Patterns in the co-occurrence of fish species in streams: the role of site suitability,morphology and phylogeny versus species interactions

A number of studies at large scales have pointed out that abiotic factors and recolonization dynamics appear to be more important than biotic interactions in structuring stream-fish assemblages. In contrast, experimental and field studies at small scales show the importance of competition among stream fishes. However, given the highly variable nature of stream systems over time, competition may not be intense enough to generate large-scale complementary distributions via competitive exclusion. Complementary distribution is a recurrent pattern observed in fish communities across stream gradients, though it is not clear which instances of this pattern are due to competitive interactions and which to individual species requirements. In this study, I introduce a series of null models developed to provide a more robust evaluation of species associations by facilitating the distinction between different processes that may shape species distributions and community assembly. These null models were applied to test whether conspicuous patterns in species co-occurrences are more consistent with their differences in habitat use, morphological features and/or phylogenetic constraints, or with species interactions in fish communities in the streams of a watershed in eastern Brazil. I concluded that patterns in species co-occurrences within the studied system are driven by common species-habitat relationships and species interactions may not play a significant role in structuring these communities. I suggest that large-scale studies, where adequate designs and robust analytical tools are applied, can contribute substantially to understanding the importance of different types of processes in structuring stream-fish communities.     

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.     

Body size distribution in flea communities harboured by Siberian small mammals as affected by host species,host sex and scale: scale matters the most

The distribution of body sizes of co-existing species at different scales reflects the scale-dependency of rules governing community assembly. Investigation of among-scale variation in community assembly is impeded by the methodological difficulties of establishing scale boundaries. Studying body size distribution in parasites allows us to avoid the problem of defining scale because parasite communities have clear boundaries and are represented by infracommunities (an assemblage harboured by an individual host), component communities (an assemblage harboured by a host population in a locality), and compound communities (an assemblage harboured by a host community in a locality). We studied body size distribution of fleas parasitic on small mammals in Western Siberia using null models. We asked whether body size ratios (i.e., size differences among coexisting species) in these communities demonstrate non-random segregated or aggregated patterns and whether these patterns differ between (a) host species, (b) host sexes and (c) infra-, component, and compound communities. No effect of host sex on the pattern of body size distribution was found at either scale, whereas an effect of host species was found in infracommunities only. We found a tendency of flea infracommunities toward segregation, whereas body size distributions in component and compound communities were consistently aggregated. We propose that the former could be caused by apparent competition (=?negative indirect interactions among fleas due to shared natural enemy, i.e. a host), whereas we the latter could be explained by host- and environment-associated filtering (=?factors restricting co-occurring species to a certain subset that share certain traits). We conclude that, counterintuitively, flea communities at the lowest hierarchical scale are mainly governed by evolutionary mechanisms, whereas communities at higher scale are assembled via ecological processes.     

Compartments in a marine food web associated with phylogeny, body mass, and habitat structure

A long-standing question in community ecology is whether food webs are organized in compartments, where species within the same compartment interact frequently among themselves, but show fewer interactions with species from other compartments. Finding evidence for this community organization is important since compartmentalization may strongly affect food web robustness to perturbation. However, few studies have found unequivocal evidence of compartments, and none has quantified the suite of mechanisms generating such a structure. Here, we combine computational tools from the physics of complex networks with phylogenetic statistical methods to show that a large marine food web is organized in compartments, and that body size, phylogeny, and spatial structure are jointly associated with such a compartmentalized structure. Sharks account for the majority of predatory interactions within their compartments. Phylogenetically closely related shark species tend to occupy different compartments and have divergent trophic levels, suggesting that competition may play an important role structuring some of these compartments. Current overfishing of sharks has the potential to change the structural properties, which might eventually affect the stability of the food web.     

Metazoan parasite infracommunities of Florida pompano (Trachinotus carolinus) from the coast of the Yucatan Peninsula,Mexico

Metazoan parasite infracommunities of the Florida pompano (Trachinotus carolinus) were studied in terms of species composition, species richness, diversity, numerical dominance, and similarity. Seventy-five fishes were collected from 4 localities along the Yucatan Peninsula coast and 24 parasite species recovered. Most were digeneans (8 species) and nematodes (7). Other species were monogeneans (3). aspidogastreans (2), cestodes (1), acanthocephalans (1), and crustaceans (2). Only 4 species were common in at least I locality. Mean values for species richness, abundance, diversity, numerical dominance, and similarity in total (all species in the individual fish), gastrointestinal, and ectoparasite infracommunities were within ranges observed for most helminth infracommunities of marine fishes from temperate and tropical latitudes. These infracommunities had low species richness, abundance, diversity, and predictability (except ectoparasite infracommunities) and high dominance. Within the predictable element (common species), the specialist monogenean Pseudobicotylophora atlantica was the main reason for the increase in predictability because it was the only common species at all 4 localities. Host feeding habits, the distribution of intermediate hosts and infective stages, the local species pool, and a phylogenetic component seem to be determining the characteristics of these metazoan parasite infracommunities.     

Community structure, abundance, and morphology

The role of interspecific competition in structuring communities has been a highly debated issue for the last two decades. The nonrandom nature of morphological patterns within communities has been at the center of this controversy. Null models addressing community-wide dispersions in morphology have produced equivocal results and may be based on assumptions that are too restrictive (e.g., competitive exclusion or displacement). If morphological distinctiveness allows species to escape competitive pressures and exhibit higher densities, then a positive relationship should exist between morphological dissimilarity and abundance. We develop a suite of models that evaluates patterns in abundance that are associated with the morphological proximity of a species to other competitors. We evaluated the relationship between morphological distance and abundance from a variety of morphological perspectives, from those representing strictly diffuse interactions to those representing only interactions between a species and its nearest neighbor in morphological space. These models were sufficiently powerful to detect positive associations between abundance and morphological differences in a nocturnal desert rodent guild for which the effects of competition on structure are well established. Models such as these may be more useful than traditional models evaluating morphological dispersions for many reasons. They do not require that communities reach equilibrium before competitive interactions give rise to deterministic structure. They do not suffer from limitations of potentially inaccurate faunal pools or from phylogenetic constraints. Lastly, they may be used as a diagnostic tool in comparative studies to determine the degree to which competitive interactions structure communities.     

Indirect effects in community ecology: Their definition, study and importance

The diversity of indirect interactions that can occur within communities is large. Recent research on indirect interactions is scattered in the literature under numerous labels. The definition of indirect effects is an important aspect of their study, and clarifies some of the subtle differences among indirect effects found in natural communities. Choosing which species to study, how to manipulate species and for what duration, which attributes to measure and, finally, which analytical techniques to use are all problems facing the community ecologist. Ultimately, we are striving for the best means of determining the relative importance of direct and indirect effects in structuring communities.     

Ectoparasite community structure on three closely related seabird hosts: a multiscale approach combining ecological and genetic data

Parasite communities can be structured at different spatial scales depending on the level of organization of the hosts; hence, examining this structure should be a multiscale process. We investigated ectoparasite community structure in three closely related seabird hosts, the Mediterranean Cory's shearwater Calonectris diomedea diomedea , the Atlantic Cory's shearwater C. d. borealis and the Cape Verde shearwater C. edwardsii . This community was composed of three lice ( Halipeurus abnormis , Austromenopon echinatum and Saemundssonia peusi) and one flea species ( Xenopsylla gratiosa ), and was considered at the infra-, component and regional community levels. We examined temporal and spatial structuring of the infracommunities, the influence of host aggregation and body condition on the component community, and the effect of genetic and geographic connectivity among host populations on the regional community. Ectoparasite infracommunities showed substantial species overlaps in temporal patterns of abundance, but species were spatially segregated within the host body. Within component communities, all ectoparasite species showed an aggregated distribution in abundance. However, aggregation patterns and their relationships with the spatial distribution of hosts within the breeding colony differed among ectoparasite species, mainly reflecting ecological differences between fleas and lice. At the regional scale, similarity in ectoparasite communities correlated with geographic distances among host colonies, but not with genetic distances. This result suggests differences in climate and habitat characteristics among host localities as a major determinant of regional communities, rather than host connectivity. Taken together, our results highlight the importance of the geographic distribution of host breeding colonies and the spatial segregation within the host body as key factors in determining ectoparasite community structure in Calonectris shearwaters.     

The role of glycoproteins in neural development, function, and disease

Glycoproteins play key roles in the development, structuring, and subsequent functioning of the nervous system. However, the complex glycosylation process is a critical component in the biosynthesis of CNS glycoproteins that may be susceptible to the actions of toxicological agents or may be altered by genetic defects. This review will provide an outline of the complexity of this glycosylation process and of some of the key neural glycoproteins that play particular roles in neural development and in synaptic plasticity, in the mature CNS. Finally, the potential of glycoproteins as targets for CNS disorders will be discussed.     

Interactions between imprinting effects: summary and review

Mice with uniparental disomies (uniparental duplications) for defined regions of certain chromosomes, or certain disomies, show a range of developmental abnormalities most of which affect growth. These defects can be attributed to incorrect dosages of maternal or paternal copies of imprinted genes lying within the regions involved. Combinations of certain partial disomies result in interactions between the imprinting effects that seemingly independently affect foetal and/or placental growth in different ways or modify neonatal and postnatal development. The findings are generally in accord with the 'conflict hypothesis' for the evolution of genomic imprinting but do not demonstrate common growth axes within which imprinted genes may interact. Instead, it would seem that any gene that favours embryonic/foetal development, at consequent cost to the mother, will have been subject to evolutionary selection for only paternal allele expression. Reciprocally, any gene that reduces embryonic/foetal growth to limit disadvantage to the mother will have been selected for only maternal allele expression. It is concluded that survival of the placenta is core to the evolution of imprinting.     

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.     

Functional groups of ecosystem engineers: a proposed classification with comments on current issues

Ecologists have long known that certain organisms fundamentally modify, create, or define habitats by altering the habitat's physical properties. In the past 15 years, these processes have been formally defined as "ecosystem engineering", reflecting a growing consensus that environmental structuring by organisms represents a fundamental class of ecological interactions occurring in most, if not all, ecosystems. Yet, the precise definition and scope of ecosystem engineering remains debated, as one should expect given the complexity, enormity, and variability of ecological systems. Here I briefly comment on a few specific current points of contention in the ecosystem engineering concept. I then suggest that ecosystem engineering can be profitably subdivided into four narrower functional categories reflecting four broad mechanisms by which ecosystem engineering occurs: structural engineers, bioturbators, chemical engineers, and light engineers. Finally, I suggest some conceptual model frameworks that could apply broadly within these functional groups.     

Can helminth community patterns be amplified when transferred by predation from intermediate to definitive hosts?

Helminth communities in definitive hosts are formed by the acquisition of packets of larvae arriving each time an intermediate host is consumed. It is thus possible that associations between parasite species or other aspects of community structure get transferred from intermediate to definitive hosts. Earlier computer simulations showed that associations between 2 parasite species, in particular positive associations, could be transferred up the food chain. Here, we alter some of the assumptions of previous models and generate new simulations of several ways in which source infracommunities in intermediate hosts can be transferred to target infracommunities in definitive hosts. In particular, we introduced nonrandom selection of intermediate hosts by predatory definitive hosts, to mimic the phenomenon of host manipulation by parasites; this consisted in biasing predation toward intermediate hosts harboring a certain parasite species. Overall, our results show that positive covariances between 2 parasite species can not only be transferred but can also be amplified during transmission to definitive hosts; significant covariance between parasite species can even appear in the definitive hosts when none existed in the intermediate hosts. Negative covariance was not as readily transferred to definitive hosts and amplified, in part because of properties of the presence-absence covariance index. Amplification of covariance results from intermediate host manipulation as well as from other processes taking place during transmission. These results suggest that the patterns of association between helminth species in definitive hosts cannot be taken to reflect the processes acting inside those hosts: they may simply be inherited, with amplification, from intermediate hosts.     

Nestedness, anti-nestedness, and the relationship between prevalence and intensity in ectoparasite assemblages of marine fish: a spatial model of species coexistence

Nested species subset patterns consist in a hierarchical structure of species composition in related assemblages, with the species found in depauperate assemblages representing non-random subsets of progressively richer ones. This pattern has been found at the infracommunity level in about a third of the fish ectoparasite assemblages studied to date. Here we present evidence for another non-random structural pattern in assemblages of fish ectoparasites, anti-nestedness, which corresponds to situations in which parasite species are always absent from infracommunities richer than the most depauperate one in which they occur. We show that this pattern is exactly as common as nestedness, and that anti-nested assemblages are characterised by significantly lower prevalence and mean intensities of parasites than nested assemblages. In addition, we found a positive relationship between the prevalence and the mean intensity of parasites across the different assemblages. We propose a link between the nestedness/anti-nestedness continuum and the prevalence-intensity relationship that may involve colonisation-extinction processes. The results presented here suggest that, although nestedness may not be common in parasite communities, other departures from random species assembly are possible, and that some form of structure may be present in many communities. The continuum between nestedness and anti-nestedness also has implications for recent models of species coexistence in communities.     

Gene flow and geographically structured coevolution

Many of the dynamic properties of coevolution may occur at the level of interacting populations, with local adaptation acting as a force of diversification, as migration between populations homogenizes these isolated interactions. This interplay between local adaptation and migration may be particularly important in structuring interactions that vary from mutualism to antagonism across the range of an interacting set of species, such as those between some plants and their insect herbivores, mammals and trypanosome parasites, and bacteria and plasmids that confer antibiotic resistance. Here we present a simple geographically structured genetic model of a coevolutionary interaction that varies between mutualism and antagonism among communities linked by migration. Inclusion of geographic structure with gene flow alters the outcomes of local interactions and allows the maintenance of allelic polymorphism across all communities under a range of selection intensities and rates of migration. Furthermore, inclusion of geographic structure with gene flow allows fixed mutualisms to be evolutionarily stable within both communities, even when selection on the interaction is antagonistic within one community. Moreover, the model demonstrates that the inclusion of geographic structure with gene flow may lead to considerable local maladaptation and trait mismatching as predicted by the geographic mosaic theory of coevolution.     

Patterns of infracommunity species richness in eels, Anguilla anguilla

Between October 1999 and October 2001, a total of 510 European eels Anguilla anguilla were captured in 13 different samples from the rivers Thames (five locations) and Test (one location) in southern England. The relationship between parasite component community species richness (CCR) and maximum infracommunity species richness (ICRmax) compared with that previously observed in bird and mammal hosts. Specifically, the maximum number of parasite species occurring in infracommunities equalled or exceeded half the number of parasite species in the component community at that time, across a wide range of CCR values (2-9 parasite species). Furthermore, the frequency distribution of infracommunity richness (ICR) suggested that the species composition of infracommunities is probably random. These findings suggest that intestinal macroparasite infracommunities in eels are unsaturated and potentially species rich assemblages and, in these respects, share a fundamental similarity with the infracommunities of birds and mammals.     

Complex role of predators in structuring soft-sediment macrobenthic communities: Implications of changes in spatial scale for experimental studies

In estuarine and coastal soft-sediment systems, the role that predators play in structuring communities appears to be variable. Attributes of a particular predator that influence its role in structuring the community include: the rate of prey consumption; the behaviour, morphology and mobility of the predator; and, in soft- sediment communities, sediment disturbances associated with feeding. Reviews of field experiments designed to assess the role of predators in influencing the structure and function of soft-sediment communities have concluded that many of the predators are generalists and there is usually a lack of competitive exclusion. Thus predation structures communities by many complex and indirect interactions that are often difficult to predict and generalize. Variations in the apparent strength and role of predation in structuring benthic communities may depend on a variety of ecosystem characteristics and/or aspects of study design. In this paper, I consider whether we have been conducting our experiments at the appropriate scales. Five case studies from Manukau Harbour (New Zealand) illustrate how small changes in the spatial scale can affect results, due to predator perceptions and prey mobility. The results of these studies demonstrate the need to identify scales at which predator effects are likely to be important and to fit experiments within the dynamics and heterogeneity of the system being studied. To do this, we need basic information on the natural history, behaviour and spatial and temporal variability of both predators and prey communities. We also need to be specific about scales of measurement when matching theoretical predictions to field observations/experiments. Finally, to enhance our ability to generalize from specific studies, we need to gather data that will enable us to both predict and test the importance of predation over a range of spatial and temporal scales.     

Species interactions within a fouling diatom community: roles of nutrients, initial inoculum and competitive strategies

Diatoms constitute an important component of the fouling community. Although a lot of work has dealt with the fouling diatom community structure, work on the species interactions within the community is still meagre. In this regard, a study was carried out by transferring natural diatom biofilms into controlled conditions in order to understand the roles of nutrients, initial cell inoculum and seasonal variation in species composition in structuring the fouling diatom community. This community exhibited seasonal variation during the monsoon, post-monsoon and pre-monsoon periods. During each of these seasons, diatom species interactions varied depending upon the species composition. It was observed that excess nutrients favoured those species with comparatively higher growth rates, thereby suppressing the growth of other co-existing species. This competitive trait was found to be effective at an appropriate cell density ratio of the competitive and target species. Understanding such pathways will be useful for modelling the interactions between diatom species in various habitats under different resource conditions.     

Assessing moth migration and population structuring in Helicoverpa armigera (Lepidoptera: Noctuidae) at the regional scale: example from the Darling Downs, Australia

Analysis of gene flow and migration of Helicoverpa armigera (Hübner) in a major cropping region of Australia identified substantial genetic structuring, migration events, and significant population genotype changes over the 38-mo sample period from November 1999 to January 2003. Five highly variable microsatellite markers were used to analyze 916 individuals from 77 collections across 10 localities in the Darling Downs. The molecular data indicate that in some years (e.g., April 2002-March 2003), low levels of H. armigera migration and high differentiation between populations occurred, whereas in other years (e.g., April 2001-March 2002), there were higher levels of adult moth movement resulting in little local structuring of populations. Analysis of populations in other Australian cropping regions provided insight into the quantity and direction of immigration of H. armigera adults into the Darling Downs growing region of Australia. These data provide evidence adult moth movement differs from season to season, highlighting the importance of studies in groups such as the Lepidoptera extending over consecutive years, because short-term sampling may be misleading when population dynamics and migration change so significantly. This research demonstrates the importance of maintaining a coordinated insecticide resistance management strategy, because in some years H. armigera populations may be independent within a region and thus significantly influenced by local management practices; however, periods with high migration will occur and resistance may rapidly spread.