The chemosensory ability of the predatory leech Whitmania laevis (Arhynchobdellida: Haemopidae) for prey searching

Although prey-detecting and searching abilities of predatory leeches of rhynchobdellid or the Erpobdelliformes of arhynchobdellid species have been studied in the past, hirudiniformes leeches are rarely mentioned. In this study, we investigated the chemosensory ability for prey-detecting and searching in Whitmania laevis, a hirudiniformes species that mainly preys on freshwater snails, and examined if such ability aided in their prey selection. Five sympatric snail species, i.e., apple snail Pomacea canaliculata, thiarid snail Thiara tuberculata, viviparid snail Sinotaia quadrata, ear pond snail Radix auricularia swinhoei and tadpole snail Physa acuta were used as prey. Our results showed that W. laevis has the chemosensory ability to detect the waterborne odors of snails. However, they follow the snails by their mucus trails, and not by the odor that the snails leave in the water. Of these five snail species, W. laevis only followed the trails of the thiarid snails, ear pond snails and tadpole snails, and did not show a different response to the trails produced by snails of different sizes. Our results suggest that W. laevis can use waterborne odors to detect the existence of prey. They rely on mucus trails to follow their preferred prey, but do not distinguish between snails of a preferred size by their mucus trails. In addition, when following the trail of a preferred snail, W. laevis exhibits a newly described searching behavior, i.e., head tapping, and may use it to locate a snail trail and increase its probability of finding the trail-laying snail nearby.     

Tidal regime dictates the cascading consumptive and nonconsumptive effects of multiple predators on a marsh plant

Prey perception of predators can dictate how prey behaviorally balance the need to avoid being eaten with the need to consume resources, and this perception and consequent behavior can be strongly influenced by physical processes. Physical factors, however, can also alter the density and diversity of predators that pursue prey. Thus, it remains uncertain to what extent variable risk perception and antipredator behavior vs. variation in predator consumption of prey underlie prey-resource dynamics and give rise to large-scale patterns in natural systems. In an experimental food web where tidal inundation of marsh controls which predators access prey, crab and conch (predators) influenced the survivorship and antipredator behavior of snails (prey) irrespective of whether tidal inundation occurred on a diurnal or mixed semidiurnal schedule. Specifically, cues of either predator caused snails to ascend marsh leaves; snail survivorship was reduced more by unrestrained crabs than by unrestrained conchs; and snail survivorship was lowest with multiple predators than with any single predator despite interference. In contrast to these tidally consistent direct consumptive and nonconsumptive effects, indirect predator effects differed with tidal regime: snail grazing of marsh leaves in the presence of predators increased in the diurnal tide but decreased in the mixed semidiurnal tidal schedule, overwhelming the differences in snail density that resulted from direct predation. In addition, results suggest that snails may increase their foraging to compensate for stress-induced metabolic demand in the presence of predator cues. Patterns from natural marshes spanning a tidal inundation gradient (from diurnal to mixed semidiurnal tides) across 400 km of coastline were consistent with experimental results: despite minimal spatial variation in densities of predators, snails, abiotic stressors, and marsh productivity, snail grazing on marsh plants increased and plant biomass decreased on shorelines exposed to a diurnal tide. Because both the field and experimental results can be explained by tidal-induced variation in risk perception and snail behavior rather than by changes in snail density, this study reinforces the importance of nonconsumptive predator effects in complex natural systems and at large spatial scales.     

Intertidal distribution and long-term movements of Littorina irrorata (Mollusca: Gastropoda)

The distribution of Littorina irrorata Say on a low-energy barrier beach on the northern Gulf of Mexico is described, and correlated with the presence of Spartina alterniflora and other plants in the upper intertidal zone. The movements of 66 individually tagged snails were followed in the S. alterniflora zone for an average of 226 days, during which time an average of 10.6 positions were recorded per snail. The snails travelled an average total path distance of at least 995 cm, but due to contorted paths, ended up an average resultant distance of only 399 cm away from their original positions. They moved an estimated resultant distance of approximately 20 to 25 cm per activity period. Despite a slight offshore movement during the fall and winter, the snails moved more parallel to the shoreline than perpendicular to it.     

Controls of spatial variation in the prevalence of trematode parasites infecting a marine snail

Geographic variability in abundance can be driven by multiple physical and biological factors operating at multiple scales. To understand the determinants of larval trematode prevalence within populations of the marine snail host Littorina littorea, we quantified many physical and biological variables at 28 New England intertidal sites. A hierarchical, mixed-effects model identified the abundance of gulls (the final hosts and dispersive agents of infective trematode stages) and snail size (a proxy for time of exposure) as the primary factors associated with trematode prevalence. The predominant influence of these variables coupled with routinely low infection rates (21 of the 28 populations exhibited prevalence <12%) suggest broad-scale recruitment limitation of trematodes. Although infection rates were spatially variable, formal analyses detected no regional spatial gradients in either trematode prevalence or independent environmental variables. Trematode prevalence appears to be predominantly determined by local site characteristics favoring high gull abundance.     

Understanding the net effects of pesticides on amphibian trematode infections

Anthropogenic factors can have simultaneous positive and negative effects on parasite transmission, and thus it is important to quantify their net effects on disease risk. Net effects will be a product of changes in the survival and traits (e.g., susceptibility, infectivity) of both hosts and parasites. In separate laboratory experiments, we exposed cercariae of the trematode Echinostoma trivolvis, and its first and second intermediate hosts, snails (Planorbella trivolvis) and green frog tadpoles (Rana clamitans), respectively, to one of four common pesticides (atrazine, glyphosate, carbaryl, and malathion) at standardized, ecologically relevant concentrations (201.0, 3700.0, 33.5, and 9.6 microg/L, respectively). We measured effects of pesticide exposure on six mechanisms important to this host-parasite interaction: (1) survival of E. trivolvis cercariae over 26 hours, (2) tadpole survival over two weeks, (3) snail survival over four weeks, (4) snail growth and fecundity, (5) cercarial infectivity, and (6) tadpole susceptibility to a fixed number of cercariae. Pesticides, in general, caused significantly greater mortality of E. trivolvis cercariae than did control treatments, but atrazine was the lone chemical to significantly reduce cercarial survival (LC50 value = 267 mg/L) and then only at concentrations greater than commonly found in aquatic ecosystems (> or =200 microg/L). None of the pesticides significantly enhanced E. trivolvis virulence, decreased tadpole survival, or reduced snail survival, growth, or fecundity. Sublethal exposure of the cercariae to the pesticides (4 h) did not significantly affect trematode encystment in R. clamitans. In contrast, sublethal exposure of R. clamitans to each of the four pesticides increased their susceptibility as measured by the percentage of cercariae that encysted. The reduction in exposure to trematodes due to pesticide-induced cercarial mortality (a density-mediated effect) was smaller than the pesticide-induced increase in amphibian susceptibility (a trait-mediated effect), suggesting that the net effect of exposure to environmentally realistic levels of pesticides will be to elevate amphibian trematode infections. These findings highlight the importance of elucidating the lethal and sublethal effects of anthropogenic factors on both hosts and parasites to understand the mechanisms underlying changes in parasite transmission and virulence, an approach that is especially needed for amphibians, a taxon experiencing global disease-related declines.     

Using parasites to inform ecological history: comparisons among three congeneric marine snails

Species introduced to novel regions often leave behind many parasite species. Signatures of parasite release could thus be used to resolve cryptogenic (uncertain) origins such as that of Littorina littorea, a European marine snail whose history in North America has been debated for over 100 years. Through extensive field and literature surveys, we examined species richness of parasitic trematodes infecting this snail and two co-occurring congeners, L. saxatilis and L. obtusata, both considered native throughout the North Atlantic. Of the three snails, only L. littorea possessed significantly fewer trematode species in North America, and all North American trematodes infecting the three Littorina spp. were a nested subset of Europe. Surprisingly, several of L. littorea's missing trematodes in North America infected the other Littorina congeners. Most likely, long separation of these trematodes from their former host resulted in divergence of the parasites' recognition of L. littorea. Overall, these patterns of parasitism suggest a recent invasion from Europe to North America for L. littorea and an older, natural expansion from Europe to North America for L. saxatilis and L. obtusata.     

Ecology of a corallivorous gastropod,<Emphasis Type="Italic"> Coralliophila abbreviata</Emphasis>, on two scleractinian hosts. I: Population structure of snails and corals

Despite their potential importance in structuring reef communities, invertebrate corallivores and their population structures are poorly understood. We found distinct differences in the population structures (length-frequency distribution and sex ratio) of the corallivorous gastropod Coralliophila abbreviata residing on two coral-host taxa, Montastraea spp. and Acropora palmata, in the Florida Keys. In each of two survey years, around 50% of the Montastraea spp. colonies were infested, with a mean snail density of eight snails per infested colony (range 1–45), while around 20% of A. palmata colonies harbored three snails per infested colony (range 1–23). Variation in patterns of snail occurrence was also observed within a host taxon. A. palmata occurred in low- and high-density stands (0.4 and 1 colony m^–2, respectively, at the initial survey) at different sites. Hurricane Georges struck the area in September 1998. When resurveyed in 1999, density of colonies in low-density stands had decreased by 75% to 0.1 colonies m^–2. This decrease was accompanied by a doubling in the proportion of colonies infested with snails (from 19% to 46%) and an increase in snail density per infested colony (from 3.7±3.3 SD to 5.4±4.6 SD) as snails apparently concentrated on surviving A. palmata. In contrast, sites with high density A. palmata stands (thickets) retained colony densities of about ~1 colony m^–2 among years, while snail infestation increased only from 9% to 14% of colonies surveyed and snail density essentially remained unchanged (from 2.7±1.8 to 2.9±1.9 snails per infested colony). Snails collected from Montastraea spp. were shorter than those from A. palmata in low-density stands and were longest on A. palmata in thickets. On both host taxa, female snails were longer than males. The sex ratio of snails on Montastraea spp. hosts was even (1:1), while that of snails on A. palmata was skewed (70% males). Factors that could explain observed differences in size structure and sex ratio between Coralliophila populations on the two coral host taxa include: differential susceptibility to predators, influence of host tissue nutritional quality and/or secondary metabolite content, and genetic differences (cryptic species). The host-specific characteristics of C. abbreviata populations imply that the impact of gastropods on reef communities will vary with the coral species composition.     

Invasive Non‐Native Species’ Provision of Refugia for Endangered Native Species

Abstract: The influence of non‐native species on native ecosystems is not predicted easily when interspecific interactions are complex. Species removal can result in unexpected and undesired changes to other ecosystem components. I examined whether invasive non‐native species may both harm and provide refugia for endangered native species. The invasive non‐native plant Casuarina stricta has damaged the native flora and caused decline of the snail fauna on the Ogasawara Islands, Japan. On Anijima in 2006 and 2009, I examined endemic land snails in the genus Ogasawarana. I compared the density of live specimens and frequency of predation scars (from black rats[Rattus rattus]) on empty shells in native vegetation and Casuarina forests. The density of land snails was greater in native vegetation than in Casuarina forests in 2006. Nevertheless, radical declines in the density of land snails occurred in native vegetation since 2006 in association with increasing predation by black rats. In contrast, abundance of Ogasawarana did not decline in the Casuarina forest, where shells with predation scars from rats were rare. As a result, the density of snails was greater in the Casuarina forest than in native vegetation. Removal of Casuarina was associated with an increased proportion of shells with predation scars from rats and a decrease in the density of Ogasawarana. The thick and dense litter of Casuarina appears to provide refugia for native land snails by protecting them from predation by rats; thus, eradication of rats should precede eradication of Casuarina. Adaptive strategies, particularly those that consider the removal order of non‐native species, are crucial to minimizing the unintended effects of eradication on native species. In addition, my results suggested that in some cases a given non‐native species can be used to mitigate the impacts of other non‐native species on native species.     

Growth of the queen conch Strombus gigas,with a discussion of the practicality of its mariculture

Data on the growth of the large marine snail Strombus gigas was obtained from specimens collected as veligers in the plankton and reared through metamorphosis, from larger individuals (5.5 cm) reared in a mariculture system, and from field tagrecapture experiments. Using the von Bertalanffy growth equation, I estimate that 1, 2, and 3 year-old juvenile snails are 10.8, 17, and 20.5 cm in maximum shell length, respectively. The snails reach the flaring-lip stage after 3 years and have a mean longevity of another 3 years. Approximately 12% of the total weight of a juvenile snail is marketable meat. Measurements of meat weight, shell length, and total weight are highly correlated with one another, thereby providing reliable means of assessing meat yields from living snails. The mariculture of S. gigas is feasible, but because of the snails' slow rate of growth it may not be economically practical at this time. Perhaps local fisheries in the Caribbean Sea could be reestablished and/or maintained by seeding subtidal algal flats with hatchery-reared juvenile snails.     

Movements of <Emphasis Type="Italic">Ilyanassa obsoleta</Emphasis> (Gastropoda) on an intertidal sandflat

The gastropod Ilyanassa obsoleta (Say) is native to the east coast of North America where it is locally abundant on sandflats, mudflats, and in saltmarsh creeks. The local disturbances created by snails and their movements affect soft-sediment community composition. Movements of individually marked snails were followed on an intertidal sandflat on Cape Henlopen, Delaware, U.S.A. In June 1991, 1,200 snails that had tested as trematode-uninfected were released and over 5 months 554 were sighted 971 times. Mean daily net distance moved was 1.7 m, but snails often moved 10–20 m day⁻¹ and one snail was 180 m distant after 130 days. Net dispersal of the released population was attained in ≈10 days, by which time, a typical distance from release was 15–20 m. Snails were not found crossing sandbars and most moved away from shore into a tidal gully. In June 1993, 500 snails, both uninfected and trematode-infected, were released at the same position and over 6 months, 350 snails were sighted 949 times. Sandbars were again barriers to movement, but their changed positions allowed wider dispersal. Net dispersal was complete in ≈20 days by which time a typical distance from release was 30–40 m. Mean daily net distance moved was 2.2 m, but within 10 days snails had moved 50–100 m. In both years, following initial dispersal, snails (infected or not) took up random directions from move to move. Infected and uninfected snails dispersed equal distances, but had different mean final dispersal directions. Dispersal of I. obsoleta individuals was extensive and affected by shifting sandbar positions and parasitism. Recognizing this will be important in appreciating the ecological dynamics of this gastropod and in determining its effects on soft-bottom communities.Electronic Supplementary Material Supplementary material is available for this article at     

Species diversity reduces parasite infection through cross-generational effects on host abundance

With growing interest in the effects of biodiversity on disease, there is a critical need for studies that empirically identify the mechanisms underlying the diversity-disease relationship. Here, we combined wetland surveys of host community structure with mechanistic experiments involving a multi-host parasite to evaluate competing explanations for the dilution effect. Sampling of 320 wetlands in California indicated that snail host communities were strongly nested, with competent hosts for the trematode Ribeiroia ondatrae predominating in low-richness assemblages and unsuitable hosts increasingly present in more diverse communities. Moreover, competent host density was negatively associated with increases in snail species richness. These patterns in host community assembly support a key prerequisite underlying the dilution effect. Results of multigenerational mesocosm experiments designed to mimic field-observed community assemblages allowed us to evaluate the relative importance of host density and diversity in influencing parasite infection success. Increases in snail species richness (from one to four species) had sharply negative effects on the density of infected hosts (-90% reduction). However, this effect was indirect; competition associated with non-host species led to a 95% reduction in host density (susceptible host regulation), owing primarily to a reduction in host reproduction. Among susceptible hosts, there were no differences in infection prevalence as a function of community structure, indicating a lack of support for a direct effect of diversity on infection (encounter reduction). In monospecific conditions, higher initial host densities increased infection among adult hosts; however, compensatory reproduction in the low-density treatments equalized the final number of infected hosts by the next generation, underscoring the relevance of multigenerational studies in understanding the dilution effect. These findings highlight the role of interspecific competition in mediating the relationship between species richness and parasite infection and emphasize the importance of field-informed experimental research in understanding mechanisms underlying the diversity-disease relationship.     

Morphologie,Entwicklung und Ökologie digener Trematoden-Larven der StrandschneckeLittorina littorea

More than 6000 individuals of the snailLittorina littorea L., from 6 different localities on the coast of the German Bay (North Sea), were examined for larval trematodes by crushing their shells, an additional 6000 by isolation in small quantities of sea water. Six species of larval trematodes were found inL. littorea and described: Notocotylid larva —Cercaria lebouri Stunkard, 1932; Cercaria ofHimasthla elongata (Mehlis, 1831); Cercaria ofPodocotyle atomon (Rudolphi, 1802); Cercaria ofRenicola roscovita (Stunkard, 1932) n. comb.; Metacercaria ofMicrophallus pygmaeus (Levinsen, 1881); Cercaria ofCryptocotyle lingua (Creplin, 1825). Considering the large number of investigated snails, the trematode fauna ofL. littorea in this area is assumed to be completely known. The life cycles ofHimasthla elongata andRenicola roscovita are described. Larvae of the different trematode species show differential preferences for distinct size groups of snail hosts. Juvenile snails are not infected. No correlation exists between infection rates and sex of the hosts. Quantitative aspects of multiple infections are calculated and discussed. The incidence of larval trematodes in the investigated localities differs. Information is presented on seasonal variations of infestations and their possible causes.     

Local variation of within-host clonal diversity coupled with genetic homogeneity in a marine trematode

Despite their ubiquity and importance to intertidal ecosystems, information is currently lacking regarding the genetic diversity of trematode parasites within coastal organisms and the distribution of their genetic variation among intertidal habitats. In this study, we quantified the clonal diversity of the coastal marine trematode Maritrema novaezealandensis within Zeacumantus subcarinatus snail hosts from three coastal bays in Otago Harbour, New Zealand, using five microsatellite loci to determine if differences exist in the frequency of occurrence of multi-clone infections. In addition, we examined gene flow among M. novaezealandensis collected from the three bays. The frequency of mixed-clone infections varied fourfold among bays and no genetic differentiation was detected among intertidal bays. Across the coastal bays studied, M. novaezealandensis comprises a single population that is potentially infecting multiple Z. subcarinatus populations with varying life history traits.     

生石灰对福寿螺的毒性效应研究

通过室内毒性试验,探索了生石灰对不同大小福寿螺的毒性效应。试验中将福寿螺按照壳高（H）分为小螺（5 mm≤H〈15 mm）、中螺（15 mm≤H〈25 mm）、大螺（25 mm≤H〈35 mm）和超大螺（35 mm≤H〈45 mm）,结果表明：（1）生石灰的控螺效应显著,尤其是对中小福寿螺具很强的抑制和杀灭作用;生石灰处理6 d,除了最低质量浓度0.05 g.L-1处理造成35%死亡率,其他处理死亡率均在70%以上,其中0.20、0.50、1.00和2.00 g.L-1处理中超过90%的福寿螺死亡。（2）生石灰对不同大小福寿螺的半抑制质量浓度（24 h）大小顺序为超大螺〉中螺、大螺〉小螺,半致死质量浓度（24 h）大小顺序为超大螺〉大螺〉中螺〉小螺;（3）生石灰对福寿螺的毒性以抑制效应为主,生石灰对福寿螺的平均半抑制质量浓度为0.06～0.22 g.L-1,低于平均半致死质量浓度0.11～1.09 g.L-1;（4）0.50 g.L-1和2.00 g.L-1质量浓度的石灰处理12 h后,福寿螺鰓组织SOD酶活性呈极显著下降,分别为对照的33%和38%。研究表明,生石灰可用于防治入侵生物福寿螺,在实践中可部分替代杀螺剂的使用。     

Endangered light-footed clapper rail affects parasite community structure in coastal wetlands.

An extinction necessarily affects community members that have obligate relationships with the extinct species. Indirect or cascading effects can lead to even broader changes at the community or ecosystem level. However, it is not clear whether generalist parasites should be affected by the extinction of one of their hosts. We tested the prediction that loss of a host species could affect the structure of a generalist parasite community by investigating the role of endangered Light-footed Clapper Rails (Rallus longirostris levipes) in structuring trematode communities in four tidal wetlands in southern California, U.S.A. (Carpinteria Salt Marsh, Mugu Lagoon) and Mexico (Estero de Punta Banda, Bahia Falsa-San Quintin). We used larval trematode parasites in first intermediate host snails (Cerithidea californica) as windows into the adult trematodes that parasitize Clapper Rails. Within and among wetlands, we found positive associations between Clapper Rails and four trematode species, particularly in the vegetated marsh habitat where Clapper Rails typically occur. This suggests that further loss of Clapper Rails is likely to affect the abundance of several competitively dominant trematode species in wetlands with California horn snails, with possible indirect effects on the trematode community and changes in the impacts of these parasites on fishes and invertebrates.     

Habitat choice in the intertidal snail Tegula funebralis

Intraspecific variation in habitat preference was studied in the black turban snail Tegula funebralis (Adams, 1854), at two locations on the northern U.S. Pacific Coast. Studies in 1977 using a mass-marking technique showed that most snails found either above or in permanent tidepools at low tide return to their original habitats within a few days after experimental habitat reversal. This return is not due to homing behavior, but is apparently based on the recognition of ecological characteristics of the two habitats. Experiments in 1978 with individually-marked snails suggest that they prefer specific intertidal levels, and not merely above-pool or in-pool habitats. Theoretical models predict that this behavior could play a major role in the maintenance of genetic polymorphism in a species like T. funebralis, whose intertidal environment is characterized by extreme spatial heterogeneity.     

Does risk of intraspecific interactions induce shifts in prey-size preference in aquatic predators?

Interactions between foragers may seriously affect individual foraging efficiency. In a laboratory study of handling time, prey value and prey-size preference in northern pike and signal crayfish, we show that risk of intraspecific interactions between predators does not affect handling time or value of prey. However, the presence of agonistic intraspecific interactors shifts prey-size preference in these predators. Neither northern pike nor signal crayfish foraging alone show a prey-size preference, while pike foraging among conspecifics prefer small prey, and crayfish foraging in groups prefer large prey. We ascribe the different outcomes in prey preference to differences in susceptibility to interactions: northern pike under risk avoid large prey to avoid long handling times and the associated risk of interactions, while signal crayfish foraging among conspecifics may defend themselves and their prey during handling, and thus select prey to maximise investment. In addition, the value of pike prey (roach) is low for very small prey, maximises for small prey, and then decreases monotonically for larger prey, while crayfish prey (pond snail) value is low for very small prey, has a maximum at small prey, but does not decrease as much for larger prey. Therefore, a large and easily detected snail prey provides a crayfish with as much value as a small prey. We conclude that interaction risk and predator density affect prey-size preference differently in these aquatic predators, and therefore has different potential effects on prey-size structure and population and community dynamics. Received: 4 October 1999 / Revised: 20 March 2000 / Accepted: 27 May 2000     

Effects of Polinices duplicatus (Gastropoda: Naticidae) on infaunal community structure at Barnstable Harbor,Massachusetts, USA

Effects of the predatory naticid snail Polinices duplicatus (Say) on species composition, diversity and density, of the infauna of intertidal sand-flats at Barnstable Harbor, Massachusetts, USA, were studied using field experiments. Responses of molluscs (prey of P. duplicatus) and nonmolluscs (nonprey) were considered separately to distinguish between the effects of feeding and sediment disturbance during foraging. The fauna of 0.25 m² predator-exclusion cages, coarse-mesh cages, sieved areas, and controls was followed for 1 yr. Species associations within cages from which predators were excluded were denser, more diverse, and richer in molluscs than those in other treatments. Larger areas (3×3 m) of natural bottom were fenced and maintained as snail exclosures and enclosures during two feeding seasons. For both molluscs and nonmolluscs, diversity (H'), number of species, evenness (SD), and density all decreased with increasing snail density. Intense predation pressure on molluscs and selective feeding on thin-shelled bivalves, which were rare, removed individuals and species of molluscs from the community. Comparison of samples taken inside and outside trails made by snails showed that disturbance of the surface sediment layers by snails decreased the abundance of spionid polychaetes and total nonmolluscs. The sipunculan Phascolopsis gouldi and the bivalve Gemma gemma dominated the community in all experimental treatments and were little affected by the activity of Polinices duplicatus. Predation and disturbance by snails lowered community diversity by removing individuals of the less abundant species, and generally maintained population densities below the level where strong competition would occur.     

Range of ecological impact of highway construction in the Longitudinal Range-Gorge Region,China

The major features of different road sections were identified. Methods quantitatively determining the ecological impact of highway construction were discussed, and a demonstration was presented. The Dabao highway (from Dali to Baoshan) and the Sixiao highway (from Simao to Xiaomengyang) passing through the Longitudinal Range-Gorge Region were used as examples in this paper. For the normal road sections having no important species to protect, a method of ecosystem health assessment was introduced to calculate the overall range of the ecological impact of highway construction. For the road sections having plants or soils of interest, indexes were selected and the range of impact was determined using mathematical methods such as regression analysis and variance analysis. The range of impact on animal populations was also discussed in terms of the minimum living areas required by animal populations and the fragmentation caused by highway construction. The results indicate that the zones impacted by highway construction were composed of both regular and anomalous figures, the range of impacts for different landforms of the two sample highways were substantially different; highway construction had dramatic effects on roadside ecosystem health; the impact on soil factors of farmland was greater than 200 m, and the distance may exceed 1000 m when important large animals were considered.     

Using Parasitic Trematode Larvae to Quantify an Elusive Vertebrate Host

Abstract: Digenean trematode parasites require multiple host species to complete their life cycles, and their abundance can often be strongly correlated with the abundance of their host species. Species richness and abundance of parasites in easily sampled host species may yield an accurate estimate of the species richness and abundance of other hosts in a parasite's life cycle that are difficult to survey directly. Accordingly, we investigated whether prevalence and mean abundance of trematodes could be used to estimate the abundance of one of their host species, diamondback terrapins (Malaclemys terrapin), which are difficult to sample and are designated as near threatened (by the International Union for Conservation of Nature [IUCN Red List]) along some U.S. coasts. As an adult the trematode Pleurogonius malaclemys is specific to terrapins. Its larval stages live first inside mud snails (Ilyanassa obsoleta) and are subsequently shed into the environment where they form external metacercarial cysts on hard surfaces such as snail opercula. The life cycle of P. malaclemys is completed when terrapins ingest these cysts. At 12 sites along the coast of Georgia (U.S.A.), we determined the prevalence of internal P. malaclemys larvae in mud snails (proportion of infected snails in a population) and the prevalence and mean abundance of external trematode cysts. We examined whether these data were correlated with terrapin abundance, which we estimated with mark‐recapture methods. The abundance of external cysts and salinity explained ≥59% of the variability in terrapin abundance. We suggest that dependent linkages between the life stages of multihost parasites make them reliable predictors of host species’ abundance, including hosts with abundances that are challenging to quantify directly.