine capacity to discriminate conspecific eggs and to eject them (44.2% of foreign eggs ejected) while incurring very low rejection costs (4.2% of own eggs ejected). This result contradicts those previously found in high-density house sparrow populations in which very high rejection costs and very high clutch desertion rates were reported, probably as a consequence of intraspecific competition and infanticide provoked by the high breeding density. The house sparrow has only rarely been reported as the host of an interspecific brood parasite, which implies that it is a newly described example of an altricial species in which egg ejection has evolved and is maintained in response to intraspecific brood parasitism.

Does competition stress decrease allelopathic potential?

In natural communities, plants compete in different ways, among them chemical interactions in the form of allelopathy. Whereas the effects of abiotic stresses (temperature, light, nutrients, etc.) on the production of allelochemicals are well known, only few studies deal with the impact of the stress induced by competition. When they do so, these studies are done under experimental conditions. The aim of this study is to evaluate the effect of intra-specific competition on the production of allelochemicals and biomass of Pinus halepensis Mill. in a natural forest using three levels of density. Phenolics and aliphatic acids were extracted from pine needles, analysed and quantified by GC-MS. Trunks, branches, needles and necromass were measured. We observed an increase in allelochemical content at low or medium level of competition and a decrease at high competition level. Moreover trees in competition allocate proportionally more biomass to the trunk and less to foliage and branches. This study provides evidence of substantial changes in allocation between the primary and the secondary metabolism.

Understanding the relative roles of disturbance and species interactions in shaping Mississippi River island plant communities

In order to better understand the factors governing community assembly on riverine islands, we conducted a greenhouse experiment that examined the effects of species interactions in combination with flooding that mimicked hydrologic gradients found on Mississippi River islands. Specifically, we examined the effects of inter and intra-specific competition and three disturbance treatments (drained, drought, and flood) on the growth performance of three island-ubiquitous herbaceous plant species (Amaranthus palmeri, Cyperus strigosus. and Xanthium strumarium). We predicted a decrease in interspecific competitive effects in disturbance treatments. Specifically, we expected A palmeri to be most affected by flooding treatment based on its facultative upland status suggesting that it should be a poor tolerator of flooded conditions. X. strumarium and C. strigosus both facultative wetland species in their status, should tolerate flooding, but we predict that C. strigosus should have greater stress tolerance due to its perennial life history. Because we find these species coexisting at the same elevation zones on riverine islands, we also predicted stronger intraspecific competition effects. Our results indicate that disturbance indeed affected competitive interactions, but the effects were species-specific. X. strumarium showed stronger competitive effects when grown with conspecifics while C. strigosus experienced greater competition with heterospecifics under drought treatments. Based on its wetland indicator status and life history characteristics, we expected A palmeri to exhibit a facilitative effect, as it is typically an upland species, but instead it was impacted most by interspecific competition. Our study indicates that factors other than competition, facilitation, and flood tolerance (e.g., priority effects) may be controlling island plant community dynamics, further studies are required.

Contribution of contrasting plant hierarchies to the response to N fertilizer in maize

The typical size structuring process that occurs as a consequence of intra-specific competition in maize promotes the appearance of contrasting plant hierarchies (i.e. dominated and dominant individuals). This process may become more intense under low nitrogen (N) availability. The alleviation of plant competition by N addition may reduce plant yield variability through a differential response to N in individuals of contrasting hierarchies. In this work, the response to N of dominated and dominant plants from stands with contrasting N supply (0 to 140–200 kg N ha −1) was examined on 11 experiments carried out in Paraná, Argentina (31°50′S; 60°31′W) in a broad range of growing conditions that included the variation of the year, genotype, plant population and sowing date. Our objectives were: (i) to evaluate the response to N in contrasting plant hierarchies of maize, (ii) to quantify the contribution of dominated and dominant plants to the response to N of the overall stand, and (iii) to study the effect of N on relationships between plant hierarchies and stand variability. Response to N of yield per plant was associated with biomass per plant in non-fertilized controls, tending to be higher in plants with low biomass. The response to N of yield per unit area (i.e., considering all individuals of the stand) was related to the response to N of dominant and dominated plants ( P < 0.0001). However, at a higher level of response to N of grain yield per unit area (>50–60%), dominant plants had a considerable lower response than dominated plants, whereas at a lower level of response (<30%), the contribution of contrasting plant hierarchies was similar. In stands with similar plant biomass between hierarchies, the differences in the response to N between plant types tended to be negligible. The coefficient of variation of yield per plant was reduced ( P < 0.05) by effect of N in 4 out of 11 experiments, although it tended to be consistently lower in fertilized treatments. When the differences between the biomass of dominated and dominant plants were ample we found the highest response to N at the stand level, as a result of the higher increase in grain yield per plant in dominated plants than in dominant ones. The response to N in each plant hierarchy was differentially associated with increases in shoot biomass, harvest index, kernel number per plant and kernel weight.

Optimization of breeding output for larval stage of Anopheles gambiae (Diptera: Culicidae): prospects for the creation and maintenance of laboratory colony from wild isolates

Domesticating anopheline species from wild isolates provides an important laboratory tool but requires detailed knowledge of their natural biology and ecology, especially the natural breeding habitats of immature stages. The aim of this study was to determine the optimal values of some parameters of Anopheles gambiae larval development, so as to design a standard rearing protocol of highland isolates, which would ensure: the biggest fourth instars, the highest pupae productivity, the shortest duration of the larval stage and the best synchronization of pupation. The density of larvae, the size of breeding water and the quantity of food supplied were tested for their effect on larval growth. Moreover, three cheap foodstuffs were selected and tested for their capability to improve the breeding yield versus TetraMin® as the standard control. The larval density was a very sensitive parameter. Its optimal value, which was found to be ≈1 cm-2 surface area, yielded a daily pupation peak of 38.7% on day 8 post-oviposition, and a global pupae productivity of 78.7% over a duration range of three days. Anopheles gambiae's larval growth, survival and developmental synchronization were density-dependent, and this species responded to overcrowding by producing smaller fourth instars and fewer pupae, over elongated immature lifetime and duration range of pupae occurrence, as a consequence of intraspecific competition. While shallow breeding waters (<3 cm) produced a higher number of pupae than deeper ones, no effect of the breeding habitat's absolute surface area on larval development was observed. Increasing the daily food supply improved the pupae productivity but also boosted the water pollution level (which was assessed by the biological oxygen demand (BOD) and the chemical oxygen demand (COD)) up to a limit depending on the food quality, above which a rapid increase in larval mortality was recorded. The food quality that could substitute the manufactured baby fish food was obtained with weighed mixture of 1 wheat+1 shrimp+2 fish. On establishing an anopheline mosquito colony in the laboratory, special care should be taken to design and maintain the appropriate optimal values of larval density, water depth, daily diet quantity and nutritional quality.

Niche construction in the light of niche theory

Ecological niche construction, the process whereby an organism improves its environment to enhance its growth and persistence, is an important missing element of niche theory. Niche theory has mainly focused on niche-deteriorating processes, such as resource consumption, predation and competition, which have negative effects on population growth. Here, we integrate niche construction explicitly into modern niche theory. We use a graphical approach to analyse how a species' niche-improving impacts interplay with niche-deteriorating impacts to modify its response to the environment. In a model of two consumers that compete for one limiting resource and one predator, we show how niche construction modifies the traditional niche-deteriorating impacts of its agent or of competing species, and hence the potential for species coexistence. By altering the balance between intraspecific and interspecific competitive effects, niche construction can either generate net interspecific facilitation or strengthen interspecific competition. The adaptive benefit derived from niche construction also strongly affects the realized niche of a niche-constructing species.

Intraspecific competition affects population size and resource allocation in an ant dispersing by colony fission

Intraspecific competition is a pervasive phenomenon with important ecological and evolutionary consequences, yet its effect in natural populations remains controversial. Although numerous studies suggest that in many cases populations across all organisms are limited by density-dependent processes, this conclusion often relies on correlative data. Here, using an experimental approach, we examined the effect of intraspecific competition on population regulation of the ant Aphaenogaster senilis. In this species females are philopatric while males disperse by flying over relatively long distances. All colonies were removed from 15 experimental plots, except for one focal colony in each plot, while 15 other plots remained unmanipulated. After the first reproductive season, nest density in the experimental plots returned to a level nonsignificantly different from that in the control plots, which was not expected if the populations were indeed regulated by density-independent phenomena. In both the control plots and the experimental plots colonies remained overdispersed throughout the experiment, suggesting colony mutual exclusion. Nests outside the plots rapidly extended their foraging span, but we did not detect any significant inward migration into the experimental plots. Experimental reduction in density did not significantly affect the focal colonies' biomass, measured just before the first reproductive season. However, the ratio of males to workers-pupae biomasses was smaller in experimental plots, suggesting that colonies there had redirected part of the resources normally allocated to male production to the production instead of new workers. Microsatellite analysis indicated that, after the reproductive season, many colonies in the experimental plots were headed by a young queen that was the mother of the brood but not of the old workers, indicating that reduction in colony density stimulated fission of the remaining colonies. Finally, at the end of the experiment, 14 months after experimental reduction in density, colonies that derived from fission were smaller in the experimental than in the control plots, suggesting that the former had undergone fission at a smaller size than in control plots, which presumably allowed them to colonize the emptied areas. We conclude that colonies adjust resource allocation and colony fission to the degree of intraspecific competition.

Determining the strength of exploitative competition from an introduced fish: roles of density, biomass and body size

Britton JR, Cucherousset J, Grey J, Gozlan RE. Determining the strength of exploitative competition from an introduced fish: roles of density, biomass and body size. Ecology of Freshwater Fish 2011: 20: 74–79. © 2010 John Wiley &amp; Sons A/SAbstract – As species introductions can result in increased resource competition for coexisting species in the receiving ecosystems, the effects of increased exploitative competition for limited food resources from an introduced fish (Pseudorasbora parva) on a coexisting fish (Cyprinus carpio) were tested experimentally using a substitutive–additive design. Additive treatments revealed that the growth of C. carpio was significantly suppressed following the introduction of P. parva with the magnitude of growth suppression directly proportional to P. parva density and biomass. A substitutive treatment that tested for the effect of intraspecific competition revealed that when C. carpio were introduced at a similar biomass to P. parva, there was no significant difference in the extent of the suppressed growth. At the same density, however, the effect of C. carpio (higher biomass) on growth was significantly above that of P. parva (lower biomass). This was independent of the initial body sizes of the introduced fishes. Thus, the interspecific competition imposed by P. parva was only as strong as the intraspecific competition of C. carpio when present at a similar biomass.

Species identity and negative density dependence lead to convergence in designed plant mixtures of twelve species

Competition effects on community development are difficult to quantify in species-rich plant communities due to the complexity of possible interactions. We used multispecies mixtures to investigate how species identity and competitive interactions influence the development of plant communities. Given the same set of species with differing initial abundance in various communities, we tested whether communities would become more similar (converge) or dissimilar (diverge) over time depending on the relative importance of species identity and competition. Twenty-four experimental communities were established by planting seedlings of twelve wetland species at different relative abundances and absolute densities. The development of the communities was monitored over three years, and yearly changes in biomass were modelled as a linear function of the species biomass at the start of each period. After three years, a clear dominance structure had developed, with four species making up 80% of the aboveground biomass. In all three years, community dynamics was driven by differences in relative growth rates among the species (i.e. an effect of species identity). However, in the second and third years negative density dependence was also important, with changes in the relative abundance of the most abundant species being negatively related to their biomass at the start of the period. Multiple species interactions – though generally weaker than effects of species identity and intraspecific competition – became increasingly important and also contributed to the dominance pattern. It is concluded that species identity and negative density dependence of the dominant species were the most important factors causing the experimental plant communities to converge. We suggest that model systems composed of several species offer a useful method for investigating the influence of functional traits upon community dynamics.

Bushkiller (Cayratia japonica) Growth in Interspecific and Intraspecific Competition

Bushkiller was evaluated under inter- and intraspecific competition. In experiment 1, bushkiller, trumpetcreeper, and wild grape were greenhouse-grown alone and in two or three species mixtures in pots. Of the three species, bushkiller grew the tallest and had the greatest final biomass when grown alone. When all three species were grown together, bushkiller grew over twice the height of trumpetcreeper, over three times the height of wild grape, and over four times the biomass of either competing species. Plots of height over time showed that competition did not affect bushkiller or wild grape growth rate, but trumpetcreeper growth was reduced when grown with bushkiller. In experiment 2, bushkiller was grown in cultures of one, two, and three plants per pot to determine intraspecific competition effects on growth. Final height of bushkiller was not affected by intraspecific competition; however, bushkiller biomass decreased with increasing competition.

Fitness-Associated Sexual Reproduction in a Filamentous Fungus

Sex is a long-standing evolutionary enigma. Although the majority of eukaryotes reproduce sexually at least sometimes [1-3], the evolution of sex from an asexual ancestor has been difficult to explain because it requires sexually reproducing lineages to overcome the manifold costs of sex, including the destruction of favorable gene combinations created by selection [4, 5]. Conditions for the evolution of sex are much broader if individuals can reproduce either sexually or asexually (i.e., facultative sex) and allocate disproportionately more resources to sex when their fitness is low (fitness-associated-sex or FAS [6-10]). Although facultatively sexual organisms have been shown to engage in more sex when stressed [11], direct evidence for FAS is lacking. We provide evidence using 53 genotypes of the filamentous fungus Aspergillus nidulans in a reciprocal transplant experiment across three environments. Different genotypes achieved highest fitness in different environments and genotypes invested relatively more in sex in environments in which their fitness was lower, showing that allocation to sexual reproduction is a function of how well-adapted a genotype is to its environment. FAS in A. nidulans is unlikely to have evolved as a strategy to resist or avoid stress because asexual spores are more dispersive and equally resistant [12, 13].

Coexistence in the intertidal: interactions between the non-indigenous New Zealand mud snail Potamopyrgus antipodarum and the native estuarine isopod Gnorimosphaeroma insulare

Nonindigenous species (NIS) that achieve high densities in their recipient communities are expected to have negative effects on native competitors. However, recent reviews have shown that competitors, unlike predators and parasites, have rarely been documented to cause extinctions. There is a need for better understanding of competitive interactions between nonindigenous species and native competitors across systems where NIS reach high densities. In this paper we examine competitive interactions between the exotic, invasive New Zealand mud snail Potamopyrgus antipodarum and native estuarine invertebrates. P. antipodarum is often described as a freshwater snail but can occur in brackish water as well; it has been established in the Columbia River Estuary for over ten years. We addressed competition in this estuarine system using three approaches: field surveys of the benthic invertebrate community, stable isotope analysis of overlap in resource use by common invertebrates, and a laboratory competition experiment that tested the strength of competition between P. antipodarum and the isopod, Gnorimosphaeroma insulare, which overlap in resource use. We found no evidence of negative competitive impacts of P. antipodarum on native benthic invertebrates in this estuarine system. Densities of P. antipodarum and common native epibenthic invertebrates are positively correlated in the intertidal, likely due to shared habitat preferences. The competition experiment showed that the effect of interspecific competition from P. antipodarum on the foraging and survival of G. insulare was significantly less than the effect of intraspecific competition between isopods. The presence of the isopod G. insulare reduced foraging in P. antipodarum, but this change did not result in reduced survival and growth of snails. Hence, interspecific competition is weak despite high densities achieved by the invading species. Finally, we discuss several possible explanations for the observation that P. antipodarum does not have an obvious negative competitive impact on native benthic invertebrates.