On the relative importance of history and ecology in structuring communities of desert small animals

Several recent studies have compared small mammal community structure across multiple deserts on different continents. These studies have tacitly assumed that variation in community structure was greater between continents than within, and so have not evaluated variation across desert regions within continents. I evaluated several metrics of community structure and a model of community assembly for four desert regions in North America - the Great Basin, Mojave. Sonoran. and Chihuahuan Deserts in order to explicitly compare these metrics across these deserts. Additionally. I compared these results with similar analyses conducted on two desert regions in central Asia - the Gobi Desert and the Turan Desert Region to evaluate the relative magnitude of intra- vs inter-continental variation. Although the patterns observed are complex, they demonstrated marked heterogeneity in desert small mammal communities within North America. However, this heterogeneity is much less than that observed in inter-continental comparisons, in which Asian and North American deserts differ markedly. These results agree with other recent studies providing limited or no support for the existence of substantial convergence in community characteristics or ecological function across geographically distant regions. Rather, the results support the hypothesis that the common evolutionary history of faunas in globally disjunct landmasses has had a stronger influence on the evolution of communities and faunas than do regional variations in climate, physiography, etc. Whereas a common ecological setting may have large impacts on some facets of organismal structure (e.g., bipedalism in desert small mammals), common evolutionary history appears to have a more profound influence on local dynamics.     

Scaling coexistence and assemblage patterns of desert small mammals

Scaling biodiversity patterns has been recognized lately as a very important issue in the search of global processes; however coexistence and assemblage patterns are typically approached at a single spatial scale. Here, we examined coexistence and co-occurrence patterns of desert small mammal communities across different spatial scales in the search of general community patterns. We sampled small mammals in Monte desert (Argentina, South America) for small spatial scales and reviewed published papers from other worldwide deserts for large spatial scale analyses. We used classic community estimators (Shannon, Richness), rank abundance curves and fitting distributions to analyze species coexistence and co-occurrence patterns. Assemblage patterns were analyzed evaluating nestedness across spatial scales and among deserts. Worldwide desert small mammal assemblages are characterized mainly by low species richness and high variation in species composition. The central Monte desert of Argentina showed a consistent assemblage pattern across spatial scales, with a generalist species being the most abundant and widely distributed, accompanied by other subordinate and more narrowly distributed species. All Monte desert communities were significantly nested, with nestedness increasing with scale from patch to regional. Assemblage and coexistence patterns were similar when comparing worldwide deserts despite differences in total richness and faunal singularity. The degree of nestedness varied among worldwide deserts; however all of them showed a consistent nested pattern. Differences in the degree of nestedness could be a result of different regulating factors depending on the desert and scale. These results highlight the importance of including multiscale approaches when dealing with processes that structure desert communities.     

Patterns in the local assembly of Egyptian rodent faunas: Areography and species combinations

We implicate ecological processes in assembly patterns of Egyptian desert rodents using spatial variation of distribution and composition of local assemblages. We also compare our assemblages to prior analyses of North American and Australian small mammal communities in terms of species richness and representation of trophic groups.We studied patterns of occurrence of 29 rodent species among 335 collecting events in 308 sites using museum specimen records resulting from a country-wide survey. The studied taxa vary greatly in their natural histories and ecology. Fully 69% of studied species (20) were localized in 30 or fewer sites (9% of all sites). Site incidence was closely correlated with the geographic range size of species, while local abundance of a species showed little relationship to its geographic range. The species richness of local assemblages ranged from a lone species at 73 sites to 11 species at 3 sites. A total of 214 different combinations were recorded of which 164 (77%) were unique to a single site. G. gerbillus was both the most abundant and most ubiquitous species. Coexistence with other species was positively correlated with incidence and geographic range size. Body mass distribution was remarkably uniform for the fauna as a whole, and influenced the geographic distributions and abundance of individual species.Our sites have low species richness and substantial variability in species composition, which also characterize desert rodent communities elsewhere. Habitat requirements, exclusive distributions of sibling species, low primary productivity, and Egypt's location all influenced species assembly. The Egyptian and Australian deserts supported higher proportions of low richness assemblages compared to North America. As in North America, Egyptian sites were dominated by granivorous species.     

Desert vines: a comparison of Australia with other areas

Aim To characterize the Australian desert vine flora and to compare it with that of deserts in other continents. Location The Stony Deserts, the Simpson Desert and the four main deserts of Western Australia. Methods The Western Australian Herbarium data base and published papers were used to develop vine checklists for each Australian desert studied. A literature search was used to classify the families and genera into phytogeographical elements and to provide data for intercontinental comparisons. Results Thirty‐seven vine species are listed for the six Australian deserts studied. They constitute from 0.8 to 2.7% of the vascular flora, which is within the normal range for arid zone floras. Comparing Australian data with those from the Sonoran and Chihuahuan deserts (North America), for non‐vine taxa, very different phylogenetic lines are present. However, for vines, three of the four most important families are the same in each case, viz. Convolvulaceae, Fabaceae and Asclepiadaceae. This reflects large pantropical and cosmopolitan families shared between all three data sets. At the generic level, in Australia pantropical taxa and taxa from the Old World tropics far outnumber endemic ones, as do pantropical and neotropical genera in North America. Herbaceous vines predominate in Australian deserts as they do in North American ones, but nevertheless, the percentage of woody vines is higher in Australia (32%) than in North America (highest value of 24%). Earlier views that Australian deserts are rich in annual vines are not supported. Main conclusions For many life‐forms, the Australian flora is composed of very different phylogenetic lines to the floras of other continents. However, for desert vines, at the level of family and even of genus, there are surprising similarities between Australia and even a continent as distant as North America, because of shared pantropical and cosmopolitan taxa.     

Trophic relationships and their importance for biotopic distribution of grasshoppers (Orthoptera,Acridoidea) in semi-deserts and deserts of the lower Volga river area

The trophic associations of grasshoppers in the semi-deserts and deserts of the lower Volga area are analyzed. Most of the species studied are narrow oligophages or monophages; they constitute four groups trophically associated with edificators of the commonest desert plant communities (hygro- and mesophytic grasses, xerophytic grasses, wormwoods, and chenopods). The trophic specialization is shown to contribute significantly to the formation of biotopic association and structure of grasshopper assemblages.     

Small mammal seed consumption in the Karoo,South Africa: further evidence for divergence in desert biotic processes

Summary Annual seed consumption by a small mammal community in the semi-arid Karoo, South Africa, was calculated using population, biomass and dietary estimates and published estimates of field metabolic rate. The community comprised a macroscelid elephant shrew Macroscelides proboscideus, and two rodents Gerbillurus paeba and Desmodillus auricularis. Only G. paeba consumed seed during the study, and then in low amounts (annual mean = 0.4% of diet, insects = 53%, herbage = 47%), with M. proboscideus and D. auriocularis consuming mainly insects (88%) and herbage (90%), respectively. Energy requirements for the G. paeba population averaged 568 kJ ha^–1 day^–1. Total seed consumption was estimated to be 140 g ha^–1, or 155 000 seeds ha^–1 which represents 0.5% of the annual seed production for this site. These levels of granivory by small mammals are significantly lower than those recorded for North American deserts, and raises the question as to why granivory varies between these systems. These findings also refute the hypothesis that as a result of the harshness and similarity of physical conditions in deserts, the properties of desert communities are convergent.     

Testing multiple assembly rule models in avian communities on islands of an inundated lake,Zhejiang Province,China

Aim A fundamental question in community ecology is whether general assembly rules determine the structure of natural communities. Although many types of assembly rules have been described, including Diamond’s assembly rules, constant body‐size ratios, favoured states, and nestedness, few studies have tested multiple assembly rule models simultaneously. Therefore, little is known about the relative importance of potential underlying factors such as interspecific competition, inter‐guild competition, selective extinction and habitat nestedness in structuring community composition. Here, we test the above four assembly rule models and examine the causal basis for the observed patterns using bird data collected on islands of an inundated lake. Location Thousand Island Lake, China. Methods  We collected data on presence–absence matrices, body size and functional groups for bird assemblages on 42 islands from 2007 to 2009. To test the above four assembly rule models, we used null model analyses to compare observed species co‐occurrence patterns, body‐size distributions and functional group distributions with randomly generated assemblages. To ensure that the results were not biased by the inclusion of species with extremely different ecologies, we conducted separate analyses for the entire assemblage and for various subset matrices classified according to foraging guilds. Results The bird assemblages did not support predictions by several competitively structured assembly rule models, including Diamond’s assembly rules, constant body‐size ratios, and favoured states. In contrast, bird assemblages were highly significantly nested and were apparently shaped by extinction processes mediated through area effects and habitat nestedness. The nestedness of bird assemblages was not a result of passive sampling or selective colonization. These results were very consistent, regardless of whether the entire assemblage or the subset matrices were analysed. Main conclusions Our results suggest that bird assemblages were shaped by extinction processes mediated through area effects and habitat nestedness, rather than by interspecific or inter‐guild competition. From a conservation point of view, our results indicate that we should protect both the largest islands with the most species‐rich communities and habitat‐rich islands in order to maximize the number of species preserved.     

Trophic Niche Differentiation and Microhabitat Utilization in a Species‐rich Montane Forest Small Mammal Community of Eastern Madagascar

Unraveling the mechanisms facilitating species coexistence in communities is a central theme in ecology. Species‐rich tropical mammal communities provide excellent settings to explore such mechanisms as they often harbor numerous congeneric species with close phylogenetic relationships. Explicit tests for the mechanisms that allow syntopic occurrence in these assemblages, however, is often hampered because of the difficulty in obtaining detailed ecological data on the organisms making up the community. Using stable nitrogen and carbon ratios of hair samples, we examine whether trophic niche differentiation and microhabitat segregation explain the coexistence of 21 small mammal species at a montane humid forest site in eastern Madagascar. Overall, the community was trophically diverse and covered wide isotopic space. This diversity was based on: (1) a multi‐layered trophic community structure with mainly frugivorous‐granivorous rodents (subfamily Nesomyinae) as primary consumers and insectivorous tenrecs (family Tenrecidae) as secondary and tertiary consumers; (2) trophic segregation of rodents and tenrecs with the latter occupying different microhabitats; and (3) a dense and regular packing of species in the community. The 12 locally occurring Microgale shrew tenrecs (subfamily Oryzorictinae) showed high trophic redundancy, but were maximally spaced from each other within the trophic space covered by the genus. Results of stable isotope analysis suggest that in combination the differentiation of microhabitats and trophic niches explain the coexistence of small mammals in this community. Congeneric species appeared to be under more intense competition compared with non‐congeneric species and their coexistence can only partly be explained by trophic and microhabitat niche segregation.     

The role of local versus biogeographical processes in influencing diversity and body‐size variation in mammal assemblages

Our objective was to estimate and analyze the body‐size distribution parameters of terrestrial mammal assemblages at different spatial scales, and to determine whether these parameters are controlled by local ecological processes or by larger‐scale ones. Based on 93 local assemblages, plus the complete mammal assemblage from three continents (Africa, North, and South America), we estimated three key distribution parameters (diversity/size slope, skewness, and modal size) and compared the values to those expected if size distributions are mainly controlled by local interactions. Mammal diversity decreased much faster as body size increased than predicted by fractal niche theory, both at continental and at local scales, with continental distributions showing steeper slopes than the localities within them. South America showed a steeper slope (after controlling for species diversity), compared to Africa and North America, at local and continental scales. We also found that skewness and modal body size can show strikingly different correlations with predictor variables, such as species richness and median size, depending on the use of untransformed versus log‐transformed data, due to changes in the distribution density generated by log‐transformation. The main differences in slope, skewness, and modal size between local and continental scales appear to arise from the same biogeographical process, where small‐sized species increase in diversity much faster (due to higher spatial turnover rates) than large‐sized species. This process, which can operate even in the absence of competitive saturation at local scales, generates continental assemblages with steeper slopes, smaller modal sizes, and higher right skewness (toward small‐sized species) compared to local communities. In addition, historical factors can also affect the size distribution slopes, which are significantly steeper, in South American mammal assemblages (probably due to stronger megafauna extinction events in South America) than those in North America and Africa.     

Stochastic and deterministic processes interact in the assembly of desert microbial communities on a global scale

Extreme arid regions in the worlds'' major deserts are typified by quartz pavement terrain. Cryptic hypolithic communities colonize the ventral surface of quartz rocks and this habitat is characterized by a relative lack of environmental and trophic complexity. Combined with readily identifiable major environmental stressors this provides a tractable model system for determining the relative role of stochastic and deterministic drivers in community assembly. Through analyzing an original, worldwide data set of 16S rRNA-gene defined bacterial communities from the most extreme deserts on the Earth, we show that functional assemblages within the communities were subject to different assembly influences. Null models applied to the photosynthetic assemblage revealed that stochastic processes exerted most effect on the assemblage, although the level of community dissimilarity varied between continents in a manner not always consistent with neutral models. The heterotrophic assemblages displayed signatures of niche processes across four continents, whereas in other cases they conformed to neutral predictions. Importantly, for continents where neutrality was either rejected or accepted, assembly drivers differed between the two functional groups. This study demonstrates that multi-trophic microbial systems may not be fully described by a single set of niche or neutral assembly rules and that stochasticity is likely a major determinant of such systems, with significant variation in the influence of these determinants on a global scale.     

小兴安岭阔叶红松林地表甲虫Beta多样性

Beta多样性用来衡量集群内物种组成的变异性,可以被分解为空间物种转换和物种集群镶嵌两个组分,是揭示群落构建机制的重要基础。目前开展了较多的地上生态系统beta多样性研究,然而地下生态系统beta多样性进展缓慢。以小兴安岭凉水和丰林自然保护区为研究地区,于2015年8、10月采用陷阱法对阔叶红松林进行调查,揭示地表甲虫(步甲科、隐翅虫科、葬甲科)的beta多样性。结果表明:(1)凉水共发现39种、856只地表甲虫,丰林共发现43种、1182只地表甲虫。8月凉水明显具有较高的全部甲虫(三个科的总和)物种多样性和丰富度,10月正好相反。(2)凉水和丰林之间地表甲虫beta多样性的差异仅发现于8月的步甲科和葬甲科之间。(3)凉水和丰林地表甲虫的beta多样性主要由空间物种转换组成,物种集群镶嵌对beta多样性的贡献很小,说明地表甲虫物种组成变异主要由本地物种之间较高的转换引起。研究表明小兴安岭阔叶红松林地表甲虫的beta多样性主要由空间物种转换组成,在揭示群落构建机制过程中,其内部物种交换和环境调控不容忽视。     

Nestedness and migratory status of avian assemblages in North America and Europe

We examine patterns of nestedness and species incidence for the resident and migrant components of avifaunas in North America and Europe. While all assemblages were significantly nested, there were no significant differences between North American and European avifaunas overall in nestedness or incidence. Residents did not differ from migrants in their adherence to a nested distribution, but did exhibit significantly higher incidences when continental affiliation was ignored.¶We develop a new nestedness index that examines each species' relative contribution to an assemblage's overall pattern of nestedness. The relative nestedness index exhibits a quadratic relationship with incidence such that species with low incidences and species with high incidences generally increase the overall level of nestedness, while species with intermediate incidence tend to decrease nestedness.     

Diversity of rodent and shrew assemblages in different vegetation types of the savannah biome in South Africa: no evidence for nested subsets or competition

Identifying nonrandom species composition patterns predicted by assembly rules has been a central theme in community ecology. Few studies have investigated the prevalence of multiple drivers on species composition patterns in small mammal assemblages in the Old World. This study investigated seasonal changes in rodent and shrew diversity in eleven savannah vegetation types in South Africa. We tested whether species composition patterns are nonrandom with respect to predictions from Diamond's assembly rules, niche limitation hypothesis and nestedness hypothesis. Species richness estimators indicated that inventories for the rodents (80%) and shrews (100%) were relatively complete. Rodent (n = 11 species) diversity and shrew (n = 5 species) diversity were highest in summer and lowest in autumn. Rodent richness was highest in the Terminalia sericea bushveld and woodlands and lowest in the Drypetes arguta sand forest, whilst shrew richness was highest in the T. sericea bushveld and woodlands and lowest in the Acacia nilotica/Dichrostachys cinerea open shrub savannah. We found no support for the predictions of competition and nestedness hypotheses and suggest that this was probably due to the high seasonal and annual variability in rodent and shrew diversity.     

Soil fungal diversity and assembly along a xeric stress gradient in the central Namib Desert

The Namib Desert of south-western Africa is one of the oldest deserts in the world and possesses unique geographical, biological and climatic features. While research through the last decade has generated a comprehensive survey of the prokaryotic communities in Namib Desert soils, little is yet known about the diversity and function of edaphic fungal communities, and even less of their responses to aridity. In this study, we have characterized soil fungal community diversity across the longitudinal xeric gradient across the Namib desert (for convenience, divided into the western fog zone, the central low-rainfall zone and the eastern high-rainfall zone), using internal transcribed sequence (ITS) metabarcoding. Ascomycota, Basidiomycota and Chytridiomycota consistently dominated the Namib Desert edaphic fungal communities and a core mycobiome composed of only 15 taxa, dominated by members of the class Dothideomycetes (Ascomycota), was identified. However, fungal community structures were significantly different in the fog, low-rainfall and high-rainfall zones. Furthermore, Namib Desert gravel plain fungal community assembly was driven by both deterministic and stochastic processes; the latter dominating in the all three xeric zones. We also present data that suggest that the inland limit of fog penetration represents an ecological barrier to fungal dispersal across the Namib Desert.     

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.     

Nested assemblages resulting from host size variation: the case of endoparasite communities in fish hosts

Nested species subsets are a common pattern in many types of communities found in insular or fragmented habitats. Nestedness occurs in some communities of ectoparasites of fish, as does the exact opposite departure from random assembly, anti-nestedness. Here, we looked for nested and anti-nested patterns in the species composition of communities of internal parasites of 23 fish populations from two localities in Finland. We also compared various community parameters of nested and anti-nested assemblages of parasites, and determined whether nestedness may result simply from a size-related accumulation of parasite species by feeding fish hosts. Nested parasite communities were characterised by higher prevalence (proportion of infected fish) and intensities of infection (number of parasites per fish) than anti-nested communities; the two types of non-random communities did not differ with respect to parasite species richness, however. In addition, the correlation between fish size and the number of parasite species harboured by individual fish was much stronger in nested assemblages than in anti-nested ones, where it was often nil. These results were shown not to be artefacts of sampling effort or host phylogeny. They apply to both assemblages of adult and larval parasites, which were treated separately. Since species of larval parasites are extremely unlikely to interact with one another in fish hosts, the establishment of nestedness appears independent of the potential action of interspecific interactions. The species composition of these parasite communities is not determined from within the community, but rather by the extrinsic influence of host feeding rates and how they amplify differences among parasite species in probabilities of colonisation or extinction. Nested patterns occur in parasite communities whose fish hosts accumulate parasites in a predictable fashion proportional to their size, whereas anti-nested communities occur in parasite communities whose fish hosts do not, possibly because of dietary specialisation preventing them from sampling the entire pool of parasite species available locally. Thus, nestedness in parasite communities may result from processes somewhat different from those generating nested patterns in free-living communities.     

Arthropod communities on creosote bush (<Emphasis Type="Italic">Larrea tridentata</Emphasis>) in desert patches of varying degrees of urbanization

The goal of the present study was to investigate the influence of urbanization on the richness, abundance and composition of arthropod communities associated with creosote bush, Larrea tridentata [DC] Cov., in Phoenix, Arizona. Arthropod communities were sampled in two desert types varying in degree of urbanization including fringe deserts (relatively undisturbed expanses of desert outside of Phoenix) and urban deserts (patches of desert within the urban core of Phoenix). Two studies were conducted including (1) a seasonal study (conducted at two fringe desert and two urban desert sites over a nine-month period), and (2) a snapshot study (conducted at multiple fringe desert and urban desert sites over an eight-day period). Results from both studies demonstrated that overall richness and abundance of creosote bush arthropod communities were lower in urban deserts than in fringe deserts. Additionally, creosote bush arthropod community composition varied greatly, both temporally and spatially. These differences in richness and abundance between fringe deserts and urban deserts suggest that the creosote bush arthropod community may be a useful focal biotic community to monitor when assaying for environmental change due to urbanization in arid habitats.     

Identifying Pleistocene refugia in North American cold deserts using phylogeographic analyses and ecological niche modelling

Aim We analysed phylogeographic patterns and ecological niche models (ENMs) of the widespread velvet ant (Hymenoptera: Mutillidae) Sphaeorpthalma difficilis to understand the history of diversification in the Nearctic deserts and to identify areas that may have been cold‐desert refugia during the Pleistocene. These areas should be targeted for conservation because of their climatic stability throughout historical climate change events. Location North American arid regions. Methods The two internal transcribed spacer regions (ITS1 and ITS2) were sequenced and analysed using Bayesian techniques to uncover phylogeographic patterns of relatedness among S. difficilis populations. History of diversification was estimated using parsimony‐based and maximum likelihood character reconstructions. Molecular dating analyses were implemented in the program r8s and were calibrated with Dominican amber fossils. ENMs were developed based on current climate data and projected onto Pleistocene climate surfaces. Results The analyses suggest that S. difficilis had a complex history of Pleistocene range expansion and contraction that led to the formation of genetically distinct populations inhabiting distinct arid regions. ENMs and phylogeographic patterns indicate that several cold‐desert refugia existed in North America, particularly in the Colorado Plateau and parts of the Great Basin Desert. Main conclusions Analyses of S. difficilis are used to identify potential Pleistocene refugia in the North American cold deserts. Because these areas represent climatically stable locations, they are critical for the long‐term persistence of biodiversity. This research provides evidence that in addition to desert‐like conditions persisting through the ice age in parts of the Nearctic warm deserts, many areas maintained desert‐like characteristics in the regional cold deserts. Further work is needed to elucidate options for preserving biodiversity in these cold‐desert refugia.     

Invertebrate community response to fire and rodent activity in the Mojave and Great Basin Deserts

Recent increases in the frequency and size of desert wildfires bring into question the impacts of fire on desert invertebrate communities. Furthermore, consumer communities can strongly impact invertebrates through predation and top‐down effects on plant community assembly. We experimentally applied burn and rodent exclusion treatments in a full factorial design at sites in both the Mojave and Great Basin deserts to examine the impact that fire and rodent consumers have on invertebrate communities. Pitfall traps were used to survey invertebrates from April through September 2016 to determine changes in abundance, richness, and diversity of invertebrate communities in response to fire and rodent treatments. Generally speaking, rodent exclusion had very little effect on invertebrate abundance or ant abundance, richness or diversity. The one exception was ant abundance, which was higher in rodent access plots than in rodent exclusion plots in June 2016, but only at the Great Basin site. Fire had little effect on the abundances of invertebrate groups at either desert site, with the exception of a negative effect on flying‐forager abundance at our Great Basin site. However, fire reduced ant species richness and Shannon's diversity at both desert sites. Fire did appear to indirectly affect ant community composition by altering plant community composition. Structural equation models suggest that fire increased invasive plant cover, which negatively impacted ant species richness and Shannon's diversity, a pattern that was consistent at both desert sites. These results suggest that invertebrate communities demonstrate some resilience to fire and invasions but increasing fire and spread of invasive due to invasive grass fire cycles may put increasing pressure on the stability of invertebrate communities.     

A neutral‐niche theory of nestedness in mutualistic networks

Recently, there has been a vigorous interest in community ecology about the structure of mutualistic networks and its importance for species persistence and coevolution. However, the mechanisms shaping mutualistic networks have been rarely explored. Here we extend for the first time the neutral theory of biodiversity to a multi trophic system. We focus on nestedness, a distinctive pattern of mutualistic community assembly showing two characteristics, namely, asymmetrical specialization (specialists interacting with generalists) and a generalist core (generalists interacting with generalists). We investigate the importance of relative species abundance (RSA) for the nested assembly of plant–animal mutualistic networks. Our results show that neutral mutualistic communities give rise to networks considerably more nested than real communities. RSA explains 60–70% of nested patterns in two real communities studied here, while 30–40% of nestedness is still unexplained. The nested pattern in real communities is better explained when we introduce interaction‐specific species traits such as forbidden links and intensity of dependence (relative importance of fruits for the diet of a frugivore) in our analysis. The fact that neutral mutualistic communities exhibit a perfectly nested structure and do not show a random or compartmentalized structure, underlines the importance of RSA in the assembly of mutualistic networks.