Pollinator‐mediated assemblage processes in California wildflowers

Community assembly is the result of multiple ecological and evolutionary forces that influence species coexistence. For flowering plants, pollinators are often essential for plant reproduction and establishment, and pollinator‐mediated interactions may influence plant community composition. Here, we use null models and community phylogenetic analyses of co‐occurrence patterns to determine the role of pollinator‐mediated processes in structuring plant communities dominated by congeners. We surveyed three species‐rich genera (Limnanthes, Mimulus and Clarkia) with centres of diversity in the Sierra Nevada of California. Each genus contains species that co‐flower and share pollinators, and each has a robust phylogeny. Within each genus, we surveyed 44–48 communities at three spatial scales, measured floral and vegetative traits and tested for segregation or aggregation of: (i) species, (ii) floral traits (which are likely to be influenced by pollinators), and (iii) vegetative traits (which are likely affected by other environmental factors). We detected both aggregation and segregation of floral traits that were uncorrelated with vegetative trait patterns; we infer that pollinators have shaped the community assembly although the mechanisms may be varied (competition, facilitation, or filtering). We also found that mating system differences may play an important role in allowing species co‐occurrence. Together, it appears that pollinators influence community assemblage in these three clades.     

Checkerboard score–area relationships reveal spatial scales of plant community structure

Identifying the spatial scale at which particular mechanisms influence plant community assembly is crucial to understanding the mechanisms structuring communities. It has long been recognized that many elements of community structure are sensitive to area; however the majority of studies examining patterns of community structure use a single relatively small sampling area. As different assembly mechanisms likely cause patterns at different scales we investigate how plant species co‐occurrence patterns change with sampling unit scale. We use the checkerboard score as an index of species segregation, and examine species C‐score1–sampling area patterns in two ways. First, we show via numerical simulation that the C‐score–area relationship is necessarily hump shaped with respect to sample plot area. Second we examine empirical C‐score–area relationships in arctic tundra, grassland, boreal forest and tropical forest communities. The minimum sampling scale where species co‐occurrence patterns were significantly different from the null model expectation was at 0.1 m² in the tundra, 0.2 m² in grassland, and 0.2 ha in both the boreal and tropical forests. Species were most segregated in their co‐occurrence (maximum C‐score) at 0.3 m² in the tundra (0.54 3 0.54 m quadrats), 1.5 m² in the grassland (1.2 3 1.2 m quadrats), 0.26 ha in the tropical forest (71 3 71 m quadrats), and a maximum was not reached at the largest sampling scale of 1.4 ha in the boreal forest. The most important finding is that the dominant scales of community structure in these systems are large relative to plant body size, and hence we infer that the dominant mechanisms structuring these communities must be at similarly large scales. This provides a method for identifying the spatial scales at which communities are maximally structured; ecologists can use this information to develop hypotheses and experiments to test scale‐specific mechanisms that structure communities.     

Drought response of five conifer species under contrasting water availability suggests high vulnerability of Norway spruce and European larch

The ability of tree species to cope with anticipated decrease in water availability is still poorly understood. We evaluated the potential of Norway spruce, Scots pine, European larch, black pine, and Douglas‐fir to withstand drought in a drier future climate by analyzing their past growth and physiological responses at a xeric and a mesic site in Central Europe using dendroecological methods. Earlywood, latewood, and total ring width, as well as the δ¹³C and δ¹⁸O in early‐ and latewood were measured and statistically related to a multiscalar soil water deficit index from 1961 to 2009. At the xeric site, δ¹³C values of all species were strongly linked to water deficits that lasted longer than 11 months, indicating a long‐term cumulative effect on the carbon pool. Trees at the xeric site were particularly sensitive to soil water recharge in the preceding autumn and early spring. The native species European larch and Norway spruce, growing close to their dry distribution limit at the xeric site, were found to be the most vulnerable species to soil water deficits. At the mesic site, summer water availability was critical for all species, whereas water availability prior to the growing season was less important. Trees at the mesic were more vulnerable to water deficits of shorter duration than the xeric site. We conclude that if summers become drier, trees growing on mesic sites will undergo significant growth reductions, whereas at their dry distribution limit in the Alps, tree growth of the highly sensitive spruce and larch may collapse, likely inducing dieback and compromising the provision of ecosystem services. However, the magnitude of these changes will be mediated strongly by soil water recharge in winter and thus water availability at the beginning of the growing season.     

Positive interactions under nurse-plants: spatial scale, stress gradients and benefactor size

Positive interactions often play an important role in structuring plant communities and increasing biological diversity. Using three scales of resolution, we examine the importance of a long-lived desert tree, ironwood (Olneya tesota), in structuring plant communities and promoting biological diversity in the Sonoran Desert. We examined the positive effects of Olneya canopies of different sizes on plant communities in mesic and xeric habitats throughout the central Gulf Coast subregion of Sonora, Mexico. In xeric sites, Olneya canopies had strong positive effects on plant richness and abundance, and small positive effects on the size of plants, underscoring the role of facilitation in extreme environments. In mesic sites, Olneya canopies had very little effect on perennials and a negative effect on ephemeral richness, suggesting predominantly competitive effects in this less stressful environment. Overall, Olneya canopies increased biological diversity where abiotic stress was high, but did not increase diversity in more mesic areas. Thus Olneya canopies caused consistent shifts in plant-community structure among xeric and mesic sites, but not when these landscapes were combined. Benefactor size also mediated positive interactions, with larger Olneya canopies supporting larger perennials in both xeric and mesic sites. Thus stress gradients and benefactor size both influenced the balance of facilitative and competitive effects under nurse-plant canopies, and the spatial scale at which facilitative effects shape community structure.     

Competitive dominance in the organisation of Mediterranean ant communities

1. Competition by dominant species is thought to be key to structuring ant communities. However, recent findings suggest that the effect of dominant species on communities is less pronounced than previously assumed. 2. The aim of the present study was to identify the role of dominant ants in the organisation of Mediterranean communities, particularly the role of competition in invaded and uninvaded communities. The effects on ant assemblages of two dominant ants, the invasive Argentine ant and the native ant, Tapinoma nigerrimum Nylander, were assessed. 3. The abundances of both dominant ants were significantly correlated with a decrease in native ant richness at traps. However, only the invasive ant was associated with a reduction in diversity and abundance of other ant species at site scale. In the presence of T. nigerrimum, species co‐occurrence patterns were segregated or random. Community structure in both the dominant‐free and the Argentine ant sites showed random patterns of species co‐occurrence. 4. The present findings indicate that dominant ants regulate small‐scale diversity by competition. However, at the broader scale of the assemblage, T. nigerrimum may only affect species distribution, having no apparent effect on community composition. Moreover, we find no evidence that inter‐specific competition shapes species distribution in coastal Mediterranean communities free of dominant ants. 5. These results show that dominant species may affect ant assemblages but that the nature and the intensity of such effects are species and scale dependent. This confirms the hypothesis that competitive dominance may be only one of a range of factors that structure ant communities.     

Intraspecific facilitation: a missing process along increasing stress gradients – insights from simulated shrub populations

In recent years many field studies have been conducted to assess the relative importance of facilitation and competition in structuring vegetation communities in different environments. Herein, we present a simulation model which systematically explores the relative importance of intra‐specific facilitation and competition between adult shrubs and seedlings for spatial pattern formation. A grid‐based simulation model was constructed and calibrated using data collected in the field from Sarcopoterium spinosum populations in Israel to simulate population dynamics along a rainfall gradient. A series of simulation experiments was conducted in which manipulations of seedling survival probabilities were carried out to assess the relative importance of these processes in generating spatial patterns. Increased survival probabilities of first‐year shrubs in open areas were used to simulate competition effects, while increased survival probabilities in the vicinity of shrubs were used to simulate facilitation effects. Simulation results were then compared to shrub spatial patterns observed in the field. The results indicate that facilitation is not an important process in generating intra‐specific spatial patterns. Rather, in mesic environments with high precipitation, competition is the dominant process generating spatial patterns, resulting in regular spacing of shrubs, similarly to the patterns observed in the field (L(h) values<0). In arid sites, where precipitation values are lower, and stress conditions are higher, the dominant process generating spatial patterns was random mortality due to drought conditions. The resulting spatial patterns in this case are random (L(h)～0), whereas observed field populations exhibited clumped patterns (L(h)>0). We conclude that as stress conditions increase, the importance of intraspecific neighborhood interactions decrease whereas the importance of environmental factors increase in dictating intra‐specific spatial pattern formation. Consequently in mesic environments intra‐specific competition among adults determines the emerging patterns, while intraspecific facilitation is a negligible process.     

Patterns of functional clonal traits and clonal growth modes in contrasting grasslands in the central Apennines,Italy

Abstract. Aim: Patterns of plant functional traits related to clonality (clonal growth modes; CGM) in plant communities were studied and hypotheses on the importance of the selected traits in plant communities supported by soils differing in moisture and nutrient status were tested. Material and Methods: Selected plant functional traits, such as the position of the mother‐daughter plants connections, length of spacers, frequency of multiplication, persistency of ramets connections, presence of storage organs and bud protection were studied in two contrasting plant communities (xeric and mesic abandoned pastures) typical of central Apennines, Italy. Results and Discussion: Clonality was shown to be of great importance in both mesic and xeric grasslands. The major differences between the two communities were due to the dominant CGMs: turf graminoids (having effective protection of growth meristems in dense tussocks) dominated xeric grasslands, while rhizomatous graminoids (typical of competitive resource‐rich environments) dominated mesic grasslands. Below‐ground CGOs (clonal growth organs), shorter spacers, higher multiplication potential, permanent ramet connection, large bud bank and increased importance of bud protection were found to be of importance in water stressed xeric grassland. Contrary to our expectations, the mesic (less stressed) grasslands have the higher number of clonal plants possessing storage organs.     

Soil environmental heterogeneity allows spatial co‐occurrence of competitor earthworm species in a gallery forest of the Colombian ‘Llanos’

Disentangling how communities of soil organisms are deterministically structured by abiotic and biotic factors is of utmost relevance, and few data sets on co‐occurrence patterns exist in soil ecology compared to other disciplines. In this study, we assessed species spatial co‐occurrence and niche overlap together with the heterogeneity of selected soil properties in a gallery forest (GF) of the Colombian Llanos. We used null‐model analysis to test for non‐random patterns of species co‐occurrence and body size in assemblages of earthworms and whether the pattern observed was the result of environmental heterogeneity or biotic processes structuring the community at small scales by means of co‐inertia analysis (CoIA). The results showed that earthworm species co‐occurred more frequently than expected by chance at short distances, and CoIA highlighted a significant specific relationship between earthworm species and soil variables. The effect of soil environmental heterogeneity on one litter‐feeding species but also the impact of soil‐feeding species on soil physical properties was revealed. Correlogram analysis on the first axis extracted in the CoIA showed the scale of the common structure shared by the fauna and soil variable tables. The earthworm community was not deterministically structured by competition and co‐occurrence of competing species was facilitated by soil environmental heterogeneity at small scales in the GF. Our results agreed with the coexistence aggregation model which suggests that spatial aggregation of competitors at patchily distributed resources (environment) can facilitate species coexistence.     

Nonrandom co-occurrence patterns of rainforest chameleons

Null‐model analysis of co‐occurrence patterns is a powerful tool to identify ‘structure’ in community ecology data sets. We evaluated the community structure of chameleons in rainforest regions of Nigeria and Cameroon using available data in the literature, including peer‐reviewed articles and unpublished environmental reports to industries. We performed Monte Carlo simulations (5000 iterations, using the sequential swap algorithm) under several model assumptions to derive co‐occurrence patterns among species. Food and spatial (habitat) segregation patterns in both lowland rainforest and montane forest were investigated. We subjected four indices of co‐occurrence patterns (C‐ratio, number of checkerboard species pairs, number of species combinations, and V‐score) to randomization procedures. Overall, the chameleon communities do not show random organization, but instead exhibit precise deterministic patterns. In lowland rainforest, chameleon communities are assembled deterministically along the food niche resource axis, but not along the habitat niche resource axis. The opposite holds for chameleon communities in montane rainforest. We predict that these patterns can be generalized to other regions of tropical Africa, thus helping to determine the general structure of chameleon communities in tropical African forests.     

Ecological assembly rules in plant communities—approaches,patterns and prospects

Understanding how communities of living organisms assemble has been a central question in ecology since the early days of the discipline. Disentangling the different processes involved in community assembly is not only interesting in itself but also crucial for an understanding of how communities will behave under future environmental scenarios. The traditional concept of assembly rules reflects the notion that species do not co‐occur randomly but are restricted in their co‐occurrence by interspecific competition. This concept can be redefined in a more general framework where the co‐occurrence of species is a product of chance, historical patterns of speciation and migration, dispersal, abiotic environmental factors, and biotic interactions, with none of these processes being mutually exclusive. Here we present a survey and meta‐analyses of 59 papers that compare observed patterns in plant communities with null models simulating random patterns of species assembly. According to the type of data under study and the different methods that are applied to detect community assembly, we distinguish four main types of approach in the published literature: species co‐occurrence, niche limitation, guild proportionality and limiting similarity. Results from our meta‐analyses suggest that non‐random co‐occurrence of plant species is not a widespread phenomenon. However, whether this finding reflects the individualistic nature of plant communities or is caused by methodological shortcomings associated with the studies considered cannot be discerned from the available metadata. We advocate that more thorough surveys be conducted using a set of standardized methods to test for the existence of assembly rules in data sets spanning larger biological and geographical scales than have been considered until now. We underpin this general advice with guidelines that should be considered in future assembly rules research. This will enable us to draw more accurate and general conclusions about the non‐random aspect of assembly in plant communities.     

A comprehensive framework for the study of species co‐occurrences,nestedness and turnover

Binary presence–absence matrices (rows = species, columns = sites) are often used to quantify patterns of species co‐occurrence, and to infer possible biotic interactions from these patterns. Previous classifications of co‐occurrence patterns as nested, segregated, or modular have led to contradictory results and conclusions. These analyses usually do not incorporate the functional traits of the species or the environmental characteristics of the sites, even though the outcomes of species interactions often depend on trait expression and site quality. Here we address this shortcoming by developing a method that incorporates realized functional and environmental niches, and relates them to species co‐occurrence patterns. These niches are defined from n‐dimensional ellipsoids, and calculated from the n eigenvectors and eigenvalues of the variance–covariance matrix of measured environmental or trait variables. Average niche overlap among species and the spatial distribution of niches define a triangle plot with vertices of species segregation (low niche overlap), nestedness (high niche overlap), and modular co‐occurrence (clusters of overlapping niches). Applying this framework to temperate understorey plant communities in southwest Poland, we found a consistent modular structure of species occurrences, a pattern not detected by conventional presence–absence analysis. These results suggest that, in our case study, habitat filtering is the most important process structuring understorey plant communities. Furthermore, they demonstrate how incorporating trait and environmental data into co‐occurrence analysis improves pattern detection and provides a stronger theoretical framework for understanding community structure.     

The vertical distribution of below-ground biomass in grassland communities in relation to grazing regime and habitat characteristics

Abstract. 40 sites, representing different pasture types in Northwest Spain, were sampled in respect of their floristic composition, distribution of above and below-ground biomass and environmental and physical variables. Five plant community types were identified by classification techniques of plant species composition. These communities were then characterized in terms of the percentage of ground covered by herbaceous and shrub vegetation, stones, rocks and gaps as well as their topographic location and characteristics of the shallow soil (pH, organic matter, nitrogen and calcium content). Bio-mass was assessed in terms of above-ground structures, surface crowns and three below-ground layers to a depth of 10 cm. Three types of grazing regime were distinguished: Concentrated Intense Grazing in early spring (CIG), Extended Intense Grazing throughout the spring (EIG), and Non-Intense Grazing (NIG). Grazing regime showed the highest association with plant community type and three broad categories were identified: xeric stressed pastures, which nevertheless received CIG, mesic pastures with EIG, and three kinds of NIG mesic pastures. The xeric communities had the highest proportion of aboveground biomass, as a consequence of their greater proportion of woody perennials. These xeric communities displayed a more gradual reduction in below-ground biomass with depth than mesic pastures, a likely consequence of the low water content in the upper soil layers. The mesic communities had a high concentration of below-ground biomass in the upper layers when they were intensely grazed. However, when grazing was low (i.e. NIG situations), these communities had greater variability in biomass profiles than any of the other pasture types. Possible causes of the patterns in biomass distribution of the intensely grazed pastures are discussed.     

Patterns of species co-occurrence and density compensation: a test for interspecific competition in bat ectoparasite infracommunities

Parasites constitute an ideal system with which to investigate patterns and mechanisms of community structure and dynamics. Nevertheless, despite their prevalence in natural systems, parasites have been examined less often than other organisms traditionally used for testing hypotheses of community assembly. In the present study, we investigate possible effects of competitive interactions on patterns of distribution (co‐occurrence) and density among a group of streblid bat flies parasitic on short‐tailed fruit bats, Carollia perspicillata. Using null model analyses of species co‐occurrence, we did not find evidence that competition affects the distribution of bat fly species across hosts. Moreover, when non‐infested hosts were included, analyses showed evidence for interspecific aggregation, rather than for the segregation predicted by competition theory. Partial Pearson correlations among bat fly species densities showed no evidence of negative covariation in two of three cases. In the species pair for which a significant negative correlation was found, a visual analysis of plotted covariation indicated a constraint line, suggesting that competition between these two species might become operational only in some infracommunities when abundances of bat flies approach a maximum set by one or more limiting resources. Moreover, when a community‐wide estimation of the significance of density compensation was calculated, the result was not significant. Overall, we find no evidence that competition influences the distribution of bat flies on their hosts, and mixed support for effects of competition on the densities of species. These results are consistent with the idea that competition plays a role in structuring natural communities, but in many systems its effects are context‐dependent and might not be important relative to other factors. Wider analyses across taxonomic and environmental gradients and a detailed consideration of the different hypothesized effects of competition are necessary to fully understand the importance of competition on natural communities.     

Frequency and intensity of facilitation reveal opposing patterns along a stress gradient

Disentangling the different processes structuring ecological communities is a long‐standing challenge. In species‐rich ecosystems, most emphasis has so far been given to environmental filtering and competition processes, while facilitative interactions between species remain insufficiently studied. Here, we propose an analysis framework that not only allows for identifying pairs of facilitating and facilitated species, but also estimates the strength of facilitation and its variation along environmental gradients. Our framework combines the analysis of both co‐occurrence and co‐abundance patterns using a moving window approach along environmental gradients to control for potentially confounding effects of environmental filtering in the co‐abundance analysis. We first validate our new approach against community assembly simulations, and exemplify its potential on a large 1,134 plant community plots dataset. Our results generally show that facilitation intensity was strongest under cold stress, whereas the proportion of facilitating and facilitated species was higher under drought stress. Moreover, the functional distance between individual facilitated species and their facilitating species significantly changed along the temperature–moisture gradient, and seemed to influence facilitation intensity, although no general positive or general negative trend was discernible among species. The main advantages of our robust framework are as follows: It enables detecting facilitating and facilitated species in species‐rich systems, and it allows identifying the directionality and intensity of facilitation in species pairs as well as its variation across long environmental gradients. It thus opens numerous opportunities for incorporating functional (and phylogenetic) information in the analysis of facilitation patterns. Our case study indicated high complexity in facilitative interactions across the stress gradient and revealed new evidence that facilitation, similarly to competition, can operate between functionally similar and dissimilar species. Extending the analyses to other taxa and ecosystems will foster our understanding how complex interspecific interactions promote biodiversity.     

Geographic variations of seasonality and coexistence in communities: The role of diversity and climate

One of the most conspicuous and widely analyzed patterns in ecology is the latitudinal gradient in species richness. Over the 200 years since its recognition, several hypotheses have accumulated in order to account for spatial variations in diversity. Geographic variations in seasonality have been repeatedly proposed as a determinant of community richness. However, the geographic structure of community seasonality has not yet been analyzed. In the present work we evaluated three hypotheses that account for variations in the temporal structuring of communities: first, environmental seasonality determines community seasonality; second, community richness determines its degree of structuring; and third, the presence of an increase in species segregation with latitude, reflected in a pattern of species negative co‐occurrence. The hypotheses were evaluated using path analysis on 29 amphibian communities from South America, connecting latitude, environmental conditions, diversity, seasonality, and coexistence structure – nestedness and negative co‐occurrence – within communities. Latitude positively affects community seasonality through an increase in temperature seasonality, but a weak negative direct effect suggests that other variables not considered in the model – such as the strength of biotic interactions – could also be involved. Both latitude and diversity (directly and indirectly) determine an increase in negative co‐occurrence and nestedness. This suggests that groups of species that are mutually nested in time are internally segregated. Further, the strength of this structure is determined by community diversity and latitude. Temporal structuring of a community is associated with latitude and diversity, pointing to the existence of a systematic change in community organization far beyond, but probably interrelated, with the recognized latitudinal trend in richness. The available information and analysis supported the three hypotheses evaluated.     

Competitive interactions change the pattern of species co‐occurrences under neutral dispersal

Non‐random patterns of species segregation and aggregation within ecological communities are often interpreted as evidence for interspecific interactions. However, it is unclear whether theoretical models can predict such patterns and how environmental factors may modify the effects of species interactions on species co‐occurrence. Here we extend a spatially explicit neutral model by including competitive effects on birth and death probabilities to assess whether competition alone is able to produce non‐random patterns of species co‐occurrence. We show that transitive and intransitive competitive hierarchies alone (in the absence of environmental heterogeneity) are indeed able to generate non‐random patterns with commonly used metrics and null models. Moreover, even weak levels of intransitive competition can increase local species richness. However, there is no simple rule or consistent directional change towards aggregation or segregation caused by competitive interactions. Instead, the spatial pattern depends on both the type of species interaction and the strength of dispersal. We conclude that co‐occurrence analysis alone may not able to identify the underlying processes that generate the patterns.     

Competition from sea to mountain: Interactions and aggregation in low‐diversity monogenean and endohelminth communities in twospot livebearer Pseudoxiphophorus bimaculatus (Teleostei: Poeciliidae) populations in a neotropical river

1. The role of interspecific interactions in structuring low‐diversity helminth communities is a controversial topic in parasite ecology research. Most parasitic communities of fish are species‐poor; thus, interspecific interactions are believed to be unimportant in structuring these communities.
2. We explored the factors that might contribute to the richness and coexistence of helminth parasites of a poeciliid fish in a neotropical river.
3. Repeatability of community structure was examined in parasitic communities among 11 populations of twospot livebearer Pseudoxiphophorus bimaculatus in the La Antigua River basin, Veracruz, Mexico. We examined the species saturation of parasitic communities and explored the patterns of species co‐occurrence. We also quantified the associations between parasitic species pairs and analyzed the correlations between helminth species abundance to look for repeated patterns among the study populations.
4. Our results suggest that interspecific competition could occur in species‐poor communities, aggregation plays a role in determining local richness, and intraspecific aggregation allows the coexistence of species by reducing the overall intensity of interspecific competition.

     

Oribatid mites reveal that competition for resources and trophic structure combine to regulate the assembly of diverse soil animal communities

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Neutral processes and species sorting in benthic microalgal community assembly: effects of tidal resuspension

Benthic microalgae (BMA) provide vital food resources for heterotrophs and stabilize sediments with their extracellular secretions. A central goal in ecology is to understand how processes such as species interactions and dispersal, contribute to observed patterns of species abundance and distribution. Our objectives were to assess the effects of sediment resuspension on microalgal community structure. We tested whether taxa‐abundance distributions could be predicted using neutral community models (NCMs) and also specific hypotheses about passive migration: (i) As migration decreases in sediment patches, BMA α‐diversity will decrease, and (ii) As migration decreases, BMA community dissimilarity (β‐diversity) will increase. Co‐occurrence indices (checkerboard score and variance ratio) were also computed to test for deterministic factors, such as competition and niche differentiation, in shaping communities. Two intertidal sites (mudflat and sand bar) differing in resuspension regime were sampled throughout the tidal cycle. Fluorometry and denaturing gradient gel electrophoresis were utilized to investigate diatom community structure. Observed taxa‐abundances fit those predicted from NCMs reasonably well (R² of 0.68–0.93), although comparisons of observed local communities to artificial randomly assembled communities rejected the null hypothesis that diatom communities were assembled solely by stochastic processes. No co‐occurrence tests indicated a significant role for competitive exclusion or niche partitioning in microalgal community assembly. In general, predictions about relationships between migration and species diversity were supported for local community dynamics. BMA at low tide (lowest migration) exhibited reduced α‐diversity as compared to periods of immersion at both mudflat and sand bar sites. β‐diversity was higher during low tide emersion on the mudflat, but did not differ temporally at the sand bar site. In between‐site metacommunity comparisons, low‐ and high‐resuspension sites exhibited distinct community compositions while the low‐energy mudflats contained higher microalgal biomass and greater α‐diversity. To our knowledge this is the first study to test the relevance of neutral processes in structuring marine microalgal communities. Our results demonstrate a prominent role for stochastic factors in structuring local BMA community assembly, although unidentified nonrandom processes also appear to play some role. High passive migration, in particular, appears to help maintain species diversity and structure communities in both sand and muddy habitats.     

Co‐occurrence and phylogenetic distance in communities of mammalian ectoparasites: limiting similarity versus environmental filtering

Similarity between species plays a key role in the processes governing community assembly. The co‐occurrence of highly similar species may be unlikely if their similar needs lead to intense competition (limiting similarity). On the other hand, persistence in a particular habitat may require certain traits, such that communities end up consisting of species sharing the same traits (environmental filtering). Relatively little information exists on the relative importance of these processes in structuring parasite communities. Assuming that phylogenetic relatedness reflects ecological similarity, we tested whether the co‐occurrence of pairs of flea species (Siphonaptera) on the same host individuals was explained by the phylogenetic distance between them, among 40 different samples of mammalian hosts (rodents and shrews) from different species, areas or seasons. Our results indicate that frequency of co‐occurrence between flea species increased with decreasing phylogenetic distance between them in 37 out of 40 community samples, with 14 of these correlations being statistically significant. A meta‐analysis across all samples confirmed the overall trend for closely related species to co‐occur more frequently on the same individual hosts than expected by chance, independently of the identity of the host species or of environmental conditions. These findings suggest that competition between closely related, and therefore presumably ecologically similar, species is not important in shaping flea communities. Instead, if only fleas with certain behavioural, ecological and physiological properties can encounter and exploit a given host, and if phylogenetic relationships determine trait similarity among flea species, then a process akin to environmental filtering, or host filtering, could favour the co‐occurrence of related species on the same host.