Levels Of Nestedness Research Articles

Impact of tillage intensity and an invasive grass on plant-pollinator networks in riparian agroecosystems

Insects are a main component of a stable ecosystem and influence crop production. Pollinators, in particular, by regulating pollination between flowers, shape and secure plant communities around crop fields. They are also essential for food provisioning by sustaining the pollination of crops. Studies in recent decades have been covering the mixture of effects various factors have on pollinators. However, there is still a gap in understanding how different taxa in pollinator guilds respond to these pressures and how is network structure affected by it. Therefore, here we tried to further uncover the complex relationship between habitat suitability and pollinator biodiversity in an agro-riparian matrix. Using bipartite network analysis and geographical information systems (GIS) in four sites with different land cover configuration, we assessed faunal and floral diversity, land cover differences and network metrics. Hymenoptera were responsible for most of the visits, while also exhibiting higher levels of richness across sites. Lepidoptera on the other hand, were the order less represented. Overall, tillage intensity and Arundo donax were determinant in differences in the number of visits for both orders, with Diptera and Coleoptera being less affected. However, results regarding visitation rates reported tillage’s higher explanatory power, in comparison with A. donax. Intensive sites showed significantly higher levels of nestedness and lower specialization and diversity across taxa. The results suggest that intensive agriculture and exotic species infestation can impact pollinator taxa differently, thus influencing network structure. By scoping network metrics, we can better determine what species are more affected by agricultural expansion, assess ecosystem resilience and consequently, determine appropriate conservation measures to maintain stable plant-pollinator networks.

Global effects of land-use intensity and exotic plants on the structure and phylogenetic signal of plant-herbivore networks

Interactions between plants and herbivorous insects are often phylogenetically structured, with closely related insect species using similar sets of species or lineages of plants, while phylogenetically closer plants tend to share high proportions of their herbivore insect species. Notably, these phylogenetic constraints in plant-herbivore interactions tend to be more pronounced among internal plant-feeding herbivores (i.e., endophages) than among external feeders (i.e., exophages). In the context of growing human-induced habitat conversion and the global proliferation of exotic species, it is crucial to understand how ecological networks respond to land-use intensification and the increasing presence of exotic plants. In this study, we analyzed plant-herbivore network data from various locations of the World to ascertain the degree to which land-use intensity and the prevalence of exotic plants induce predictable changes in their network topology - measured by levels of nestedness and modularity - and phylogenetic structures. Additionally, we investigated whether the intimacy of plant-herbivore interactions, contrasting endophagous with exophagous networks, modulate changes in network structure. Our findings reveal that most plant-herbivore networks are characterized by significant phylogenetic and topological structures. However, neither these structures did not show consistent changes in response to increased levels of land-use intensify. On the other hand, for the networks composed of endophagous herbivores, the level of nestedness was higher in the presence of a high proportion of exotic plants. Additionally, for networks of exophagous herbivores, we observed an increase in the phylogenetic structure of interactions due to exotic host dominance. These results underscore the differential impacts of exotic species and land-use intensity on the phylogenetic and topological structures of plant-herbivore networks.

Structure of a metacommunity of urban bees: Species diversity and spatio-temporal modularity

As the globe gets more urbanised, the question about how natural biodiversity is structured in cities becomes increasingly pertinent. To contribute to an answer, we studied species richness and spatio-temporal structure of bees in a North European metropolitan area. A gradient of 13 sites in the city of Aarhus, Denmark, was censused from April to September 2016. Forty species, i.e. 29 solitary species (40 % of all individuals), ten Bombus species (28 %), and Apis mellifera (32 %), were sampled monthly in pan traps. (i) Information about species traits was extracted from the literature, and trait values were correlated and used to characterize the fauna. Most were soil-nesters, pollen generalists, and common. (ii) Habitat diversity within five concentric circles with trap at the centre and radii from 50 m to 1000 m was related to bee α diversity. The relationship was significant only within 1,000 m for all bees and for bumblebees. Solitary bee diversity was uncorrelated with habitat diversity at all spatial levels. (iii) Spatio-temporal structure was analysed as two networks, one for bees linked to sites, and one for bees linked to months. Link patterns were analysed for levels of nestedness, modularity, and spatio-temporal β diversity. The two networks were weakly and non-significantly nested, but strongly modular, being composed of five and four modules of co-occurring bees, respectively. (iv) Finally, we studied total β diversity, βTOTAL, being the sum of species turnover, βTURN, and species loss/gain or nestedness, βNEST. For both site and season, βTURN was higher than βNEST, and site βTOTAL was higher than season βTOTAL. One reason for this metacommunity structure may be a high spatio-temporal habitat patchiness, sustaining a rich biodiversity. Thus, a few large areas may not compensate for the loss of several small patches. Consequently, establishment of many green, even small habitats is recommended.

Temporal origin of nestedness in interaction networks

Abstract Nestedness is a common property of communication, finance, trade, and ecological networks. In networks with high levels of nestedness, the link positions of low-degree nodes (those with few links) form nested subsets of the link positions of high-degree nodes (those with many links), leading to matrix representations with characteristic upper triangular or staircase patterns. Recent theoretical work has connected nestedness to the functionality of complex systems and has suggested that it is a structural by-product of the skewed degree distributions often seen in empirical data. However, mechanisms for generating nestedness remain poorly understood, limiting the connections that can be made between system processes and observed network structures. Here, we show that a simple probabilistic model based on phenology—the timing of copresences among interaction partners—can produce nested structures and correctly predict around two-thirds of interactions in two fish market networks and around one-third of interactions in 22 plant–pollinator networks. Notably, the links most readily explained by frequent actor copresences appear to form a backbone of nested interactions, with the remaining interactions attributable to opportunistic interactions or preferences for particular interaction partners that are not routinely available.

Contagion dynamics on hypergraphs with nested hyperedges.

In complex social systems encoded as hypergraphs, higher-order (i.e., group) interactions taking place among more than two individuals are represented by hyperedges. One of the higher-order correlation structures native to hypergraphs is the nestedness: Some hyperedges can be entirely contained (that is, nested) within another larger hyperedge, which itself can also be nested further in a hierarchical manner. Yet the effect of such hierarchical structure of hyperedges on the dynamics has remained unexplored. In this context, here we propose a random nested-hypergraph model with a tunable level of nestedness and investigate the effects of nestedness on a higher-order susceptible-infected-susceptible process. By developing an analytic framework called the facet approximation, we obtain the steady-state fraction of infected nodes on the random nested-hypergraph model more accurately than existing methods. Our results show that the hyperedge-nestedness affects the phase diagram significantly. Monte Carlo simulations support the analytical results.

A waterfowl seed‐dispersal network from the Neotropical region is nested and modular

AbstractSeed dispersal by vertebrates is fundamental for the persistence of plant species, forming networks of interactions that are often nested and modular. Networks involving angiosperms and frugivorous birds are relatively well‐studied in the Neotropical region, but there are no previous studies of networks involving waterbirds. Here, we describe the structure of a Neotropical waterfowl seed‐dispersal network and identify the species that have an important role for the network structure. We used information on 40 plant taxa found in fecal samples of five common waterfowl species to calculate the nestedness (NODF), weighted nestedness (WNODF), modularity, and weighted modularity of the network. We found that the network was nested, with yellow‐billed teal showing the highest contribution both to nestedness and weighted nestedness. Twenty‐four plant species contributed positively to weighted nestedness, with Salzmann's mille graines presenting the highest influence both to nestedness and weighted nestedness. The network was modular, but the weighted modularity was not significant. These results need to be considered with caution due to incomplete interaction sampling for two species. Ringed teal, Brazilian teal, and yellow‐billed teal were considered hub modular species. Among plants, beak sedges and water snowflake were considered modular hub species, while Salzmann's mille graines and spikerush were network connectors. The structure of this Neotropical waterbird seed‐dispersal network differed from the only previous waterfowl network study, from Europe, which found similar level of nestedness but no significant modularity. We include several possible explanations for this discrepancy and identified priorities for future research into waterbird–plant interaction networks.Abstract in Portuguese is available with online material.

Four reasons for scepticism about a human major transition in social individuality.

The 'major transitions in evolution' are mainly about the rise of hierarchy, new individuals arising at ever higher levels of nestedness, in particular the eukaryotic cell arising from prokaryotes, multicellular individuals from solitary protists and individuated societies from multicellular individuals. Some lists include human societies as a major transition, but based on a comparison with the non-human transitions, there are reasons for scepticism. (i) The foundation of the major transitions is hierarchy, but the cross-cutting interactions in human societies undermine hierarchical structure. (ii) Natural selection operates in three modes-stability, growth and reproductive success-and only the third produces the complex adaptations seen in fully individuated higher levels. But human societies probably evolve mainly in the stability and growth modes. (iii) Highly individuated entities are marked by division of labour and commitment to morphological differentiation, but in humans differentiation is mostly behavioural and mostly reversible. (iv) As higher-level individuals arise, selection drains complexity, drains parts, from lower-level individuals. But there is little evidence of a drain in humans. In sum, a comparison with the other transitions gives reasons to doubt that human social individuation has proceeded very far, or if it has, to doubt that it is a transition of the same sort. This article is part of the theme issue 'Human socio-cultural evolution in light of evolutionary transitions'.

Nestedness interacts with subnetwork structures and interconnection patterns to affect community dynamics in ecological multilayer networks

Ecological networks describe ecological interactions among species in ecosystems. In natural ecosystems, plant-mutualist (PM) and plant-herbivore (PH) networks are two of the most documented bipartite ecological networks, which are often interconnected through shared plants to form multilayer networks (here referred to as ecological networks with multiple interaction types). Recent developments in multilayer networks have challenged the effects of topological properties on biodiversity and stability once found in ecological networks with a single interaction type. In this study, my goal was to theoretically test the effects of the nested topology of subnetworks (i.e. plant-mutualist and plant-herbivore networks) on the local stability and persistence of the entire community and determine how their effect sizes were dependent on subnetwork structures and interconnection patterns. I used a simple algorithm to construct plant-mutualist or plant-herbivore networks with different levels of nestedness while fixing connectance and network size. By artificially interconnecting plant-mutualist and plant-herbivore networks through shared plants, I also manipulated the inter-subnetwork connection patterns as positive, negative and no correlations between the number of interacting partners of shared plants of two subnetworks. Community dynamics were simulated to show how subnetwork nestedness interacted with other network properties to affect local stability and persistence of multilayer networks. I found that low nestedness of both plant-mutualist and plant-herbivore subnetworks promoted stability and persistence. Effect sizes of the focal PM- or PH-subnetwork nestedness were positively associated with the nestedness levels of the interconnected subnetworks. A positive correlation between the mutualistic and herbivory generalism of plants also led to higher (signed) effect sizes of subnetwork nestedness. Further analyses showed that the effect sizes of the subnetwork nestedness also depended on subnetwork complexity and intraguild competition intensity. Finally, the modularity of interconnected subnetworks had little association with the effect sizes of subnetwork nestedness irrespective of interconnection patterns. The results demonstrate that the effects of topological structures (such as nestedness) on community dynamics in single-interaction networks may be altered by the architectures of multilayer networks, which highlights the need to study the interactions between the architectures of within- and inter-subnetworks in affecting ecosystem stability and biodiversity.

Interaction generalisation and demographic feedbacks drive the resilience of plant-insect networks to extinctions.

Understanding the processes driving ecological resilience, that is the extent to which systems retain their structure while absorbing perturbations, is a central challenge for theoretical and applied ecologists. Plant-insect assemblages are well-suited for the study of ecological resilience as they are species-rich and encompass a variety of ecological interactions that correspond to essential ecosystem functions. Mechanisms affecting community response to perturbations depend on both the natural history and structure of ecological interactions. Natural history attributes of the interspecific interactions, for example whether they are mutualistic or antagonistic, may affect the ecological resilience by controlling the demographic feedbacks driving ecological dynamics at the community level. Interaction generalisation may also affect resilience, by defining opportunities for interaction rewiring, the extent to which species are able to switch interactions in fluctuating environments. These natural history attributes may also interact with network structure to affect ecological resilience. Using adaptive network models, we investigated the resilience of plant-pollinator and plant-herbivore networks to species loss. We specifically investigated how fundamental natural history differences between these systems, namely the demographic consequences of the interaction and their level of generalisation-mediating rewiring opportunities-affect the resilience of dynamic ecological networks to extinctions. We also create a general benchmark for the effect of network structure on resilience simulating extinctions on theoretical networks with controlled structures. When network structure was static, pollination networks were less resilient than herbivory networks; this is related to their high levels of nestedness and the reciprocally positive feedbacks that define mutualisms, which made co-extinction cascades more likely and longer in plant-pollinator assemblages. When considering interaction rewiring, the high generalisation and the structure of pollination networks boosted their resilience to extinctions, which approached those of herbivory networks. Simulation results using theoretical networks suggested that the empirical structure of herbivory networks may protect them from collapse. Elucidating the ecological and evolutionary processes driving interaction rewiring is key to understanding the resilience of plant-insect assemblages. Accounting for rewiring requires ecologists to combine natural history with network models that incorporate feedbacks between species abundances, traits and interactions. This combination will elucidate how perturbations propagate at community level, reshaping biodiversity structure and ecosystem functions.

From structure to function in mutualistic interaction networks: Topologically important frugivores have greater potential as seed dispersers.

Networks of mutualistic interactions between animals and plants are considered a pivotal part of ecological communities. However, mutualistic networks are rarely studied from the perspective of species-specific roles, and it remains to be established whether those animal species more relevant for network structure also contribute more to the ecological functions derived from interactions. Here, we relate the contribution to seed dispersal of vertebrate species with their topological role in frugivore-plant interaction networks. For one year in two localities with remnant patches of Colombian tropical dry forest, we sampled abundance, morphology, behaviour and fruit consumption from fleshy-fruited plants of various frugivore species. We assessed the network topological role of each frugivore species by integrating their degree of generalization in interactions with plants with their contributions to network nestedness and modularity. We estimated the potential contribution of each frugivore species to community-wide seed dispersal, on the basis of a set of frugivore ecological, morphological and behavioural characteristics important for seed dispersal, together with frugivore abundance and frugivory degree. The various frugivore species showed strong differences in their network structural roles, with generalist species contributing the most to network modularity and nestedness. Frugivores also showed strong variability in terms of potential contribution to seed dispersal, depending on the specific combinations of frugivore abundance, frugivory degree and the different traits and behaviours. For both localities, the seed dispersal potential of a frugivore species responded positively to its contribution to network structure, evidencing that the most important frugivore species in the network topology were also those making the strongest contribution as seed dispersers. Contribution to network structure was correlated with frugivore abundance, diet and behavioural characteristics. This suggests that the species-level link between structure and function is due to the fact that the occurrence of frugivore-plant interactions depends largely on the characteristics of the frugivore involved, which also condition its ultimate role in seed dispersal.

Community structure and population dynamics of small mammals in an urban-sylvatic interface area in Rio de Janeiro, Brazil

The Atlantic Forest is one of the most disturbed Brazilian biomes, with 183 out of 298 species of mammals occurring in the state of Rio de Janeiro. In this study, we aimed to characterize the diversity, community structure, and habitat use of small mammals in the FIOCRUZ Atlantic Forest Campus (CFMA), including areas of Pedra Branca State Park (PBSP, subunit Pau da Fome), state of Rio de Janeiro. We also compared species diversity and composition between two moments 15 years apart (2001 and 2012–2015) and analyzed the population dynamics of the marsupialDidelphisaurita(Wied-Neuwied, 1826). Small mammal captures were made in different habitats: sylvatic-urban interface areas near human dwellings, disturbed forest, and preserved forest areas. Five marsupial species and four rodent species were captured in both periods. There was a reduction in species richness and β diversity between the two periods, indicating that disturbances in the environment over the years may have affected the small mammal community structure. The most altered environment showed the greatest species richness and abundance, while the forest areas showed the smallest values, which may be explained by the loss of mammal species, mainly specialist species in forested areas. We identified three groups of species according to habitat preferences: one related to environments with a higher density of vegetation in upper strata –Marmosaparaguayana(Tate, 1931) andMonodelphisamericana(Müller, 1776), another related to a higher density in lower forest strata –Akodoncursor(Winge, 1887), and another with no association with the investigated habitat variables –D.auritaandOligoryzomysnigripes(Olfers, 1818). The small mammal community structure showed a low level of nestedness in both sampling periods. This study is the first report to evaluate the community structure of small mammals in the sylvatic-urban interface area of Pedra Branca State Park, the largest forest reserve within an urban area in Brazil. The surveys indicate that the small mammal diversity was low in both sampling periods and in both areas, and a species loss in the Pau da Fome locality was observed, despite it is a conservation unit. The greater species abundance and richness in the most disturbed areas suggest an increase of factors favoring the occurrence of synanthropic and opportunistic species.

Why are some plant-pollinator networks more nested than others?

Empirical studies have found that the mutualistic interactions forming the structure of plant-pollinator networks are typically more nested than expected by chance alone. Additionally, theoretical studies have shown a positive association between the nested structure of mutualistic networks and community persistence. Yet, it has been shown that some plant-pollinator networks may be more nested than others, raising the interesting question of which factors are responsible for such enhanced nested structure. It has been argued that ordered network structures may increase the persistence of ecological communities under less predictable environments. This suggests that nested structures of plant-pollinator networks could be more advantageous under highly seasonal environments. While several studies have investigated the link between nestedness and various environmental variables, unfortunately, there has been no unified answer to validate these predictions. Here, we move from the problem of describing network structures to the problem of comparing network structures. We develop comparative statistics, and apply them to investigate the association between the nested structure of 59 plant-pollinator networks and the temperature seasonality present in their locations. We demonstrate that higher levels of nestedness are associated with a higher temperature seasonality. We show that the previous lack of agreement came from an extended practice of using standardized measures of nestedness that cannot be compared across different networks. Importantly, our observations complement theory showing that more nested network structures can increase the range of environmental conditions compatible with species coexistence in mutualistic systems, also known as structural stability. This increase in nestedness should be more advantageous and occur more often in locations subject to random environmental perturbations, which could be driven by highly changing or seasonal environments. This synthesis of theory and observations could prove relevant for a better understanding of the ecological processes driving the assembly and persistence of ecological communities.

The mossy north: an inverse latitudinal diversity gradient in European bryophytes.

It remains hotly debated whether latitudinal diversity gradients are common across taxonomic groups and whether a single mechanism can explain such gradients. Investigating species richness (SR) patterns of European land plants, we determine whether SR increases with decreasing latitude, as predicted by theory, and whether the assembly mechanisms differ among taxonomic groups. SR increases towards the south in spermatophytes, but towards the north in ferns and bryophytes. SR patterns in spermatophytes are consistent with their patterns of beta diversity, with high levels of nestedness and turnover in the north and in the south, respectively, indicating species exclusion towards the north and increased opportunities for speciation in the south. Liverworts exhibit the highest levels of nestedness, suggesting that they represent the most sensitive group to the impact of past climate change. Nevertheless, although the extent of liverwort species turnover in the south is substantially and significantly lower than in spermatophytes, liverworts share with the latter a higher nestedness in the north and a higher turn-over in the south, in contrast to mosses and ferns. The extent to which the similarity in the patterns displayed by spermatophytes and liverworts reflects a similar assembly mechanism remains, however, to be demonstrated.

Diatom Cooccurrence Shows Less Segregation than Predicted from Niche Modeling.

Species cooccurrence patterns give significant insights into the processes shaping communities. While biotic interactions have been widely studied using cooccurrence analyses in animals and larger plants, studies about cooccurrences among micro-organisms are still relatively rare. We examined stream diatom cooccurrences in France through a national database of samples. In order to test the relative influence of environmental, biotic and spatial constraints on species’ incidence distribution, cooccurrence and nestedness patterns of real communities were compared with the patterns generated from a set of standard and environmentally constrained null models. Real communities showed a higher level of segregation than the most conservative standard null models, but a general aggregation of cooccurrences when compared to environmentally constrained null models. We did not find any evidence of limiting similarity between cooccurring species. Aggregations of species cooccurrences were associated with the high levels of nestedness. Altogether, these results suggested that biotic interactions were not structuring cooccurrences of diatom species at our study scale. Instead, the patterns were more likely to be related with colonization patterns, mass effect, and local temporal dynamics of diatom biofilms. We further highlight that the association of standard and environmentally constrained null models may give realistic insight into the cooccurrence patterns of microbial communities.

Nestedness of hoopoes' bacterial communities: symbionts from the uropygial gland to the eggshell

How microbial symbionts are established and maintain on their hosts is a leading question with important consequences for the understanding of the evolution and functioning of mutualistic relationships. The acquisition by hosts of mutualistic microbial symbionts can be considered as colonization processes of environments (i.e., host) by symbionts. Colonization processes can be explored by characterizing the nestedness of communities, but this approach has rarely been applied to communities of microbial symbionts. We used this approach here, and estimated the nestedness of bacterial communities of hoopoes (Upupa epops), a species with symbiotic bacteria in their uropygial gland that are expected to colonize eggshells where they protect embryos from pathogens. Bacterial communities were characterized by ARISA (Automated rRNA Intergenetic Spacer Region) and studied the nestedness characteristics of bacterial communities living in the uropygial secretion, bill, belly and eggshells of each sampled female hoopoes. We detected a consistent nested pattern of bacterial communities of hoopoes; from the uropygial gland to the eggshell. We also found evidence of study year and reproductive events influencing the level of nestedness of bacterial communities of hoopoes. These results indicate that bacterial communities of eggshells and body parts of female hoopoes are at least partially conditioned by the symbiotic community in the uropygial gland. We discuss the importance of these results for understanding this host–microbial mutualism functioning and evolution.

Network perspectives of ectomycorrhizal associations

Network analysis has successfully been applied to investigate ecological interactions. Most information about the structure of ecological networks is derived from aboveground interactions of plants with pollinators and seed dispersers. Here we examine ectomycorrhizal interactions between plants and fungi in a network perspective that has received little attention so far. For network analyses, we examined the main network properties-nestedness and modularity. Certain ectomycorrhizal fungal communities displayed modularity, which is directly attributable to partner selectivity and, thus, context dependent. Our data also showed that ectomycorrhizal networks exhibit non-nested or anti-nested patterns, which is in contrast to other mutualistic interactions. The low level of nestedness may indicate that specific ectomycorrhizal plant species do not favour generalist ectomycorrhizal fungi over specialists and vice versa. This can stem from a strong selection pressure of host in choosing its mycobionts as suggested from the substantial host phylogeny effect on ectomycorrhizal fungal richness and community. Whether the low level of nestedness in ectomycorrhizal associations is due to methodological or ecological factors requires further investigation.

Compositional dissimilarity patterns of reptiles and amphibians in insular systems around the world

AbstractSeveral studies have shown that taxa with poor dispersal ability have a higher level of compositional dissimilarity than good dispersers. However, compositional dissimilarity patterns between islands with respect to dispersal ability of taxa have never been investigated before. In this study, we investigated compositional dissimilarity patterns of three taxonomic groups, namely amphibians, lizards, and snakes, differing in their dispersal abilities, in various insular systems around the world. We compiled presence–absence matrices, based on which we calculated several metacommunity indices to check for differences among taxonomic groups and island types (oceanic and continental shelf) using classical statistical tests and generalized linear mixed‐effects models. According to our results, compositional dissimilarity was positively affected by the isolation of the insular system, in accordance to theory. In particular, oceanic systems were characterized by a high level of compositional dissimilarity between islands and subsequently by a low level of nestedness. SIEs may be generating these patterns causing distortions from expected levels of nestedness. Similar to our predictions, compositional dissimilarity patterns were also dependent on taxon‐specific dispersal ability, with good dispersers showing lower levels of between‐island compositional dissimilarity than poor dispersers in continental shelf systems. However, this pattern was not observed in oceanic systems. In conclusion, compositional dissimilarity in insular systems is dependent on both taxon and island type.

The Dynamics of Nestedness Predicts the Evolution of Industrial Ecosystems

In economic systems, the mix of products that countries make or export has been shown to be a strong leading indicator of economic growth. Hence, methods to characterize and predict the structure of the network connecting countries to the products that they export are relevant for understanding the dynamics of economic development. Here we study the presence and absence of industries in international and domestic economies and show that these networks are significantly nested. This means that the less filled rows and columns of these networks' adjacency matrices tend to be subsets of the fuller rows and columns. Moreover, we show that their nestedness remains constant over time and that it is sustained by both, a bias for industries that deviate from the networks' nestedness to disappear, and a bias for the industries that are missing according to nestedness to appear. This makes the appearance and disappearance of individual industries in each location predictable. We interpret the high level of nestedness observed in these networks in the context of the neutral model of development introduced by Hidalgo and Hausmann (2009). We show that the model can reproduce the high level of nestedness observed in these networks only when we assume a high level of heterogeneity in the distribution of capabilities available in countries and required by products. In the context of the neutral model, this implies that the high level of nestedness observed in these economic networks emerges as a combination of both, the complementarity of inputs and heterogeneity in the number of capabilities available in countries and required by products. The stability of nestedness in industrial ecosystems, and the predictability implied by it, demonstrates the importance of the study of network properties in the evolution of economic networks.

Ecological impacts of tropical forest fragmentation: how consistent are patterns in species richness and nestedness?

Large areas of tropical forest now exist as remnants scattered across agricultural landscapes, and so understanding the impacts of forest fragmentation is important for biodiversity conservation. We examined species richness and nestedness among tropical forest remnants in birds (meta-analysis of published studies) and insects (field data for fruit-feeding Lepidoptera (butterflies and moths) and ants). Species-area relationships were evident in all four taxa, and avian and insect assemblages in remnants typically were nested subsets of those in larger areas. Avian carnivores and nectarivores and predatory ants were more nested than other guilds, implying that the sequential loss of species was more predictable in these groups, and that fragmentation alters the trophic organization of communities. For butterflies, the ordering of fragments to achieve maximum nestedness was by fragment area, suggesting that differences among fragments were driven mainly by extinction. In contrast for moths, maximum nestedness was achieved by ordering species by wing length; species with longer wings (implying better dispersal) were more likely to occur at all sites, including low diversity sites, suggesting that differences among fragments were driven more strongly by colonization. Although all four taxa exhibited high levels of nestedness, patterns of species turnover were also idiosyncratic, and thus even species-poor sites contributed to landscape-scale biodiversity, particularly for insects.

Climatic history and dispersal ability explain the relative importance of turnover and nestedness components of beta diversity

ABSTRACTAim We tested whether the geographic variation in the proportion of beta diversity attributed to nestedness or turnover components was explained by the effect of past glaciation events. Specifically, we tested the hypothesis that most of the beta diversity in regions retaining ice until recent periods was due to nestedness. Additionally, we tested whether the variation was influenced by thermal tolerance and the dispersal ability of species.Location This study analysed data from the New World.Methods We used presence/absence data for amphibians, birds and mammals of the New World. We calculated beta diversity among each 1°× 1° cell and the adjacent cells using the Sorensen dissimilarity index that expresses the total beta diversity. Furthermore, we partitioned it into turnover and nestedness components. The relative importance of the two latter components was expressed as the proportion of total beta diversity explained by nestedness (βratio). We calculated the correlation between βratio and the time each cell was free of ice since the last glaciation (cell age). To control the effects of spatial autocorrelation, we calculated geographically effective degrees of freedom.Results The proportion of beta diversity attributed to nestedness was negatively correlated with cell age. Moreover, this effect was stronger for amphibians than mammals, and stronger for mammals than birds.Main conclusions Our results are in accordance with the hypothesis that the nestedness component of beta diversity is more important in areas affected by glaciations until recent time. The beta diversity in high latitudes is the result of past extinctions and recent recolonization, which result in higher levels of nestedness. This process is more evident for vertebrates with lower dispersal ability and lower temperature tolerance.