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Apparent Competition (apparent + competition)
Selected AbstractsEnemy-mediated apparent competition: empirical patterns and the evidenceOIKOS, Issue 2 2000Enrique J. Chaneton Apparent competition arises when two victim species negatively affect each other (,,,,) by enhancing the equilibrium density or changing the foraging behaviour of a shared natural enemy. Shared enemies can also mediate non-reciprocal (,,,0) indirect effects, i.e. indirect amensalism, whenever one prey species is not affected by the presence of alternative prey. We review 34 studies on terrestrial and freshwater systems to evaluate the extent to which apparent competition has been perceived as a reciprocal (,,,,) or non-reciprocal (,,,0) interaction. We found only three studies showing reciprocal effects between apparent competitors. Indirect amensalism was documented in 10 studies and could be inferred for 16 other cases (76% in total). The remaining five studies provided insufficient data to determine the form of indirect interaction. The apparent prevalence of non-reciprocal enemy-mediated interactions resembles that observed for resource-based interspecific competition. Amensal indirect effects via shared predation may result from differences in population size, nutritional value, susceptibility to attack, or asynchronous dynamics of alternative prey, or the predator's feeding preferences. Moreover, experimental protocols may confound the actual form of apparent competition through short-term observations, incomplete designs, or biased consideration of conspicuous interactions, leading to reciprocal effects being overlooked. We conclude that, at present, it is still difficult to determine the relative role of apparent competition vs indirect amensalism in natural food webs because most published studies have failed to document in full interactions via shared enemies. [source] Endangered, apparently: the role of apparent competition in endangered species conservationANIMAL CONSERVATION, Issue 4 2010N. J. DeCesare Abstract Conservation biologists have reported growing evidence of food-web interactions as causes of species endangerment. Apparent competition is an indirect interaction among prey species mediated by a shared predator, and has been increasingly linked to declines of prey species across taxa. We review theoretical and empirical studies of apparent competition, with specific attention to the mechanisms of asymmetry among apparently competing prey species. Asymmetry is theoretically driven by niche overlap, species fitness traits, spatial heterogeneity and generalist predator behavior. In real-world systems, human-induced changes to ecosystems such as habitat alteration and introduced species may be ultimate sources of species endangerment. However, apparent competition is shown to be a proximate mechanism when resultant changes introduce or subsidize abundant primary prey for predator populations. Demonstration of apparent competition is difficult due to the indirect relationships between prey and predator species and the potential for concurrent exploitative competition or other community effects. However, general conclusions are drawn concerning the characteristics of prey and predator species likely to exhibit asymmetric apparent competition, and the options for recovering endangered species. While short-term management may be required to avoid imminent extinction in systems demonstrating apparent competition, we propose adaptive conservation efforts directed at long-term recovery. [source] Biotic indirect effects: a neglected concept in invasion biologyDIVERSITY AND DISTRIBUTIONS, Issue 4 2006Eve M. White ABSTRACT Indirect effects involve more than two species and are defined as how one species alters the effect that another species has on a third. These complex interactions are often overlooked in studies of interactions between alien and native species, and their role in influencing biological invasions has been rarely considered. Based on a comprehensive review of the invasion biology literature, we examine the evidence for the occurrence of four of the most commonly documented indirect effects (apparent competition, indirect mutualism/commensalism, exploitative competition, and trophic cascades) in the invasion process. Studies investigating indirect effects in the context of invasion biology are relatively rare, but have been increasing in recent years, and there are sufficient examples to indicate that this kind of interaction is likely to be more common than is currently recognized. Whether indirect interactions are mediated by an alien or a native species, and whether they occur between ecologically similar or dissimilar alien and native species, depends in part on the type of interaction considered and no predictable patterns were detected in the literature. Further empirical studies will help to elucidate such patterns. At this stage, the inherent unpredictability of indirect interactions means that their impacts in relation to invasions are particularly challenging for land managers to deal with, and their role in invasions is a complex, but is a valuable area of investigation for researchers. [source] Structure and vertical stratification of plant galler,parasitoid food webs in two tropical forestsECOLOGICAL ENTOMOLOGY, Issue 3 2009MIGUEL R. PANIAGUA Abstract 1.,Networks of feeding interactions among insect herbivores and natural enemies such as parasitoids, describe the structure of these assemblages and may be critically linked to their dynamics and stability. The present paper describes the first quantitative study of parasitoids associated with gall-inducing insect assemblages in the tropics, and the first investigation of vertical stratification in quantitative food web structure. 2.,Galls and associated parasitoids were sampled in the understorey and canopy of Parque Natural Metropolitano in the Pacific forest, and in the understorey of San Lorenzo Protected Area in the Caribbean forest of Panama. Quantitative host,parasitoid food webs were constructed for each assemblage, including 34 gall maker species, 28 host plants, and 57 parasitoid species. 3.,Species richness was higher in the understorey for parasitoids, but higher in the canopy for gall makers. There was an almost complete turnover in gall maker and parasitoid assemblage composition between strata, and the few parasitoid species shared between strata were associated with the same host species. 4.,Most parasitoid species were host specific, and the few polyphagous parasitoid species were restricted to the understorey. 5.,These results suggest that, in contrast to better-studied leaf miner,parasitoid assemblages, the influence of apparent competition mediated by shared parasitoids as a structuring factor is likely to be minimal in the understorey and practically absent in the canopy, increasing the potential for coexistence of parasitoid species. 6.,High parasitoid beta diversity and high host specificity, particularly in the poorly studied canopy, indicate that tropical forests may be even more species rich in hymenopteran parasitoids than previously suspected. [source] Refuge-mediated apparent competition in plant,consumer interactionsECOLOGY LETTERS, Issue 1 2010John L. Orrock Abstract At the intersection of consumer behaviour and plant competition is the concept of refuge-mediated apparent competition: an indirect interaction whereby plants provide a refuge for a shared consumer, subsequently increasing consumer pressure on another plant species. Here, we use a simple model and empirical examples to develop and illustrate the concept of refuge-mediated apparent competition. We find that the likelihood that an inferior competitor will succeed via refuge-mediated apparent competition is greater when competitors have similar resource requirements and when consumers exhibit a strong response to the refuge and high attack rates on the superior competitor. Refuge-mediated apparent competition may create an emergent Allee effect, such that a species invades only if it is sufficiently abundant to alter consumer impact on resident species. This indirect interaction may help explain unresolved patterns observed in biological invasion, such as the different physical structure of invasive exotic plants, the lag phase, and the failure of restoration efforts. Given the ubiquity of refuge-seeking behaviour by consumers and the ability of consumers to alter the outcome of direct competition among plants, refuge-mediated apparent competition may be an underappreciated mechanism affecting the composition and diversity of plant communities. Ecology Letters (2010) 13: 11,20 [source] Ecology of invasive mosquitoes: effects on resident species and on human healthECOLOGY LETTERS, Issue 5 2005Steven A. Juliano Abstract Investigations of biological invasions focus on patterns and processes that are related to introduction, establishment, spread and impacts of introduced species. This review focuses on the ecological interactions operating during invasions by the most prominent group of insect vectors of disease, mosquitoes. First, we review characteristics of non-native mosquito species that have established viable populations, and those invasive species that have spread widely and had major impacts, testing whether biotic characteristics are associated with the transition from established non-native to invasive. Second, we review the roles of interspecific competition, apparent competition, predation, intraguild predation and climatic limitation as causes of impacts on residents or as barriers to invasion. We concentrate on the best-studied invasive mosquito, Aedes albopictus, evaluating the application of basic ecological theory to invasions by Aedes albopictus. We develop a model based on observations of Aedes albopictus for effects of resource competition and predation as barriers to invasion, evaluating which community and ecosystem characteristics favour invasion. Third, we evaluate the ways in which invasive mosquitoes have contributed to outbreaks of human and animal disease, considering specifically whether invasive mosquitoes create novel health threats, or modify disease transmission for existing pathogen,host systems. [source] Weak parasitoid-mediated apparent competition between two Phyllonorycter (Lepidoptera: Gracillariidae) leaf miner species on a deciduous oak Quercus dentataENTOMOLOGICAL SCIENCE, Issue 3 2009Takashi NAKAMURA Abstract Parasitoid assemblages and the rates of parasitism on tissue-feeding larvae of two Phyllonorycter leaf miner species, P. persimilis and P. leucocorona, were studied from the autumn generation in 2002 to the summer generation in 2005 to understand whether parasitoids mediate interactions between the two leaf miner species. Fourteen species of parasitoids emerged from P. persimilis and 11 emerged from P. leucocorona. The parasitism rate was high: 24.1,92.6% in P. persimilis and 58.9,81.7% in P. leucocorona. Thus, parasitism was a major mortality factor in the present Phyllonorycter species. The parasitoid composition was distinctly different between the two host species, although most parasitoids were able to parasitize both leaf miner species. The analysis based on the quantitative parasitoid overlap revealed that the present parasitoids could mediate interactions between the present leaf miner species, but their effects would be weak. This is attributable to parasitoid's preferential uses of either of the leaf miners as a host. [source] Field experiments testing for apparent competition between primary parasitoids mediated by secondary parasitoidsJOURNAL OF ANIMAL ECOLOGY, Issue 2 2001Rebecca J. Morris Summary 1,Two field manipulation experiments were performed to look for apparent competition between primary parasitoids of aphids, mediated by shared secondary parasitoids. In each case the frequency of secondary parasitism on a ,focal' species was compared in the presence or absence of a ,treatment' species and its aphid host. 2,In both experiments the focal species was Aphidius ervi (Haliday) attacking Acyrthosiphon pisum (Harris) (pea aphid). In the first experiment the treatment species was Aphidius rhopalosiphi (De Stefani-Perez) attacking Sitobion avenae (Fabricius) (a grass aphid) and in the second Aphidius microlophii (Pennachio & Tremblay) attacking Microlophium carnosum (Buckton) (nettle aphid). 3,No significant effects of either the presence of A. rhopalosiphi or of its host on secondary parasitism of A. ervi were detected in the first experiment. 4,In the second experiment, rates of secondary parasitism were estimated twice: a significant effect of treatment was found on the first occasion but not the second. In the former, parasitism was highest in the presence of the treatment species and its host, intermediate when just the host was present, and lowest when both were absent. 5,We discuss the evidence for apparent competition in host-parasitoid communities. [source] Modelling the spatial dynamics of parapoxvirus disease in red and grey squirrels: a possible cause of the decline in the red squirrel in the UK?JOURNAL OF APPLIED ECOLOGY, Issue 6 2000S.P. Rushton Summary 1. ,A stochastic individual-based model for simulating the dynamics of an infectious disease in sympatric red and grey squirrel populations is described. The model simulates the spread of parapoxvirus between squirrels in fragmented populations based on the dispersal of infected animals, the probability of encounters between individuals, exposure to the virus and subsequent mortality. 2. ,The disease model was integrated with a spatially explicit population dynamics model that simulated red and grey squirrel populations in real landscapes, using habitat information held in a geographical information system. Latin hypercube sampling was used to create a range of realistic life-history and infection scenarios and the model was used to investigate the dynamics of red and grey squirrels in Norfolk between 1966 and 1980. 3. ,The model predicted that parapoxvirus, like interspecific competition, could have led to the extinction of the red squirrel in Norfolk. The results suggest that the red squirrel,grey squirrel,parapoxvirus interaction represents a system of apparent competition mediated by an infectious agent, as seen in other interactions between resident and exotic species. 4. ,The need for further epidemiological research on the virus is emphasized. We believe that the combined effects on disease transmission of habitat, behaviour and grey squirrels acting as reservoir hosts will lead to a patchy prevalence and sporadic incidence of parapoxvirus disease in red squirrels and a more rapid local replacement by grey squirrels. 5. ,These results have implications for conservation management of the red squirrel in the UK. Schemes in which animals are translocated or given supplementary feeding may enhance disease spread by bringing infected animals into contact with others. [source] Barley yellow dwarf viruses (BYDVs) preserved in herbarium specimens illuminate historical disease ecology of invasive and native grassesJOURNAL OF ECOLOGY, Issue 6 2007CAROLYN M. MALMSTROM Summary 1In plant invasion ecology, viruses and other pathogens are often considered in terms of the enemy release hypothesis, which predicts that plants become invasive in new ranges if they escape pathogens from their home range. However, pathogens may sometimes facilitate host spread rather than hinder it. 2Previously, we hypothesized that apparent competition mediated by barley and cereal yellow dwarf viruses (Luteoviridae: BYDVs, CYDVs) may have facilitated historical grassland invasion in California, USA, where Eurasian grasses displaced native grasses in the 18th and 19th centuries (the disease facilitation hypotheses). However, this could have happened only if the viruses were present during the invasion, which is unknown. 3To investigate the historical ecology of BYDVs in California grasses, we analysed preserved virus infections in herbarium specimens and used the historical virus sequences to determine rough time estimates of relevant phylogenetic events. 4The historical viral RNA sequences we identified in invasive and native grasses date from 1917 and are among the oldest recovered from plants thus far and the oldest from North America. 5Herbarium evidence and phylogenetic analysis suggest that BYDVs were likely to have been present in wild grasses during the California grassland invasion and to have shared some functional characteristics with present-day isolates, supporting the disease facilitation hypothesis. 6We found evidence of virus spread from California to Australia (or, less likely, from Australia to California) in the late 19th century, when much horticultural exchange occurred, as well as potential correspondence in the timing of virus diversification events and the beginning of extensive human exchange between the Old and New Worlds. 7Synthesis. Increasing evidence indicates that viruses are important in the ecology of grasslands and may, in some cases, mediate apparent competition among species. Historical data provide essential insight into plant virus ecology and suggest the need to examine human influence on plant virus diversification and spread within natural ecosystems. [source] Virus infection and grazing exert counteracting influences on survivorship of native bunchgrass seedlings competing with invasive exoticsJOURNAL OF ECOLOGY, Issue 2 2006C. M. MALMSTROM Summary 1,Invasive annual grasses introduced by European settlers have largely displaced native grassland vegetation in California and now form dense stands that constrain the establishment of native perennial bunchgrass seedlings. Bunchgrass seedlings face additional pressures from both livestock grazing and barley and cereal yellow dwarf viruses (B/CYDVs), which infect both young and established grasses throughout the state. 2,Previous work suggested that B/CYDVs could mediate apparent competition between invasive exotic grasses and native bunchgrasses in California. 3,To investigate the potential significance of virus-mediated mortality for early survivorship of bunchgrass seedlings, we compared the separate and combined effects of virus infection, competition and simulated grazing in a field experiment. We infected two species of young bunchgrasses that show different sensitivity to B/CYDV infection, subjected them to competition with three different densities of exotic annuals crossed with two clipping treatments, and monitored their growth and first-year survivorship. 4,Although virus infection alone did not reduce first-year survivorship, it halved the survivorship of bunchgrasses competing with exotics. Within an environment in which competition strongly reduces seedling survivorship (as in natural grasslands), virus infection therefore has the power to cause additional seedling mortality and alter patterns of establishment. 5,Surprisingly, clipping did not reduce bunchgrass survivorship further, but rather doubled it and disproportionately increased survivorship of infected bunchgrasses. 6,Together with previous work, these findings show that B/CYDVs can be potentially powerful elements influencing species interactions in natural grasslands. 7,More generally, our findings demonstrate the potential significance of multitrophic interactions in virus ecology. Although sometimes treated collectively as plant ,predators', viruses and herbivores may exert influences that are distinctly different, even counteracting. [source] Dogs Canis familiaris as carnivores: their role and function in intraguild competitionMAMMAL REVIEW, Issue 4 2009ABI TAMIM VANAK ABSTRACT 1Dogs Canis familiaris are the world's most common carnivore and are known to interact with wildlife as predators, prey, competitors, and disease reservoirs or vectors. 2Despite these varied roles in the community, the interaction of dogs with sympatric wild carnivore species is poorly understood. We review how dogs have been classified in the literature, and illustrate how the location and ranging behaviour of dogs are important factors in predicting their interactions with wild prey and carnivores. 3We detail evidence of dogs as intraguild competitors with sympatric carnivores in the context of exploitative, interference and apparent competition. 4Dogs can have localized impacts on prey populations, but in general they are not exploitative competitors with carnivores. Rather, most dog populations are highly dependent on human-derived food and gain a relatively small proportion of their diet from wild prey. However, because of human-derived food subsidies, dogs can occur at high population densities and thus could potentially outcompete native carnivores, especially when prey is limited. 5Dogs can be effective interference competitors, especially with medium-sized and small carnivores. Dogs may fill the role of an interactive medium-sized canid within the carnivore community, especially in areas where the native large carnivore community is depauperate. 6Dogs can also be reservoirs of pathogens, because most populations around the world are free-ranging and unvaccinated. Diseases such as rabies and canine distemper have resulted in severe population declines in several endangered carnivores coexisting with high-density dog populations. Dogs can therefore be viewed as pathogen-mediated apparent competitors, capable of facilitating large-scale population declines in carnivores. 7Based on this information, we propose conceptual models that use dog population size and ranging patterns to predict the potential for dogs to be intraguild competitors. We discuss how interactions between dogs and carnivores might influence native carnivore communities. [source] Smaller Microtus vole species competitively superior in the absence of predatorsOIKOS, Issue 1 2007Elina Koivisto Interspecific competition is assumed to generate negative effects on coexisting species, possibly including slower population growth and lower survival. The field vole (Microtus agrestis) and the sibling vole (M. rossiaemeridionalis) are sympatric close relatives which compete for similar resources. Previous non-experimental studies suggest that the smaller sibling vole is a superior competitor, yet more vulnerable to predation than the larger field vole. We studied the effects of coexistence on population densities, reproductive parameters, and survival in these two species by means of experimentation in large, predator-free outdoor enclosures. While populations of both species reached higher densities in the absence of the other, field voles appeared to suffer more from interspecific competition than sibling voles. The proportion of young individuals in the population was higher in the sibling vole than in the field vole at the end of the experiment. The presence of a coexisting species reduced the survival of field voles. Sibling voles, on the other hand, appeared to suffer more from intraspecific competition than interspecific competition. On a population level, the sibling vole seems to be a superior competitor in the absence of predators due to better survival and possibly a higher reproductive capacity. However, predation probably has a profound influence on the interspecific dynamics of these two species indicating that in natural surroundings apparent competition (i.e. competition via shared predators) is stronger than direct competition. [source] From simple rules to cycling in community assemblyOIKOS, Issue 2 2004Sebastian J. Schreiber Simulation studies of community assembly have frequently observed two related phenomena: (1) the humpty dumpty effect in which communities can not be reconstructed by "sequential" invasions (i.e. single species invasions separated by long intervals of time) and (2) cycling between sub-communities. To better understand the mechanisms underlying these phenomena, we analyze a system consisting of two predators and two prey competing for a shared resource. We show how simple dominance rules (i.e. R* and P* rules) lead to cycling between sub-communities consisting of predator,prey pairs; predator and prey invasions alternatively lead to prey displacement via apparent competition and predator displacement via exploitative competition. We also show that these cycles are often dynamically unstable in the population phase space. More specifically, while for too slow invasion rates (i.e. "sequential" invasions) the system cycles indefinitely, faster invasion rates lead to coexistence of all species. In the later case, the assembly dynamics exhibit transient cycling between predator-prey subcommunities and the length of these transients decreases with the invasion rate and increases with habitat productivity. [source] The interaction between bird predation and plant cover in determining habitat occupancy of darkling beetlesOIKOS, Issue 1 2001Elli Groner Tenebrionid beetles in the Negev Desert exhibit size-related habitat segregation, with larger species found in denser cover. Size-dependent predation by birds has been suggested as the mechanism behind this habitat segregation. Two predictions of this hypothesis were tested: (1) plant cover reduces the predation efficiency of birds upon large tenebrionids, and (2) birds prefer larger species. Both predictions were supported: plant cover reduced predation rate by the most common spring and summer predatory birds: white storks (Ciconia ciconia) and stone curlews (Burhinus oedicnemus), in cage experiments. Results from preference experiments suggest that tenebrionid species can be divided according to their profitability as prey. Large species are the most profitable, medium-sized species are less profitable but still acceptable and small species are unprofitable and therefore ignored. Field observations demonstrated that the well-vegetated wadi habitats are dominated by large and small species whereas acceptable, medium-sized species are under-represented in this habitat. The results of the cage experiments indicate possible apparent competition between the large profitable and the medium acceptable tenebrionid species in the wadis. Aggregative response of predators in the profitable habitat is suggested as the mechanism leading to truncated distribution of prey species. Large profitable species are refuge-dependent, medium-sized acceptable species use enemy free space and small species are predator independent. [source] Enemy-mediated apparent competition: empirical patterns and the evidenceOIKOS, Issue 2 2000Enrique J. Chaneton Apparent competition arises when two victim species negatively affect each other (,,,,) by enhancing the equilibrium density or changing the foraging behaviour of a shared natural enemy. Shared enemies can also mediate non-reciprocal (,,,0) indirect effects, i.e. indirect amensalism, whenever one prey species is not affected by the presence of alternative prey. We review 34 studies on terrestrial and freshwater systems to evaluate the extent to which apparent competition has been perceived as a reciprocal (,,,,) or non-reciprocal (,,,0) interaction. We found only three studies showing reciprocal effects between apparent competitors. Indirect amensalism was documented in 10 studies and could be inferred for 16 other cases (76% in total). The remaining five studies provided insufficient data to determine the form of indirect interaction. The apparent prevalence of non-reciprocal enemy-mediated interactions resembles that observed for resource-based interspecific competition. Amensal indirect effects via shared predation may result from differences in population size, nutritional value, susceptibility to attack, or asynchronous dynamics of alternative prey, or the predator's feeding preferences. Moreover, experimental protocols may confound the actual form of apparent competition through short-term observations, incomplete designs, or biased consideration of conspicuous interactions, leading to reciprocal effects being overlooked. We conclude that, at present, it is still difficult to determine the relative role of apparent competition vs indirect amensalism in natural food webs because most published studies have failed to document in full interactions via shared enemies. [source] Endangered, apparently: the role of apparent competition in endangered species conservationANIMAL CONSERVATION, Issue 4 2010N. J. DeCesare Abstract Conservation biologists have reported growing evidence of food-web interactions as causes of species endangerment. Apparent competition is an indirect interaction among prey species mediated by a shared predator, and has been increasingly linked to declines of prey species across taxa. We review theoretical and empirical studies of apparent competition, with specific attention to the mechanisms of asymmetry among apparently competing prey species. Asymmetry is theoretically driven by niche overlap, species fitness traits, spatial heterogeneity and generalist predator behavior. In real-world systems, human-induced changes to ecosystems such as habitat alteration and introduced species may be ultimate sources of species endangerment. However, apparent competition is shown to be a proximate mechanism when resultant changes introduce or subsidize abundant primary prey for predator populations. Demonstration of apparent competition is difficult due to the indirect relationships between prey and predator species and the potential for concurrent exploitative competition or other community effects. However, general conclusions are drawn concerning the characteristics of prey and predator species likely to exhibit asymmetric apparent competition, and the options for recovering endangered species. While short-term management may be required to avoid imminent extinction in systems demonstrating apparent competition, we propose adaptive conservation efforts directed at long-term recovery. [source] Test of the enemy release hypothesis: The native magpie moth prefers a native fireweed (Senecio pinnatifolius) to its introduced congener (S. madagascariensis)AUSTRAL ECOLOGY, Issue 1 2008EVE M. WHITE Abstract The enemy release hypothesis predicts that native herbivores will either prefer or cause more damage to native than introduced plant species. We tested this using preference and performance experiments in the laboratory and surveys of leaf damage caused by the magpie moth Nyctemera amica on a co-occuring native and introduced species of fireweed (Senecio) in eastern Australia. In the laboratory, ovipositing females and feeding larvae preferred the native S. pinnatifolius over the introduced S. madagascariensis. Larvae performed equally well on foliage of S. pinnatifolius and S. madagascariensis: pupal weights did not differ between insects reared on the two species, but growth rates were significantly faster on S. pinnatifolius. In the field, foliage damage was significantly greater on native S. pinnatifolius than introduced S. madagascariensis. These results support the enemy release hypothesis, and suggest that the failure of native consumers to switch to introduced species contributes to their invasive success. Both plant species experienced reduced, rather than increased, levels of herbivory when growing in mixed populations, as opposed to pure stands in the field; thus, there was no evidence that apparent competition occurred. [source] Presence of Lythrum salicaria enhances the bodyguard effects of the parasitoid Asecodes mento for Filipendula ulmariaOIKOS, Issue 3 2007Johan A. Stenberg This paper reports significant effects of a co-occurring plant species (Lythrum salicaria, Lythraceae) on the reproductive success of the perennial herb Filipendula ulmaria (Rosaceae). We studied 15 Filipendula populations in the Skeppsvik Archipelago; seven of which were monospecific and eight mixed with Lythrum. All the Filipendula populations studied harbored the chrysomelid beetle Galerucella tenella, and in 2005 seed set was strongly negatively correlated with the percentage leaf area consumed. Moreover, data from 2004 showed that 25,100% of the G. tenella larvae were parasitized by the hymenopteran parasitoid Asecodes mento, and we found a strong cascading top-down effect of parasitism in 2004 on Filipendula seed set in 2005. In 2004, parasitism (at the population level) was negatively correlated with percentage leaf area consumed and positively correlated with seed set in 2005. The parasitoid Asecodes also parasitized G. calmariensis, which is monophagous on Lythrum. Mixed populations of Filipendula and Lythrum supported higher densities of their shared ,bodyguard'Asecodes. Further, Y-tube bioassays showed that floriferous Filipendula attracted more than twice as many gravid Asecodes females as floriferous Lythrum. Taken together, these findings suggest that coexistence of the two plants results in ,associational resistance' for Filipendula and ,associational susceptibility' for Lythrum. This scenario was supported for Filipendula since, for this species, we found lower leaf consumption followed by higher seed production in mixed than in monospecific populations. Considered together, our results show that bodyguards may increase the reproductive fitness of a perennial herb, and that the strength of the cascading ,bodyguard' effect can be strongly influenced by co-occurring plants through ,apparent competition'. This is the first paper to demonstrate that, in the wild, plant species may use odors to compete for ,bodyguards', thereby causing asymmetrical ,apparent competition' between the herbivores involved. Our data emphasize the need to consider community factors in studies of trophic interactions. [source] | |||||||||||||||||||||||||