Aggregation patterns of helminth populations in the introduced fish, Liza haematocheilus (Teleostei: Mugilidae): disentangling host–parasite relationships

A number of hypotheses exist to explain aggregated distributions, but they have seldom been used to investigate differences in parasite spatial distribution between native and introduced hosts. We applied two aggregation models, the negative binomial distribution and Taylor’s power law, to study the aggregation patterns of helminth populations from Liza haematocheilus across its native (Sea of Japan) and introduced (Sea of Azov) distribution ranges. In accordance with the enemy release hypothesis, we predicted that parasite populations in the introduced host range would be less aggregated than in the native host area, because aggregation is tightly constrained by abundance. Contrary to our expectation, aggregation of parasite populations was higher in the introduced host range. However, the analyses suggested that the effect of host introduction on parasite aggregation depends on whether parasite species, or higher level taxonomic groups, were acquired in or carried into the new area. The revealed similarity in the aggregation parameters of co-introduced monogeneans can be attributed to the repeatability and identity of the host–parasite systems. In contrast, the degree of aggregation differed markedly between regions for higher level taxa, which are represented by the native parasites in the Sea of Japan versus the acquired species in the Sea of Azov. We propose that the host species plays a crucial role in regulating infra-population sizes of acquired parasites due to the high rate of host-induced mortality. A large part of the introduced host population may remain uninfected due to their resistance to native naïve parasites. The core concept of our study is that the comparative analysis of aggregation patterns of parasites in communities and populations, and macroecological relationships, can provide a useful tool to reveal cryptic relationships in host–parasite systems of invasive hosts and their parasites.     

Realized vs apparent reduction in enemies of the European starling

Release from parasites, pathogens or predators (i.e. enemies) is a widely cited ‘rule of thumb’ to explain the proliferation of nonindigenous species in their introduced regions (i.e. the ‘enemy release hypothesis’, or ERH). Indeed, profound effects of some parasites and predators on host populations are well documented. However, some support for the ERH comes from studies that find a reduction in the species richness of enemies in the introduced range, relative to the native range, of particular hosts. For example, data on helminth parasites of the European starling in both its native Eurasia and in North America support a reduction of parasites in the latter. However, North American ‘founder’ starlings were likely not chosen randomly from across Eurasia. This could result in an overestimation of enemy release since enemies affect their hosts on a population level. We control for the effects of subsampling colonists and find, contrary to previous reports, no evidence that introduced populations of starlings experienced a reduction in the species richness of helminth parasites after colonization of North America. These results highlight the importance of choosing appropriate contrast groups in biogeographical analyses of biological invasions to minimize the confounding effects of ‘propagule biases’.     

Has the introduction of brown trout altered disease patterns in native New Zealand fish?

1. It is well recognised that non-indigenous species (NIS) can affect native communities via the 'spillover' of introduced parasites. However, two other potentially important processes, the 'spillback' of native parasites from a competent NIS host, where the latter acts as a reservoir leading to amplified infection in native hosts, and the 'dilution' of parasitism by a NIS host acting as a sink for native parasites, have either not been tested or largely overlooked.
2. We surveyed the helminth parasite fauna of native New Zealand fish in Otago streams that varied in the abundance of introduced brown trout Salmo trutta , to look for evidence of spillback and/or dilution. Spillover is not an issue in this system, with trout introduced as parasite-free eggs.
3. Seven native parasite species were present across 12 sites; significant inverse relationships with an index of trout abundance (i.e. dilution) were documented for three species infecting the native upland bully Gobiomorphus breviceps , and one species infecting the native roundhead galaxias Galaxias anomalus .
4. An inverse relationship between bully energy status and infection intensity of one parasite species suggests that parasite dilution could have positive effects on bully populations. Our failure to detect similar relationships for the other parasites does not preclude the possibility that dilution is beneficial to native fish, since parasites may have subtle or unmeasured impacts.
5. The parasite dilution patterns reported are compelling in that they occurred across several native host and parasite species; as such they have important implications for invasion ecology, providing an interesting contrast to the largely negative impacts reported for NIS. Mechanisms potentially responsible for the patterns observed are discussed.     

Parasites in the northern Wadden Sea: a conservative ecosystem component over 4 decades

We investigated potential changes in the metazoan endoparasite fauna in the northern Wadden Sea during the past 4 decades by compiling published studies, reports and original data. During the time considered, the parasite fauna has remained basically the same. Only a few changes in parasite species presence occurred that resulted from changes in host distribution and abundance. The introduction of potential host species had little effect on the parasite community because no alien parasites were concomitantly introduced and the native parasites show low prevalence and intensity in these novel hosts. Eutrophication and effects of phased-out hunting may not have had clear bottom–up or top–down effects on the parasite community because of various confounding factors. Parasites depending on several host species may only be subject to strong population changes if all hosts are affected in a unidirectional way. This, however, is rather unlikely to happen in a coastal ecosystem subject to multiple pressures. Hence, parasites appear to be a relatively conservative component of the northern Wadden Sea.     

Parasites of the shore crab Carcinus maenas (L.): implications for reproductive potential and invasion success

The European shore crab, Carcinus maenas, is one of the most successful marine invasive species. Its success has been in part attributed to the loss of parasites, rekindling an interest in host-parasite interactions and impacts on host fitness in this crab. In the present study, we investigated C. maenas populations from Europe, South Africa and Australia for parasites, and assessed their impact on the fitness of male crabs. For the shore crab, testes weight along with success in mating competition is traded off against other life-history traits. We therefore used this parameter as an indicator both for reproductive fitness and a possible resource trade-off in response to parasite infestation. In the native range, crabs infested with Sacculina carcini showed significantly lower testes weight than uninfected crabs. However, helminth parasites did not generally cause reduced testes weights. Crab populations from South Africa and Australia were either parasitized at very low prevalences, or were completely parasite free. However, no population level effect of this parasite release was reflected in testes weight. These findings do not support a severe fitness impact of helminth parasites on C. maenas, which questions the role of parasites on its population dynamics, both in the native area and for invasive success.     

Parasites of Perccottus glenii Dybowski, 1877 (Actinopterygii: Odontobutidae) in the native and the introduced host range: Abundance–occupancy and abundance–variance relationships

The Chinese sleeper Perccottus glenii Dybowski, 1877 is an invasive fish species rapidly expanding in Siberia and Europe. Its native range encompasses the Far East region of Russia, northeastern China and northern North Korea. We studied species composition, prevalence, mean abundance and variance of mean abundance of macroparasites of the Chinese sleeper in the native and the introduced range. The species composition of the parasite component communities differed considerably in the native and the introduced range. The frequency distributions of prevalence, mean abundance and variance of mean abundance of the parasites did not demonstrate any significant differentiation between the two parts of the host range. However, an analysis of the abundance–occupancy and the abundance–variance relationships revealed that the parasite component communities in the two parts of the host range were quite distinct. In the native range, prevalence increased faster and variance increased more slowly with the increasing abundance of the parasites than in the introduced range. These features are mostly associated with considerably increased prevalence, abundance and aggregation of the host-specific cestode Nippotaenia mogurndae in recipient water bodies as compared with the native habitats.     

Higher parasite richness, abundance and impact in native versus introduced cichlid fishes

Empirical studies suggest that most exotic species have fewer parasite species in their introduced range relative to their native range. However, it is less clear how, ecologically, the loss of parasite species translates into a measurable advantage for invaders relative to native species in the new community. We compared parasitism at three levels (species richness, abundance and impact) for a pair of native and introduced cichlid fishes which compete for resources in the Panama Canal watershed. The introduced Nile tilapia, Oreochromis niloticus, was infected by a single parasite species from its native range, but shared eight native parasite species with the native Vieja maculicauda. Despite acquiring new parasites in its introduced range, O. niloticus had both lower parasite species richness and lower parasite abundance compared with its native competitor. There was also a significant negative association between parasite load (abundance per individual fish) and host condition for the native fish, but no such association for the invader. The effects of parasites on the native fish varied across sites and types of parasites, suggesting that release from parasites may benefit the invader, but that the magnitude of release may depend upon interactions between the host, parasites and the environment.     

Helminth communities of the lesser sandeel Ammodytes tobianus L. off the west coast of Ireland

The helminth parasites present in 412 lesser sandeels (Ammodytes tobianus) taken from June 1996 to May 1997 from the Aran Islands on the west coast of Ireland were examined. Ten helminth parasite species were recorded, and more than 92% of the sandeels were infected with at least 1 helminth species. Seven of the species were digeneans, including Brachyphallus crenatus, Hemiurus communis, Derogenes varicus, Lecithaster gibbosus, Opechona bacillaris, Cryptocotyle lingua, and Galactosomum lacteum; 2 nematodes, including Hysterothylacium sp. and Contracaecum sp.; and 1 cestode, Scolex pleuronectis. Three of the 7 digenean species were either larvae or immature. Only 2 species, the digeneans G. lacteum and H. communis, had prevalences greater than 50%. The dominant species was G. lacteum, accounting for 67% of all parasites present. The relationship between spawned groups, host length, and season versus the abundance, prevalence, species richness, and the total number of parasites in the infracommunities was investigated. No difference was found between the parasite communities of the 2 spawning races of the host population. Mean abundance and prevalence of the different parasite species showed seasonal variation. Numbers of parasite species and numbers of parasites increased with fish length. The role of A. tobianus as an intermediate host for helminths was assessed; it was determined that most were infectious to birds or mammals, with the majority of the parasite species being autogenic (infectious to fish). The mean number of parasites per fish was nearly a quarter of the value recorded for A. tobianus in the North Sea, where a much higher intensity of infection was recorded.     

Is invasion success explained by the enemy release hypothesis?

A recent trend in invasion ecology relates the success of non‐indigenous species (NIS) to reduced control by enemies such as pathogens, parasites and predators (i.e. the enemy release hypothesis, ERH). Despite the demonstrated importance of enemies to host population dynamics, studies of the ERH are split – biogeographical analyses primarily show a reduction in the diversity of enemies in the introduced range compared with the native range, while community studies imply that NIS are no less affected by enemies than native species in the invaded community. A broad review of the invasion literature implies at least eight non‐exclusive explanations for this enigma. In addition, we argue that the ERH has often been accepted uncritically wherever (i) NIS often appear larger, more fecund, or somehow ‘better’ than either congeners in the introduced region, or conspecifics in the native range; and (ii) known enemies are conspicuously absent from the introduced range. However, all NIS, regardless of their abundance or impact, will lose natural enemies at a biogeographical scale. Given the complexity of processes that underlie biological invasions, we argue against a simple relationship between enemy ‘release’ and the vigour, abundance or impact of NIS.     

Living with the enemy: parasites and pathogens of the ladybird <Emphasis Type="Italic">Harmonia axyridis</Emphasis>

Harmonia axyridis is an invasive alien predator in many countries across the world. The rapid establishment and spread of this species is of concern because of the threat it poses to biodiversity as a generalist predator. Understanding the mechanisms that contribute to the success of this species as an invader is not only intriguing but also critical to our understanding of the processes governing such invasions. The enemy release hypothesis (ERH) could explain the rapid population growth of many invasive alien species. However, empirical evidence in support of the ERH is lacking. An alternative hypothesis that could explain rapid population growth is evolution of increased competitive ability (EICA). Here we provide an overview of the parasites and pathogens of coccinellids with a particular focus on H. axyridis as a host. We examine the differential susceptibility of host species and highlight the resilience of H. axyridis in comparison to other coccinellids. We recognise the paucity and limitations of available information and suggest that studies, within a life-table framework, comparing life history traits of H. axyridis in both the native and introduced ranges are necessary. We predict that H. axyridis could benefit from both enemy release and EICA within the introduced range but require further empirical evidence.     

BIODIVERSITY RESEARCH: Parasites of exotic species in invaded areas: does lower diversity mean lower epizootic impact?

Aim Exotic species may serve as vectors for the introduction of parasites from their native range and may also become infected by parasites already present in invaded areas, but the total number of parasites infecting such exotic species in their invaded areas is typically less than that in their native range. We tested whether the diversity of parasites associated with exotic species in the native and invaded areas is related to the epizootic impact these parasites cause. Location Global. Methods We examined the diversity and epizootic impact of 384 parasite taxa associated with 22 exotic freshwater invertebrate species. The epizootic impact of each parasite was rated based on whether it had been documented to cause a major pathological impact on a large proportion of an infected host population (other than the invader under consideration). Results The total number of parasites associated with an exotic host in its native range was about twice that of all parasites associated with it in its entire invaded range. This was mainly because of the loss in the invaded areas of low impact parasites, whereas the average number of high impact parasites per host in these areas did not differ statistically from that in the native range. Main conclusions Our study suggests similar levels of adverse impact of parasites of exotic species in both their native and invaded areas. In addition to the introduction of highly pathogenic exotic parasites, other mechanisms that may be involved include (1) acquisition by the invaders of new high impact parasites in the invaded ranges, (2) high abundance of the invaders in their new ranges and (3) susceptibility of novel hosts to exotic parasites because of the ‘naive host syndrome’.     

High parasite infection level in non‐native invasive species: it is just a matter of time

The enemy release hypothesis is often used to explain the success of non‐native species invasions. Growing evidence indicates that parasite or pathogen species richness increases over time in invasive non‐native species; however, this increase should not directly translate into release from enemy pressure as infection intensity of parasites (number of parasites per host) has a more profound impact on host fitness. The changes in intensity of parasitic infections in invasive non‐native species have not yet been thoroughly analysed in newly colonized areas. The goal of this study was to determine whether gastrointestinal parasite (nematode and trematode) infection intensity has increased with time since the populations of American mink Neovison vison were established and how host demographic parameters affect infection intensity. We tested the enemy release hypothesis by substituting space for time, evaluating parasite abundance in American mink at six sites along a chronosequence of mink invasion history. Nematode and trematode abundance increased with time since mink introduction, from a few parasites on average per mink after 16 yr, to 200–250 parasites per mink after 34 yr. The rate of increase in parasite abundance varied among demographic groups of mink (sex and age). Both nematodes and trematodes were more abundant in males than in females, and in subadults than in adults. Higher nematode abundance negatively affected body condition of mink. Our results provide evidence that non‐native species are released from enemy pressure only in the first phase of invasion, and that infection is modulated by host demographics and season. These results contribute to the evaluation of the long‐term patterns of parasite accumulation in invasive non‐native species after their colonization of new territories.     

Spatial variation in population density across the geographical range in helminth parasites of yellow perch Perca flavescens

The abundance of a species is not constant across its geographical range; it has often been assumed to decrease from the centre of a species’ range toward its margins. The central assumption of this “favourable centre” model is tested for the first time with parasites, using different species of helminth parasites exploiting fish as definitive hosts. Data on prevalence (percentage of hosts that are infected) and abundance (mean no. parasites per host) were compiled for 8 helminth species occurring in 23 populations of yellow perch Perca flavescens, from continental North America. For each parasite species, correlations were computed between latitude and both local prevalence and abundance values. In addition, the relationships between the relative prevalence or abundance in one locality and the distance between that locality and the one where the maximum value was reported, were assessed separately for each species to determine whether abundance tends to decrease away from the presumed centre of the range, where it peaks. For both the cestode Proteocephalus pearsei and the acanthocephalan Leptorhynchoides thecatus, there was a positive relationship between prevalence or abundance and the latitude of the sampled population. There was also a significant negative relationship between relative prevalence and the distance from the locality showing the maximum value in P. pearsei, but no such pattern was observed for the other 7 parasite species. Since this single significant decrease in prevalence with increasing distance from the peak value may be confounded by a latitudinal gradient, it appears that the distribution of abundance in parasites of perch does not follow the favourable centre model. This means that the environmental variables affecting the density of parasites (host availability, abiotic conditions) do not show pronounced spatial autocorrelation, with nearby sites not necessarily providing more similar conditions for the growth of parasite populations than distant sites.     

Regional Variation in Parasite Species Richness and Abundance in the Introduced Range of the Invasive Lionfish,Pterois volitans

Parasites can play an important role in biological invasions. While introduced species often lose parasites from their native range, they can also accumulate novel parasites in their new range. The accumulation of parasites by introduced species likely varies spatially, and more parasites may shift to new hosts where parasite diversity is high. Considering that parasitism and disease are generally more prevalent at lower latitudes, the accumulation of parasites by introduced hosts may be greater in tropical regions. The Indo-Pacific lionfish (Pterois volitans) has become widely distributed across the Western Atlantic. In this study, we compared parasitism across thirteen locations in four regions, spanning seventeen degrees of latitude in the lionfish''s introduced range to examine potential spatial variation in parasitism. In addition, as an initial step to explore how indirect effects of parasitism might influence interactions between lionfish and ecologically similar native hosts, we also compared parasitism in lionfish and two co-occurring native fish species, the graysby grouper, Cephalopholis cruentata, and the lizardfish, Synodus intermedius, in the southernmost region, Panama. Our results show that accumulation of native parasites on lionfish varies across broad spatial scales, and that colonization by ectoparasites was highest in Panama, relative to the other study sites. Endoparasite richness and abundance, on the other hand, were highest in Belize where lionfish were infected by twice as many endoparasite species as lionfish in other regions. The prevalence of all but two parasite species infecting lionfish was below 25%, and we did not detect an association between parasite abundance and host condition, suggesting a limited direct effect of parasites on lionfish, even where parasitism was highest. Further, parasite species richness and abundance were significantly higher in both native fishes compared to lionfish, and parasite abundance was negatively associated with the condition index of the native grouper but not that of the lionfish or lizardfish. While two co-occurring native fishes were more heavily parasitized compared to lionfish in Panama any indirect benefits of differential parasitism requires further investigation. Future parasitological surveys of lionfish across the eastern coast of North America and the Lesser Antilles would further resolve geographic patterns of parasitism in invasive lionfish.     

Parasites and invasions: a biogeographic examination of parasites and hosts in native and introduced ranges

Aim To use a comparative approach to understand parasite demographic patterns in native versus introduced populations, evaluating the potential roles of host invasion history and parasite life history. Location North American east and west coasts with a focus on San Francisco Bay (SFB). Methods Species richness and prevalence of trematode parasites were examined in the native and introduced ranges of two gastropod host species, Ilyanassa obsoleta and Littorina saxatilis. We divided the native range into the putative source area for introduction and areas to the north and south; we also sampled the overlapping introduced range in SFB. We dissected 14,781 snails from 103 populations and recorded the prevalence and identity of trematode parasites. We compared trematode species richness and prevalence across the hosts’ introduced and native ranges, and evaluated the influence of host availability on observed patterns. Results Relative to the native range, both I. obsoleta and L. saxatilis have escaped (lost) parasites in SFB, and L. saxatilis demonstrated a greater reduction of trematode diversity and infection prevalence than I. obsoleta. This was not due to sampling inequalities between the hosts. Instead, rarefaction curves suggested complete capture of trematode species in native source and SFB subregions, except for L. saxatilis in SFB, where infection was extremely rare. For I. obsoleta, infection prevalence of trematodes using fish definitive hosts was significantly lower in SFB compared to the native range, unlike those using bird hosts. Host availability partly explained the presence of introduced trematodes in SFB. Main conclusions Differential losses of parasite richness and prevalence for the two gastropod host species in their introduced range is probably the result of several mechanistic factors: time since introduction, propagule pressure, vector of introduction, and host availability. Moreover, the recent occurrence of L. saxatilis’ invasion and its active introduction vector suggest that its parasite diversity and distribution will probably increase over time. Our study suggests that host invasion history and parasite life history play key roles in the extent and diversity of trematodes transferred to introduced populations. Our results also provide vital information for understanding community‐level influences of parasite introductions, as well as for disease ecology in general.     

Parasite spillover: indirect effects of invasive Burmese pythons

Identification of the origin of parasites of nonindigenous species (NIS) can be complex. NIS may introduce parasites from their native range and acquire parasites from within their invaded range. Determination of whether parasites are non‐native or native can be complicated when parasite genera occur within both the NIS’ native range and its introduced range. We explored potential for spillover and spillback of lung parasites infecting Burmese pythons (Python bivittatus) in their invasive range (Florida). We collected 498 indigenous snakes of 26 species and 805 Burmese pythons during 2004–2016 and examined them for lung parasites. We used morphology to identify three genera of pentastome parasites, Raillietiella, a cosmopolitan form, and Porocephalus and Kiricephalus, both New World forms. We sequenced these parasites at one mitochondrial and one nuclear locus and showed that each genus is represented by a single species, R. orientalis, P. crotali, and K. coarctatus. Pythons are host to R. orientalis and P. crotali, but not K. coarctatus; native snakes are host to all three species. Sequence data show that pythons introduced R. orientalis to North America, where this parasite now infects native snakes. Additionally, our data suggest that pythons are competent hosts to P. crotali, a widespread parasite native to North and South America that was previously hypothesized to infect only viperid snakes. Our results indicate invasive Burmese pythons have affected parasite‐host dynamics of native snakes in ways that are consistent with parasite spillover and demonstrate the potential for indirect effects during invasions. Additionally, we show that pythons have acquired a parasite native to their introduced range, which is the initial condition necessary for parasite spillback.     

Can the enemy release hypothesis explain the success of invasive alien predators and parasitoids?

Biological invasions are ecologically and economically costly. Understanding the major mechanisms that contribute to an alien species becoming invasive is seen as essential for limiting the effects of invasive alien species. However, there are a number of fundamental questions that need addressing such as why some communities are more vulnerable to invasion than others and, indeed, why some alien species become widespread and abundant. The enemy release hypothesis (ERH) is widely evoked to explain the establishment and proliferation of an alien species. ERH predicts that an alien species introduced to a new region should experience a decrease in regulation by natural enemies which will lead to an increase in the distribution and abundance of the alien species. At the centre of this theory is the assumption that natural enemies are important regulators of populations. Additionally, the theory implies that such natural enemies have a stronger regulatory effect on native species than they do on alien species in the introduced range, and this disparity in enemy regulation results in increased population growth of the alien species. However, empirical evidence for the role of the ERH in invasion success is lacking, particularly for invertebrates. Many studies equate a reduction in the number of natural enemies associated with an alien species to release without studying population effects. Further insight is required in relation to the effects of specific natural enemies on alien and native species (particularly their ability to regulate populations). We review the role of ecological models in exploring ERH. We suggest that recent developments in molecular technologies offer considerable promise for investigating ERH in a community context.     

Potential spread of introduced black rat (Rattus rattus) parasites to endemic deer mice (Peromyscus maniculatus) on the California Channel Islands

Introduced species have the potential to outperform natives in two primary ways: via increased rates of predation and competition, and via the introduction of new parasites against which native species often lack effective immune defences. To assess the extent to which invasive species' parasites spread to native hosts, we compared the composition of helminth parasites found in introduced black rat ( Rattus rattus ) and endemic deer mouse ( Peromyscus maniculatus ) populations on a subset of the California Channel Islands. Results suggest that the whipworm, Trichuris muris , may have spread from introduced black rats to endemic island deer mice and has continued to thrive in one island population where rats were recently eradicated. These results yield two important conservation messages: (1) although the parasites introduced with invasive species may be few, they should not be ignored as they can spread to native species, and (2) introduced parasites have the potential to remain in a system even after their founding host is extirpated. These findings underscore the importance of parasitological surveys in invasive species research and baseline data for ecosystems where exotic species are likely to invade.     

Community structure of helminth parasites of the tuna, Euthynnus affinis, from the Visakhapatnam coast, Bay of Bengal

An analysis is made of the community structure of the helminth parasites of the tuna Euthynnus affinis collected off the coast of Visakhapatnam, Bay of Bengal. The helminth fauna comprised 23 species, consisting of two monogeneans, 18 digeneans, one larval cestode, one nematode and one acanthocephalan. Didymozoids, represented by 14 species, were the dominant members of the parasite spectrum. The parasite assemblage is characterized by species richness as well as a high diversity. Most of the parasites found, the didymozoids in particular, were host specialists, exhibiting a high degree of host specificity. The fauna comprised three core species, four satellite species and many secondary species, with no potential for interaction among them but the degree of predictability is high in that each infracommunity comprised two or three core species superimposed by a few secondary species. Many host factors such as the varied diet, high vagility, long life span and endothermy appear to have contributed to the development of the species rich and diverse parasite communities in the tuna. There was evidence for a decrease in parasite density and an increase in diversity with increase in host size, indicating that host size has a profound influence on the component community structure.     

Variation in infection parameters among populations within parasite species: intrinsic properties versus local factors

Within any parasite species, variation among populations in standard infection parameters (prevalence, intensity and abundance) is an accepted fact. The proportion of hosts infected and the mean number of parasites per host are not fixed values across the entire geographic range of any parasite species. The question is whether this inter-population variation occurs within a narrow, species-specific range and is thus driven mainly by the biological features of the parasite, or whether it is substantial and unpredictable, leaving population parameters at the mercy of local conditions. Here, the repeatability of estimates of prevalence, intensity and abundance of infection was assessed across populations of the same parasite species, for 77 metazoan parasite species of Canadian freshwater fishes. Overall, parameter values from different populations of the same parasite species were more similar to each other and more different from those of other species, than expected by chance alone. Much of the variation in parameter values in the dataset was associated with differences between parasite species, rather than with differences among populations within species. This was particularly true for intensity and abundance of infection; in contrast, prevalence values, while somewhat repeatable among populations of the same species, still showed considerable variation. Among the higher taxa investigated (monogeneans, trematodes, cestodes, nematodes, acanthocephalans, copepods), there was no evidence that species of one taxon display intrinsically greater variation in population parameters than species of other taxa. Overall, the results suggest that intensity and abundance of infection are real species characters, though somewhat variable. This conclusion supports the view that the biological features of parasite species can potentially override local environmental conditions in driving parasite population dynamics.