Structure and temporal variation of trematode and gastropod communities in a freshwater ecosystem.

The infro- and component community dynamics of digenetic trematodes in a freshwater gastropod community were examined over a 33-month period. The gastropod and trematode communities were composed of 17 and 10 species respectively. A total of 9,831 snails was collected; among them, 192 belonging to 14 species were infected by larval trematodes. The size of infected snails was significantly greater than that of healthy ones, and the increase of prevalence with size/age was interpreted as related to the increased probability of ultimately becoming parasitized. The trematode community was rich in allogenic species, but the most frequent trematode (cercariaeum) was autogenic and generalist (a range of 12 snail host species). There was a significantly positive relationship between the frequency of trematode species in the community and the number of first intermediate host species. A great temporal heterogeneity occurred in the prevalence of the snails, mainly attributed to the great temporal fluctuations of snail host populations and the variability of freshwater ecological conditions. The data on the occurrence of larval trematodes in 14 host species over the 33-month study allowed indicate a significant negative correlation between the abundance of gastropods and the prevalence of trematodes.     

Anthropogenic Pressure on a Molluscan–Trematode Community over a Long-term Period in the Basingstoke Canal, UK, and Its Implications for Ecosystem Health

A long-term study of a molluscan–trematode community in the Basingstoke Canal, UK, was undertaken over the period 1970–2003 which coincided with a significant restoration program involving substantial anthropogenic changes to the canal’s ecosystem. The aim of this study was to correlate these changes with alterations in the molluscan–trematode community, focusing on fluctuations in the prevalence and species richness of the trematode population as a good indicator, in terms of biodiversity, of ecosystem health. Molluscs were sampled at four time periods (1970, 1982, 1983, 2003) and examined for trematode parasites. The molluscan population over the sampling period demonstrated a limited stability with few species being present during all time periods. During 1982 and 1983, when restoration of the sampling site was being undertaken, Lymnaea stagnalis dominated the molluscan fauna, possibly because it was best able to take advantage of the disturbed conditions, while other species such as Bithynia tentaculata were absent. In the trematode population, the prevalence and diversity of both cercarial and metacercarial infections was high in 1970 prior to the period of restoration work. However, during 1982 and 1983, the trematode community was significantly reduced with cercarial prevalence declining to less than 1%. By 2003, more than 12 years after restoration had been completed, the trematode population had shown only a modest recovery. Trematodes use a wide range of different host species for completing their life-cycles and may be used as bioindicators of ecosystem diversity and health. An interpretation of the structure of the molluscan–trematode community over the sampling period suggests that the restoration project had an initial negative effect on ecosystem health, in terms of biodiversity, but this has subsequently only shown modest signs of recovery suggesting continuing problems in ecological functioning and stability.     

Spatial and temporal variations in the trematode component community of the mudsnail Hydrobia ventrosa in relation to the occurrence of waterfowl as definitive hosts

The component community of larval trematodes infecting the mudsnail Hydrobia ventrosa (Montagu) was examined in coastal lagoons of the southern Baltic Sea among different host subpopulations in relation to the structure of the waterfowl community. The 10 trematode species observed represent the families Notocotylidae (1), Echinostomatidae (1 or 2), Heterophyidae (2). Monorchidae (1). Microphallidae (3 or 4), Psilostomatidae (1), and Hemiuridae (1). Eight of these species infect waterfowl as adults. The structure of the trematode communities was similar between sampling sites. Seven trematode taxa were commonly found at all sampling sites. Prevalence values of the 6 most abundant taxa, which infect birds as final hosts, were significantly different between neither sampling sites nor across year. Overall trematode prevalence in H. ventrosa fluctuated seasonally. Prevalence usually peaked in summer between July and September or October. Low prevalences were observed in late winter and early spring. In contrast, the seasonal maximum in waterfowl numbers differed between areas because of significant spatial differences in the bird community structure. The species composition of the component trematode community of H. ventrosa in the coastal lagoons of the southern Baltic Sea is more or less independent of the species composition of the waterfowl community. This independence presumably results from the lack of host specificity in most of the observed trematode species. Otherwise, the low host specificity in combination with the enormous waterfowl diversity in the coastal lagoons might explain the stability of the prevalence pattern of the component trematode community.     

Host immunization shapes interspecific associations in trematode parasites

1. Individuals of free-living organisms are commonly infected by multiple parasite species. Under such circumstances, positive or negative associations between the species are possible because of direct or indirect interactions, details in parasite transmission ecology and host-mediated factors. One possible mechanism underlying these processes is host immunity, but its role in shaping these associations has rarely been tackled experimentally.
2. In this study, we tested the effect of host immunization on associations between trematode parasites infecting eyes of fish. We first analysed the associations between three species ( Diplostomum spathaceum , Diplostomum gasterostei and Tylodelphys clavata ) in wild hosts, roach ( Rutilus rutilus ) and perch ( Perca fluviatilis ). Second, using rainbow trout ( Oncorhynchus mykiss ) as a model fish species, we experimentally investigated how sequential immunization of the host (i.e. one parasite species infects and immunizes the host first) could affect the associations between two of the species.
3. The results indicated that most of the associations were positive in wild hosts, which supports between-individual variation in host susceptibility, rather than competitive exclusion between the parasite species. However, positive associations were more common in roach than in perch, possibly reflecting differences in ecological conditions of exposure between the host species. The experimental data showed that positive associations between two of the species were eroded by host immunization against one of the parasite species.
4. We conclude that sequential immunization of hosts has a marked effect on interspecific parasite associations and basically can determine if positive associations are detected or not. This implies that correlative results suggesting non-interactive community structure in general may be obscured by the sequence of previous parasite exposure and corresponding dynamics of host immunization.     

Species co-occurrences in ectoparasite infracommunities: Accounting for confounding factors associated with space,time, and host community composition

1. We explored patterns of co-occurrence of ectoparasite species on individual hosts from Central Europe (Slovakia) and South America (Argentina) within and between higher taxa while controlling for confounding factors associated with variation between host individuals and host species, as well as spatial and temporal variation. We used a recently developed statistical approach, the hierarchical modelling of species communities. 2. Although the probability of pairwise associations of the majority of species in both regions did not differ from random, all significant species associations in Slovakia were positive, whereas the only three significant species associations in Argentina were negative. On average, associations between ectoparasite species belonging to different higher taxa were positive in Slovakia and negative in Argentina. 3. At the host species level, both positive and negative associations between species were detected in all higher taxa. This was also true for ectoparasite co-occurrences in the same site, habitat, or year; although the number of co-occurring species pairs with high posterior probability was much greater in Slovakia than in Argentina. 4. We conclude that consideration of species co-occurrences across the entire multi-host assemblage and control for confounding spatial and temporal factors provided important insights into parasite community structure.     

Spatial and temporal structure of the trematode component community in Valvata macrostoma (Gastropoda, Prosobranchia)

We conducted the first comprehensive study on the spatiotemporal structure of trematode communities in the large-mouthed valve snail, Valvata macrostoma. A total of 1103 snails were examined monthly between May and October 2007 from Lake Konnevesi, Central Finland, from a shallow (1-2 m deep) and an offshore site (5-6 m deep), located ca. 50-70 m apart. Snails were infected by 10 trematode species. The species composition and prevalence were strikingly different between the sites with high species diversity in the shallow site (all 10 species; total prevalence of sporocysts/rediae 12.1%, metacercariae 55.4%) compared to the deeper site (3 species; prevalence 15.0% and 1.9%, respectively). This difference persisted throughout our study and is probably related to the spatial distribution of bird definitive hosts, whereas the seasonal parasite dynamics are likely to be affected by changes in the age-structure of the snail population. The probability of sporocyst infections increased with snail size, but no such trend was observed in redial or metacercarial infections which decreased with host size. Our results show that generally well-described spatiotemporal differences in trematode infection of molluscs can emerge in very narrow spatial and temporal scales, which emphasizes the importance of these factors in community studies.     

Aggregative structure is the rule in communities of fleas: null model analysis

We used null models to examine patterns of species co‐occurrences in 59 communities of fleas parasitic on small mammals from 4 biogeographic realms (Afrotropics, Nearctic, Neotropics, and Palaearctic). We compared frequencies of co‐occurrences of flea species across host species with those expected by chance, using a null model approach. We used 4 tests for non‐randomness to identify pairs of species (within a community) that demonstrate significant positive or negative co‐occurrence. The majority of flea communities were non‐randomly assembled. Patterns of flea co‐occurrences on the same host species indicated aggregation but not segregation of flea species (except for the flea community of Madagascar). Although only a small fraction of species pairs were associated significantly (264 of 10, 943 species pairs according to the most liberal criterion), most of these associations were positive (except for 2 negatively associated species pairs). Significantly associated pairs were represented mainly by non‐congeneric species. The degree of non‐randomness of the entire flea community was similar among biogeographic regions, but the strength of pair‐wise association varied geographically, being the highest in the Afrotropics and the lowest in the European region of the Palaearctic.     

Changes in semi-arid plant species associations along a livestock grazing gradient

In semi-arid ecosystems, vegetation is heterogeneously distributed, with plant species often associating in patches. These associations between species are not constant, but depend on the particular response of each species to environmental factors. Here, we investigated how plant species associations change in response to livestock grazing in a semi-arid ecosystem, Cabo de Gata-Níjar Natural Park in South East Spain. We established linear point-intercept transects at four sites with different grazing intensity, and recorded all species at each point. We investigated plant associations by comparing the number of times that each pair of species occurred at the same spatial point (co-occurrences), with the expected number of times based on species abundances. We also assessed associations for each shrub and grass species by considering all their pairs of associations and for the whole plant community by considering all pairs of associations on each site. At all sites, the plant community had a negative pattern of association, with fewer co-occurrences than expected. Negative association in the plant community increased at maximum grazing intensity. Most species associated as expected, particularly grass species, and positive associations were most important at intermediate grazing intensities. No species changed its type of association along the grazing gradient. We conclude that in the present plant community, grazing-resistant species compete among themselves and segregate in space. Some shrub species act as refuges for grazing-sensitive species that benefit from being spatially associated with shrub species, particularly at intermediate grazing intensities where positive associations were highest. At high grazing intensity, these shrubs can no longer persist and positive associations decrease due to the disappearance of refuges. Spatial associations between plant species and their response to grazing help identify the factors that organize plant communities, and may contribute to improving management of semi-arid ecosystems.     

Associations between shorebird abundance and parasites in the sand crab, Emerita analoga, along the California coast

I investigated spatial variation in the prevalence and abundance of 4 species of parasites in the sand crab, Emerita analoga, on 8 sandy beaches along 800 km of the California coast, to assess the importance of bird abundance for the distribution of parasites among sand crab populations. I collected sand crabs and counted shorebirds and gulls at each beach during June and November 1994. Sand crabs served as intermediate hosts for 4 species of parasites, including a trematode, Spelotrema nicolli (Cable and Hunnienen, 1938); an acanthocephalan, Polymorphus kenti (Van Cleave, 1947); a nematode, Proleptus sp., and an unidentified trypanorhynch tapeworm. Among sand crab populations, there was substantial spatial variation in the prevalence and abundance of each parasite species. No latitudinal pattern was apparent for any of the 4 species observed. Temporally, parasite prevalence and abundance was significantly different between dates for all 4 parasites. Specifically, sand crab populations experienced higher trematode, nematode, and trypanorhynch prevalence and abundance in November than in June. In contrast, prevalence and abundance of acanthocephalans were higher in June than in November. There were strong positive associations between bird abundance and prevalence of parasitic infection for trematodes and acanthocephalans for some dates but not for nematodes or trypanorhynchs, which use elasmobranchs as definitive hosts. The spatial variation in prevalence and abundance of trematodes and acanthocephalans observed among sand crab populations may be attributed to the distribution and abundance of shorebirds and gulls that serve as definitive hosts.     

鹞落坪国家级自然保护区吊罐井物种分布格局与种间关联的多尺度分析

为了探讨群落结构形成机制,采用点格局分析方法,对鹞落坪国家级自然保护区吊罐井核心区1hm2样地内36个优势物种在0~50m尺度上的空间分布格局和种间关联性进行了研究。结果显示,36个物种的空间分布格局在小尺度上主要呈聚集分布,在较大尺度上呈随机分布的物种比例最高;对36个物种种间关联性的分析发现,无关联类型是该样地物种间相互作用的主要表现形式,并且在10m尺度上无关联种对的比例最低,正相关和负相关的种对在所有尺度上的比例均未超过20%。群落结构的次生性、生境异质性和扩散限制性是导致该样地物种分布和种间关系格局的主要原因。本研究结果有助于了解物种共存的尺度依赖性特征及其形成机制。     

Spatiotemporal patterns of avian host–parasite interactions in the face of biogeographical range expansions

Exploration of interactions between hosts and parasitic symbionts is important for our understanding of the temporal and spatial distribution of organisms. For example, host colonization of new geographical regions may alter levels of infections and parasite specificity, and even allow hosts to escape from co‐evolved parasites, consequently shaping spatial distributions and community structure of both host and parasite. Here we investigate the effect of host colonization of new regions and the elevational distribution of host–parasite associations between birds and their vector‐transmitted haemosporidian blood parasites in two geological and geographical settings: mountains of New Guinea and the Canary Islands. Our results demonstrate that bird communities in younger regions have significantly lower levels of parasitism compared to those of older regions. Furthermore, host–parasite network analyses demonstrate that blood parasites may respond differently after arriving to a new region, through adaptations that allow for either expanding (Canary Islands) or retaining (New Guinea) their host niches. The spatial prevalence patterns along elevational gradients differed in the two regions, suggesting that region‐specific biotic (e.g., host community) and abiotic factors (e.g., temperature) govern prevalence patterns. Our findings suggest that the spatiotemporal range dynamics in host–parasite systems are driven by multiple factors, but that host and parasite community compositions and colonization histories are of particular importance.     

海南霸王岭油丹所在群落的优势种群种间联结性研究

基于2×2联列表,以X2统计量、Ochiai联结测度指数和多物种关联指数为参数,分析了海南霸王岭油丹所在群落中油丹(Alseodaphne hainanensis)等17个主要种群(物种)的种间联结性特征。结果表明, 17个主要种群(物种)总体水平上表现为不显著的正联结。17个物种组成的136个种对中：当取样面积在100 m2时,有72对表现正关联,63对为负关联,1对无关联。X2检验有2对表现显著正关联,有3对表现显著或极显著负关联。有125个种对的0＜OI＜0.6,仅有9个种对的OI≥0.6, 另有2个种对OI=0;当取样面积为50 m2时,有59对表现正关联,70对为负关联,7对无关联。X2检验有6对表现显著或极显著正关联,有2对表现显著负关联。有129个种对的0＜OI＜0.6,而OI≥0.6的种对没有, 另有7个种对OI=0。不同优势种群下的种间联结性存在一定的差异,而以线枝蒲桃(Syzygium araiocladum)等为优势种群的群落种间联结性较高,群落结构较稳定。     

Component community of larval trematodes in the mudsnail Hydrobia ventrosa: temporal variations in prevalence in relation to host life history

Temporal variations in the prevalence of larval trematodes in the short-lived prosobranch mudsnail Hydrobia ventrosa (Montagu) were investigated in relation to host life history and season for 4 successive years in temperate windflats of the southern Baltic Sea. The component community of trematode larvae in H. ventrosa comprises at least 10 species; families (and species) represented include Notocotylidae (1), Echinostomatidae (1 or 2), Heterophyidae (2), Monorchidae (1), Microphallidae (3 or 4), Psilostomatidae (1), and Hemiuridae (1). The notocotylid Paramonostomum alveatum was the most prevalent species, followed by the microphallids Maritrema subdolum and Microphallus sp. Trematode prevalence in H. ventrosa fluctuated seasonally. Prevalence usually peaked in summer between July and September-October and decreased in late winter-early spring. This seasonal change is chiefly explained by the life history patterns of the semelparous snail host. Hydrobia ventrosa has a maximum life span of about 2 yr and reproduces between June and November of its second calendar year. The first trematode infections appeared annually in May when the most abundant cohort of H. ventrosa, the second-calendar-year snails, mature. The prevalence continued to increase until August-September, throughout the reproductive period of the second-calendar-year snails, Prevalence decreased during winter, when most of the second-calendar-year snails died after reproduction. On the basis of longterm laboratory experiments, it has been shown that the late autumn-winter mortality was not the result of trematode infections. Seasonal patterns of prevalence were similar among the trematode species except for the monorchid Asymphylodora demeli, the only one using fish definitive hosts. Species-specific differences in the seasonal occurrence of prepatent infections and the predominance of certain larval stages in winter are interpreted as different strategies of the trematode species to survive the harsh winter conditions, or to survive the death of the first intermediate host in autumn-winter, or both.     

Prevalence and species richness of trematode parasites only partially recovers after the 2011 Tohoku,Japan, earthquake tsunami

Trematode parasites have complex life cycles and use a variety of host species across different trophic levels. Thus, they can be used as indicators of disturbance and recovery of coastal ecosystems. Estuaries on the Pacific coast of northeastern Japan were heavily affected by the 2011 Tohoku earthquake tsunami. To evaluate the effect of the tsunami on the trematode community, we examined trematodes in the mud snail, Batillaria attramentaria, at five study sites (three sites severely exposed to the tsunami and two sites sheltered from the tsunami) in Sendai Bay for 2 years prior to and 8 years after the tsunami. While the trematode prevalence decreased at all study sites, the species richness decreased only at the sites exposed to the tsunami. Although parasitism increased over the study period post-tsunami, the community had not fully recovered 8 years after the event. Trematode community structure has changed every year since the tsunami and has not stabilised. This could be explained by the alteration of first and second intermediate host communities. Our study suggests that it will take more time for the recovery of the trematode community and the associated coastal ecosystem in the Tohoku region.     

喀斯特常绿落叶阔叶林个体死亡对空间格局及种间关联性的影响

树木死亡作为森林生态系统中一个重要的生态学过程,研究其如何影响物种的空间分布格局及种间关联性,可以为揭示树木死亡机制及群落动态变化规律提供重要参考。本研究根据木论25 hm²喀斯特常绿落叶阔叶林动态监测样地两次木本植物调查数据,采用点格局分析方法,分析了树木死亡前后空间格局和种间关系的变化。结果表明: 样地内个体死亡前后主要表现为聚集分布,但死亡后聚集分布的物种比例较死亡前略微下降,小尺度上呈现随机分布的物种比例有所增加,个体的死亡呈非随机性死亡。在物种水平上,13个优势种的死树和活树之间在0～30 m各尺度上呈显著正关联,表明样地内优势物种个体之间种内和种间竞争作用不激烈;树木死亡前后种间关联性主要为正关联,且大部分物种种间关系在死亡前后并未发生变化,表明群落已发展到相对稳定的阶段;死亡后,在1～30 m尺度上表现为正关联的物种对比例有所增加,负关联和无关联的物种对在大部分尺度上有所减少,说明个体死亡发生后,各个物种之间的竞争压力在一定程度上有所缓解。     

Environmental factors influencing trematode prevalence in grey tree frog (Hyla versicolor) tadpoles in southern Ontario

The emergence or increased prevalence of various parasites may be linked to alterations in host-parasite interactions caused by environmental changes. We investigated prevalence of trematode infections in grey tree frog (Hyla versicolor) tadpoles from ponds in nonagricultural settings versus ponds adjacent to cornfields in southern Ontario. We found that agricultural activity was a significant factor in determining the percentage of tadpoles infected by 1 or more trematodes from 1 or more species (combined trematode infection). However, we found no associations between combined trematode infection and forest cover; pond size; road density; and measures of water quality, such as nitrate level and the presence of the herbicide atrazine. Although combined trematode prevalence was associated only with agricultural activity, prevalence of Alaria species showed a positive association with forest cover. This latter result probably reflects the importance of habitat suitability for the canid definitive hosts of this trematode species.     

The role of spatial and temporal heterogeneity and competition in structuring trematode communities in the great pond snail, Lymnaea stagnalis (L.)

We assessed how spatial and temporal heterogeneity and competition structure larval trematode communities in the pulmonate snail Lymnaea stagnalis . To postulate a dominance hierarchy, mark-release-recapture was used to monitor replacements of trematode species within snails over time. In addition, we sampled the trematode community in snails in different ponds in 3 consecutive years. A total of 7,623 snails (10,382 capture events) was sampled in 7 fishponds in the Jind?ich?v Hradec and T?eboň areas in South Bohemia (Czech Republic) from August 2006 to October 2008. Overall, 39% of snails were infected by a community of 14 trematode species; 7% of snails were infected with more than 1 trematode species (constituting 16 double- and 4 triple-species combinations). Results of the null-model analyses suggested that spatial heterogeneity in recruitment among ponds isolated trematode species from each other, whereas seasonal pulses in recruitment increased species interactions in some ponds. Competitive exclusion among trematodes led to a rarity of multiple infections compared to null-model expectations. Competitive relationships among trematode species were hypothesized as a dominance hierarchy based on direct evidence of replacement and invasion and on indirect evidence. Seven top dominant species with putatively similar competitive abilities (6 rediae and 1 sporocyst species) reduced the prevalence of the other trematode species developing in sporocysts only.     

Range limits and parasite prevalence in a freshwater snail

Geographical range limits are thought to be set by species' physiological or ecological adaptation to abiotic factors, but the importance of biotic factors such as parasitism in determining range limits has not been well explored. In this study the prevalence of trematode parasitism in populations of a freshwater gastropod snail, Lymnaea stagnalis, increased sharply as this species approached its western UK range limit. The likelihood of trematode infection increased with snail size, but high prevalence at the range edge was not a result of interpopulation variation in snail size. Changes in population growth rates resulting from high rates of parasitism at the range edge could contribute to range limitation. The mechanism driving high rates of parasitism at the range edge is not clear, but changes in abiotic factors towards the range limit may influence snail life history and immune response to trematode infection, indirectly altering the prevalence of parasites in marginal host populations.     

Foliar fungal endophyte community structure is independent of phylogenetic relatedness in an Asteraceae common garden

Phylogenetic distance among host species represents a proxy for host traits that act as biotic filters to shape host‐associated microbiome community structure. However, teasing apart potential biotic assembly mechanisms, such as host specificity or local species interactions, from abiotic factors, such as environmental specificity or dispersal barriers, in hyperdiverse, horizontally transmitted microbiomes remains a challenge. In this study, we tested whether host phylogenetic relatedness among 18 native Asteraceae plant species and spatial distance between replicated plots in a common garden affects foliar fungal endophyte (FFE) community structure. We found that FFE community structure varied significantly among host species, as well as host tribes, but not among host subfamilies. However, FFE community dissimilarity between host individuals was not significantly correlated with phylogenetic distance between host species. There was a significant effect of spatial distance among host individuals on FFE community dissimilarity within the common garden. The significant differences in FFE community structure among host species, but lack of a significant host phylogenetic effect, suggest functional differences among host species not accounted for by host phylogenetic distance, such as metabolic traits or phenology, may drive FFE community dissimilarity. Overall, our results indicate that host species identity and the spatial distance between plants can determine the similarity of their microbiomes, even across a single experimental field, but that host phylogeny is not closely tied to FFE community divergence in native Asteraceae.