Coevolution of resource trade-offs driving species interactions in a host–parasite network: an exploratory model

Patterns of specialization asymmetry, where specialist species interact mainly with generalists while generalists interact with both generalists and specialists, are often observed in mutualistic and antagonistic bipartite ecological networks. These have been explained in terms of the relative abundance of species, using a null model that assigns links in proportion to abundance, but doubts have been raised as to whether this offers a complete explanation. In particular, host–parasite networks offer a variety of examples in which the reverse patterns are observed. We propose that the link between specificity and species richness may also be driven by the coevolution of hosts and parasites, as hosts allocate resources to optimize defense against parasites, and parasites to optimize attack on hosts. In this hypothesis, species interactions are a result of resource allocations. This novel concept, linking together many different arguments for network structures, is introduced through the adaptive dynamics of a simple ecological toy system of two hosts and two parasites. We analyze the toy model and its functionality, demonstrating that coevolution leads to specialization asymmetry in networks with closely related parasites or fast host mutation rates, but not in networks with more distantly related species. Having constructed the toy model and tested its applicability, our model can now be expanded to the full problem of a larger system.     

Abundance and generalisation in mutualistic networks: solving the chicken‐and‐egg dilemma

A frequent observation in plant–animal mutualistic networks is that abundant species tend to be more generalised, interacting with a broader range of interaction partners than rare species. Uncovering the causal relationship between abundance and generalisation has been hindered by a chicken‐and‐egg dilemma: is generalisation a by‐product of being abundant, or does high abundance result from generalisation? Here, we analyse a database of plant–pollinator and plant–seed disperser networks, and provide strong evidence that the causal link between abundance and generalisation is uni‐directional. Specifically, species appear to be generalists because they are more abundant, but the converse, that is that species become more abundant because they are generalists, is not supported by our analysis. Furthermore, null model analyses suggest that abundant species interact with many other species simply because they are more likely to encounter potential interaction partners.     

Species composition of dabbling duck assemblages: ecomorphological patterns compared with null models

Ecomorphological patterns of breeding dabbling duck (Anas spp.) assemblages were studied in six regions in northern Europe. Observed spacings among species in terms of bill lamellar density and body length were compared with expected spacings based on null models incorporating different levels of constraints (regional species pools, species relative abundances, lake size and habitat requirements of species). Deviations of observed spacings from expected ones were compared with prey abundance and prey size diversity in the lakes. Observed spacings in terms of body length, but not in terms of bill lamellar density, were greater than expected on the basis of null models. The most abundant species were generally relatively more different than less abundant species in terms of body length but not in terms of bill lamellar density. Deviations between observed and expected spacings in terms of body length were more like those predicted by the competition hypothesis in lakes with low food abundance than in lakes with high food abundance. Patterns in bill lamellar spacings were not related to food abundance nor to food size diversity. In general, patterns in body length spacings were consistent with the competition hypothesis whether the null model used in comparisons was constrained or not.     

Abundance drives broad patterns of generalisation in plant–hummingbird pollination networks

Abundant pollinators are often more generalised than rare pollinators. This could be because abundant species have more chance encounters with potential interaction partners. On the other hand, generalised species could have a competitive advantage over specialists, leading to higher abundance. Determining the direction of the abundance–generalisation relationship is therefore a ‘chicken‐and‐egg’ dilemma. Here we determine the direction of the relationship between abundance and generalisation in plant–hummingbird pollination networks across the Americas. We find evidence that hummingbird pollinators are generalised because they are abundant, and little evidence that hummingbirds are abundant because they are generalised. Additionally, most patterns of species‐level abundance and generalisation were well explained by a null model that assumed interaction neutrality (interaction probabilities defined by species relative abundances). These results suggest that neutral processes play a key role in driving broad patterns of generalisation in animal pollinators across large spatial scales.     

Is the abundance of species determined by their functional traits? A new method with a test using plant communities

The relation between functional traits and abundance of species has the potential to provide evidence on the mechanisms that structure local ecological communities. The niche-limitation/limiting-similarity hypothesis, derived from MacArthur and Levins’ original concept, predicts that species that are similar to others in terms of functional traits will suffer greater competition and hence be less abundant. On the other hand, the environment-filtering/habitat-optimum hypothesis predicts that groups of species with functional traits that are close to the optimum for that environment, and are therefore similar to other species, will be more abundant. We propose a new niche-assembly model for predicting the relative abundance of species in communities from their functional traits, which can detect the patterns that would be expected from either of these hypotheses. The model was fitted to eight plant communities sampled in the Lake Ohau district of New Zealand. For seven of the sites, the patterns could not be distinguished from that expected under a null model. However, in one site there was highly significant departure from the null model in the direction expected from the niche-limitation hypothesis. The site was probably the most productive of those examined. It is possible that competition for light rather than belowground resources, or faster recovery from disturbance, allowed greater predictability. Surprisingly, the predictability was seen when just the presences of a species’ neighbours in trait space were taken into account, but not when the potential effects of those neighbours were weighted by their abundance. For three of the four model types, the effects of species on each other were consistently negative: a significant trend. These results contradict the various neutral models of ecological communities.     

Species abundance distributions and numerical dominance in gastrointestinal helminth communities of fish hosts

The abundances of different species in a parasite community are never similar: there is typically one or a few numerically dominant species and many species with low abundance. Here, we determine whether basic features of parasite communities are associated with strong dominance by one or a few species, among 39 component communities of gastrointestinal helminths in marine fishes from Brazil. First, we tested whether the shape of the species abundance distribution in these communities fits that predicted by several theoretical models, using a goodness-of-fit procedure. Only the canonical lognormal model could be rejected for 5 out of 39 communities; all other comparisons of observed and predicted abundance distributions showed no significant differences, although this may be due to limited statistical power. Second, we used the ratio between the abundance of the most abundant species and either the second or third most abundant species, as indices of dominance; these show, for instance, that the dominant species in a community is typically twice, but sometimes over ten times, as abundant as the next most abundant species. We found that these ratios were not influenced by either the community's species richness, the mean number of individual parasites per host, or the taxonomic identity of the dominant species. However, the abundance ratio between the first and third most abundant species in a community was significantly correlated with an independent index of species interactivity, based on the likelihood that the different parasite species in a component community co-occur in the same host individuals: the difference in abundance between the dominant and third most abundant species was greater in communities characterized by weak interactions. These findings suggest that strong interactions may lead to greater evenness in the abundance of species, and that numerical dominance is more likely to result from interspecific differences in recruitment rates.     

Abundance and nestedness in interaction networks

Nestedness is a useful metric that characterizes the generalist–specialist balance in ecological communities. Although several nestedness indices have been proposed, few have explored how species abundance per se affects their performance and the ability to detect true interaction networks. We here develop a mathematical framework that takes into account abundance in estimates of nestedness. We use an analytical approach to relate abundance and nestedness. In our null model the probability of interaction among species is determined solely as function of their abundances. Assuming a power-law abundance model we analytically find the nestedness index and its coefficient of variability. We find that the sloping abundance distribution of our null model generates more nested structures. On the other hand steeper abundances lead to higher coefficients of variability. Both results suggest that nestedness analysis should be evaluated and explanations sought carefully.     

Asymmetric specialization and extinction risk in plant–flower visitor webs: a matter of morphology or abundance?

A recently discovered feature of plant–flower visitor webs is the asymmetric specialization of the interaction partners: specialized plants interact mainly with generalized flower visitors and specialized flower visitors mainly with generalized plants. Little is known about the factors leading to this asymmetry and their consequences for the extinction risk of species. Previous studies have proposed random interactions proportional to species abundance as an explanation. However, the simulation models used in these studies did not include potential biological constraints. In the present study, we tested the potential role of both morphological constraints and species abundance in promoting asymmetric specialization. We compared actual field data of a Mediterranean plant–flower visitor web with predictions of Monte Carlo simulations including different combinations of the potential factors structuring the web. Our simulations showed that both nectar-holder depth and abundance were able to produce asymmetry; but that the expected degree of asymmetry was stronger if based on both. Both factors can predict the number of interaction partners, but only nectar-holder depth was able to predict the degree of asymmetry of a certain species. What is more, without the size threshold the influence of abundance would disappear over time. Thus, asymmetric specialization seems to be the result of a size threshold and, only among the allowed interactions above this size threshold, a result of random interactions proportional to abundance. The simulations also showed that asymmetric specialization could not be the reason that the extinction risk of specialists and generalists is equalized, as suggested in the literature. In asymmetric webs specialists clearly had higher short-term extinction risks. In fact, primarily generalist visitors seem to profit from asymmetric specialization. In our web, specialists were less abundant than generalists. Therefore, including abundance in the simulation models increased the difference between specialists and generalists even more. Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Manifold influences of phylogenetic structure on a plant–herbivore network

Ecologists are increasingly aware of the interplay between evolutionary history and ecological processes in shaping current species interaction patterns. The inclusion of phylogenetic relationships in studies of species interaction networks has shown that closely related species commonly interact with sets of similar species. Notably, the degree of phylogenetic conservatism in antagonistic ecological interactions is frequently stronger among species at lower trophic levels than among those at higher trophic levels. One hypothesis that accounts for this asymmetry is that competition among consumer species promotes resource partitioning and offsets the maintenance of dietary similarity by phylogenetic inertia. Here, we used a regional plant–herbivore network comprised of Asteraceae species and flower‐head endophagous insects to evaluate how the strength of phylogenetic conservatism in species interactions differs between the two trophic levels. We also addressed whether the asymmetry in the strength of the phylogenetic signal between plants and animals depends on the overall degree of relatedness among the herbivores. We show that, beyond the previously reported compositional similarity, closely related species also share a greater proportion of counterpart phylogenetic history, both for resource and consumer species. Comparison of the patterns found in the entire network with those found in subnetworks composed of more phylogenetically restricted groups of herbivores provides evidence that resource partitioning occurs mostly at deeper phylogenetic levels, so that a positive phylogenetic signal in antagonist similarity is detectable even between closely related consumers in monophyletic subnetworks. The asymmetry in signal strength between trophic levels is most apparent in the way network modules reflect resource phylogeny, both for the entire network and for subnetworks. Taken together, these results suggest that evolutionary processes, such as phylogenetic conservatism and independent colonization history of the insect groups may be the main forces generating the phylogenetic structure observed in this particular plant–herbivore network system.     

Simple assumptions predicts prey selection by piscivorous fishes

Studies on trophic interactions permits the use of community-wide network analyses to evaluate the consequences of human interventions in natural communities. In this paper, we aimed to get insights into the underlying mechanism of prey selection for four piscivorous species, and evaluate behavioral responses to prey selection after an impoundment. We assemble six food web models to search for the hypothesis that best predict observed prey selection pattern of piscivorous fishes combining the following assumptions: (i) predation window, defined as the size range of prey species consumed by a piscivorous fish; (ii) prey strategies to avoid predation (iii) and prey abundance. We tested the probability of each hypothesis to reproduce two empirical data, one before and one after an impoundment with minimum assumptions. Before impoundment, we found that predators presented switching behavior, preying preferably on abundant prey; while after impoundment, predators consumed prey within its predation window. Those results explained better than the null hypotesis and all other assumptions; and corroborate with both theoretical and empirical studies. We conclude that different assumptions drives piscivorous fish behavior in different environments; and modelling procedures can be used to assess gaps in trophic interactions of fish communities.     

Comparing species interaction networks along environmental gradients

Knowledge of species composition and their interactions, in the form of interaction networks, is required to understand processes shaping their distribution over time and space. As such, comparing ecological networks along environmental gradients represents a promising new research avenue to understand the organization of life. Variation in the position and intensity of links within networks along environmental gradients may be driven by turnover in species composition, by variation in species abundances and by abiotic influences on species interactions. While investigating changes in species composition has a long tradition, so far only a limited number of studies have examined changes in species interactions between networks, often with differing approaches. Here, we review studies investigating variation in network structures along environmental gradients, highlighting how methodological decisions about standardization can influence their conclusions. Due to their complexity, variation among ecological networks is frequently studied using properties that summarize the distribution or topology of interactions such as number of links, connectance, or modularity. These properties can either be compared directly or using a procedure of standardization. While measures of network structure can be directly related to changes along environmental gradients, standardization is frequently used to facilitate interpretation of variation in network properties by controlling for some co‐variables, or via null models. Null models allow comparing the deviation of empirical networks from random expectations and are expected to provide a more mechanistic understanding of the factors shaping ecological networks when they are coupled with functional traits. As an illustration, we compare approaches to quantify the role of trait matching in driving the structure of plant–hummingbird mutualistic networks, i.e. a direct comparison, standardized by null models and hypothesis‐based metaweb. Overall, our analysis warns against a comparison of studies that rely on distinct forms of standardization, as they are likely to highlight different signals. Fostering a better understanding of the analytical tools available and the signal they detect will help produce deeper insights into how and why ecological networks vary along environmental gradients.     

The abundance effect on network nestedness is stronger for parasitic than herbivory interactions

多度对寄生型网络嵌套结构的影响大于食草型网络 因为物种多度显著影响种间互作频率,分析食物网结构对物种多样性和稳定性影响时,应基于能够反映物种真实偏好的网络(即偏好网),而不是直接观察得到的网络(即观察网)。食草网络中(植物为低营养级)的植物资源多度大于寄生网络中(动物为低营养级)寄主资源多度,因此我们假设：寄生网络的结构比食草网络的结构更易受到多度效应的影响。为验证这一假设,我们从已发表的文献中收集了80 个定量观察网络(包括34个植物-食草昆虫网络和46个寄生网络),应用有效多度模型去除物种多度对观察网络的影响,从而得出偏好网络。然后,我们应用weighted NODF和spectral radius两个嵌套系数表征网络嵌套性,分析了观察网和偏好网的物种链接数分布、相互作用均匀度、加权连通度和稳健性的差异。结果表明,在偏好网中,寄生网络的嵌套程度要低于食草网络,这可能是因为去除多度影响增加了种间作用频率的均匀度。偏好网的加权连通度和稳健性显著高于相应的观察网,表明偏好网比观察网具有更高的网络稳定性。未来的食物网研究不仅应关注互惠和拮抗网络的结构差异,还应该关注食草和寄生等不同类型拮抗食物网的结构差异。     

Detecting temporal trends in species assemblages with bootstrapping procedures and hierarchical models

Quantifying patterns of temporal trends in species assemblages is an important analytical challenge in community ecology. We describe methods of analysis that can be applied to a matrix of counts of individuals that is organized by species (rows) and time-ordered sampling periods (columns). We first developed a bootstrapping procedure to test the null hypothesis of random sampling from a stationary species abundance distribution with temporally varying sampling probabilities. This procedure can be modified to account for undetected species. We next developed a hierarchical model to estimate species-specific trends in abundance while accounting for species-specific probabilities of detection. We analysed two long-term datasets on stream fishes and grassland insects to demonstrate these methods. For both assemblages, the bootstrap test indicated that temporal trends in abundance were more heterogeneous than expected under the null model. We used the hierarchical model to estimate trends in abundance and identified sets of species in each assemblage that were steadily increasing, decreasing or remaining constant in abundance over more than a decade of standardized annual surveys. Our methods of analysis are broadly applicable to other ecological datasets, and they represent an advance over most existing procedures, which do not incorporate effects of incomplete sampling and imperfect detection.     

Spatial predictability and resource specialization of bees (Hymenoptera: Apoidea) at a superabundant, widespread resource

For reciprocal specialization (coevolution) to occur among floral visitors and their host plants the interactions must be temporally and spatially persistent. However, studies repeatedly have shown that species composition and relative abundance of floral visitors vary dramatically at all spatial and temporal scales. We test the hypothesis that, on average, pollen specialist bee species occur more predictably at their floral hosts than pollen generalist bee species. Taxonomic floral specialization reaches its extreme among species of solitary, pollen-collecting bees, yet few studies have considered how pollen specialization by floral visitors influences their spatial constancy. We test this hypothesis using an unusually diverse bee guild that visits creosote bush (Larrea tridentatd), the most widespread, dominant plant of the warm deserts of North America. Twenty-two strict pollen specialist and 80 + generalist bee species visit Larrea for its floral resources. The sites we sampled were separated by 0.5 to > 1450 km, and spanned three distinct deserts and four vegetation zones. We found that species of Larrea pollen specialist bees occurred at more sites and tended to be more abundant than generalists. Surprisingly, spatial turnover was high for both pollen specialist and generalist bee species at all distances, and species composition of samples from sites 1–5 km apart varied as much as repeat samples made at single sites. Nevertheless, the pattern of bee species turnover was not haphazard. As distance among sites increased faunal similarity of sites decreased. Faunal similarities among sites within 250 km of each other were generally greater than if randomly distributed over all sites (the null model). No single ecological category of species (widespread, localized, Larrea pollen specialist, floral generalist) accounted for this spatial predictability. Evidently, concordant local distribution patterns of many ecologically diverse species contribute to the non-random spatial pattern. The ecological dominance of creosote bush does not confer obvious ecological advantages to its specialist floral visitors. Spatial turnover is comparable to that found for bee guilds from other biogeographic regions of the world and is not therefore limited to those bee species that inhabit highly seasonal climates, such as deserts. Philopatry and differences in bloom predictability among sites are probably more important causes for spatial turnover of bee species than are interspecific competition for nest sites or floral resources.     

Null model tests of clustering of species, negative co-occurrence patterns and nestedness in meta-communities

We propose tests for patterns in meta-community structure. The tests for clustering and nestedness of the occurrences of species and negative co-occurrence patterns provide four important innovations. Firstly, they are not restricted to the analysis of communities along one-dimensional gradients or to the main axis of variation. Secondly, abundance data can also be considered in the null model whereas most previous approaches could consider only presence/absence data. And thirdly, habitat suitability and spatial autocorrelation can be incorporated in the null model so that patterns that might be caused by biotic interactions can be distinguished from patterns which are the result of differences in the suitability or accessibility of sites for the examined organisms. Finally, the test for nestedness is also appropriate if there is more than one set of nested subsets. A re-analysis of 35 data sets with these tests showed the importance of considering the autocorrelation of the occurrences of species in analyses of meta-community structure and demonstrated the advantage of abundance data for tests of clustering of species. With abundance data it could be shown that there is a significant clustering of species, i.e. there are positive associations of species in most meta-communities, even if an environmentally or spatially constrained null model is used for the test. Co-occurrence patterns that might indicate interspecific competition were found in many of the analysed presence/absence data sets. Surprisingly the analysis of abundance data sets provides less evidence for interspecific competition. A hierarchical organization of communities, i.e. nestedness, turned out to be a rare pattern, if the autocorrelation of the occurrences of species is considered.     

Pattern in the Co-occurrence of Fishes Inhabiting the Coral Reefs of Bonaire,Netherlands Antilles

Synopsis We conducted an analysis of species associations using fish diversity and abundance surveys conducted in Bonaire Marine Park by recreational divers. We used data from the REEF (Reef Environmental Education Foundation) Fish Survey Project to compute Bray–Curtis similarity coefficients for all species pairs for the 100 most abundant species. We quantified relationships between species using hierarchical agglomerative clustering and non-metric multidimensional scaling (MDS) of the matrix of Bray–Curtis similarity coefficients. We identified three clusters of species from the analysis. MDS results showed species clusters occupied distinct regions across a continuous gradient of species in two-dimensional space, rather than form distinct clusters. While differences in habitat requirements can explain some of the pattern in pairwise species interactions, these results suggest that there are significant direct and indirect behavioral interactions mediating the distribution and abundance of species. Studies conducted to elucidate patterns of species-habitat relationships have been central to conservation planning for marine protected areas (MPAs). However, the role of behavioral interactions between species driving the dynamics of species composition within MPA networks, designed for representation of biological diversity, should be considered when selecting sites in order to be effective.     

Bryozoan assemblages on hard substrata: species abundance distribution and competition for space

Bryozoans are colonial invertebrates that often dominate epibenthic assemblages on the lower surfaces of hard substrata. Competition among neighbouring organisms is usually a critical process regulating biodiversity, and hard substrata have proved to be a suitable model habitat to analyse spatial interactions. We explored the relationships among abundance, species richness, diversity, competitive ability, coverage, available surface, depth and substratum size in an assemblage of bryozoans encrusting pebbles and cobbles in a bank off the eastern mouth of the Strait of Magellan. We also tested whether overgrowth competition can be regarded as hierarchical, and whether the species abundance distribution shows a mode of rare species and a decreasing frequency of increasingly abundant species. Abundance, species richness, diversity and overgrowth competition were highest on the largest substrata. Smaller pebbles tended to be encrusted by the commonest bryozoans, while the rarest species were mainly found on relatively larger clasts. A high proportion of the lower surfaces of most substrata was available for growth. Diversity values of relatively shallow stations were lower than expected under Caswell’s neutral model. Interspecific competition was hierarchical, but the abundance of colonies was not related to the competitive ability of each species. The species abundance distribution was bimodal, with a main mode of rare species and a second one partly composed of relatively abundant bryozoans with poor competitive abilities. This study shows that even in an encrusting assemblage where competition is hierarchical, the weakest competitors persist and the dominant species are far from being capable of monopolizing space.     

EQUIVALENCE OF COMPETITORS IN PLANT COMMUNITIES: A NULL HYPOTHESIS AND A FIELD EXPERIMENTAL APPROACH

One of the underlying assumptions of both theoretical and empirical community ecology is that the processes determining community composition and abundance of species are interactions specific to particular pairs of species. However, we argue that, in sessile plants at least, competitive interactions are not usually species-specific and that there exists a large degree of equivalence of the effect of species of similar growth form on the ability of any particular species to establish within a community. This null hypothesis of equivalence of competitive effects is based on three characteristics of plants: homogeneity of resource requirements among autotrophs; low encounter probabilities between individuals of any particular species pair; and the predominance of size asymmetries between competing individuals (e.g., seedling-adult interactions.) We present an experimental design to quantify competitive interactions among plant species under field conditions and therefore enable statistical comparisons of competitive abilities among species. The competitive effect of one “neighbor” species on one “target” species is measured as the slope of a regression of performance of target individuals on biomass (or other measure of amount) of its immediate neighbors. Use of the design to test for equivalence of competitive effects and other advantages are described.     

Interactions of gall-forming species at different plant spatial scales

Interspecific competition has been intensely studied as an organizing force in insect herbivore communities that can be mediated by changes in resource availability. We analyzed patterns of interspecific association of three species of gall-forming insects at shoot length class and shrub levels for Bauhinia brevipes through a null model program. Results show that shoots galled by three species were distributed independently among shoot length classes over 3-years, hence, no evidence of competition for shoots was found. Nevertheless, at the plant level our results suggest that there was a positive association. We found no evidence of any reciprocal negative effect because the density of species did not differ among shoot length classes. We suggest that this lack of pattern was probably due to: (a) host-plant resistance mediating interactions; (b) higher abundance of plant resource available, or (c) free-feeding herbivores mediating interactions by manipulating the resources used by gall-forming species.     

Addressing common pitfalls does not provide more support to geographical and ecological abundant‐centre hypotheses

A long‐standing hypothesis in biogeography is that a species’ abundance is highest at the centre of its geographical or environmental space and decreases toward the edges. Several studies tested this hypothesis and provided mixed results and overall weak support to the theory. Most studies, however, are affected by several limitations related to the sample size, the comparability among abundance measures, the definition of species geographic range and corresponding environmental space, and the proxy variables used to represent centrality/marginality gradients. Here we test the abundant‐centre hypothesis on 108 bird and mammal species and embrace the plural nature of the hypothesis by considering 9 geographic and ecological centrality/marginality measures. We analyse the species‐specific effect sizes using a meta‐analytical approach, and test whether the support for the hypothesis is mediated by species dispersal abilities, and the geographic and environmental coverage of the data. The summary effect sizes estimated for the 9 measures are largely inconsistent with the theoretical expectations and show a significant amount of residual heterogeneity. Variables such as dispersal distance, geographic and environmental coverage of the data, appear important in explaining the variation observed between different species, but the results are contrary to those originally hypothesized, and inconsistent across centrality/marginality measures and the datasets used. We show that addressing common pitfalls in previous studies does not provide more support to the abundant‐centre hypothesis, with support being very dependent on the centrality/marginality measure tested, the geographic extent considered for the test, and geographic and environmental coverage of the data. The abundant‐centre hypothesis so far remains an appealing speculation with little and variable empirical support.