Assessing the potential impact of an invasive species on a Mediterranean amphibian assemblage: a morphological and ecological approach

The introduction of exotic species is a major cause of ecological disturbance and has recently been shown to promote the decline of some amphibian populations. In Western Europe, several amphibian species have been introduced in recent decades, among them Discoglossus pictus, impact of which in native amphibian communities is still unknown. We studied the potential impact of D. pictus by analysing the degree of niche overlap, assuming the possible existence of competitive interactions with native species. We have studied the structure of the anuran assemblage at local level, defining the morphospace occupied by native species and the habitat occupied by the different ecomorphs. The analysis of distance matrices suggested that there was a covariation between morphological characters and habitat selection. We determined the position of D. pictus within the morphospace of the native anuran community, patterns of co-occurrence among alien, and native species and habitat selection. The potential effect of climate on local assemblages had been controlled based on data obtained from climate models. These analysis showed that D. pictus was clustered with the benthic guild, selected positively small ephemeral ponds and was a thermophilic species. Based on these results, a greater degree of niche overlap was expected with Bufo calamita and Pelodytes punctatus. The definition of morphological groups can be useful to understand the invaded assemblage structure and the potential effect of an alien frog on native communities.     

The impact of vertic soil cracks on submerged macrophyte diaspore bank depth distribution in Mediterranean temporary wetlands

In temporary wetlands, a majority of diaspores are located in the upper layers of the soil. In general, it is assumed that the vertical movement of diaspores is caused by animal disturbance, and that reduction in this activity concentrates the diaspores in the topsoil. The occurrence of seasonal cracks and the dispersion of topsoil during dry periods are processes that have been underestimated in diaspore bank studies. Our results indicated that diaspore size can partially explain the density of diaspores of submerged macrophytes observed at four discrete soil depths in Doñana National Park temporary marshes (SW Spain). The top layers of the soil exhibit the largest number of species. Spore density of Riella spp., a liverwort, increases with depth, creating an important reserve of viable diaspores in deep soil. Downward movement of diaspores in soil cracks during dry periods, and the extended longevity of Riella diaspores together explain this inverse depth distribution.     

The impact of vertic soil cracks on submerged macrophyte diaspore bank depth distribution in Mediterranean temporary wetlands

In temporary wetlands, a majority of diaspores are located in the upper layers of the soil. In general, it is assumed that the vertical movement of diaspores is caused by animal disturbance, and that reduction in this activity concentrates the diaspores in the topsoil. The occurrence of seasonal cracks and the dispersion of topsoil during dry periods are processes that have been underestimated in diaspore bank studies. Our results indicated that diaspore size can partially explain the density of diaspores of submerged macrophytes observed at four discrete soil depths in Doñana National Park temporary marshes (SW Spain). The top layers of the soil exhibit the largest number of species. Spore density of Riella spp., a liverwort, increases with depth, creating an important reserve of viable diaspores in deep soil. Downward movement of diaspores in soil cracks during dry periods, and the extended longevity of Riella diaspores together explain this inverse depth distribution.     

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.     

The distribution of the crayfish, Austropotamobius pallipes, in British waters

SUMMARY. The distribution of Austropotamobius pallipes , the only freshwater crayfish indigenous to the British Isles, is examined using over 300 records collected since 1970. The majority of records are for England where crayfish are found in areas associated with base-rich easily weathered substrata. Waters containing crayfish tend to be in the pH range 7–9, with calcium levels above 5 mg l⁻¹. Crayfish are found in a wide variety of habitats ranging from deep water-filled quarries with a soft substratum to shallow fast-flowing streams.     

Invasive crayfish in Europe: the impact of Procambarus clarkii on the littoral community of a Mediterranean lake

1. Despite the growing number of non‐indigenous crayfish species introduced worldwide, little scientific attention has been given to their effects on the recipient communities. Here, we quantified the impact of adult red swamp crayfish (Procambarus clarkii) on key components of the littoral community of a mesotrophic lake in central Italy. We used two densities of crayfish plus a no‐crayfish control, and two complementary methods –in situ enclosures and a laboratory study. 2. Both experiments showed that P. clarkii strongly affects the community even at a low density (4 m⁻²), although a high crayfish density (8 m⁻²) showed a more pronounced impact in a shorter time. 3. In enclosures, P. clarkii quickly consumed the pond snail Haitia acuta, notwithstanding its anti‐predator behaviour, but not the mosquitofish Gambusia affinis. The biomass of the hydrophytes Nymphoides peltata and Potamogeton spp. was strongly reduced by crayfish grazing, coupled with their non‐consumptive plant clipping and uprooting, which were particularly intense in the case of Potamogeton crispus. In contrast, Utricularia australis was avoided by P. clarkii, even when other resources were exhausted. 4. The introduction of P. clarkii could reduce biodiversity and increase biotic homogenisation in a short time.     

Selective foraging by the crayfish Orconectes virilis and its impact on macroinvertebrates

SUMMARY. 1. The impact of crayfish predation on the abundance of macroinvertebrates was examined under semi-natural conditions. Female (Experiment 1) or male (Experiment 2) crayfish (Orconectes virilis) were held for 5 weeks in twelve small pools (4.67 m² surface area) at biomasses of 0. 5, 10 or 18 g m^?2 (live weight). The pools were stocked with known densities of macroinvertebrates. 2. Crayfish significantly affected the abundance of macroinvertebrates in the pools. Differences in the effects of crayfish on macroinvertebrates were related to crayfish sex, the presence of age-0 crayfish, and the species of macroinvertebrate. 3. The abundance of snails (Stagnicola elodes and Physa gyrina) was greatly reduced, in comparison with controls, by biomass of female crayfish ≥10 g m^?2 and by biomasses of male crayfish ≥5 g m^?2. The total density of non-molluscan invertebrates was inversely correlated with the biomass of female crayfish but the total biomass of non-molluscan invertebrates did not differ between treatments. This is consistent with our observation that small invertebrates (<2 mg wet weight) were less numerous, and large amphipods (32–64 mg) were more numerous, in pools stocked with female crayfish. In contrast, male crayfish had little apparent effect on the abundance of non-molluscan invertebrates. 4. Age-0 crayfish hatched at the end of Experiment 1 and were present in each pool at the start of Experiment 2. Surprisingly, male crayfish preyed little on age-0 crayfish. At the end of Experiment 2, the densities of age-0 crayfish varied between six and 116 individuals m^?2 and there was a strong inverse correlation between the mean biomass and density of age-0 crayfish recovered from the pools. This suggests age-0 crayfish were food limited in the pools and may explain the dominance of oligochaetes (which largely escape predation by burrowing) in the invertebrate community at the end of Experiment 2. 5. These results indicate that even relatively low densities of crayfish could greatly affect the abundance of macroinvertebrates in lakes. The introduction of crayfish into lakes (most lakes in Alberta currently have no crayfish) could substantially affect abundance and species composition of the macroinvertebrate community and, ultimately, the fish populations.     

How consumption and fragmentation of macrophytes by the invasive crayfish Procambarus clarkii shape the macrophyte communities of temporary ponds

Crayfish are major herbivores in freshwaters, and their activity may change invaded freshwater ecosystems. Macrophyte removal by crayfish results from consumption and fragmentation of plant material and may depend upon the palatability of each species. We studied the impact of Procambarus clarkii on the macrophyte community of Mediterranean temporary ponds, testing in laboratory if consumption and fragmentation of five macrophyte species were correlated in palatability tests and in a preference test. We performed an experiment in a natural pond, where we introduced crayfish and estimated its effects on macrophyte biomass, number of rooted stalks and number of floating cut stalks. P. clarkii consumed preferably Juncus heterophyllus in both tests and avoided Carex divisa and Ranunculus peltatus in the preference test. In the presence of preferred species, consumption and fragmentation of the non-preferred species were heavily reduced. In the field trial, the “Crayfish” compartment had 31% less macrophyte biomass, 41% less rooted stalks and 92% more floating cut stalks. Crayfish may remove macrophyte species from the community sequentially, from the most to the least preferred species. Impacts of crayfish in temporary ponds may depend on time of invasion and on the composition of the macrophyte community.     

Population dynamics of an epibiont Ostracoda on the invasive red swamp crayfish Procambarus clarkii in a western Mediterranean wetland

Recently, the American entocytherid ostracod Ankylocythere sinuosa was discovered for the first time in Europe to inhabit widely distributed populations of the invasive American crayfish Procambarus clarkii in the Iberian Peninsula. Based on this finding, the aim was to describe the population dynamics of exotic entocytherids for the first time beyond their original area, and to analyse the main variables modulating temporal demographic patterns. We monitored a population of A. sinuosa and its host, P. clarkii, in the Pego-Oliva wetland (Eastern Iberian Peninsula) monthly for 1 year. Crayfish entocytherid loads strongly related to crayfish weight, moult-related exoskeleton hardening stage and water temperature. The proportion of earliest juvenile entocytherid instars, an indicator of recent hatching periods, also related to water temperature and conductivity. According to our results, the most important factors affecting entocytherid dynamics are individual crayfish moulting events, which diminish ostracod load, together with water temperature and ionic concentration, both influencing the life cycle of the exotic epibiont ostracod A. sinuosa.     

Abundance and distribution of the endangered loggerhead turtle in Spanish Mediterranean waters and the conservation implications

During 2 years (2001–2003), we performed seasonal aerial surveys in the central Spanish Mediterranean following the transect line methodology in order to determine the abundance and distribution patterns of loggerhead turtles Caretta caretta . We surveyed a total of 16 700 km, accounting for 770 turtle sightings. Loggerhead turtles were present with high abundance all year round. No seasonal differences in abundance were found, except in spring 2001, where the density of turtles was higher than in the other seasons. Our results show that the Western Mediterranean is not a 'summer' feeding area as proposed previously, as a high number of turtles are present throughout the year. The average surface density of turtles in the whole study area was 0.21 turtles km⁻² [95% confidence interval (CI): 0.17–0.25], and the mean abundance was 6653 turtles (95% CI: 5514–8027). The data relate to the number of turtles on the surface only, as diving turtles escape observation. Correcting our estimations of diving behaviour data in the area, the absolute abundance was 18 954 turtles (95% CI: 6679–53 786). Bearing in mind that around 25 000 loggerheads are caught per year in the Spanish Mediterranean, our results indicate that accidental captures seem to be a significant threat for this species, and conservation measures have to be implemented to avoid a non-sustainable situation.     

Community level impacts of invasive mosquitofish may exacerbate the impact to a threatened amphibian

Invasive fish threaten many native freshwater fauna. However, it can be difficult to determine how invasive fish impact animals with complex life cycles as interaction may be driven by either predation of aquatic larvae or avoidance of fish‐occupied waterbodies by the terrestrial adult stage. Mosquitofish (Gambusia spp.) are highly successful and aggressive invaders that negatively impact numerous aquatic fauna. One species potentially threatened by Gambusia holbrooki is the green and golden bell frog (Litoria aurea). However, G. holbrooki's role in this frog's decline was unclear due to declines driven by the chytrid fungal disease and the continued co‐existence of these fish and frogs in multiple locations. To clarify the extent to which Gambusia is impacting L. aurea, we conducted 3 years of field surveys across a deltaic wetland system in south‐east Australia. We measured the presence and abundance of aquatic taxa including G. holbrooki, and L. aurea frogs and tadpoles, along with habitat parameters at the landscape and microhabitat scale. Generalized linear models were used to explore patterns in the abundance and distributions of L. aurea and G. holbrooki. We found strong negative associations between G. holbrooki and tadpoles of most species, including L. aurea, but no apparent avoidance of G. holbrooki by adult frogs. Native invertebrate predators (Odonata and Coleoptera) were also absent from G. holbrooki‐occupied ponds. Due to the apparent naivety of adult frogs toward G. holbrooki, the separation of G. holbrooki and tadpoles, plus the abundance of alternative predators in G. holbrooki‐free ponds, we conclude that the impact of G. holbrooki on L. aurea recruitment is likely substantial and warrants management action.     

Integrating detection probabilities in species distribution models of amphibians breeding in Mediterranean temporary ponds

Aim The imperfect detection of species may lead to erroneous conclusions about species–environment relationships. Accuracy in species detection usually requires temporal replication at sampling sites, a time‐consuming and costly monitoring scheme. Here, we applied a lower‐cost alternative based on a double‐sampling approach to incorporate the reliability of species detection into regression‐based species distribution modelling. Location Doñana National Park (south‐western Spain). Methods Using species‐specific monthly detection probabilities, we estimated the detection reliability as the probability of having detected the species given the species‐specific survey time. Such reliability estimates were used to account explicitly for data uncertainty by weighting each absence. We illustrated how this novel framework can be used to evaluate four competing hypotheses as to what constitutes primary environmental control of amphibian distribution: breeding habitat, aestivating habitat, spatial distribution of surrounding habitats and/or major ecosystems zonation. The study was conducted on six pond‐breeding amphibian species during a 4‐year period. Results Non‐detections should not be considered equivalent to real absences, as their reliability varied considerably. The occurrence of Hyla meridionalis and Triturus pygmaeus was related to a particular major ecosystem of the study area, where suitable habitat for these species seemed to be widely available. Characteristics of the breeding habitat (area and hydroperiod) were of high importance for the occurrence of Pelobates cultripes and Pleurodeles waltl. Terrestrial characteristics were the most important predictors of the occurrence of Discoglossus galganoi and Lissotriton boscai, along with spatial distribution of breeding habitats for the last species. Main conclusions We did not find a single best supported hypothesis valid for all species, which stresses the importance of multiscale and multifactor approaches. More importantly, this study shows that estimating the reliability of non‐detection records, an exercise that had been previously seen as a naïve goal in species distribution modelling, is feasible and could be promoted in future studies, at least in comparable systems.     

Effectiveness of dam construction to contain the invasive crayfish Procambarus clarkii in a Mediterranean mountain stream

Invasive crayfish are a worldwide problem for aquatic ecosystems. So far, no control method has proven successful. Field data from invaded streams have suggested that physical structures may hamper the spread rate of invasive crayfish into new areas. However, given the difficulties for carrying out dam construction in natural habitats, data about their effectiveness in incipient invading populations are practically nonexistent. This paper evaluates the effectiveness of small dams to contain the advance of an incipient population of the invasive Procambarus clarkii in a mountain stream harboring the European southernmost population of the endangered Austropotamobius pallipes. The management strategy involved the construction of 3 artificial small (<3 m high) dams that were complemented with specific design solutions to increase their theoretical efficacy. Catch Per Unit Effort (CPUE) obtained during monitoring surveys carried out for 4 years were used as an indicator of containment effectiveness. The dams successfully prevented Procambarus clarkii from extending its range into the area occupied by A. pallipes. In addition, regardless of whether the construction of new dams isolated the leading edge of the invading population between two dams, the CPUE declined to below detectable limits. Therefore, dams are suggested to effectively stop, or at least hinder, the advance of invasive crayfish. Long-term monitoring is recommended to verify the on-going success of the dams in preventing upstream invasion.     

Characteristics of the soil seed bank in Mediterranean temporary ponds and its role in ecosystem dynamics

Species in temporary ponds overcome periods of unfavorable weather conditions by building up a large seed bank. With this strategy, the species diversity of ponds is preserved and information on their dynamics and structure is retained. Little is known about the characteristics, spatial patterns and role in the vegetation dynamics of the soil seed banks of Mediterranean temporary ponds, which are regarded as priority habitats under protection. We studied two sites of western Crete: Omalos, a mountain plateau at 1,060 m a.s.l. and Elafonisos, located near the coast at 60 m a.s.l. The seed bank was surveyed along transects using the germination method. Aboveground vegetation was measured on quadrats along the same transects. Canonical Correspondence Analysis (CCA) was run to define the zonation patterns. High density and species richness were recorded in both sites, with an average of 75,662 seeds/m² found in Omalos and 22,941 seeds/m² in Elafonisos. The community composition of both sites was remarkably different but in both locations perennial species were inconspicuous while annuals, prevailed in the seed banks. An important array of protected or rare species as well as several others which were absent from the vegetation were hosted in the soil seed banks, thereby rendering a low similarity between their composition. Soil seed banks in these ecosystems indicated a spatial heterogeneity that mirrored the aboveground vegetation distribution, sorted along the moisture gradient by their tolerance to flooding. Soil seed banks play a key role in the vegetation recovery after summer drought. The acts of preserving the soil seed bank and ensuring a transient flooding regime are essential to protect the unique vegetation communities of Mediterranean temporary ponds.     

Rainfall facilitates the spread, and time alters the impact, of the invasive Argentine ant

Climate change may exacerbate invasions by making conditions more favorable to introduced species relative to native species. Here we used data obtained during a long-term biannual survey of the distribution of ant species in a 481-ha preserve in northern California to assess the influence of interannual variation in rainfall on the spread of invasive Argentine ants, Linepithema humile, and the displacement of native ant species. Since the survey began in 1993, Argentine ants have expanded their range into 74 new hectares. Many invaded hectares were later abandoned, so the range of Argentine ants increased in some years and decreased in others. Rainfall predicted both range expansion and interannual changes in the distribution of Argentine ants: high rainfall, particularly in summer months, promoted their spread in the summer. This suggests that an increase in rainfall will promote a wider distribution of Argentine ants and increase their spread into new areas in California. Surprisingly, the distribution of two native ant species also increased following high rainfall, but only in areas of the preserve that were invaded by L. humile. Rainfall did not have a negative impact on total native ant species richness in invaded areas. Instead, native ant species richness in invaded areas increased significantly over the 13 years of observation. This suggests that the impact of Argentine ants on naïve ant communities may be most severe early in the invasion process.     

臭氧分布及对呼吸系统的影响

臭氧是氧气的同素异形体,在常温条件下它是一种有特殊臭味的淡蓝色气体。臭氧现已成为我国主要的一种环境污染物。臭氧的分布存在着明显的时间特异性和地区特异性,与气象条件、污染物的输送与扩散以及局地光化学反应强度等因素有着重要的关系。同时在人群流行病学和动物实验中发现了臭氧可以提高慢性阻塞性肺疾病、哮喘等呼吸系统疾病的发病率和病死率,同时提示了老年人、有呼吸系统疾病病史的人群和儿童都是易受臭氧影响的高危人群。国内外学者也对臭氧对机体损伤的机制进行了研究,主要集中在臭氧的致炎作用、氧化应激作用、造成机体气道高反应性和对细胞DNA的损伤等方面。本文主要就国内外对臭氧在时空分布、对机体呼吸系统影响和臭氧造成机体损伤的机制这3方面作一简要综述。     

Muddied waters: suspended sediment impacts on gill structure and aerobic scope in an endangered native and an invasive freshwater crayfish

Suspended sediment (SS) loadings in freshwater habitats have increased over the past century and SS is now a significant environmental stressor. Greater tolerance to environmental stressors has been proposed as a factor in the success of aquatic invasive species. Further, parasites may interact with environmental stressors to increase host susceptibility to loss of fitness and mortality. We compared the effects of SS exposure on the gill structure and aerobic scope of the endangered white-clawed crayfish (Austropotamobius pallipes), and the invasive signal crayfish (Pacifastacus leniusculus), and assessed impacts in relation to parasite burden. SS caused gill fouling and reduction in aerobic scope in both species, though A. pallipes was more susceptible than invasive P. leniusculus. The parasite Branchiobdella astaci, a crayfish worm that infests the gills, interacted with the sediment to affect gill structure whereas infection with the microsporidian parasite Thelohania contejeani had no effect on crayfish response to SS. Juvenile P. lenisuculus had a higher standard metabolic rate than A. pallipes, which may be linked to competitive advantages such as higher growth rate and behavioural dominance. Conservation of A. pallipes often involves relocation of threatened populations to isolated stillwaters; our findings suggest that SS levels should be assessed before relocation.