Crayfish invasion facilitates dispersal of plants and invertebrates by gulls

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Early assessment of the impact of alien species: differential consequences of an invasive crayfish on adult and larval amphibians

Aim Early assessment of the impact of invasive alien species is crucial to set up timely management, but often the impact is evident when it is too late for action. We evaluated relationships between the alien crayfish, Procambarus clarkii, the distribution of native amphibians, and the abundance of their larvae. We assessed whether considering measures of reproductive success provide a more prompt measure of impact than considering just species distribution. Location One hundred and twenty‐five wetlands in Northern Italy, in an area recently invaded by P. clarkii. Methods We surveyed wetlands to assess the presence of breeding activity of amphibians and the distribution of P. clarkii. We measured the abundance of amphibian larvae before metamorphosis through pipe sampling. We built models analysing the relationships between amphibian and crayfish distribution, while taking into account spatial autocorrelation and environmental features. Analyses were performed at both the species level (generalized linear models and spatial eigenvector mapping) and community level (constrained redundancy analysis). Results In terms of breeding site distribution, only two amphibians (Lissotriton vulgaris and Hyla intermedia) were negatively associated with P. clarkii, while the relationships between other the species and P. clarkii were positive or not significant. However, larval abundance for all amphibian species was negatively associated with the alien crayfish. Analyses performed at community and single species levels yielded consistent results. Main conclusions Procambarus clarkii impacts amphibians through different processes. Newts probably avoid invaded wetlands for breeding. Other species attempt breeding in wetlands with crayfish, but suffer very low success. Considering distribution data alone would not provide a correct picture of the impact of this alien species; measures of reproductive success may allow a more accurate assessment of the impact.     

Growth, size and age at maturity of the agile frog (Rana dalmatina) in an Iberian Peninsula population

The mean age of a population of agile frogs (Rana dalmatina) from the Iberian Peninsula was estimated using mark and recapture and skeletochronology. Life-history parameters, including growth rate, body length, age and size at maturity, sexual dimorphism and longevity, were studied. The regression between age and snout-vent length (SVL) was highly significant in both sexes. Males reached sexual maturity at two years of age, although sometimes they can reach it at only one year of age. The average SVL at maturity was 51.75 mm (standard error (SE) = 0.71; n = 45). Females reached sexual maturity at two years of age with an average SVL of 62.14 mm (SE = 2.20; n = 14). A subset of the female population reached sexual maturity at three years of age. Growth was rapid until sexual maturity was reached. There was an overlap of SVL between different age classes. Growth was continuous, fulfilling the conditions of Von Bertalanffy's model. The growth coefficient (K) was 0.840 in males and 0.625 in females. The maximum SVL was greater in females (73.00 mm) than in males (59.50 mm). Sexual dimorphism was significantly biased towards females in all age classes. The maximum longevity observed was 6 years in females and 8 years in males. Management strategies for agile frogs should take into account factors such as these life-history characteristics.     

Novel genetic sex markers reveal high frequency of sex reversal in wild populations of the agile frog (Rana dalmatina) associated with anthropogenic land use

Populations of ectothermic vertebrates are vulnerable to environmental pollution and climate change because certain chemicals and extreme temperatures can cause sex reversal during early ontogeny (i.e. genetically female individuals develop male phenotype or vice versa), which may distort population sex ratios. However, we have troublingly little information on sex reversals in natural populations, due to unavailability of genetic sex markers. Here, we developed a genetic sexing method based on sex‐linked single nucleotide polymorphism loci to study the prevalence and fitness consequences of sex reversal in agile frogs (Rana dalmatina). Out of 125 juveniles raised in laboratory without exposure to sex‐reversing stimuli, 6 showed male phenotype but female genotype according to our markers. These individuals exhibited several signs of poor physiological condition, suggesting stress‐induced sex reversal and inferior fitness prospects. Among 162 adults from 11 wild populations in North‐Central Hungary, 20% of phenotypic males had female genotype according to our markers. These individuals occurred more frequently in areas of anthropogenic land use; this association was attributable to agriculture and less strongly to urban land use. Female‐to‐male sex‐reversed adults had similar body mass as normal males. We recorded no events of male‐to‐female sex reversal either in the laboratory or in the wild. These results support recent suspicions that sex reversal is widespread in nature, and suggest that human‐induced environmental changes may contribute to its pervasiveness. Furthermore, our findings indicate that sex reversal is associated with stress and poor health in early life, but sex‐reversed individuals surviving to adulthood may participate in breeding.     

浙江东部牛蛙的自然种群及潜在危害

牛蛙(Rana catesbeiana)的入侵已成为一些本地两栖类种群数量下降或灭绝的主要因素之一。1959年牛蛙被引入中国大陆,但直到1990年才开始在国内大范围饲养。牛蛙的自然种群在国内一直未见报道。我们于2003年9—11月调查了浙江省东部5区3县的牛蛙种群,发现在岱山、秀山、六横及佛渡岛上有牛蛙分布,共捕获23只牛蛙,包括4只成年个体,18个亚成体和1只蝌蚪。于2004年1月和2月在岱山捕获28只蝌蚪。结果显示：(1)牛蛙的蝌蚪能在当地越冬成活,说明牛蛙在上述4岛成功地建立了自然种群;(2)牛蛙主要分布于水库、池塘和溪流,牛蛙的生境与6种当地蛙有部分重叠;(3)养殖时管理不善造成的牛蛙逃逸,以及牛蛙价格过低而导致养殖户将牛蛙弃置野外可能是牛蛙入侵的主要原因。牛蛙对当地蛙的潜在威胁很大,开展牛蛙的分布、食性和种群动态研究是当务之急。     

Antipredator responses to alarm pheromone in groups of young and/or old thrips larvae

Many prey species suffer from different predators in the course of their ontogeny. Hence, the alarm signal a small prey individual sends can have a different meaning than the signal a large prey individual sends, both for small and for large receivers. Larvae of Western Flower Thrips face predators that attack only small larvae, or predators that attack small larvae and large larvae. Furthermore, thrips larvae release a two‐component alarm pheromone, which varies in composition with larval age. Here, we study whether their response to alarm pheromone varies with composition of the pheromone. First, we confirmed that large and small larvae respond when nearby larvae of both sizes were prodded with a brush to induce alarm pheromone excretion. Subsequently, we tested whether thrips larvae of a given size respond differentially to alarm pheromone excreted by a small or large companion larva. We analyzed two types of behavior used in direct defense against a predator and one type of escape response. Only small (not large) larvae attempted to escape more frequently in response to excretions from a large larva. This difference in response could have been due to the alarm pheromone or to the companion larva in the vicinity. We subsequently tested for, but did not find, an effect of size of the companion larva on the behavior of the test larva when exposed to synthetic pheromone mimicking that of a large larva. Finally, we tested how pheromone composition affects antipredator behavior by exposing thrips larvae to synthetic pheromones differing in amount and ratio of the two components. Only for small larvae, we found significant changes in escape behavior with pheromone amount, and a trend with the ratio. Overall, we conclude that small thrips larvae respond differentially to alarm pheromones excreted by small and large larvae and that this differential response is due to differences in pheromone quantity and possibly also quality. Our results suggest that responses to alarm signals can vary with the chemical composition of those alarm signals.     

Reactions of passerine birds to aposematic and non-aposematic firebugs (Pyrrhocoris apterus ; Heteroptera)

In spite of the existence of many experimental studies on the function of warning coloration in insects, little is known about the universality of reactions of different predators towards a particular warning signal. Reactions of nine passerine bird species, namely Parus major , Parus caeruleus , Aegithalos caudatus , Erithacus rubecula , Turdus merula , Sylvia atricapilla , Fringilla coelebs , Carduelis chloris and Emberiza citrinella , to the firebug Pyrrhocoris apterus wildtype (brachypterous adults) and its artificially obtained (painted) brown non-aposematic variant were compared. Most insectivorous birds (great tits, blue tits, robins and blackcaps) distinguished between aposematic and non-aposematic bugs, attacking the former less often. Partly granivorous buntings and finches did not distinguish between them, and attacked both variants equally. As all the birds were caught in the wild, the results can be interpreted in terms of the presence of a higher proportion of experienced individuals among insectivorous than among omnivorous species. Two insectivorous species differed from others. The heaviest blackbird attacked and killed aposematic as well as non-aposematic firebugs, and, in contrast, the lightest long-tailed tit avoided both variants. © 2003 The Linnean Society of London, Biological Journal of the Linnean Society , 2003, 78 , 517–525.     

Antennal responses of Cydia pomonella (L.) exposed to surfaces treated with methoxyfenozide

Electroantennogram (EAG) measurements were recorded from the antennae of male and female codling moth, Cydia pomonella L., to determine whether adult moths exposed to surfaces treated with the ecdysteroid agonist methoxyfenozide experience a decline in their antennal reception and thus olfactory sensitivity. Such a phenomenon would offer a possible mechanism for the previously reported decreased responsiveness from moths treated with methoxyfenozide to pheromone‐ and plant volatile‐based monitoring lures. Mean EAG data revealed that the antennae from methoxyfenozide‐treated male moths appear to be just as sensitive to various doses of synthetic codlemone as the antennae from the control and surfactant‐treated moths, but they appeared to be less sensitive to the pheromone component 12OH (collected from female effluvia) than the control male antennae. Mean male EAG responses to the pheromone components E8,E10‐12Al and codlemone collected from methoxyfenozide‐treated females were significantly less than the responses towards those two pheromone components collected from the control and surfactant‐treated females. Female moth exposure to methoxyfenozide did not negatively impact the sensitivity of female antennae to the plant volatile pear ester, but it did towards the apple volatile butyl hexanoate. Data from this study show that adult C. pomonella exposure to methoxyfenozide‐treated surfaces does appear to negatively impact, in a minor way, the (i) olfactory sensitivity (or detection) of male antennae towards some components of the female sex pheromone, (ii) the female antennal sensitivity towards a key apple volatile and (iii) the attractiveness of female pheromone effluvia.     

Discovery and characterization of novel genetic markers for use in the management of Lahontan cutthroat trout (Oncorhynchus clarkii henshawi)

The Lahontan cutthroat trout (Oncorhynchus clarkii henshawi) is threatened by habitat destruction, over‐harvest and hybridization with nonnative trout. Currently, three Geographic Management Units (GMUs) are recognized within the taxon. Here, we describe a suite of 68 single‐nucleotide polymorphism (SNP) genetic markers for use in the study and management of Lahontan cutthroat trout and a closely related subspecies, the Paiute cutthroat trout (O. c. seleneris). These include markers variable within the two subspecies (n = 35), diagnostic for the two subspecies (n = 23) and diagnostic for Yellowstone cutthroat trout (O. c. bouvieri) and other closely related subspecies (n = 10). Sixty‐three markers were discovered by Sanger sequencing of 171 EST loci in an ascertainment panel including Lahontan cutthroat trout from four populations representing all GMUs. Five markers were identified in a secondary sequencing effort with a single population of Lahontan cutthroat trout. TaqMan assays were validated on six Lahontan cutthroat trout populations and a diverse panel of other trout. Over 90% of the markers variable in Lahontan cutthroat trout were polymorphic in at least two populations, and 66% were variable within all three GMUs. All Lahontan diagnostic markers were also fixed for the Lahontan allele in Paiute cutthroat trout. Most of the Yellowstone diagnostic markers can also be used for this purpose in other cutthroat trout subspecies. This is the first set of SNP markers to be developed for Lahontan cutthroat trout, and will be an important tool for conservation and management.     

Predator responses to novel haemolymph defences of western corn rootworm (Diabrotica virgifera) larvae

Many herbivorous arthropods use defensive chemistry to discourage predators from attacking. This chemistry relies on the ability of predators to rapidly learn to recognize and avoid offensive stimuli. Western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), employs multifaceted chemical defences in its haemolymph, which may contribute significantly to its success as a major economic pest. Here, we test the hypothesis that agrobiont predators can rapidly learn to recognize and avoid WCR larvae, and will thereby reduce their contribution to WCR suppression. In controlled feeding assays, the effectiveness of WCR haemolymph defences varied across three predator taxa (crickets, centipedes, and ants). Centipedes (Chilopoda: Lithobiidae) were minimally affected by WCR defences, but crickets [Gryllus pennsylvanicus Burmeister (Orthoptera: Gryllidae)] spent less time feeding on WCR than on an undefended control prey, house fly maggots. However, we uncovered no evidence indicating that experienced crickets rapidly learn to avoid WCR larvae, indicating that haemolymph defences offer few, if any, survival benefits for WCR. Colonies of ants [Lasius neoniger Emery (Hymenoptera: Formicidae)] switched from low worker participation in initial attacks on WCR to higher worker participation in subsequent attacks, indicating an attempt to overcome, rather than avoid, WCR haemolymph defences. These results suggest that a diverse assemblage of natural enemies will show a diverse array of behavioural responses to toxic pest prey, and highlight the importance of behavioural diversity in driving the function of natural enemy assemblages.     

Reproductive potential and seedling establishment of the invasive alien tree Schinus molle (Anacardiaceae) in South Africa

Schinus molle (Peruvian pepper tree) was introduced to South Africa more than 150 years ago and was widely planted, mainly along roads. Only in the last two decades has the species become naturalized and invasive in some parts of its new range, notably in semi‐arid savannas. Research is being undertaken to predict its potential for further invasion in South Africa. We studied production, dispersal and predation of seeds, seed banks, and seedling establishment in relation to land uses at three sites, namely ungrazed savanna once used as a military training ground; a savanna grazed by native game; and an ungrazed mine dump. We found that seed production and seed rain density of S. molle varied greatly between study sites, but was high at all sites (384 864–1 233 690 seeds per tree per year; 3877–9477 seeds per square metre per year). We found seeds dispersed to distances of up to 320 m from female trees, and most seeds were deposited within 50 m of putative source trees. Annual seed rain density below canopies of Acacia tortillis, the dominant native tree at all sites, was significantly lower in grazed savanna. The quality of seed rain was much reduced by endophagous predators. Seed survival in the soil was low, with no survival recorded beyond 1 year. Propagule pressure to drive the rate of recruitment: densities of seedlings and sapling densities were higher in ungrazed savanna and the ungrazed mine dump than in grazed savanna, as reflected by large numbers of young individuals, but adult : seedling ratios did not differ between savanna sites. Frequent and abundant seed production, together with effective dispersal of viable S. molle seed by birds to suitable establishment sites below trees of other species to overcome predation effects, facilitates invasion. Disturbance enhances invasion, probably by reducing competition from native plants.     

Genomic footprints of a biological invasion: Introduction from Asia and dispersal in Europe of the topmouth gudgeon (Pseudorasbora parva)

Facilitated by the intensification of global trading, the introduction and dispersal of species to areas in which they are historically non‐native is nowadays common. From an evolutionary standpoint, invasions are paradoxical: not only non‐native environments could be different from native ones for which introduced individuals would be ill‐adapted, but also small founding population size should be associated with reduced adaptive potential. As such, biological invasions are considered valuable real‐time evolutionary experiments. Here, we investigated the population structure and adaptive potential of the highly invasive topmouth gudgeon (Pseudorasbora parva) across Europe and East Asia. We RAD‐sequenced 301 specimens from sixteen populations and three distinct within‐catchment invaded regions as well as two locations in the native range. With 13,785 single nucleotide polymorphisms, we provide conclusive evidence for a genome‐wide signature of two distinct invasion events, in Slovakia and Turkey, each originating from a specific area in the native range. A third invaded area, in France, appears to be the result of dispersal within the invasive range. Few loci showed signs of selection, the vast majority of which being identified in the Slovakian region. Functional annotation suggests that faster early stage development, resistance to pollution and immunocompetence contribute to the invasion success of the local habitats. By showing that populations in the invasive range have different evolutionary histories, our study reinforces the idea that populations, rather than species, are the units to consider in invasion biology.     

Little association of biological trait values with environmental variables in invasive alien round goby (Neogobius melanostomus)

The relative importance of species‐specific biological trait characteristics and environmental factors in invasions of nonindigenous species remains controversial because both have mostly been studied independently. Thus, the main objective of this study was to examine the correlation of biological traits with environmental variation in the globally invasive round goby Neogobius melanostomus from the upper Danube River. Based on a sample of 653 specimens along a continuous 200 km river pathway, links between nine environmental factors (substrate‐type, six water measurements, and the communities of fishes and macroinvertebrates) and seven biological traits (nutritional and energetic status, trade‐offs of parasite resistance and resource allocation, and three growth proxies) were analyzed. Biological trait values of N. melanostomus hardly correlated with the environment, could not explain invasion progress and imply a general low overall importance for invasion success. Instead, alternative individual life‐history trajectories appear to determine invasion success. This is in line with up to 15% of all specimens having outlying biological trait values of potential adaptive value, suggesting a considerable importance of adaptive trait variation among single individuals for the whole invasion progress. This “individual trait utility hypothesis” gives an alternative explanation for success of invasive species by single individuals carrying particular traits, and it should be specifically targeted and analyzed at currently invaded sites.