Differential vulnerability of two sympatric tadpoles to an invasive crayfish predator

Many amphibian population declines have been associated with the introduction of alien aquatic predators. Here, we explore the vulnerability of tadpoles of two sympatric Japanese species [Pelophylax nigromaculatus (PN) and Rhacophorus schlegelii (RS)] to the invasive crayfish Procambarus clarkii. We first examined the behavioral responses of the tadpoles to the cues of caged, fed crayfish predator in a controlled laboratory experiment, and subsequently tested their survival when together in the presence of free-ranging predator in outdoor mesocosms that simulated natural ponds. Only PN reduced activity level to the cues of the predator, but this apparent behavioral defense recorded in the laboratory did not result into higher survival in outdoor mesocosms. In mesocosms, PN exhibited higher biomass increment but experienced higher mortality in predator environments. The mechanism mediating mortality remains unclear though. These results indicate that sympatric prey may differentially respond and be disproportionally vulnerable to novel predators. Our study illustrates the possible contribution of a life-history trait influencing risk of predation in newly invaded systems.     

Variations in parasitism in sympatric populations of three invasive leaf miners

Abstract:  Parasitism was investigated in sympatric populations of three invasive gracillariid leaf miners in Europe, Cameraria ohridella , Phyllonorycter robiniella and Phyllonorycter platani to test the hypotheses that C. ohridella is less heavily parasitized by native parasitoids and attacked by fewer species than the two other invasive species. In all regions investigated, C. ohridella showed a lower parasitism rate, and its parasitoid complex was poorer in species than those of either Phyllonorycter spp. Comparisons were made between sympatric populations of C. ohridella on its main host tree, Aesculus hippocastanum , and an occasional host, Acer pseudoplatanus . Parasitism rates were similarly low and composed of the same parasitoid species on both trees. In contrast, a sympatric population of Phyllonorycter geniculella , a native species mining A. pseudoplatanus , was heavily parasitized by a totally different parasitoid complex. These results suggest that the low parasitism in C. ohridella by native polyphagous leaf miner parasitoids is due neither to its host tree, nor to a problem of synchronization between the phenology of the pest and that of its parasitoids. Instead, it probably results from the inability of the native parasitoids to locate, attack, or develop on a new host that does not have any native congener in Europe.     

Invasion of vacant niches and subsequent sympatric speciation.

An individual-based simulation study was conducted to examine the population dynamics of 'invasion of a vacant niche' and subsequent speciation (by reproductive isolation) when food resources are randomly distributed spatially within the habitat and the frequencies of different food types are bimodally distributed (i.e. smaller and larger sizes of food being most abundant). The initially vacant niche was that of unused larger sizes of food. When phenotypic variation for resource use (i.e. food sizes) was small in the initial population, and each female could choose a mate from anywhere in the habitat, the population could not invade the vacant niche. But when the dispersal distance of the offspring and the area within which a female could choose a mate were small (i.e. the genetic neighbourhood size was small), the population could, in most cases, evolve to use both smaller and larger food sizes and form sister species sympatrically, with each species utilizing one of the two niches (small and large sizes of food). When phenotypic variation in resource use in the initial population was large, the population could, in most cases, invade the vacant niche by evolving to use both smaller and larger sizes of food. The probability of speciation increased as the dispersal distance of offspring decreased. The results indicate that populations whose individuals have small Wright's genetic neighbourhoods may often exploit a vacant niche and diversify sympatrically in the process.     

Trophic niche and habitat shifts of sympatric Gerreidae

The diet and mouth growth rates of three Gerreidae species (Eugerres brasilianus, Eucinostomus melanopterus and Diapterus rhombeus) were assessed at different ontogenetic phases (juveniles, sub‐adults and adults) in order to detect allometric growth, and whether they are related to habitat and seasonal changes in the Goiana Estuary, north‐east Brazil. The importance of each prey for each ontogenetic phase was described using the index of relative importance. The three species showed seasonal ontogenetic shifts in diet and allometric growth of mouth morphology. They also had an exclusively zoobenthic diet, comprising mainly Polychaeta, Copepoda, Ostracoda, Gastropoda and Bivalvia. Mouth development showed a possible influence on diet changes for E. melanopterus. Significant interactions (P < 0·01) were detected among seasons, areas and ontogenetic phases for the most important prey for E. brasilianus and E. melanopterus. Diet overlaps are evidence of intra and interspecific competition among gerreids for specific prey. A conceptual model of the competition and seasonal diet shifts among ontogenetic phases of gerreids is given. The sediment ingested due to the feeding mechanisms of Gerreidae species could also partially explain the ingestion of synthetic items observed for all ontogenetic phases, which indicates one of a myriad effects of human activities (e.g. artisanal fishery) in this estuary.     

Overlap of temporal niches among four sympatric species of shrews

Hypotheses about the dependence of circadian activity from metabolic rate and the segregation of temporal niches among competing species were verified by the study of activity patterns in a shrew community of two semiaquatic species,Neomys anomalus Cabrera, 1907 andN. fodiens (Pennant, 1771), and two terrestrial species,Sorex araneus Linnaeus, 1758 andS. minutus Linnaeus, 1766, co-existing in wet habitats of Białowieża Forest (E Poland). In ten trapping sessions, performed in early summer between 1991 and 2000, traps were open 24 hours continuously and patrolled at 1:00, 5:00, 10:00, 15:00, and 20:00. All the shrew species were most active between 20:00 and 1:00, and least active around mid-day (10:00–15:00). However, activity of the twoSorex species was lower than that of the twoNeomys species in the period 20:00–1:00, but higher in the period 15:00–20:00. BothNeomys species displayed clearly nocturnal, unimodal patterns of activity. In contrast, activity of bothSorex species was relatively evenly distributed over 24 hours and they increased their activity earlier (ie after 15:00) than bothNeomys species (after 20:00). These results confirm the idea that small shrew species with higher metabolic rate have more frequent and more equally distributed activity bouts than large species. Overlap of temporal niches was the highest within genera (99.29% between bothNeomys species and 98.36% between bothSorex species), the lowest betweenN. fodiens andS. araneus (88.26%) andS. minutus (89.34%), and intermediate betweenN. anomalus and bothSorex species (91.78 and 93.34%, respectively). Such high interspecific overlaps in activity suggest a joint-action of other mechanisms that separate ecological niches of these species also in other dimensions (eg food, microhabitat).     

Trophic niches and feeding relationships of shorebirds in southern Brazil

Niche theory predicts that sympatric species should differ in some ecological characteristic, to allow co-existence and reduce competition for key resources. Food is critical on wintering grounds and stopover areas for migratory species that need to accumulate reserves in order to complete their migration. Wetlands of the Rio Grande do Sul coastal plain, in southern Brazil, host several species of shorebirds with similar morphology, foraging methods and diet. When these species are in sympatry, some trophic niche overlap is expected. Diets and trophic niches of migratory and resident shorebirds were investigated during the austral summer on Torotama Island, Lagoa dos Patos Estuary, Brazil. Complementary methods were used to determine the trophic ecology of three shorebird species; diet was determined through analysis of feces and food samples, using stable isotopes of carbon and nitrogen. The local invertebrate community was sampled to determine potential prey and ascertain feeding preferences of birds. Coleoptera was the most abundant taxon in the feces of all shorebirds. Trophic niche overlap in the diets was high, with the widest trophic niche found for the buff-breasted sandpiper Calidris subruficollis. Isotopic mixing models indicated differences in the main food sources of shorebirds. The isotopic niche breadth was widest for the American golden-plover Pluvialis dominica. These species, as well as the resident southern lapwing Vanellus chilensis, consumed some prey in higher proportions over others, although they had generalist diets. Migratory species with generalist habits benefit from heterogeneous environments such as floodplains during the non-breeding season.     

Darwin's finches and their diet niches: the sympatric coexistence of imperfect generalists

Adaptive radiation can be strongly influenced by interspecific competition for resources, which can lead to diverse outcomes ranging from competitive exclusion to character displacement. In each case, sympatric species are expected to evolve into distinct ecological niches, such as different food types, yet this expectation is not always met when such species are examined in nature. The most common hypotheses to account for the coexistence of species with substantial diet overlap rest on temporal variation in niches (often diets). Yet spatial variation in niche overlap might also be important, pointing to the need for spatiotemporal analyses of diet and diet overlap between closely related species persisting in sympatry. We here perform such an analysis by characterizing the diets of, and diet overlap among, four sympatric Darwin's ground finch species at three sites and over 5 years on a single Galápagos island (Santa Cruz). We find that the different species have broadly similar and overlapping diets – they are to some extent generalists and opportunists – yet we also find that each species retains some ‘private’ resources for which their morphologies are best suited. Importantly, use of these private resources increased considerably, and diet overlap decreased accordingly, when the availability of preferred shared foods, such as arthropods, was reduced during drought conditions. Spatial variation in food resources was also important. These results together suggest that the ground finches are ‘imperfect generalists’ that use overlapping resources under benign conditions (in space or time), but then retreat to resources for which they are best adapted during periods of food limitation. These conditions likely promote local and regional coexistence.     

Trophic niche width and overlap of two sympatric gulls in the southwestern mediterranean

The diets of two potential competitor species, Audouin's Larus audouinii and yellow-legged gulls Larus cachinnans, were examined while they bred at the Chafarinas Islands during 1993, 1994, and 1995. Data were collected during two commercial fishing regimes: (1) trawling and purse seine fisheries, and (2) diurnal trawlers only. Since the food supply for the gulls in this area was heavily reliant on the activity of purse seine fisheries, these contrasting situations allowed us to analyze short-term effects, induced by daily changes in food supply, on niche width, dietary shift, and niche overlap between the two species. Overall, both species relied mainly on fish for food, especially Clupeiforms, in the case of Audouin's gull irrespective of the fishing situation, and in the case of the yellow-legged gull, only when purse seine fishing was in operation. When purse seine boats did not operate (food shortage), yellow-legged gulls broadened their niche, consuming equal amounts of all the feeding resources, and they showed a dietary shift toward a greater consumption of prey from refuse tips. In contrast, Audouin's gulls did not change their niche width, but showed a slight dietary shift away from the consumption of epipelagic fish, compensated by an increase in reliance on benthic-mesopelagic resources. Niche overlap was clearly higher on days when both fishing fleets operated, probably because a superabundant food resource facilitates high overlap without affecting coexistence between the two species. Since our study was developed on the basis of daily variations in food supply, and competition effects are to be expected on a longer-term basis, these changes can be seen as the outcome of the coexistence of two species in stable competitive equilibrium. Received: 4 October 1996 / Accepted: 20 May 1997     

Behavioural determinants of agonistic success in invasive crayfish

Ecosystems today increasingly suffer invasions by multiple invasive species, some of which may share similar advantageous life history traits and ecological niche. In such cases, direct competition can influence invasion success of both species, and provide insights into competition without co-evolution in species equally novel to the environment. We used two widespread crayfish invaders of freshwater ecosystems of Europe, signal crayfish (Pacifastacus leniusculus) and spiny cheek crayfish (Orconectes limosus), to investigate how behavioural decisions in agonistic encounters contribute to competitive advantages in the absence of adaptation to either opponents or an environment. In direct competition against novel but comparable opponents, the key factor for establishing clear dominance of P. leniusculus in interspecific bouts was its greater tendency towards continued engagement in high-intensity fights. With O. limosus individuals consistently retreating from staged bouts as fights became more intense, P. leniusculus individuals did not need to adapt their strategy to be successful, suggesting that their agonistic behaviour intrinsically predisposed them to win. While both species are detrimental to invaded ecosystems, our results indicate that aggressive behaviour of P. leniusculus against unfamiliar opponents could allow it to more easily outcompete other comparable species and consequently present a potentially greater threat for native ecosystems.     

Habitat use by five sympatric Australian freshwater crayfish species (Parastacidae)

1. The Grampians National Park in Victoria is a ‘hot spot’ for freshwater crayfish diversity, with seven species from six genera occurring in sympatry. Few studies have examined how multiple species of freshwater crayfish co‐exist across landscapes consisting of a mosaic of perennial and seasonal habitats. Despite their endemicity and likely key role in freshwaters, the ecology and biology of these crayfish remains unknown. 2. This study determined the distribution and habitat use of five crayfish species (Euastacus bispinosus, Cherax destructor, Geocharax falcata, Gramastacus insolitus and Engaeus lyelli). Seasonal sampling surveys ascertained whether crayfish distribution was related to habitat type, environmental or physicochemical variables, catchment or season. 3. Distribution was directly related to habitat type and the environmental and physicochemical variables that characterised habitats. Engaeus lyelli, G. falcata and G. insolitus occurred predominantly in floodplain wetlands and flooded vegetation habitats, E. bispinosus occurred only in flowing soft‐sediment channels and C. destructor was found in all catchments and habitat types studied. Gramastacus insolitus co‐occurred with G. falcata at all sites except two, so no distinct habitat separations were apparent for these two species. 4. A high percentage cover of boulders was the best indicator of crayfish absence, and discriminated between habitat types and crayfish species: it was probably a surrogate for a larger range of environmental and physicochemical variables. Catchment and season did not affect crayfish distribution. 5. These crayfish species varied in their degree of habitat specialisation from strongly generalist (C. destructor) to occupying only a specific habitat type (E. bispinosus). Some species appeared specialised for seasonal wetlands (G. insolitus and G. falcata). Overlap in site occupancy also varied: G. insolitus and G. falcata distributions were strongly associated, whereas C. destructor appeared to occur opportunistically across habitats, both alone and co‐occurring with all the other species. 6. Management strategies to conserve multiple species of crayfish co‐existing within landscapes will need to incorporate a range of perennial and seasonal habitat types to ensure sufficient space is available for species to maintain different occupancy patterns. Given that water resources are under increasing pressure and are strongly regulated within the Grampians National Park, this may present a conservation challenge to water managers in this location.     

Effects of water clarity and other environmental factors on trophic niches of two sympatric piscivores

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可可西里地区藏羚羊、藏原羚和藏野驴的营养生态位

2004年8月,在可可西里自然保护区的楚玛尔河至五道梁一带观察藏羚羊、藏原羚和藏野驴的行为活动,并收集其粪便,运用粪便显微分析法对其食性进行研究,计算3种动物的营养生态位宽度和营养生态位重叠指数.结果表明：藏羚羊、藏原羚和藏野驴所采食的植物种类基本相似,但在食谱中所占的比例不同,禾本科植物在藏羚羊、藏原羚和藏野驴的食谱中所占的比例分别为58.7%、44.57%和9228%.藏羚羊、藏原羚和藏野驴的营养生态位宽度分别为0.878、0.735和0.695.藏羚羊和藏野驴、藏羚羊和藏原羚、藏野驴和藏原羚的营养生态位重叠值(FT)分别为0.869、0.985和0.785.结合藏羚羊、藏原羚和藏野驴的生态习性,从营养生态位的角度探讨了它们之间竞争与共存的关系.     

Niche relations among three sympatric Mediterranean carnivores

Previous studies carried out in the Doñana National Park reported that red foxes (Vulpes vulpes) were killed by Iberian lynxes (Lynx pardinus), whereas similar-sized Eurasian badgers (Meles meles) were not. Therefore, we predicted that fox would avoid lynx predation risk by niche segregation whereas we did not expect such a segregation between badger and lynx. As an approach for evaluating our predictions, we compared their diet, activity patterns, and habitat use in an area of Doñana where the three carnivores are sympatric. Lynxes preyed almost uniquely on European rabbits (Oryctolagus cuniculus), and though badgers and foxes were omnivorous, rabbits also were a major prey, resulting in high overlaps throughout the year. However, badgers preyed largely on small rabbits, whereas lynxes and foxes preyed mainly on medium-sized rabbits. There were also interspecific differences in activity patterns. Maximum levels of activity among lynxes were during sunrise and dusk (49–67%). Foxes were most active during dusk and night (34–67%), and badgers were mainly nocturnal (53–87%). Though there were seasonal differences in the amount of activity of each species, specific activity patterns changed little throughout the year. There was a strong difference in annual habitat use by the three species (P?MAX) and the resting (P_MIN) periods. During P_MIN, foxes used the Mediterranean scrubland intensively (40% of locations on average), but during P_MAX, they used the pastureland much more intensively despite this habitat being poorer in their main prey (rabbits). As a consequence, foxes and lynxes exhibited segregation in their habitat use during the active period. Badgers also used the Mediterranean scrubland intensively, especially during P_MIN. There were no seasonal differences in habitat use for lynx and fox, but there was for badgers (P?相似文献   

同地共栖四种蝙蝠食性和回声定位信号的差异及其生态位分化

本研究于 2 0 0 2年 5月初至 2 0 0 3年 9月中旬在北京房山区霞云岭四合村蝙蝠洞进行 ,分析了共栖同一山洞四种蝙蝠的形态特征、食性和回声定位信号。大足鼠耳蝠食谱中以宽鳍等三种鱼为主 (体积百分比为5 3% ) ,回声定位主频 4 1 87± 1 0 7kHz;马铁菊头蝠主要掠捕鳞翅目昆虫 (73% ) ,恒频叫声主频 74 70± 0 13kHz ;中华鼠耳蝠以近地面或在地表活动的鞘翅目昆虫步甲类和埋葬甲类为主要食物 (6 5 4 % ) ,声脉冲主频较低 35 73± 0 92kHz;白腹管鼻蝠捕食花萤总科和瓢虫科等鞘翅目昆虫 (90 % ) ,回声定位信号主频为 5 9 4 7±1 5 0kHz。结果证实同地共栖四种蝙蝠种属特异的回声定位叫声和形态结构的差异 ,以及不同的捕食生境和捕食策略 ,导致取食生态位分离是四种蝙蝠同地共栖的原因     

Trophic specialization and morphological divergence between two sympatric species in Lake Catemaco,Mexico

The association of morphological divergence with ecological segregation among closely related species could be considered as a signal of divergent selection in ecological speciation processes. Environmental signals such as diet can trigger phenotypic evolution, making polymorphic species valuable systems for studying the evolution of trophic‐related traits. The main goal of this study was to analyze the association between morphological differences in trophic‐related traits and ecological divergence in two sympatric species, Astyanax aeneus and A. caballeroi, inhabiting Lake Catemaco, Mexico. The trophic differences of a total of 70 individuals (35 A. aeneus and 35 A. caballeroi) were examined using stable isotopes and gut content analysis; a subset of the sample was used to characterize six trophic and six ecomorphological variables. In our results, we recovered significant differences between both species in the values of stable isotopes, with higher values of δ¹⁵N for A. caballeroi than for A. aeneus. Gut content results were consistent with the stable isotope data, with a higher proportion of invertebrates in A. caballeroi (a consumption of invertebrates ten times higher than that of A. aeneus, which in turn consumed three times more vegetal material than A. caballeroi). Finally, we found significant relationship between ecomorphology and stable isotopes (r = .24, p < .01), hence, head length, preorbital length, eye diameter, and δ¹⁵N were all positively correlated; these characteristics correspond to A. caballeroi. While longer gut and gill rakers, deeper bodies, and vegetal material consumption were positively correlated and corresponded to A. aeneus. Our results are consistent with the hypothesis that morphological divergence in trophic‐related traits could be associated with niche partitioning, allowing the coexistence of closely related species and reducing interspecific competition.     

Competition and intraguild predation among three sympatric carnivores

We examined the relative roles of dominance in agonistic interactions and energetic constraints related to body size in determining local abundances of coyotes (Canis latrans, 8-20 kg), gray foxes (Urocyon cinereoargenteus, 3-5 kg) and bobcats (Felis rufus, 5-15 kg) at three study sites (hereafter referred to as NP, CP, and SP) in the Santa Monica Mountains of California. We hypothesized that the largest and behaviorally dominant species, the coyote, would exploit a wider range of resources (i.e., a higher number of habitat and/or food types) and, consequently, would occur in higher density than the other two carnivores. We evaluated our hypotheses by quantifying their diets, food overlap, habitat-specific abundances, as well as their overall relative abundance at the three study sites. We identified behavioral dominance of coyotes over foxes and bobcats in Santa Monica because 7 of 12 recorded gray fox deaths and 2 of 5 recorded bobcat deaths were due to coyote predation, and no coyotes died as a result of their interactions with bobcats or foxes. Coyotes and bobcats were present in a variety of habitats types (8 out of 9), including both open and brushy habitats, whereas gray foxes were chiefly restricted to brushy habitats. There was a negative relationship between the abundances of coyotes and gray foxes (P=0.020) across habitats, suggesting that foxes avoided habitats of high coyote predation risk. Coyote abundance was low in NP, high in CP, and intermediate in SP. Bobcat abundance changed little across study sites, and gray foxes were very abundant in NP, absent in CP, and scarce in SP; this suggests a negative relationship between coyote and fox abundances across study sites, as well. Bobcats were solely carnivorous, relying on small mammals (lagomorphs and rodents) throughout the year and at all three sites. Coyotes and gray foxes also relied on small mammals year-round at all sites, though they also ate significant amounts of fruit. Though there were strong overall interspecific differences in food habits of carnivores (P<0.0001), average seasonal food overlaps were high due to the importance of small mammals in all carnivore diets [bobcat-gray fox: 0.79ǂ.09 (SD), n=4; bobcat-coyote: 0.69ǂ.16, n=6; coyote-gray fox: 0.52ǂ.05, n=4]. As hypothesized, coyotes used more food types and more habitat types than did bobcats and gray foxes and, overall, coyotes were the most abundant of the three species and ranged more widely than did gray foxes. We propose that coyotes limit the number and distribution of gray foxes in Santa Monica Mountains, and that those two carnivores exemplified a case in which the relationship between their body size and local abundance is governed by competitive dominance of the largest species rather than by energetic equivalences. However, in the case of the intermediate-sized bobcat no such a pattern emerged, likely due to rarity or inconsistency of agonistic interactions and/or behavioral avoidance of encounters by subordinate species.     

Managing invasive crayfish: use of X-ray sterilisation of males

1. In the last decade, many attempts have been made to contain the spread of the North American crayfish Procambarus clarkii , an invader of many European water bodies, but none has been successful.
2. This study investigates the effects that ionising irradiation has on the reproduction of P. clarkii males by analysing the behaviour of the treated individuals, their reproductive output, the vitality and survival of their offspring and the damages induced to their gonads.
3. We found that a dose of 20 Gy X-rays did not compromise either the survival or the mating ability of males, but reduced the size of their testes and significantly altered spermatogenesis. The number of aborted eggs was larger in clutches sired by treated males, with a consequent reduction (by 43%) in the number of offspring.
4. These results foster hopes in our efforts to control invasive crayfish, showing the potential effectiveness of ionising irradiation to reduce male fertility in the perspective of adopting the Sterile Male Release Technique to control P. clarkii populations.     

Exploring trophic niches and parasite communities of sympatric Arctic charr and brown trout populations of southern Norway

Hydrobiologia - Catchment-scale variation between lake habitats has the potential to simultaneously influence the trophic niche and parasite community of fish hosts. In this study, we investigated...     

Complex impact of an invasive crayfish on freshwater food webs

Invasive alien species can have complex effects on native ecosystems, and interact with multiple components of food webs, making it difficult a comprehensive quantification of their direct and indirect effects. We evaluated the relationships between the invasive crayfish, Procambarus clarkii, amphibian larvae and predatory insects, to quantify crayfish impacts on multiple levels of food webs, and to evaluate whether crayfish predation of aquatic insects has indirect consequences for their preys. We used pipe sampling to assess the abundance of crayfish, amphibian larvae and their major predators (Ditiscidae, Notonectidae and larvae of Anisoptera) in invaded and uninvaded ponds within a human dominated landscape. We disentangled the multivariate effects of P. clarkii on different components of food web through a series of constrained redundancy analyses. The crayfish had a negative, direct impact on both amphibian communities and their predators. Amphibian abundance was negatively related to both predators. However, the negative, direct effects of crayfish on amphibians were much stronger than predation by native insects. Our results suggest that this crayfish impacts multiple levels of food webs, disrupting natural prey-predator relationships.