The role of interspecific interactions in the structure of gastrointestinal helminth communities has been at the core of most research in parasite community ecology, yet there is no consensus regarding their general importance. There have been two different approaches to the study of species interactions in helminths. The first one consists of measuring the responses of helminth species in concomitant infections, preferably in laboratory experiments. Any change in numbers of parasite individuals or in their use of niche space, compared with what is observed in single infections, provides solid evidence that the species are interacting. The second approach can only provide indirect, circumstantial evidence. It consists in contrasting observed patterns either in the distribution of species richness of infracommunities from wild hosts, in their species composition, or in pairwise associations between helminth species among infracommunities, with the random patterns predicted by appropriate null models. In many cases, observed patterns do not depart from predicted ones; when they do, alternative explanations are usually as plausible as invoking the effect of interactions among helminth species. The present evidence suggests that the role of species interactions in helminth community structure is often negligible, but that it must always be evaluated on a case-by-case basis.

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Interactions between species and the structure of helminth communities.

There is now unequivocal evidence for global climate change; however, its potential impacts on evolutionary processes remain unclear. Many species have responded to contemporary climate change through shifts in their geographic range. This could lead to increased sympatry between recently diverged species; likely increasing the potential for hybridization. Recently, following a series of warm winters, southern flying squirrels (Glaucomys volans) in Ontario, Canada rapidly expanded their northern range limit resulting in increased sympatry with the closely related northern flying squirrel (Glaucomys sabrinus). This provided the opportunity to test the prediction that contemporary climate change can act as a catalyst creating conditions for the formation of hybrid zones. Following extensive sampling and molecular analyses (nuclear and mitochondrial DNA), we identified the occurrence of hybridization between sympatric G. sabrinus and G. volans .T here was evidence of backcrossing but not of extensive introgession, consistent with the hypothesis of recent rather than historic hybridization. To our knowledge, this is the first report of hybrid zone formation following a range expansion induced by contemporary climate change. This is also the first report of hybridization between North American flying squirrel species.

Climate change induced hybridization in flying squirrels

Traditionally, the field of parasitology has dealt with eukaryotic animals, to the exclusion of viruses, bacteria, fungi, etc., which is the way it will be approached here. The focus of the present paper will be on certain ecological aspects of the life cycles and life-history strategies employed by the Digenea, a diverse group of platyhelminths that includes some 25,000 species. More specifically, the review will consider the nature of host/parasite interactions within molluscan intermediate hosts and the manner in which these interactions, or lack thereof, function in structuring trematode infracommunities within these molluscan intermediate hosts. Literature in this area suggests that predation/competition may be a significant structuring force for infracommunities in certain marine prosobranchs, but not others, and that temporal/spatial factors may be involved as structuring mechanisms in at least some freshwater pulmonates.

The Transmission of Digenetic Trematodes: Style, Elegance, Complexity

One hundred seventy-two frogs (Leptodactylus chaquensis) were collected from November 2002 to November 2003, in agricultural (n = 132) and nonagricultural (n = 40) areas. Both sites are near the city of Corrientes, Argentina. The main goals of this study were as follows: (1) to determine the helminth parasite community in agricultural and nonagricultural habitats; (2) to analyze the relationships between helminth parasites and site of infection, frog body size, and gender; (3) to identify and examine covariation and association of helminth communities; and (4) to determine the mean richness and diversity of parasite communities. The helminth compound community of this amphibian species consisted of 24 species: 19 in agricultural habitats and 18 in nonagricultural habitats. The mean richness, mean diversity, and evenness of helminths were significantly different between the habitats (P < 0.05). The body size of the host was the important factor in determining parasite richness. Both habitats exhibited differences in community ordination. The helminth species in the 2 habitats exhibited the same interspecific relationships, although differences were observed in the intensity of infection.

https://www.jstor.org/stable/pdfplus/40058651.pdf

Species affinity and infracommunity ordination of helminths of leptodactylus chaquensis (anura: leptodactylidae) in two contrasting environments from northeastern argentina

A total of 36 adult individuals of Leptodactylus latinasus were collected near the city of Corrientes, Province of Corrientes in Argentina. The main goals of this study were: (1) to determine the helminth parasite fauna of L. latinasus; (2) to determine the richness and diversity of parasites at the component and infracommunity levels; (3) to analyze the relationships between helminth, frog body size and sex, and (4) to identify and examine species affinity of helminth communities. The helminth component community of this frog’s population consisted of 17 species. The predominant groups of parasites were the trematodes (adults: Glypthelmins repandum, Catadiscus inopinatus and Haematoloechus longiplexus; larvae: Travtrema aff. stenocotyle, Bursotrema aff. tetracotyloides, Styphlodora sp., unknown opisthogonimid species, Petasiger sp. and unknown strigeid species), followed by the nematodes (Cosmocerca podicipinus, C. parva, C. rara, C. cruzi, Schrankiana schranki and Aplectana hylambatis); other groups of parasites were represented by only one species (unknown larval cestode species and Centrorhynchus sp.). All parasite helminth species showed an aggregated pattern of distribution. The most infected organs were kidneys, small intestine, large intestine and pharyngeal zone. The host body size was important in determining the parasites abundance of G. repandum. At the level of component community G. repandum was the species with highest prevalence of infection and Bursotrema aff. tetracotyloides was the dominant species. Helminth species showed four significant pairs of covariation and two significant pairs of association in the infracommunities of Leptodactylus latinasus.

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Helminth community structure of the oven frog Leptodactylus latinasus (Anura, Leptodactylidae) from Corrientes, Argentina

-A series of studies on the population and community ecology of several digenetic trematodes was begun in 1983 in a small, eutrophic impoundment in the Piedmont area of North Carolina. Initially, efforts were focused on the hemiurid trematode, Halipegus occidualis. Prevalence of this parasite in its snail intermediate host, Helisoma anceps, reached 60% in late June-early July, then dropped precipitously as the mature cohort of the snail host died-off. Over the next 13 yr, this annual pattern consistently repeated itself. Several years later, another pulmonate snail, Physa gyrina, colonized the pond. Either simultaneously, or closely thereafter, Halipegus eccentricus and two other species using P gyrina as an intermediate host also appeared. The trematode infracommunities in both hosts were examined; structuring forces were found not to be affected by competitive interactions among larval trematodes, but by a variety of factors, including snail vagility and size (age), microhabitat heterogeneity, distribution and temporal visitation patterns of definitive hosts, and the sequence of parasite recruitment mechanisms. Very few ( 7000 Helisoma anceps, whereas nearly 19% of all infections in 1181 P gyrina were multiple. Additional studies indicate that Halipegus occidualis cercariocysts do not remain infective for extended periods for second intermediate hosts (ostracods) as suggested in the older literature. Moreover, foraging ecology by odonate naiads is a significant factor in the recruitment and dispersion patterns of H. occidualis metacercariae. The long-term population ecology of H. occidualis and H. eccentricus was examined in mark-release-recapture studies of Rana clamitans, the definitive host. Halipegus occidualis prevalence and density were higher than those of H. eccentricus, although both were contagiously distributed in their frog definitive hosts. Halipegus occidualis exhibited large and rapid changes in density within frog hosts. Increases were probably because of the recruitment of contagiously distributed parasites from their third intermediate host, whereas declines in heavily infected definitive hosts were attributed to inflammation and tissue sloughing at the point of parasite attach-

Long-term changes in parasite population and community structure : a case history

-The effects of temperature on in vitro egg-hatching success, juvenile survival, development and behavior of Strongyloides robustus were examined. As culture temperatures declined (25-10 C), both egg hatching and rate of larval development decreased. Hatching occurred as low as 10 C. Eggs showed low resistance to freezing temperatures and no juveniles survived freezing. When exposed to a thermal gradient, filariform juveniles moved toward the higher temperature. The mean temperature at which worms were found was 37.0 ? 3.3 C, which approximates host temperature within a nest. These results suggest that dissimilarities in the prevalences and intensities of infection with S. robustus in the northern and southern flying squirrels, Glaucomys sabrinus and G. volans, respectively, are at least partly due to environmental differences between habitats. Overlap in the squirrels' nest use may facilitate passage of this parasite from one host to the other, but the survival of eggs and infective juveniles may be severely limited by low temperatures. While transmission of S. robustus is limited during winter, summer transmission alone could account for the maintenance of the parasitic infections in these hosts.

Ecological implications for flying squirrels (Glaucomys spp.) of effects of temperature on the in vitro development and behavior of Strongyloides robustus

The seasonal population dynamics of Halipegus occidualis and Halipegus eccentricus (Hemiuridae) in their amphibian host, the green frog (Rana clamitans), were examined for 3 yr (weekly, April through October). Frogs were caught, marked, and examined for H. occidualis and H. eccentricus, both of which occur as immatures and adults in the buccal cavity. Frogs were then released and allowed to continue natural recruitment and loss of the parasites. It was thus possible to monitor individual infrapopulations over successive time periods. One-hundred and forty-nine frogs were caught and released, with a total of 328 observations. Overall, the levels of both parasites among male and female frogs were similar, as were levels of infection among adult and juvenile hosts. There was, however, no correlation between the total number of H. occidualis and the body size (snout-vent length) of R. clamitans. In contrast, there was a significant correlation between the total number of H. eccentricus and frog size. Recruitment of both species began in May, peaked in June/July, and ended in July (H. eccentricus) or August (H. occidualis). In general, both the prevalence and relative density of H. occidualis was greater than that of H. eccentricus and may be related to space constraints in the buccal cavity of R. clamitans. Adults of both species were observed from April through October. Following increases in parasite recruitment, infrapopulation sizes declined in September 1992 and in August of 1993 and 1994. Examination of variance to mean ratios indicated that both species were overdispersed in the frogs. Large declines in the variance to mean ratios for H. occidualis after periods of greatest recruitment are most likely associated with the loss of larger infrapopulations, suggesting that there may be density-dependent regulation of infrapopulation size. By monitoring individual hosts using the mark-release-recapture protocol, dynamic changes in parasite infrapopulations were observed, e.g., there were losses of immature worms and rapid changes in infrapopulation sizes, observations that would not be made with typical host-parasite systems.

Seasonal population dynamics of Halipegus occidualis and Halipegus eccentricus (Digenea:Hemiuridae) in their amphibian host, Rana clamitans.

Individual infrapopulations of Halipegus occidualis and Halipegus eccentricus (Hemiuridae) in the buccal cavities of their amphibian host (Rana clamitans) were monitored for changes over time. Rates and patterns of parasite maturation, recruitment, and loss were estimated, and the probabilities of infection for definitive hosts in this system were calculated. Although variable, the time required for maturation of immature worms (once in the buccal cavity) was as little as 1 wk. Relatively rapid losses of worms from individual infrapopulations were observed, although this did not appear to inhibit additional recruitment of these trematodes. Also, it was clear that these species overwintered within their amphibian hosts. There was no statistical difference among the 3 sampling years in the probability of acquiring an initial or a subsequent infection with either parasite. However, we suggest that frogs with an existing infrapopulation may have a greater probability of adding worms if those hosts are within certain foci of infection within the pond. The probability of frogs becoming infected in this system increased in late spring and peaked in midsummer. By examining the temporal changes within individual infrapopulations, it is clear that dynamic changes in infrapopulation size and composition may occur, suggesting that rates of trematode transmission may be greater than previously estimated.

Eric J. Wetzel

Papers

Long-term changes in parasite population and community structure : a case history

Ecological implications for flying squirrels (Glaucomys spp.) of effects of temperature on the in vitro development and behavior of Strongyloides robustus

Seasonal population dynamics of Halipegus occidualis and Halipegus eccentricus (Digenea:Hemiuridae) in their amphibian host, Rana clamitans.

Infrapopulation dynamics of Halipegus occidualis and Halipegus eccentricus (Digenea: Hemiuridae): temporal changes within individual hosts.