Does past evolutionary history under different mating regimes influence the demographic dynamics of interspecific competition?

Interspecific interactions are contingent upon organism phenotypes, and thus phenotypic evolution can modify interspecific interactions and affect ecological dynamics. Recent studies have suggested that male–male competition within a species selects for capability to reproductively interfere with a closely related species. Here, we examine the effect of past evolutionary history under different mating regimes on the demographic dynamics of interspecific competition in Callosobruchus seed beetles. We used previously established experimental evolution lines of Callosobruchus chinensis that evolved under either forced lifelong monogamy or polygamy for 17 generations, and examined the demographic dynamics of competition between these C. chinensis lines and a congener, Callosobruchus maculatus. Callosobruchus chinensis was competitively excluded by C. maculatus in all trials. Time series data analyses suggested that reproductive interference from C. chinensis was relatively more important in the trials involving polygamous C. chinensis than those involving monogamous C. chinensis, in accordance with the potentially higher reproductive interference capability of polygamous C. chinensis. However, the estimated signs and magnitudes of interspecific interactions were not fully consistent with this explanation, implying the evolution of not only reproductive interference but also other interaction mechanisms. Our study thus suggests multifaceted effects of sexually selected traits on interspecific competitive dynamics.     

Predator biodiversity increases the survivorship of juvenile predators

When predator biodiversity strengthens herbivore suppression, the pattern generally is attributed to interspecific complementarity. However, the relaxation of intraspecific interference within diverse communities has received less attention as an underlying factor, and most experiments to date span much less than one predator generation. Here, working with a community of aphid predators, we compared the survivorship of juvenile predators embedded within diverse versus single-species communities of adult predators. We found that greater predator diversity improved juvenile survivorship for three of four predator taxa (the lady beetles Hippodamia convergens and Coccinella septempunctata, and the bug Nabis alternatus; but not the small bug Geocoris bullatus), whereas survivorship was relatively low when juveniles foraged among only conspecific adults. When aphid densities differed they were lowest for the diverse treatment, and so resource availability could not explain differences in juvenile survivorship. Instead, feeding trials indicated that cannibalism generally posed a greater risk to juveniles than did intraguild predation (with Geocoris again the exception). Our results suggest that the dilution of intraspecific interference may play an important, and perhaps underappreciated, role in shaping predator diversity effects. Furthermore, relatively strong cannibalism but weak intraguild predation has the potential to project diversity effects forward into subsequent generations.     

Identifying critical interactions in complex competition dynamics between bean beetles

Identifying behavioural basis of competitive relationship is essential to understand outcome of interspecific competition. However, it remains difficult to investigate demographic effect of competitive behaviour, because various kinds of behaviours may co‐occur in the competition and make the dynamics far complicated in nonlinear ways. We report that the behavioural basis of interspecific interaction can be identified, by focusing on the timescale difference from the occurrence of each behaviour to the appearance of its demographic effect. Between two bean beetles, Callosobruchus chinensis and C. maculatus, major interspecific interactions are resource competition (RC) at the larval stage and reproductive interference (RI) at the adult stage. RC has longer time lag than RI, because effect of RC appears in the adult number of the next generation through larval competition while effect of RI appears instantaneously in the adult number through early death of females. If we detect two effects with different time lags from the competition dynamics, an effect with intergenerational time lag and with no time lag would be considered as RC and RI, respectively. We applied empirical dynamic modelling approach, which is a nonlinear time series analysis for detecting causal interactions and the strength, to two published datasets of experimental competition between those beetles. Results showed the significant causality from the winner species to the loser one in both experiments, but the causality time lag differed between experiments: the causality had no time lag in the C. chinensis‐win data, while intergenerational time lag in C. maculatus‐win data. Furthermore, detection of the causality with intergenerational time lag from C. maculatus to C. chinensis in both experiments suggests interplay of constant RC and variable RI which can reverse the outcome. This study is the first successful case study that links behavioural‐level interactions to demographic‐level effects in interspecific competition.     

Combined versus Single Species Release of Predaceous Mites: Predator–Predator Interactions and Pest Suppression

Interactions such as competition, intraguild predation (IGP), and cannibalism affect the development and coexistence of predator populations and can have significance for biological control of commonly exploited pest organisms. We studied the consequences of combined versus single release of two predaceous mite species (Phytoseiidae), with differing degrees of diet specialization, on their population dynamics and the suppression of the carmine spider mite, Tetranychus cinnabarinus Boisduval (Tetranychidae), on greenhouse-grown gerbera. Population growth of the specialist predator Phytoseiulus persimilis Athias-Henriot was greater and population decline steeper when released in combination with the generalist Neoseiulus californicus McGregor than when released alone. In contrast, the N. californicus population grew and declined more gradually when released in combination with P. persimilis, compared to the single species release. The differential impact on each other's population dynamics can be primarily attributed to contrasting properties in competition, IGP, and cannibalism. At the same overall predator density and as long as prey was abundant, the specialist P. persimilis was more strongly affected by intraspecific competition than by interspecific competition with the generalist N. californicus. In contrast, interspecific competition with P. persimilis had a greater impact on N. californicus than intraspecific competition. After prey depletion, the generalist predator N. californicus was more likely to engage in IGP than was the specialist predator P. persimilis. Overall, the study demonstrates that prey specificity has significance for the quality and intensity of predator–predator interactions and indicates potential implications for biological control of spider mites. All predator releases (i.e., either species alone and both species in combination) resulted in reduction of the spider mite population to zero density. Individual release of the specialist P. persimilis led to the most rapid spider mite suppression. Nonetheless, in perennial greenhouse-grown crops P. persimilis and N. californicus could have complementary effects and a combination of the two predators could enhance long-term biological control of spider mites. The potential risks and benefits associated with the release of both species are discussed.     

Competition and intraguild egg predation among freshwater snails: re-examining the mechanism of interspecific interactions

Experimental and field studies suggest that freshwater snail species have negative effects on each other's population growth rates. Because snails share similar diets, these interactions have been interpreted as the result of exploitative competition, but they could also result from intraguild predation. Here we present three experiments aimed at testing the hypothesis that interspecific interactions among three species of freshwater gastropod (Helisoma trivolvis, Physa acuta and Stagnicola elodes) are mediated by intraguild egg predation. Foraging trials, conducted in a laboratory, showed that some snails readily prey on eggs, but the extent of egg predation depended on both the identity of the snail predator and the identity of the egg mass. Of the three species considered, Stagnicola had the largest effect on egg mortality and Physa had no effect on egg mortality. Foraging trials also showed that the eggs of Physa were the most vulnerable to predators and that the eggs of Stagnicola were largely invulnerable. A study conducted in large outdoor mesocosms assessed the occurrence of egg mortality in an environment of more extensive spatial scale and complexity. The results largely mirrored those of the laboratory study, with Stagnicola being the most voracious predator and the eggs of Physa being most vulnerable to predation. The reproductive success of Physa and Stagnicola raised in sympatry and allopatry was assessed in a mesocosm study conducted over three months. Recruitment of both species was depressed in sympatry, but patterns of growth in the survivors suggest contrasting mechanisms of suppression: Physa suppressed Stagnicola via exploitative competition, but Stagnicola suppressed Physa via egg predation. These experiments support the hypothesis that freshwater snail assemblages are structured by strong interspecific interactions and that a rich interplay of egg predation and interspecific competition underlie interactions among the members of this guild.     

A comparative study of cannibalism and predation in seven species of flour beetle

Abstract 1. The present study quantified egg and pupal cannibalism, and interspecific predation on eggs and pupae, by larvae and adults of seven species of flour beetle (Tribolium spp.) under laboratory conditions: T. anaphe, T. brevicornis, T. castaneum, T. confusum, T. destructor, T. freemani, and T. madens. 2. Variation among species in cannibalism and predation propensities did not reflect taxonomic affinities within the genus, indicating that these behaviours were shaped by ecology at species level. 3. Within species, larvae and adults displayed different propensities for cannibalism and predation, leading to the conclusion that these behaviours evolve independently in the two life stages. 4. All species behaved as intraguild predators to some degree, especially in the adult stage. 5. Three general patterns of cannibalism and predation were described by principal component mapping and cluster analysis. 6. The first group comprised three cosmopolitan pest species that were more voracious as adults than as larvae: T. castaneum, T. confusum, and T. destructor. It is proposed that stored product environments select for high adult voracity because the costs associated with emigration from such rare, but resource‐rich, habitats intensifies interference competition among adults. 7. The second group consisted of species that inhabit natural environments and that were more voracious as larvae: T. anaphe, T. freemani, and T. madens. Habitats for these species are probably numerous, but generally poor in quality, a situation that intensifies larval competition, while favouring earlier adult emigration. 8. The largest species, T. brevicornis, demonstrated inconsistent voracity between life stages and was the only species with chemically defended pupae. 9. It is proposed that consumption of eggs provides primarily nutritional benefits, whereas consumption of pupae has a more important role in interference competition.     

Sexual conflict and ecology: Species composition and male density interact to reduce male mating harassment and increase female survival

Sexual conflict is a pervasive evolutionary force that can reduce female fitness. Experimental evolution studies in the laboratory might overestimate the importance of sexual conflict because the ecological conditions in such settings typically include only a single species. Here, we experimentally manipulated conspecific male density (high or low) and species composition (sympatric or allopatric) to investigate how ecological conditions affect female survival in a sexually dimorphic insect, the banded demoiselle (Calopteryx splendens). Female survival was strongly influenced by an interaction between male density and species composition. Specifically, at low conspecific male density, female survival increased in the presence of heterospecific males (C. virgo). Behavioral mating experiments showed that interspecific interference competition reduced conspecific male mating success with large females. These findings suggest that reproductive interference competition between con‐ and heterospecific males might indirectly facilitate female survival by reducing mating harassment from conspecific males. Hence, interspecific competitors can show contrasting effects on the two sexes thereby influencing sexual conflict dynamics. Our results call for incorporation of more ecological realism in sexual conflict research, particularly how local community context and reproductive interference competition between heterospecific males can affect female fitness.     

Cannibalism and intraguild predation of eggs within a diverse predator assemblage

Greater biodiversity among aphid predators sometimes leads to greater predator reproductive success. This could occur if cannibalism of predator eggs is consistently stronger than intraguild predation, such that diversity dilutes cannibalism risk when total predator densities remain constant across diversity levels. We compared the frequency of cannibalism versus intraguild predation by adult predators of four species [the lady beetles Coccinella septempunctata L. and Hippodamia convergens Guerin-Meneville, and the predatory bugs Geocoris bullatus (Say) and Nabis alternatus Parshley] on the eggs of three predator species (all of these predators but Nabis). For both coccinellid species, egg predation averaged across all intraguild predators was less frequent than cannibalism. In contrast, Geocoris eggs were generally more likely to be consumed by intraguild predators than by conspecifics. Closer inspection of the data revealed that Geocoris consistently consumed fewer eggs than the other species, regardless of egg species. Indeed, for lady beetle eggs it was relatively infrequent egg predation by Geocoris that brought down the average across all heterospecific predators, masking the fact that adults of the two lady beetles were no more likely to act as egg cannibals than as intraguild predators. Nabis ate eggs of the two beetles at approximately equal rates, but rarely ate Geocoris eggs. Female predators generally consumed more eggs than did males, but this did not alter any of the patterns described above. Altogether, our results suggest that species-specific differences in egg predation rates determined the relative intensity of egg intraguild-predation versus cannibalism, rather than any more general trend for egg cannibalism to always exceed intraguild predation.     

Direct and indirect effect of cannibalism and intraguild predation in the two sibling Harmonia ladybird beetles

In this study, we focused on the direct (i.e., predation) and indirect (i.e., potential threat from coexisting with a larger individual) effects of cannibalism and intraguild predation (IGP) during larval stages of two sibling ladybird beetles. These effects play an important role in the coexistence of the generalist–common Harmonia axyridis and specialist–rare H. yedoensis (Coleoptera: Coccinellidae). Direct predation effect of cannibalism and IGP was asymmetric in the two sibling ladybird beetles; the fourth instar larvae of H. axyridis were better intraguild predators than cannibals, while the reverse was true in the larvae of H. yedoensis. Neither cannibalism nor IGP significantly affected female body weight in either species. Larval H. axyridis surviving exposure to cannibalism or IGP had a reduced number of ovarioles as adults, whereas adult H. yedoensis ovarioles were not affected. For the indirect effects, longer developmental times in males and females and a lower total number of ovarioles in females were detected in H. axyridis. In H. yedoensis, shorter developmental time of males, lighter adult weight and fewer total ovarioles in females were observed. Olfactometer choice experiments clarified that the fourth instar larvae of H. axyridis avoided the first instar conspecific larvae, while those of H. yedoensis were attracted to the odors from H. axyridis and conspecifics. Thus, H. axyridis has an avoidance mechanism only for cannibalism but not for IGP, whereas H. yedoensis does not have any avoidance mechanism. These different behaviors in the direct and indirect effects of cannibalism and IGP observed in the laboratory may play important roles in the coexistence of generalist–common H. axyridis and specialist–rare H. yedoensis in natural conditions, compensating for the large handicap of H. yedoensis at reproductive interference from H. axyridis.     

Biotic interactions between Eriopis connexa and Hippodamia variegata,a native and an exotic coccinellid species associated with alfalfa fields in Chile

Human‐assisted introductions, including those in the context of biological control, are considered to be one of the most important factors of global environmental change. However, the mechanisms underlying environmental changes, such as a decrease in the relative abundance of native species, are poorly understood. Since the introduction of the ladybird beetle, Hippodamia variegata (Goeze) (Coleoptera: Coccinellidae), in Chile in the 1970s for biological control of aphids, a reduction in the relative abundance of the native ladybird beetle Eriopis connexa (Germar) has been noticed. To explore the role of cannibalism, intraguild predation (IGP) or competition as possible mechanisms that might increase dominance of H. variegata over E. connexa, several laboratory experiments were carried out. The native and the exotic species were very similar in their voracity and biotic interactions. Although H. variegata was able to maintain constant reproductive performance, E. connexa reproduction decreased at lower densities of aphids, Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae). The impacts of cannibalism, IGP, and competition exerted by these species were very symmetrical in both larvae and adults. We conclude that these biotic factors may not fully explain the increase in relative abundance of H. variegata. The higher sensitivity of E. connexa to prey patch size, along with other factors such as chemical‐mediated negative interspecific interactions may be operating in alfalfa fields, changing the composition of associated coccinellid assemblages.     

How do invasive species affect native species? Experimental evidence from a carrion blowfly (Diptera: Calliphoridae) system

1. The necrobiome is a unique microcosm in which various organisms interact and compete for access to an ephemeral resource, such as carrion, that ultimately determines the structure and composition of these assemblages. 2. Blowfly species exhibit different competitive abilities which, when associated with other types of behaviour, such as predation or cannibalism, influence coexistence. Knowledge of the effects of competition between native and invasive species on development and survival is essential to understanding the dynamics of insect communities and to assess biological invasions. 3. Laboratory experiments were performed to evaluate the effect of interspecific competition on the bionomics and survival of a native (Cochliomyia macellaria) and an invasive (Chrysomya rufifacies) blowfly species at different population densities. 4. The deleterious effect of competition on the larval parameters of C. macellaria increased proportionally with increases in the larval density of C. rufifacies. When exposed to increased densities of C. rufifacies, larvae of C. macellaria accelerated their development and, as a trade‐off for this strategy, surviving adults were smaller and had reduced wing size, which were likely to reduce dispersal and reproductive capacity. 5. Larval competition – both as species‐dependent and density‐dependent phenomena – influences morphological and biological traits of surviving individuals. The impact of the invasive species has consequences at the population level, such as displacement or local population depletion of native species, a phenomenon likely to occur in other systems involving insects and ephemeral resources.     

Population dynamics of Monochamus carolinensis (Col., Cerambycidae) under laboratory conditions

The study of life history dynamics of the pine sawyer beetle Monochamus carolinensis (Olivier) is of interest because the beetles are vectors for the pinewood nematode, Bursaphelenchus xylophilus (Steiner and Buhrer) Nickle, the causal agent of pine wilt in susceptible pine trees. Previous observation of a laboratory colony of M. carolinensis suggested numbers of adult beetles reared from logs varied little in relation to oviposition densities among individual logs. Logs in laboratory colonies are routinely subjected to artificially high oviposition rates. The objectives of this study were to examine the relationship between initial oviposition rate, holding time (time between log cut and oviposition), log volume and area and adult survivorship in laboratory colony. This study examined the number of eggs, larvae, adults and generation survivorship in logs over a 2‐year period. The number of adults emerged per log was not related to the number of eggs laid. Late larval and adult numbers were significantly correlated with log size. Only 12% of the initial cohort completed development and emerged as adults. We attribute the high within‐log mortality of beetles to intraspecific competition and cannibalism.     

Coevolution of competing Callosobruchus species does not stabilize coexistence

Interspecific resource competition is expected to select for divergence in resource use, weakening interspecific relative to intraspecific competition, thus promoting stable coexistence. More broadly, because interspecific competition reduces fitness, any mechanism of interspecific competition should generate selection favoring traits that weaken interspecific competition. However, species also can adapt to competition by increasing their competitive ability, potentially destabilizing coexistence. We reared two species of bean beetles, the specialist Callosobruchus maculatus and the generalist C. chinensis, in allopatry and sympatry on a mixture of adzuki beans and lentils, and assayed mutual invasibility after four, eight, and twelve generations of evolution. Contrary to the expectation that coevolution of competitors will weaken interspecific competition, the rate of mutual invasibility did not differ between sympatry and allopatry. Rather, the invasion rate of C. chinensis, but not C. maculatus, increased with duration of evolution, as C. chinensis adapted to lentils without experiencing reduced adaptation to adzuki beans, and regardless of the presence or absence of C. maculatus. Our results highlight that evolutionary responses to interspecific competition promote stable coexistence only under specific conditions that can be difficult to produce in practice.     

Analysis of species coexistence co-mediated by resource competition and reproductive interference

Reproductive interference is any interspecific sexual interaction that reduces the reproductive success of females through promiscuous reproductive activities of heterospecific individuals. This phenomenon is ubiquitous in nature in both plants and animals, and is frequently observed in biological invasions. However, its effects on interspecific competition remain incompletely understood despite growing concern. To study the interactive effects of resource competition and reproductive interference on species coexistence and exclusion, we analyzed a unified competition model including both processes in symmetric and asymmetric scenarios. The results of our model showed that resource competition and reproductive interference act synergistically to promote competitive exclusion. We also found that when the two processes are asymmetric, the species that is superior in reproductive interference can coexist with or even exclude the species that is superior in resource competition. Therefore, coexistence is possible via an unbalanced trade-off between resource use and reproduction. Our results suggest that integration of reproductive interference and resource competition will contribute to a better understanding of interspecific competition and to more effective biodiversity conservation against management of biological invasions.     

An experimental manipulation of the intensity of interspecific competition: effects on a small marsupial

Summary An investigation was made of the effects of reducing and enhancing the intensity of competition on a small marsupial, Antechinus stuartii, from a larger and competitively dominant congener, A. swainsonii. Populations of these species were monitored in two study areas (one control, one experimental) in forest near Canberra, Australia, between February and July in 1980 and 1981. In the experimental study area in 1980 I reduced the numbers of A. swainsonii relative to A. stuartii (thus reducing the intensity of interspecific competition), but in 1981 I augmented the numbers of A. swainsonii (thus increasing the intensity of competition). No manipulations were made in the control study area, and the numbers of both species remained similar there in both years. When the intensity of interspecific competition was reduced, the A. stuartii population increased in size. Increases occurred also in individual movements, home range areas, diurnal activity and in the proportion of large terrestrial prey (larvae, Amphipoda) in the diet. An increase in use of structurally complex forest floor habitats also coincided with decreased arboreal activity. In contrast, when the intensity of competition was enhanced, most of these population and resource shifts were reversed. These findings suggest that reduction in the intensity of interspecific competition allows A. stuartii access to terrestrial sources of food favoured by A. swainsonii, whereas enhancement leads to exclusion of A. stuartii from the forest floor. Competition occurs by interference. This may result in fixed per capita competitive effects of A. swainsonii on A. stuartii, and account for the observed changes in a very broad range of population and resource parameters.     

Nutritional benefits of intraguild predation and cannibalism among generalist and specialist phytoseiid mites

1. Intraguild predation and cannibalism are common among predaceous phytoseiid mites (Acari, Phytoseiidae) but the nutritional benefits gained by these processes are poorly understood. 2. The study reported here addressed the questions of whether cannibalism and intraguild predation provide different nutritional benefits and whether the ability to utilise cannibalism and intraguild predation is linked to the diet specialisation of phytoseiid mites. Specialists tested were Phytoseiulus macropilis, Galendromus occidentalis, and Neoseiulus longispinosus; generalists tested were Amblyseius andersoni, Neoseiulus cucumeris, and Neoseiulus fallacis. 3. All generalists and the specialist P. macropilis were able to complete juvenile development with both con‐ and hetero‐specific prey. Juvenile development of generalists was shorter with heterospecific prey than with conspecific prey, whereas development of the specialist P. macropilis did not differ between prey types. Only a few N. longispinosus and G. occidentalis, both specialists, reached adulthood by cannibalism but none reached adulthood by intraguild predation. 4. All generalists were able to sustain oviposition by intraguild predation. Neoseiulus cucumeris and A. andersoni laid more eggs with heterospecific prey than with conspecific prey, whereas N. fallacis had similar oviposition rates with both prey types. No specialist sustained oviposition by intraguild predation or cannibalism. 5. Overall, generalists gained equal or more nutritional benefits by intraguild predation than by cannibalism and were able to utilise phytoseiid intraguild prey as an alternative food source. Specialists gained equal or more nutritional benefits from cannibalism than from intraguild predation. For specialists, con‐ and hetero‐specific phytoseiids may be considered only a supplemental food.     

Frequency-dependent community dynamics driven by sexual interactions

Research in community ecology has tended to focus on trophic interactions (e.g., predation, resource competition) as driving forces of community dynamics, and sexual interactions have often been overlooked. Here we discuss how sexual interactions can affect community dynamics, especially focusing on frequency-dependent dynamics of horizontal communities (i.e., communities of competing species in a single ecological guild). By combining mechanistic and phenomenological models of competition, we place sexual reproduction into the framework of modern coexistence theory. First, we review how population dynamics of two species competing for two resources can be represented by the Lotka–Volterra competition model as well as frequency dynamics, and how niche differentiation and overlap produce negative and positive frequency-dependence (i.e., stable coexistence and priority effect), respectively. Then, we explore two situations where sexual interactions change the frequency-dependence in community dynamics: (1) reproductive interference, that is, negative interspecific interactions due to incomplete species recognition in mating trials, can promote positive frequency-dependence and (2) density-dependent intraspecific adaptation load, that is, reduced population growth rates due to adaptation to intraspecific sexual (or social) interactions, produces negative frequency-dependence. We show how reproductive interference and density-dependent intraspecific adaptation load can decrease and increase niche differences in the framework of modern coexistence theory, respectively. Finally, we discuss future empirical and theoretical approaches for studying how sexual interactions and related phenomena (e.g., reproductive interference, intraspecific adaptation load, and sexual dimorphism) driven by sexual selection and conflict can affect community dynamics.     

Size and shape changes of Ips typographus L. (Coleoptera: Scolytinae) in relation to population level

1 High intraspecific competition is known to occur during Ips typographus outbreaks, and is thought to be the main factor regulating epidemic populations by affecting beetle population productivity. However, little is known about the consequences of intraspecific competition on population quality during outbreaks, although it could have consequences on beetle population dynamics. 2 Ips typographus morphological variations among localities, years and beetle population levels were investigated in 10 Norway spruce (Picea abies) stands having various beetle damage intensities. Beetle size and shape estimators based on wing length, and using isometric size and log‐shape ratios, were employed. Field‐caught beetles were compared with beetles emerging from controlled breeding at different densities, performed in the laboratory. Beetles from this colony were also used to check the influence of breeding densities on the size estimator. 3 Size variations occurred among localities and years and were consistent with the epidemic or latent status of the beetle populations. Controlled breeding confirmed the negative effect of beetle densities encountered in the field on offspring size. Two hypotheses are formulated to explain this increase of intraspecific competition during an outbreak, but our data support the effect of host quality change between latent and epidemic populations. 4 Shape variations also occurred among localities but were unrelated to beetle population levels. No groups consistent with a geographical structure were found, suggesting low genetic variation for I. typographus populations in France.     

Interactions between a hunting spider and a web-builder: consequences of intraguild predation and cannibalism for prey suppression

Abstract. 1. Antagonistic interactions among invertebrate predators such as intraguild predation and cannibalism have the potential to dampen top‐down impacts on shared prey at lower trophic levels. Two abundant spider predators, the large wolf spider Pardosa littoralis and the small sheet‐web builder Grammonota trivitatta co‐occur on the salt marshes of eastern North America where they both attack planthoppers (Prokelisia spp.), the dominant herbivores on the marsh. Experiments both in the laboratory and field were used to assess the incidence of intraguild predation and cannibalism in these spiders and elucidate how such antagonistic interactions influence planthopper suppression. 2. Functional response experiments showed that with an increase in planthopper prey density, Grammonota captured more prey but not a higher proportion of that offered. Pardosa exhibited the same response when Grammonota were offered as intraguild prey. Both functional responses were type I over the range of prey densities offered. 3. Grammonota is moderately cannibalistic, and the presence of planthopper prey reduced the incidence of cannibalism. 4. Factorial experiments in the laboratory showed that Pardosa but not Grammonota reduced planthopper prey populations when prey density was low. By contrast, at high prey densities, both Pardosa and Grammonota had significant adverse effects on planthopper populations. Moreover, there was an interactive effect such that Grammonota reduced planthopper populations relatively more when Pardosa was absent than when it was present. 5. There was direct evidence for the intraguild predation of Grammonota by Pardosa such that fewer Grammonota survived in the presence of Pardosa than when it was absent. This result occurred whether planthopper prey were abundant or not. 6. Field releases of Grammonota in open plots resulted in significant but small decreases in the density of planthopper prey, both nymphs and adults. 7. Enhancing densities of Pardosa in open plots resulted in Grammonota suppression. The intraguild predation of Grammonota at this enhanced Pardosa density, however, did not preclude Pardosa from significantly reducing planthopper populations. 8. Although there was evidence that Grammonota reduced planthopper populations and that the intraguild predation of Grammonota by Pardosa occurred, the strength of these interactions was relatively weak given the low consumption rate of planthoppers by Grammonota (< 3 day^–1) and Grammonota by Pardosa (≈ 2 day^?1). Thus, weak asymmetric intraguild predation among spiders on the marsh likely dampens but does not eliminate the ability of Pardosa to exert significant top‐down control on planthopper populations.