Interspecific competition and coexistence of the two chrysomelids,Gastrophysa atrocyanea motschulsky andGalerucella vittaticollis baly (Coleoptera: Chrysomelidae), under limited food resource conditions

Field experiments were conducted in order to investigate the mode of exploitation of food resources and the mechanism of coexistence of mixed larval populations of the two chrysomelids,Gastrophysa atrocyanea andGalerucella vittaticollis, under limited food resource conditions. The larval survival rates seemed high enough to assure coexistence when hatchlings of the two species were released in 1∶1 and 1∶3 ratios on a host plant. However, the survival rate became almost nil for both species when a 3∶1 ratio was employed, suggesting asymmetrical interspecific competition. Wasted food consumption was much higher inG. atrocyanea larvae. The population ofG. atrocyanea seemed to be regulated more by intraspecific competition, while on the other hand, the population ofG. vittaticollis was considered to be more likely affected by the interspecific competition withG. atrocyanea, depending on the initial ratio and density of the two species.     

Spatial aggregation across ephemeral resource patches in insect communities: an adaptive response to natural enemies?

Although an increase in competition is a common cost associated with intraspecific crowding, spatial aggregation across food-limited resource patches is a widespread phenomenon in many insect communities. Because intraspecific aggregation of competing insect larvae across, e.g. fruits, dung, mushrooms etc., is an important means by which many species can coexist (aggregation model of species coexistence), there is a strong need to explore the mechanisms that contribute to the maintenance of this kind of spatial resource exploitation. In the present study, by using Drosophila-parasitoid interactions as a model system, we tested the hypothesis whether intraspecific aggregation reflects an adaptive response to natural enemies. Most of the studies that have hitherto been carried out on Drosophila-parasitoid interactions used an almost two-dimensional artificial host environment, where host larvae could not escape from parasitoid attacks, and have demonstrated positive density-dependent parasitism risk. To test whether these studies captured the essence of such interactions, we used natural breeding substrates (decaying fruits). In a first step, we analysed the parasitism risk of Drosophila larvae on a three-dimensional substrate in natural fly communities in the field, and found that the risk of parasitism decreased with increasing host larval density (inverse density dependence). In a second step, we analysed the parasitism risk of Drosophila subobscura larvae on three breeding substrate types exposed to the larval parasitoids Asobara tabida and Leptopilina heterotoma. We found direct density-dependent parasitism on decaying sloes, inverse density dependence on plums, and a hump-shaped relationship between fly larval density and parasitism risk on crab apples. On crab apples and plums, fly larvae benefited from a density-dependent refuge against the parasitoids. While the proportion of larvae feeding within the fruit tissues increased with larval density, larvae within the fruit tissues were increasingly less likely to become victims of parasitoids than those exposed at the fruit surface. This suggests a facilitating effect of group-feeding larvae on reaching the spatial refuge. We conclude that spatial aggregation in Drosophila communities can at least in part be explained as a predator avoidance strategy, whereby natural enemies act as selective agents maintaining spatial patterns of resource utilisation in their host communities.     

Temperature modulates the effects of predation and competition on mosquito larvae

1. Predation risk affects interspecific competition by decreasing foraging activity and relative competitive ability. Predation risk is determined by predators' prey choice and prey responses, both of which can be influenced by temperature. Temperature is especially important for larval prey and can result in a trade‐off between predator‐induced decreases in foraging activity and growth. Interspecific competition must also be examined in relation to intraspecific density‐dependent competition; weaker interspecific competition leads to coexistence of competitors. 2. This study explored how temperature (15 and 25 °C) could affect a focal species, larvae of the mosquito Culex quinquefasciatus, by examining prey choice in a shared predator (mosquitofish; Gambusia holbrooki) and the effects of predation risk on interspecific competition with Limnodynastes peronii tadpoles. Intraspecific density‐dependent competition in C. quinquefasciatus at these temperatures was also examined. 3. At 25 °C, G. holbrooki consumption of both C. quinquefasciatus and L. peronii increased; however, the effects of interspecific competition on mosquito survival did not decrease with L. peronii exposure to predation risk. The relationship between intraspecific density‐dependent competition and interspecific competition was temperature‐dependent, with competitive dominance of L. peronii at 25 °C. Male and female mosquitoes had different temperature‐dependent responses, indicating sex‐specific intrinsic responses to starvation and differential selection pressures. At 25 °C, females were susceptible to interspecific competition by L. peronii, while males were susceptible to intraspecific competition. 4. The use of competitors as biological controls has implications for mosquito disease transmission, and these results suggest that control effectiveness may be modified by climate change.     

Intraspecific competition in ant-lion (Macroleon quinquemaculatus) larvae in the field

The number of larvae of the pit-digging ant-lion Macroleon quinquemaculatus at a food-poor site in Tanzania increased between May and October 1983 and then remained at high density until the following rainy season in March. Food availability was low and small larvae were most likely to suffer food shortage. Hunger level varied within and between instars: hunger was greatest after moults and only third-instar larvae weighing more than 100 mg were well fed. A simple model showed that exploitation competition was asymmetric and most likely to affect small larvae, by removing small prey. Exploitation was estimated to reduce food intake by 30–60%. Within-site differences in growth rate and size were associated with differences in larval density. Small larvae were much more likely to move than large larvae; the latter tended to occupy the periphery of the habitat. The reduction in food intake experienced by small larvae was a consequence of interference competition, which was the main factor operating at this site.     

Plant abandonment behavior and fitness of monarch larvae (Danaus plexippus) is not influenced by an intraspecific competitor

Integrating aspects of larval lepidopteran behavior that enhance survival into conservation plans could increase the overall impact of the efforts. We previously recommended that where possible, maintaining 2–4 ramets of closely-spaced common milkweed would support the development of at least one monarch through pupation, based on a seemingly innate behavior in which monarch larvae (Danaus plexippus) abandon their natal milkweed ramet (Asclepias sp.). Here, we explored the impact of intraspecific competition on larval ramet abandonment behavior and fitness of monarch larvae in small artificial milkweed patches.

We observed larvae reared under direct and indirect intraspecific competition, and larvae reared alone.

We found no influence of intraspecific competition; however, our study provides further support that milkweed ramet abandonment is a seemingly innate behavior. This behavior occurs before all of the available leaf biomass on a ramet is consumed and prior to the pre-pupal wandering stage.

Implications for insect conservation: Results from our study suggest that in the absence of predation, parasitism, and interspecific competition, and when sufficient plant biomass is present to support larval development, the presence of an intraspecific competitor does not influence larval behavior or fitness. Based on milkweed ramet abandonment behavior, we continue to suggest maintaining small patches of 2–4 milkweed ramets when possible.

     

Effects of food availability on larval development in the slipper limpet Crepidula onyx (Sowerby)

Effects of food availability on the larval survival and development of Crepidula onyx were studied in four experiments by feeding the larvae with different concentrations of the chrysophyte Isochrysis galbana and by starving the larvae for different periods of time. Food concentration had a clear impact on the survival, growth and development time of C. onyx veligers. Larval development occurred only at 10⁴ cells ml⁻¹ and higher algal concentrations. No shell increment was detected in the veligers cultured for 12 days at 10² cells ml⁻¹I. galbana or the blank control. At 10³ cells ml⁻¹, there was only a slight increase in shell length over 12 days. At 10⁴ cells ml⁻¹, about 40% of the larvae became competent in 18 days. At 10⁵ and 10⁶ cells ml⁻¹, more than 90% of the larvae reached competence in 7 days. Initial starvation negatively affected the larval development, but the sensitivity differed among parameters measured on day 5: lower survivorship was detected only for larvae that had suffered 3 days or longer initial starvation, whereas one-day initial starvation caused shorter shells and lower percentage of competent larvae. Three days of continuous feeding was required for 50% of the larvae to reach competence. After feeding for 3 days, most larvae could become competent to metamorphose even under starvation. The time of starvation was also critical: larvae that suffered 1-day food deprivation in the first 2 days of larval release had shorter shells and lowered percent competent larvae than those that suffered the same length of food deprivation in later stages of development. Our study thus indicates that both food concentration and short-term starvation have detrimental effects on the larval development of this species, and that once the larva has consumed certain amount of food, starvation may induce metamorphosis.     

No trade‐offs in interspecific interference ability and predation susceptibility in newt larvae

Coexistence of species with similar requirements is allowed, among others, through trade‐offs between competitive ability and other ecological traits. Although interspecific competition is based on two mechanisms, exploitation of resources and physical interference, trade‐off studies largely consider only species’ ability to exploit resources. Using a mesocosm experiment, we examined the trade‐off between interference competition ability and susceptibility to predation in larvae of two newt species, Ichthyosaura alpestris and Lissotriton vulgaris. In the presence of heterospecifics, L. vulgaris larvae slowed somatic growth and developmental rates, and experienced a higher frequency of injuries than in conspecific environments which suggests asymmetrical interspecific interference. During short‐term predation trials, L. vulgaris larvae suffered higher mortality than I. alpestris. Larvae of the smaller species, L. vulgaris, had both lower interference and antipredator performance than the larger I. alpestris, which suggests a lack of trade‐off between interference competition ability and predator susceptibility. We conclude that interference competition may produce a positive rather than negative relationship with predation susceptibility, which may contribute to the elimination of subordinate species from common habitats.     

Density of an intraguild predator mediates feeding group size, intraguild egg predation, and intra- and interspecific competition

Intraguild predation (IGP) is common in most communities, but many aspects of density-dependent interactions of IG predators with IG prey are poorly resolved. Here, we examine how the density of an IG predator can affect feeding group size, IG egg predation, and the growth responses of IG prey. We used laboratory feeding trials and outdoor mesocosm experiments to study interactions between a social intraguild predator (larvae of the wood frog; Rana sylvatica) and its prey (spotted salamander; Ambystoma maculatum). Larvae of R. sylvatica could potentially affect A. maculatum by consuming shared larval food resources or by consuming eggs and hatchlings. However, successful egg predation requires group feeding by schooling tadpoles. We established from five to 1,190 hatchlings of R. sylvatica in mesocosms, then added either 20 A. maculatum hatchlings to study interspecific competition, or a single egg mass to examine IGP. Crowding strongly suppressed the growth of R. sylvatica, and IGP was restricted to the egg stage. In the larval competition experiment, growth of A. maculatum was inversely proportional to R. sylvatica density. In the predation experiment, embryonic mortality of A. maculatum was directly proportional to the initial density of R. sylvatica and the mean number of tadpoles foraging on egg masses. IGP on eggs reduced A. maculatum hatchling density, which accelerated larval growth. Surprisingly, the density of R. sylvatica had no overall effect on A. maculatum growth because release from intraspecific competition via egg predation was balanced by increased interspecific competition. Our results demonstrate that the density of a social IG predator can strongly influence the nature and intensity of interactions with a second guild member by simultaneously altering the intensity of IGP and intra- and interspecific competition.L . A. Burley and A. T. Moyer contributed equally to this work.     

Interspecific interactions in a tri-trophic arthropod system: effects of a spider on the survival of larvae of three predatory ladybirds in relation to aphids

The effects of the crab spider, Misumenops tricuspidatus (Fabricius), on the larval survival of three ladybird species, Harmonia axyridis Pallas, Coccinella septempunctata L., and Propylea japonica L., in relation to aphids were investigated in the laboratory. Predation by the spider on the three ladybird species differed. All the larvae of C. septempunctata, none of H. axyridis, and an intermediate number of P. japonica were attacked and eaten by the spider. All the larvae of H. axyridis suffered mortality due to cannibalism or starvation in the treatments with and without a spider. In case of C. septempunctata, however, mortality in the early instars was significantly greater in the treatment with a spider than without a spider and no larvae developed into pupae due to predation. In the treatment without a spider, the majority of the larvae in the former treatment suffered mortality due to cannibalism or starvation, and only 13.3% of larvae developed into the adult stage. In the case of P. japonica, mortality was mainly attributed to predation in the treatment with a spider and only 26.7% became adult. In comparison, 86.7% of larvae survived to the adult stage in the treatment without a spider. In addition, in both H. axyridis and C. septempunctata, the development of young larvae was significantly slower in the presence of a spider, but this was not the case with the older larvae of H. axyridis, which indicates that the effect of the spider on larval development changed with the developmental stage of the larvae in this species. However, the spider had no significant effect on the developmental time of P. japonica larvae. Although both the spider and the ladybirds significantly affected the number of aphids, they did not have an additive effect on aphid abundance. The interactions between the spider and the ladybirds, such as intraguild predation or competition, caused them to reduce aphid population density less than the ladybirds did on their own. The effect of the spider on the larval performance of three predatory ladybirds was found to be unequal in terms of their vulnerability to predation and rate of larval development and it depended on the species and developmental stage of the ladybird.     

Theoretical studies on the role of food exploitation for the competition of scaamble type

A model was made to clarify the basic processes of competition to occur among larvae by the exploitation as defined byBakker (1969). It was found that this model is applicable to the experimental results on the food exploitation among Droshophila larvae obtained byBakker (1961). In the model the preimaginal stage is divided into two periods;T_f which is the time that a group of larvae spends in exhausting the food after hatching, and T_s which is the duration of the starvation period after T_f.T_f and then W_l (larval body weight) just after the end of T_f are decided by F_s (amount of food supplied per larva at larval hatching) and F_c (amount of food consumed per larva).T_f affects on the onset of T_s as well as R_l (rate of decrease in the individual body weight during T_s).W_a (weight of emerging adults) is gotten by a subtraction of R_l from W_l just after the end of T_f,R_e is affected directly by these components of W_l and R_l. As a result, W_a and R_e are expressed by functions of F_s. This model confirmed that the food exploitation lead to the competition of scramble type. Finally it was suggested that there exist some strategies which prevent ill-effects owing to the food exploitation.     

Consumer–resource dynamics of indirect interactions in a mutualism–parasitism food web module

Food web dynamics are well known to vary with indirect interactions, classic examples including apparent competition, intraguild predation, exploitative competition, and trophic cascades of food chains. Such food web modules entailing predation and competition have been the focus of much theory, whereas modules involving mutualism have received far less attention. We examined an empirically common food web module involving mutualistic (N ₂) and parasitic (N ₃) consumers exploiting a resource of a basal mutualist (N ₁), as illustrated by plants, pollinators, and nectar robbers. This mutualism–parasitism food web module is structurally similar to exploitative competition, suggesting that the module of two consumers exploiting a resource is unstable. Rather than parasitic consumers destabilizing the module through (?,?) indirect interactions, two mechanisms associated with the mutualism can actually enhance the persistence of the module. First, the positive feedback of mutualism favors coexistence in stable limit cycles, whereby (+,?) indirect interactions emerge in which increases in N ₂ have positive effects on N ₃ and increases in N ₃ have negative effects on N ₂. This (+,?) indirect interaction arising from the saturating positive feedback of mutualism has broad feasibility across many types of food web modules entailing mutualism. Second, optimization of resource exploitation by the mutualistic consumer can lead to persistence of the food web module in a stable equilibrium. The mutualism–parasitism food web module is a basic unit of food webs in which mutualism favors its persistence simply through density-dependent population dynamics, rather than parasitism destabilizing the module.     

The roles of predation,competition, and exploitation in the trophic dynamics of a warmwater stream: a model synthesis,analysis, and application

We developed a trophic dynamic model of key populations and processes in the New River, West Virginia, to identify the mechanisms most responsible for maintaining food web structure. Key populations were represented by thirteen model components and were aquatic insects; age-1 and age-2 crayfish (three species); age-1 and age-2 hellgrammites (Corydalus cornutus larvae); non-game fishes; age-0, age-1, and adult smallmouth bass (Micropterus dolomieu); age-0, age-1, and adult rock bass (Ambloplites rupestris); and age-0, age-1 to age-3, and adult flathead catfish (Pylodictis olivaris). In this system, crayfish and hellgrammites are harvested to provide bait for the recreational fishery that extensively exploits the three predatory fish species. Predation and intraspecific regulation were represented with nonlinear algorithms, and linear terms represented fishery harvests. Interspecific competition among components occurred through predation on shared prey. Error analysis of the model suggested that predation was the most important mechanism in maintaining system structure (the disposition of biomass among system components). Further, the trophic relation between each component and its prey accounted for 34–64% of the variability in food web structure, whereas predation on each component explained 1–24% of food web structure variability. Therefore, so-called ‘bottom-up’ effects were more influential than ‘top-down’ effects. Interspecific competition and intraspecific regulation had secondary roles in maintaining New River food web structure, although intraspecific regulation was most important to aquatic insects, which were not predatory in our model. Both forms of competition are probably tempered by extensive predation and exploitation in the New River system. Exploitation was a secondary structuring agent to adult smallmouth bass, which experience a high rate of harvest in the New River.     

Condition and size of damselflies: a field study of food limitation

Summary Based on evidence from field manipulations, several authors have recently suggested that interference competition among larval odonates reduces individual growth rates and biomass by reducing foraging rates. This study was designed to test the effects of food shortage on condition (relative mass per unit head width) of larval Ischnura verticalis (Odonata: Coenagrionidae) under laboratory conditions and to use these results to estimate the degree of food shortage of larvae under naturally occurring field conditions. In the laboratory, there were marked differences in condition of larvae fed diets ranging from ad libitum feeding with worms to ad libitum feeding with Daphnia 1 day out of every 8. Condition of larvae collected from May through October from 17 different sites in southern Ontario indicated that, for most of the year, larvae had conditions similar to those fed ad libitum with Daphnia in the laboratory. There was no evidence that larval condition was related to population density. Condition of larvae in most sites during July was similar to that of larvae fed poor diets in the laboratory. It is unlikely that the low conditions were due to competition as there were no correlations with density across sites and population densities during July were at their lowest. Adult head widths showed a seasonal decline from mid June to the end of the flight season. There was no evidence that head widths were related to population density although there was some evidence that head widths of males were positively related to larval condition. My results do not support the hypothesis that competition is important in affecting foraging rates and subsequent development of larvae. Contrasts between my results and other studies may stem from difficulties with the interpretation of field experiments, that densities in my study may have been low due to fish predation, and/or that I. verticalis larvae are slow moving relative to other larvae and thus less likely to interact.     

Patterns of resource use within a population of xanthid crabs occupying bryozoan colonies

Summary Heads, i.e. colonies, of the bryozoan Schizoporella pungens are discrete resource units which protect the xanthid crab Pilumnus sayi from predation. The heads were spatially clumped, temporally stable, and differed both qualitatively and quantitatively. The number of adult P. sayi was equal to the number of heads with preferred characteristics, yet a relative shortage of this resource is suggested with concomitant intraspecific competition. Adult crabs occupied heads individually except for infrequent cases of double occupancy by a male and female. Further spatial pattern among crabs was consistent with a hypothesis that males defend clumps of heads occupied by females.     

The relative importance of predation and competition in two reef fishes

Competition and predation may both strongly influence populations of reef fishes, but the importance of these processes relative to one another is poorly understood. I quantified the effects of predation and competition on the growth and survival of two temperate reef fishes, Lythrypnus dalli and Coryphopterus nicholsii, in field experiments in which I manipulated the densities of the two species and the abundance of predators (using exclosure cages) on small replicate patch reefs. I also evaluated the influence of predators on the behavior of the two species to help interpret the mechanisms of any predatory influences on growth or survival. Predation was much more important than competition (inter- or intraspecific) in Lythrypnus. For Coryphopterus, neither competition nor predation were particularly important. Behaviorally, both species responded to predators by reducing foraging rate and hiding. This altered behavior, however, had no repercussions for growth or survival of Coryphopterus. In contrast, Lythrypnus grew more slowly and suffered greater mortality when exposed to predators. Interspecific competition did not significantly influence either species. Intraspecific competition did not affect the growth of Coryphopterus, but survival tended to be lower at high densities. Growth of Lythrypnus was depressed by intraspecific competition, but survival was not, except that, in the presence of predators, survival was density dependent. In contrast to the historical emphasis placed on the role of competition, this study indicates that predation can be more important than competition in determining patterns of abundance of some reef fishes. For example, predators not only influenced foraging of both Lythrypnus and Coryphopterus, but they also reduced growth and survival of Lythrypnus, and therefore appear to help maintain the marked habitat segregation between the two species. Received: 16 June 1997 / Accepted: 3 December 1997     

Multiple infestation by seed predators: the effect of loculate fruits on intraspecific insect larval competition

Many morphological features of fruits are important factors affecting predispersal seed predation by insects. This paper analyses the predispersal seed predation process of a major predator (a Noctuidae lepidopteran larvae) in loculate fruits of a bushy perennial plant, Cistus ladanifer. The main aim of the study is to assess the potential effect of internal valvae (which partition groups of seeds) in the intraspecific competition between larvae in multiple-infested fruits.Our results show that larvae do not reject already infested fruits, but they avoid the proximity of other larvae within the fruit, keeping an average minimum distance of one locule. In multiple-infested fruits, larval mortality increases and the proportion of seeds consumed by each larvae decreases. In those situations in which valvae keep apart larvae within a fruit, these only suffer the cost of exploitation competition with a low acquisition of resources. However, when all valvae between them are pierced by the larvae, competition switches to an interference component and larval mortality increases markedly. The existence of valvae within a fruit allows larvae to diminish the cost of intraspecific competition, obtaining high life expectancies (70%), even in triple-infested fruits.     

Demonstration of interspecific competition between two sympatric egg parasitoids of Riptortus pedestris (Fabricius) in laboratory condition

Ooencyrtus nezarae Ishii (Hymenoptera: Encyrtidae) acts as a facultative hyperparasitoid of Gryon japonicum (Ashmead) (Hymenoptera: Platygastridae) sympatric parasitoid of Riptortus pedestris (Fabricius) (Hemiptera: Alydidae). A longer period of host egg exploitation by both parasitoid species would be beneficial for O. nezarae, while G. japonicum tends to be successful when the parasitoids have only a short exploitation period. We demonstrated the interspecific competition by measuring parasitism in nine combinations of host densities (10, 20, and 40 eggs) and exploitation times (1, 3, and 5 days). To reflect the gregarious-solitary dichotomy of the two species, three O. nezarae and one G. japonicum mated females were compared in addition to a one-to-one competition design. We found that O. nezarae was the better competitor when exploitation time was longer than 1 day, irrespective of host density. Total parasitism rate and progeny emergence of O. nezarae were 1.6–2.8 and 4.7–7.3 times higher than for G. japonicum in three-to-one competition design, respectively. Although G. japonicum females were more effective in host finding (as shown by their higher per capita rate of parasitism when exploitation time was short), their progeny suffered high mortality from the larval interspecific competition inside multiparasitized host eggs. These results suggest that gregarious O. nezarae is the superior competitor when host eggs are available for longer period of time while solitary G. japonicum is superior when host resources are available for only a limited time.     

Habitat selection of three chrysomelid species associated with Rumex spp.

Summary Habitat selection of three chrysomelid beetles feeding on the Rumex plants was studied in relation to the ecological requirements of the three species and the habitat characteristics arising in their unstable and often unpredictable environment. The different ecological requirements and bionomic characteristics of the three chrysomelids were associated not only with their differing degrees of feeding specialization; they also influenced habitat selection. For example, the small body size and lower food demands of Mantura clavareaui, which is a feeding specialist, allowed it to exploit any habitat, irrespective of habitat size and duration. The other specialist, Gastrophysa atrocyanea, selected larger habitats because of the relatively brief lifespan of its adults after overwintering, its reduced mortality, and greater food demand. On the other hand, Galerucella vittaticollis, which is a multivoltine, general feeder, selected the habitat that lasted longer, rather than one that was larger. This species' life cycle is not synchronized with Rumex phenology, but its adults are long-lived and it has a flexible life cycle. The different patterns in habitat selection among these three chrysomelids prevented serious overlapping of their habitats; thus, the habitat overlapping of 2 or 3 species was 34.62%, that of all 3 species 3.85%. Most of the habitats utilized by Gastrophysa, however, overlapped those of the other two species, because Gastrophysa's habitat utilization range was the narrowest of the three. In Mantura, the degree of overlap with the other two species was lowest because this beetle had the broadest range of habitat utilization. On the other hand, Galerucella adjusted the temporal, rather than the spatial coordinates of its habitat usage when it occupied habitats where the other two specialists were also present. Galerucella, unlike the other two species, is multivoltine. Consequently, it was able to coexist in habitats occupied by the others by exploiting the host plants at times when the other species were numerically few, or even absent. No direct interspecific competition among adults of the three species was observed. Their different patterns of habitat selection created by their different responses to available food allowed them to coexist on Rumex without direct competition.     

Ecosystem fragmentation drives increased diet variation in an endemic livebearing fish of the Bahamas

One consequence of human‐driven habitat degradation in general, and habitat fragmentation in particular, is loss of biodiversity. An often‐underappreciated aspect of habitat fragmentation relates to changes in the ecology of species that persist in altered habitats. In Bahamian wetlands, ecosystem fragmentation causes disruption of hydrological connectivity between inland fragmented wetlands and adjacent marine areas, with the consequent loss of marine piscivores from fragmented sections. We took advantage of this environmental gradient to investigate effects of ecosystem fragmentation on patterns of resource use in the livebearing fish Gambusia hubbsi (Family Poeciliidae), using both population‐ and individual‐level perspectives. We show that fragmentation‐induced release from predation led to increased G. hubbsi population densities, which consequently led to lower mean growth rates, likely as a result of higher intraspecific competition for food. This was accompanied by a broadening of dietary niches via increased interindividual diet variation, suggesting a negative effect of predation and a positive effect of intraspecific competition on the degree of diet variation in natural populations. Our results therefore indicate that habitat fragmentation can greatly impact the ecology of resilient populations, with potentially important ecological and evolutionary implications.     

Does the invasive horse-chestnut leaf mining moth, <Emphasis Type="Italic">Cameraria ohridella</Emphasis>, affect the native beech leaf mining weevil, <Emphasis Type="Italic">Orchestes fagi</Emphasis>, through apparent competition?

Apparent competition, through the action of shared natural enemies, is frequently suggested as a possible mechanism underlying the impact of invasive alien species on native species, but examples are rare, particularly in insects. A previous study showed that the beech leaf mining weevil, Orchestes fagi, was significantly less abundant close to horse-chestnut trees infested by the invasive horse-chestnut leaf mining moth, Cameraria ohridella, compared to control sites. Apparent competition through the sharing of natural enemies was proposed as a potential mechanism underlying this effect. To test the occurrence of apparent competition between the two leaf miner species, three observational studies and one experimental manipulation were carried out in Switzerland during 3 years. The total mortality, parasitism, predation and parasitoid diversity of larvae and pupae of O. fagi were compared between sites with and without horse-chestnut trees severely attacked by C. ohridella. Total mortality and predation rates of O. fagi were not significantly different between sites with and sites without C. ohridella. Despite a large overlap between the parasitoid complexes of the two leaf miners, parasitism of O. fagi was found to be positively influenced by the presence of horse-chestnuts infested by C. ohridella in only one of the four studies and only for 1 year. Similarly, parasitoid diversity was not higher near infested horse-chestnut trees compared to control sites. Thus, little evidence for apparent competition was found. Possible reasons, including possible insufficiencies in the experimental circumstances and design, are discussed.