Nestedness in colonization-dominated systems: helminth infracommunities of Rana vaillanti Brocchi (Anura: Ranidae) in Los Tuxtlas, Veracruz, Mexico

Colonization probabilities of parasite species often are determined by the habitat preference and vagility of host individuals. Although extinction-based interpretations have been investigated for nested subset patterns of parasite infracommunities, the low relative frequency of nestedness in colonization-dominated systems makes the determination and interpretation of nested infracommunities of broad ecological importance. In these systems, ontogenetic shifts in habitat preference or diet of the host have the potential to produce nested subset patterns of parasite infracommunities. Helminth infracommunity structure was investigated for 76 Rana vaillanti individuals collected from Laguna Escondida, Los Tuxtlas, Veracruz, Mexico, in 1998. Pooled helminth infracommunities were significantly nested, as were penetrating and ingested helminth infracommunities when considered separately. Richness, diversity, and evenness of the helminth infracommunities were not correlated with host size, and did not differ between host sexes, suggesting that the structure of infracommunities simply is a product of the interaction between host individuals and their landscape mediated by individual differences in vagility. It is hypothesized that individual differences in recruitment can produce nested subset infracommunity patterns when the habitats or habitat preferences of hosts are themselves nested.     

Parasite community structure within and across host populations of a marine pelagic fish: how repeatable is it?

The geographical variation in parasite community structure among populations of the same host species remains one of the least understood aspects of parasite community ecology. Why are parasite communities clearly structured in some host populations, and randomly assembled in others? Here, we address this fundamental question using data on the metazoan parasite communities of different host size-classes of four distinct populations of a small pelagic fish, the Argentine anchovy, Engraulis anchoita, from the South West Atlantic. Within each fish sample, fish length was correlated with both the total intensity of parasites and species richness among infracommunities. More importantly, average fish length correlated with mean infracommunity richness and mean total intensity across the fish samples, indicating that the characteristics of parasite assemblages in a fish population are strongly influenced by the size of its fish in relation to those in other populations. Nested subset patterns were observed in about half of the fish samples. This means that the presence or absence of parasite species among fish individuals is often not random; however, no repeatability of nestedness among component communities was observed. Average fish length did not influence directly the likelihood that a parasite assemblage was significantly nested. However, variables influenced by average fish length, namely mean infracommunity richness and mean total intensity, determine the probability that a nested hierarchy will be observed; host size may thus indirectly affect parasite community structure either itself or via its influence on host movement and feeding patterns. To some extent, this apparent link may be due to the sensitivity of nestedness analyses to the proportion of presence in a presence/absence matrix; this in itself is a biological feature of the parasite community, however, which is associated with mean host length.     

The ecological characteristics of idiosyncratic and nested diatoms

Nestedness represents the degree to which species assemblages are proper subsets of larger assemblages. Highly nested assemblages are characterized by a high number of species conforming to nested subset pattern and a low number of species which depart from nested pattern, that is, idiosyncratic species. The main aims of this paper were to (i) examine whether local stream habitat factors and spatial variables govern the number of nested stream diatom species at sites, (ii) to examine whether nested and idiosyncratic species differ in ecological characteristics, and (iii) to study if these distinctions have implications for turnover in stream diatom communities. Stream diatom communities showed a highly significant nested pattern, with observed matrix temperature of 28.4 degrees C. However, number of idiosyncratic species was high, constituting 44% of total species number of the survey. The number of nested species at sites was not related to any of the measured habitat factors or geographical location of the sampling sites. Idiosyncratic species were significantly (ANOVA: P=0.002) more widely distributed than nested species. Partial mantel tests showed that idiosyncratic species exhibited faster turnover along geographical and environmental distance than nested species. These data showed that although the degree of nestedness was highly significant, stream diatom communities were nevertheless characterized by a number of idiosyncratic species departing from the nested subset pattern. It seems that the compositional turnover is faster in communities that are dominated by idiosyncratic species suggesting that turnover diversity in diatoms may be governed more by the distribution of idiosyncratic species.     

Food of anadromous brown trout at sea

The food of migratory Salmo trutta in coastal marine waters along the Norwegian Skagerrak coast varied significantly with age, season and habitat of the fish. The main prey categories in terms of frequency of occurrence were fishes followed by crustaceans, surface insects and polychaetes. Seasonal variation in diet and within habitats was found, supporting the view that brown trout is an opportunistic feeder. An ontogenetic niche shift was observed with post-smolts feeding on inshore and shallow water prey communities, while larger brown trout fed mainly on pelagic fishes.     

Adult Male Density Influences Juvenile Microhabitat Use in a Territorial Lizard

Habitat choice often has strong effects on performance and fitness. For many animals, optimal habitats differ across age or size classes, and individuals shift habitat use through ontogeny. Although many studies document ontogenetic habitat shifts for various taxa, most are observational and do not identify the causal factor of size‐specific habitat variation. Field observations of the brown anole lizard (Anolis sagrei) show that juveniles perch on shorter and thinner vegetation than adults. We hypothesized that this variation is due to adult males forcing smaller juveniles to less preferred habitat. To test this assertion, we manipulated adult male densities in mesh enclosures with artificial trees to examine the response of juvenile microhabitat choice. We found that adult male density had strong effects on juvenile perch height, perch width, and substrate use, suggesting that age‐class competition contributes to the observed ontogenetic differences in habitat choice. We also found that time of day significantly affected juvenile perch height and substrate use. In many cases, our results suggest that juveniles distance themselves from adults using different microhabitats from those used in our control ‘no‐adult’ treatment. However, these findings were often body size dependent and varied depending upon time of day. This study highlights the complexity of juvenile perching behavior and demonstrates the role of intraspecific interactions in shaping habitat use by juvenile animals.     

Structure and similarity of helminth communities of six species of Australian turtles

Patterns of infracommunity structure and infra- and component community similarity were examined for helminths of 6 species of turtles, each collected from a single locality in Australia in 1993 and 1994. Elseya latisternum (N = 11) and Emydura kreffti (N = 16) were collected from northern Queensland, Emydura macquarii macquarii (N = 11) from southern Queensland, Emydura macquarii dhara (N = 11) and Chelodina longicollis (N = 11) from northern New South Wales, and Chelodina oblonga (N = 5) from Western Australia. Local parasite species richness was not correlated with host geographical range. Differences in parasite diversity among host species were related primarily to differences in evenness, a pattern attributed to local habitat characteristics, rather than species-specific differences in colonization potential. Ordination and analysis of similarity demonstrated the patterns of infracommunity structure of Chelodina spp. to be distinct from those of the other host species sampled, which showed considerable overlap among patterns of infracommunity structure. Despite overlap with the component communities of Em. kreffti and El. latisternum, the component communities of Em. m. dhara and Em. m. macquarii were more distinct from one another than either was to the component communities of Em. kreffti or El. latisternum.     

Richness,nestedness, and randomness in parasite infracommunity structure

Within a host population, parasite infracommunities vary in both richness and species composition. If interspecific interactions among parasites are important in shaping infracommunities, the structure of these assemblages is expected to differ from the one predicted by null models, i.e. from the one that would result from chance alone. Using data from the literature, I tested for discrepancies between observed and random patterns in the richness and composition of gastrointestinal helminth infracommunities of birds and mammals. Both the Poisson distribution and a more sophisticated null model, derived from prevalence of the different parasite species in the host population, usually provided a good fit to the observed distributions of infracommunity richness among hosts. This suggests that parasite species do not co-occur more or less frequently than expected by chance. In mammals, the co-occurrence of all available parasite species in the same host individual, or maximum potential infracommunity richness, was less likely to be observed when several parasite species were available; this is also a phenomenon expected from the random assembly of parasite species. Finally, there was no evidence for a nested subset pattern among parasite species in a host population: rate species were distributed independently of common ones. The overall picture emerging from these results is one in which parasite assemblages are more likely to be the product of random events than of predictable and repeatable processes.     

Spatial and seasonal patterns in the feeding habits of juvenile Lates niloticus (L.), in the Mwanza Gulf of Lake Victoria

Flexibility in the feeding habits of juvenile Nile perch (1–30 cm total length) was studied from September 1988 to September 1989 at four sites (depth range: 1–25 m) in the Mwanza Gulf of Lake Victoria. During this period haplochromine cichlids were virtually absent in the area. We looked at the combined effects of predator size, season and habitat. Stomach content analysis showed that with increase in size, the diet of Nile perch shifted from zooplankton and midge larvae, to macro-invertebrates (shrimps and dragonfly nymphs) and fish. At a size of 3–4 cm Nile perch shifted from size-selective predation on the largest cyclopoids to predation on the largest, less abundant, calanoids. Zooplanktivory ended at a size of ca. 5 cm. Although an ontogenetic shift in the diet of juvenile Nile perch was obvious at all sampling stations, the contribution of prey types appeared to be habitat related. With increasing water depth the frequency of occurrence in the diet of most prey types decreased, but that of shrimps increased. At the entrance of the gulf (20–25 m deep) shrimps were the main food source throughout the year. Halfway the gulf (12–16 m), Nile perch showed seasonality in their feeding behaviour. Shrimps were taken there especially during the rainy season (January to May) when their densities at this station were high, whereas cannibalism prevailed during the rest of the year. In an environment with Nile perch and dagaa as alternative prey, shrimps were taken almost exclusively. They could be regarded as a key prey for Nile perch between 5 and 30 cm.     

Helminth community structure in an expanding white-winged dove (Zenaida asiatica asiatica) population

Helminth communities of 171 fledged white-winged doves (Zenaida asiatica asiatica) from the expanding eastern population in Texas (USA) were examined from hosts collected 11 June to 19 September 1997 within their historical range, new breeding periphery, and an intermediate area. Eleven helminth species, representing 435 individuals, were found. Helminths occurred in three microhabitats, of which the small intestine was the most commonly occupied. Nematodes dominated numerically (76% of total worms), followed by cestodes (17%), and trematodes (7%). Infracommunities were species-poor; the most complex infracommunity consisted of three helminth species, which occurred in three host individuals, followed by two species that occurred in 13 hosts. The remaining 155 doves had one (70) or no (85) species. The overall helminth component community was species-poor and was dominated by Ascaridia columbae which occurred in 26% of the white-winged doves and accounted for 65% of all helminth individuals. Prevalence and abundance of A. columbae varied by geographic region and host age, but not by host sex. Helminth component communities varied by geographic region, host age, and host sex. These differences were primarily attributable to unique occurrences of uncommon species within specific host subpopulations. Results suggest that the white-winged doves' multimodal regional abundance pattern, sympatry with other columbids, and granivorous diet may be more important in shaping helminth community structure than the influences often associated with geographic range expansion.     

The predictability of helminth community structure in space: a comparison of fish populations from adjacent lakes

Patterns in helminth community structure can suggest that various processes are acting to shape parasite communities into organised, non-random assemblages of species. It is not clear, however, whether a pattern observed in one host population at one time would be observed again at another time, or at the same time in a different but comparable host population. Here, we test the repeatability of parasite community structure in space, and to a lesser extent time, with data on helminth parasites of two fish species, perch Perca fluviatilis and roach Rutilus rutilus, collected in different seasons from four adjacent lakes in Central Finland. Since populations of the same fish species harbour the same parasite species and were sampled in the same way, we would expect similar patterns in the structure of their helminth parasites if the same structuring processes are acting in all lakes. We found that no pairwise association between the most common helminth species were observed consistently between seasons within lakes, or among lakes during the same season. Similarly, nested subset patterns of species assembly were observed in some samples, but not consistently between seasons or among lakes. The lack of repeatability in space and between seasons shown by these analyses indicates that although helminth community structure often departs from randomness, it does not do so in a consistent and predictable manner. There may be some general, large-scale processes acting to structure helminth communities, but local or seasonal influences can often either mask their action, or play more important roles themselves.     

Examining the area effect for parasite communities of bluegill x green sunfish hybrids in five constructed ponds in Kansas

The parasite communities of bluegill x green sunfish hybrids were examined from 5 constructed ponds in Kansas in an attempt to evaluate the separate effects of habitat area and habitat heterogeneity on parasite community structure. Characterization of fish community structure and collection of hybrid fishes was conducted using an electrofishing boat. Benthic invertebrates were sampled, and substrate types examined at 30 evenly spaced points in each pond. Nonmetric multidimensional scaling of the parasite infracommunities, in concert with an analysis of similarities, indicated significant clustering of infracommunities by locality. The number and diversity of habitat types, and the richness and diversity of both fishes and benthic invertebrates, were positively correlated with the first axis of the infracommunity ordination. Pond surface area, parasite richness, and stocking pressure were negatively correlated with the first axis of the infracommunity ordination, suggesting that pond area, stocking pressure, or both was a stronger determinant of parasite community structure in these systems than habitat and host heterogeneity.     

Dine or Dash?: Ontogenetic Shift in the Response of Yellow Perch to Conspecific Alarm Cues

During their first year of growth yellow perch, Perca flavescens, undergo an ontogenetic niche shift from invertebrate feeding to piscivory. They also undergo a similar shift in their response to heterospecific alarm cues, switching from anti-predator to foraging behaviour. We conducted laboratory trials to determine whether yellow perch experience a comparable ontogenetic shift in their response to conspecific alarm cues. When exposed to either young-of-year (YOY) or adult perch skin extract, YOY perch responded with decreased time in motion and number of feeding attempts as well as increased time spent with spines erect and latency to first feeding attempt, all of which are indicative of an anti-predator response. Adult perch, when exposed to the same cues, responded with increased time spent moving and number of feeding attempts as well as decreased time spent with spines erect and latency to first feeding attempt, indicative of a foraging response. These data suggest that yellow perch undergo an ontogenetic niche shift in response to conspecific alarm cues.     

Ecological stoichiometry of Eurasian perch – intraspecific variation due to size,habitat and diet

The turnover and distribution of energy and nutrients in food webs is influenced by consumer stoichiometry. Although the stoichiometry of heterotrophs is generally considered to vary only little, there may be intraspecific variation due to factors such as habitat, resources, ontogeny and size. We examined intraspecific variation in Eurasian perch Perca fluviatilis stoichiometry, a common species that exhibits habitat and resource specialization, ontogenetic niche shifts and a large size range. This study investigated the elemental stoichiometry of a wide size range of perch from littoral and pelagic habitats. The mean C:N:P stoichiometry of whole perch was 37:9:1 (molar ratios). However, %C, %P, C:N, C:P and N:P varied with size, morphology, habitat and diet category. These factors together explained 24–40% of the variation in C:N:P stoichiometry. In contrast, perch stoichiometry was not related to diet stoichiometry, suggesting that the former is homeostatically regulated. The results suggest that the high P content of perch may result in stoichiometric constraints on the growth of non‐piscivorous perch, and that piscivory is an efficient strategy for acquiring P. Resource polymorphism, individual diet specialization and intraspecific size variation are widespread among animals. Thus changes in stoichiometry with size, habitat, morphology and resource use, and therefore also stoichiometric demands, are probably common.     

Rapid and unexpected effects of piscivore introduction on trophic position and diet of perch (Perca flavescens) in lakes recovering from acidification and metal contamination

1. Yellow perch (Perca flavescens) are often the only surviving fish species in acidified lakes. We studied four lakes along a gradient of recovery from acidification and that had different food web complexities. All had abundant yellow perch, two had low piscivore abundance, one had a well‐established piscivore population and one was manipulated by introducing piscivorous smallmouth bass (Micropterus dolomieu). We hypothesised that there would be strong effects on perch abundance, behaviour and diet induced by the presence of piscivores. 2. In the manipulated lake, the bass reduced yellow perch abundance by 75% over a 2‐year period. Concomitantly, perch use of the pelagic habitat fell from 48 to 40%. 3. In contrast to findings from less disturbed systems, yellow perch in the littoral zone of the manipulated lake did not strongly shift from zooplankton to benthic food sources after the arrival of piscivores. Diet analysis using stable carbon isotopes revealed a strong continued reliance on zooplankton in all lakes, independent of the degree of piscivory. The failure to switch to benthos in the refuge area of the littoral zone is most likely related to the depauperate benthos communities in these formerly acidified lakes. 4. Yellow perch in lakes recovering from acidification face a considerable ecological challenge as the necessary switch to benthic diet is hindered by a low abundance of benthos. The arrival of piscivores in these recovering lakes imposes further restrictions on perch access to food items. We infer that future recovery of perch populations (and higher trophic levels) will have to be preceded by the re‐establishment of diverse benthic macroinvertebrate communities in these lakes.     

Nestedness and niche‐based species loss in moorland plant communities

Communities in isolated habitat patches surrounded by inhospitable matrices often form a nested subset pattern. However, the underlying causal mechanisms and conservation implications of nestedness in regional communities remain controversial. The nested ranks of species in a nested species‐by‐site matrix may reflect a gradient of species vulnerability to extinction or of colonization ability. However, nestedness analysis has rarely been used to explore determinants of species rank; consequently, little is known of underpinning mechanisms. In this study, we examined nestedness in moorland plant communities widely interspersed within the subalpine zone of northern Japan. Moorland sites differed in area (1000–160 000 m²) and were naturally isolated from one another to various extents within an inhospitable forest matrix. We also determined whether site characteristics (physical and morphometric measures) and species characteristics (niche position and breadth, based on species’ traits) are related to nestedness. Moorland plant communities in the study area were significantly nested. The pH and moorland kernel density (proxy for spatial clustering of moorlands around the focal site) were the most important predictors of moorland site nested rank in a nestedness matrix. Niche breadths of species (measured as variation in leaf mass area and height) predicted species’ nested ranks. Selective environmental tolerances imposed by environmental harshness and selective extinction caused by declines in site carrying capacities probably account for the nested subset pattern in moorland plant communities. The nested rank of species in the nestedness matrix can therefore be translated into the potential order of species loss explainable by species niche breadths (based on variation in functional traits). Complementary understanding of the determinants of site ranking and species ranking in the nestedness matrix provides powerful insight into ecological processes underlying nestedness and into the ways by which communities are assembled or disassembled by such processes.     

Host ontogeny and the temporal decay of similarity in parasite communities of marine fish

Geographical distances between host populations are key determinants of how many parasite species they share. In principle, decay in similarity should also occur with increasing distance along any other dimension that characterizes some form of separation between communities. Here, we apply the biogeographical concept of distance decay in similarity to ontogenetic changes in the metazoan parasite communities of three species of marine fish from the Atlantic coast of South America. Using differences in body length between all possible pairs of size classes as measures of ontogenetic distances, we find that, using an index of similarity (Bray-Curtis) that takes into account the abundance of each parasite species, the similarity in parasite communities showed a very clear decay pattern; using an index (Jaccard) based on presence/absence of species only, we obtained slightly weaker but nevertheless similar patterns. As we predicted, the slope of the decay relationship was significantly steeper in the fish Cynoscion guatucupa, which goes through clear ontogenetic changes in diet and therefore in exposure to parasites, than in the other species, Engraulis anchoita and Micropogonias furnieri, which maintain a roughly similar diet throughout their lives. In addition, we found that for any given ontogenetic distance, i.e. for a given length difference between two size classes, the similarity in parasite communities was almost always higher if they were adult size classes, and almost always lower if they were juvenile size classes. This, combined with comparisons among individual fish within size classes, shows that parasite communities in juvenile fish are variable and subject to stochastic effects. We propose the distance decay approach as a rigorous and quantitative method to measure rates of community change as a function of host age, and for comparisons across host species to elucidate the role of host ecology in the development of parasite assemblages.     

Dynamics of parasite communities in an age series of Baikal omul Coregonus migratorius (Georgi, 1775)

Parasite communities of Baikal omul from Chivyrkuiskii Bay of Lake Baikal have been analyzed at levels of a host individual (infracommunity), a separate age group of a host (set of infracommunities), and a host population (component community). Significant positive correlations of parameters of species richness (number of parasite species, Margalef and Menhinick indices) with the age of Baikal omul were recorded only at the level of parasite infracommunities. The absence of linear positive correlations between the parameters of species richness and the age of Baikal omul at the level of sets of parasite infracommunities were revealed for the first time for fishes of Lake Baikal. The peculiarity of the dynamics of parasite communities of Baikal omul is determined by specific features of the host physiology and ecology, primarily by the age dynamics of the feeding spectrum.     

Host length,age, diet,parasites and stable isotopes as predictors of yellow perch (<Emphasis Type="Italic">Perca flavescens</Emphasis>Mitchill), trophic status in nutrient poor Canadian Shield lakes

We used data from parasites and stable isotopes of yellow perch, Perca flavescens, to determine trophic status in four small Canadian Shield lakes as parasites allow the identification of both prey and non-prey dietary components in the host's community. Stable C isotope ratios for all perch ranged from –34 to –19 while stable N isotope ratios ranged from 4.5 to 12.5. These ranges are larger than those observed in many other fish species. Perch age was the most significant predictor of stable C isotope ratio and perch parasite fauna was the most significant predictor of stable N ratios. In particular, parasite fauna indicative of zooplanktivorous or piscivorous perch were most accurate for predicting fish trophic position and thus stable isotope ratio. Fish length and age showed fewer significant relationships with isotope ratios than parasite infracommunity or diet and suggests that trophic category for perch cannot always be predicted based on size.     

Feeding ecology and growth of age 0 year Platichthys flesus(L.) in a vegetated and a bare sand habitat in a nutrient rich fjord

A study was carried out to investigate the diet and feeding strategies of age 0 year juvenile flounder Platichthys flesus in two different micro‐tidal habitats in the nutrient enriched Mariager Fjord on the Danish east coast. Juvenile flounder and benthic macrofauna were sampled monthly from June to October 1999 in a bare sand habitat and a habitat covered by filamentous and mat forming macroalgae. The presence of the 'opportunistic' macroalgae created a shift in the dominance of surface dwelling prey such as epifaunal amphipods to more infaunal groups such as oligochaetes and polychaetes. The diet of the flounder varied considerably between the two habitats mainly reflecting prey availability relative to their abundance, prey spatial distribution, habitat structure and ontogenetic prey shifts as a function of total length. Flounder in the vegetated site fed on a diverse diet of copepods, polychaetes and oligochaetes, whereas those caught in the bare sand site fed primarily on the amphipod Corophium volutator which was numerically dominant at this site. During the growth season, two diet shifts were observed: from copepods early in the season to macrofauna organisms and, later in the season, the inclusion of more hyperbenthic prey such as Mysidea spp; Idotea spp. and the common goby Pomatoschistus microps .     

Diet-dependent body morphology and ontogenetic reaction norms in Eurasian perch

Individual morphology and performance are directly or indirectly under the influence of variation in resource levels. To study the effects of different resource conditions and their effects on morphology and ontogenetic reaction norms in young‐of‐the‐year (YOY) perch (Perca fluviatilis), we used three different approaches. First, we examined the morphological trajectories over early ontogeny in relation to lake‐specific resources in a field study. Second, one lake that lacked perch recruitment was stocked with perch eggs from a control lake in a whole‐lake experiment to study ontogenetic reaction norms. Third, we compared the development of YOY perch in the three lakes that mainly inhabited the littoral zone with YOY perch experimentally confined to enclosures in the pelagic zone of the lakes.
Overall body morphology of the YOY perch changed both as a function of size and as a function of diet. As perch increased in size they developed a deeper body morphology corresponding to an increased proportion of benthic macroinvertebrates in their diet. In pelagic enclosures where perch were constrained to feed mainly on zooplankton they had a more fusiform body morphology than perch in the lakes that fed on a mixture of zooplankton and macroinvertebrates. Similarly, the ontogenetic reaction norm of perch was related to the diet and lake‐specific zooplankton levels in the whole‐lake experiment.
In the pelagic enclosures, perch with high growth rates had a more fusiform body morphology than slow‐growing perch, whereas the opposite was found in the lakes, where perch included more macroinvertebrates in their diets. Perch in lakes with a higher proportion of macroinvertebrates in the diet also had deeper body morphology. The opposite morphology – growth rate relationship found between perch in the pelagic versus those using the whole lake suggest a morphological trade‐off between foraging on zooplankton and foraging on macroinvertebrates. Our results suggest that YOY perch show different ontogenetic reaction norms as a function of lake‐specific resource levels, which may allow YOY recruitment to later stages. Our results further suggest that diet‐related changes in morphology are a rapid process.