Nestedness in fragmented landscapes: a case study on birds, arboreal marsupials and lizards

Aim The potential nestedness of assemblages of birds, arboreal marsupials and lizards was examined in a fragmented landscape in south‐eastern Australia. We assessed which ecological processes were related to the presence or absence of nestedness, particularly in relation to previous autoecological studies in the same study area. Location Data were collected at Buccleuch State Forest, c. 100 km to the west of the Australian Capital Territory in south‐eastern Australia. Methods Presence/absence matrices were compiled for birds (40 pine sites, 40 continuous forest sites, 43 fragments), arboreal marsupials (41 continuous forest sites, 39 fragments) and lizards (30 sites including all landscape elements) from a range of field surveys conducted since 1995. Nestedness was analysed using a standardized discrepancy measure, and statistical significance was assessed using the RANDNEST null model. For birds, species thought to be extinction‐prone were analysed separately to assess if assemblages comprising extinction‐prone species were more strongly nested than others. Also, sites with a substantial amount of Eucalyptus radiata were analysed separately to assess whether nestedness was stronger if environmental heterogeneity was minimized. Results The assemblages of lizards and arboreal marsupials were not nested, probably because of qualitative differences between species in response to environmental conditions. The assemblages of birds in fragments and pine sites were significantly nested, but nestedness was substantially stronger in fragments. For birds, nestedness appeared to be related to somewhat predictable extinction sequences, although there were many outliers in the analysis. Nestedness increased when extinction‐prone species were analysed by themselves. Nestedness decreased when environmental heterogeneity was minimized by including only sites dominated by E. radiata. Main conclusions In a given landscape, different vertebrate assemblages can respond in vastly different ways to fragmentation. Nestedness analyses can provide a useful overview of likely conservation issues in fragmented landscapes, for example by highlighting the possible roles of local extinction and immigration. However, nestedness analyses are a community‐level tool, and should be complemented by more detailed autoecological studies when applied in a conservation context.     

Nestedness of Southern Ocean island biotas: ecological perspectives on a biogeographical conundrum

Aim To use patterns of nestedness in the indigenous and non‐indigenous biotas of the Southern Ocean islands to determine the influence of dispersal ability on biogeographical patterns, and the importance of accounting for variation in dispersal ability in their subsequent interpretation, especially in the context of the Insulantarctic and multi‐regional hypotheses proposed to explain the biogeography of these islands. Location Southern Ocean islands. Methods Nestedness was determined using a new metric, d1 (a modification of discrepancy), for the indigenous and introduced seabirds, land birds, insects and vascular plants of 26 Southern Ocean islands. To assess the possible confounding effects of spatial autocorrelation on the results, islands were assigned to 11 major island groups and each group was treated as a single island in a following analysis. In addition, nestedness of the six Southern Ocean islands comprising the South Pacific Province (New Zealand islands) was analysed. All analyses were conducted for species and genera, for each of the taxa on its own, and for the complete data sets. Results Statistically significant nestedness was found in all of the taxa examined, with nestedness declining in the order seabirds > land birds > vascular plants > insects for the indigenous species. Vagility had a marked influence on nestedness and the biogeographical patterns shown by the indigenous species. This influence was borne out by additional analyses of marine taxa and small‐sized terrestrial species, both of which were more nested than the most nested group examined here, the seabirds. Assemblages of non‐indigenous species also showed nestedness, and nestedness was generally more pronounced than in the indigenous species. Surprisingly, vagility had a significant effect on nestedness in these assemblages too. Main conclusions Nestedness analyses provide a quantitative means of comparing biogeographical patterns for groups differing in vagility. These comparisons revealed that vagility has a considerable influence on biogeographical patterns and should be taken into account in analyses. Here, investigations of more vagile taxa support hypotheses for a single origin of the Southern Ocean island biota (the Insulantarctica scenario), whilst those of less mobile taxa support the more commonly held, multi‐regional hypothesis. All biogeographical analyses across the Southern Ocean (and elsewhere) will be influenced by the effects of dispersal ability, with composite analyses dominated by sedentary groups likely to favour multi‐regional scenarios, and those dominated by mobile groups favouring single origins. Mechanisms underlying nestedness in the region range from nested physiological tolerances in more mobile groups to colonization ability and patterns of speciation in less vagile taxa. Considerable nestedness in the non‐indigenous assemblages is largely a consequence of the fact that many of these species are European weedy species.     

Fine-scale structure and cross-taxon congruence of bird and beetle assemblages in an old-growth boreal forest mosaic

Aim Nestedness occurs when species present in depauperate sites are subsets of those found in species‐rich sites. The degree of congruence of site nestedness among different assemblages can inform commonalities of mechanisms structuring the assemblages. Well‐nested assemblages may still contain idiosyncratic species and sites that notably depart from the typical assemblage pattern. Idiosyncrasy can arise from multiple processes, including interspecific interactions and habitat preferences, which entail different consequences for species co‐occurrences. We investigate the influence of fine‐scale habitat variation on nestedness and idiosyncrasy patterns of beetle and bird assemblages. We examine community‐level and pairwise species co‐occurrence patterns, and highlight the potential influence of interspecific interactions for assemblage structure. Location Côte‐Nord region of Québec, Canada. Methods We sampled occurrences of ground‐dwelling beetles, flying beetles and birds at sites within old‐growth boreal forest. We examined the nestedness and idiosyncrasy of sites and sought relationships to habitat attributes. We analysed non‐random species co‐occurrence patterns at pairwise and community levels, using null model analysis and five ‘association’ indices. Results All three assemblages were significantly nested. There was limited congruence only between birds and flying beetles whose nestedness was related to canopy openness. For ground‐dwelling beetles, nestedness was related to high stand heterogeneity and sapling density, whereas site idiosyncrasy was inversely related to structural heterogeneity. For birds, site idiosyncrasy increased with canopy cover, and most idiosyncratic species were closed‐canopy specialists. In all assemblages, species idiosyncrasy was positively correlated with the frequency of negative pairwise associations. Species co‐occurrence patterns were non‐random, and for flying beetles and birds positive species pairwise associations dominated. Community‐level co‐occurrence summaries may not, however, always reflect these patterns. Main conclusions Nestedness patterns of different assemblages may not correlate, even when sampled at common locations, because of different responses to local habitat attributes. We found idiosyncrasy patterns indicating opposing habitat preferences, consistent with antagonistic interactions among species within assemblages. Analysis of such patterns can thus suggest the mechanisms generating assemblage structures, with implications for biodiversity conservation.     

Distributions of forest birds and butterflies in the Andaman islands, Bay of Bengal: nested patterns and processes

The distributional patterns of forest birds and butterflies in the Andaman islands, an oceanic chain located off SE Asia, were tested for nestedness. Both taxa were highly nested. Nestedness could be due to colonization or extinction processes, area or distance effects or nestedness of habitats. Nestedness in forest bird distributions were strongly influenced by area and habitat related factors. Habitats were significantly nested in all three island groups, however most strongly for the North Andamans. However forest bird distributions in the North Andamans, as indicated by row order in the packed matrix, was not correlated with habitat diversity, suggesting that habitat related factors alone cannot account for these patterns. Other causal influences could be passive sampling, where common and abundant species and habitats are more likely to have a widespread distribution than rare species and habitats. The nested subset pattern seen in two unrelated taxa suggests that the Andamans are extinction dominated and that the protection of forests on large islands is critical for the conservation of its biodiversity.     

BIODIVERSITY RESEARCH: Nestedness for different reasons: the distributions of birds,lizards and small mammals on islands of an inundated lake

Aim We examined whether the community compositions of birds, lizards and small mammals were nested in a fragmented landscape in the Thousand Island Lake, China. We also assessed whether the mechanisms influencing nestedness differed among these taxonomic groups. Location Thousand Island Lake, China. Methods Presence/absence matrices were compiled for birds (42 islands) and lizards (42 islands) using line‐transect methods, and for small mammals (14 islands) using live‐trapping methods from 2006 to 2009. Nestedness was analysed using BINMATNEST, and statistical significance was assessed using the conservative null model 3. We used Spearman rank correlations and partial Spearman rank correlations to examine associations of nestedness and habitat variables (area, isolation, habitat diversity and plant richness) as well as life‐history traits (body size, habitat specificity, geographical range size and area requirement) related to species extinction and immigration tendencies. Results The community compositions of birds, lizards and small mammals were all significantly nested, but the causal factors underlying nestedness differed among taxonomic groups. For birds, island area, habitat specificity and area requirement were significantly correlated with nestedness after controlling for other independent variables. For lizards, habitat heterogeneity was the single best correlate of nestedness. For small mammals, island area, habitat heterogeneity and habitat specificity were significantly correlated with nestedness. The nested patterns of birds, lizards and small mammals were not attributable to passive sampling or selective colonization. Main conclusions The processes influencing nested patterns differed among taxonomic groups. Nestedness of bird assemblages was driven by selective extinction, and lizard assemblage was caused by habitat nestedness, while nestedness of small mammals resulted from both selective extinction and habitat nestedness. Therefore, we should take taxonomic differences into account when analysing nestedness to develop conservation guidelines and refrain from using single taxa as surrogates for others.     

Nested bird and micro-habitat assemblages in a peatland archipelago

Biotic assemblages of insular habitats are nested when poor assemblages are subsets of richer ones. Nestedness of species assemblages is frequent and may result from selective extinction or frequent colonization in insular habitats. It may also be created by a nested distribution of habitats among islands or by sampling bias. We sampled 67 isolated peatlands (7–843 ha) in southern Quebec, Canada, to measure nestedness of bird species assemblages among peatlands and assess the habitat nestedness hypothesis. Species and microhabitat assemblages were both strongly nested among peatlands. Whether sites were ranked by species richness, microhabitat richness or peatland area had no effect on nestedness. However, microhabitat nestedness was significantly reduced when sites were sorted by area rather than by microhabitat richness. As expected, if bird-microhabitat associations are responsible for the nested pattern of distribution, we found a positive correlation between the contributions of bird species and microhabitats to individual site nestedness. Nevertheless, microhabitat assemblages were significantly less nested than bird species assemblages, possibly because of frequent recolonization by birds or uneven sampling among sites. Received: 12 June 1998 / Accepted: 20 September 1998     

Nestedness of desert bat assemblages: species composition patterns in insular and terrestrial landscapes

Nested patterns of community composition exist when species at depauperate sites are subsets of those occurring at sites with more species. Nested subset analysis provides a framework for analyzing species occurrences to determine non-random patterns in community composition and potentially identify mechanisms that may shape faunal assemblages. We examined nested subset structure of desert bat assemblages on 20 islands in the southern Gulf of California and at 27 sites along the Baja California peninsula coast, the presumable source pool for the insular faunas. Nested structure was analyzed using a conservative null model that accounts for expected variation in species richness and species incidence across sites (fixed row and column totals). Associations of nestedness and island traits, such as size and isolation, as well as species traits related to mobility, were assessed to determine the potential role of differential extinction and immigration abilities as mechanisms of nestedness. Bat faunas were significantly nested in both the insular and terrestrial landscape and island size was significantly correlated with nested structure, such that species on smaller islands tended to be subsets of species on larger islands, suggesting that differential extinction vulnerabilities may be important in shaping insular bat faunas. The role of species mobility and immigration abilities is less clearly associated with nestedness in this system. Nestedness in the terrestrial landscape is likely due to stochastic processes related to random placement of individuals and this may also influence nested patterns on islands, but additional data on abundances will be necessary to distinguish among these potential mechanisms.     

Endemicity biases nestedness metrics: a demonstration, explanation and solution

Nestedness is frequently investigated to understand complex patterns of species occurrences. Temperature (T) is commonly used for comparisons of nestedness of different-sized datasets. However, the assumptions made for the standardization of this metric have not been fully explored, particularly the effects of endemicity. Here we show that T incorrectly indicates an increase in nestedness with the addition of non-nested endemics to matrices that are not perfectly nested – a consequence of standardizing matrix size by the product of species and sites. This problem is common both to test matrices and to real matrices that are typically subjected to nestedness analyses. The latter are often characterized by substantial numbers of endemics and by variation in the numbers of endemics in different taxa. Standardizing by occupancy resolves this problem, which is demonstrated using a derivative of discrepancy, d1. A small modification to T, such that it standardizes matrices by occupancy, would resolve the current problems with this nestedness metric.     

Nestedness in insular floras: spatiotemporal variation and underlying mechanisms

Aims Nestedness is a characteristic of insular metacommunity structure. Relatively few studies, however, have attempted to evaluate temporal changes in nestedness, or elucidate the mechanisms underlying nestedness. I evaluated both spatial and temporal patterns of nestedness in the insular floras of four archipelagoes of small islands in the Bahamas and the potential underlying environmental gradients.Methods The NODF (a nestedness metric based on overlap and decreasing fill) and the matrix temperature measure, T, were used to quantify nestedness in insular floras on small islands near Abaco, Andros, Great Exuma and the Exuma Cays, Bahamas. Two different null models were employed for each nestedness measure. Six environmental variables were evaluated in relation to nestedness by ordering islands according to gradients and recalculating NODF scores.Important findings All archipelagoes were significantly nested. Nestedness among sites contributed more to overall nestedness than did nestedness among species. NODF scores varied among archipelagoes, but were surprisingly constant over time. Ordering islands by vegetated area yielded the highest nestedness scores for three archipelagoes; ordering islands by protection from exposure yielded the highest nestedness score for one archipelago. Nestedness scores varied little over time even though species compositions changed, indicating that extinctions occurred in a deterministic manner. The relative importance of area suggests extinction is an important mechanism in producing nestedness. Attempting to determine the relative importance of immigrations or extinctions requires some assumptions, however, and both processes are likely cumulative in most cases.     

Nestedness in fragmented landscapes: birds of the box‐ironbark forests of south‐eastern Australia

Nestedness in biota as a function of species richness – biota of depauperate assemblages being non‐random subsets of richer biotas – has been widely documented in recent years (see Wright et al. 1998 , Oecologia 113: 1–20). Ordering sites by richness maximizes nestedness indices; however, ordering by other criteria such as area or isolation may be more ecologically interpretable. We surveyed birds in true fragments (35 in all), and in “reference areas” in large extant forest blocks (30 locations), of the same range of areas (10, 20, 40, 80 ha). The avifauna was divided into “bush birds”– species dependent on forest and woodland, and “open country” species. We looked at nestedness in four data sets: “bush birds” in fragments and reference areas, and “all birds” in fragments and in reference areas. All data sets were significantly nested. Ordering by area in all cases was not significantly less nested than ordering by richness. Ordering by area in fragments was significantly greater than in reference areas, but the differences in standardized nestedness indices were small (<15%). We identified those birds that had distributions among fragments that conformed strongly with area, those that were more randomly distributed and some species that were more likely to occupy the smallest fragments. Among the latter was a hyperaggressive, invasive, colonial native species (noisy miner Manorina melanocephala). A suite of small, insectivorous birds were more likely to strongly conform with expected distributions in relation to area, which was consistent with observations of their vulnerability to the effects of the noisy miner in smaller fragments.     

A comparative analysis of nested subset patterns of species composition

We present a broad comparative assessment of nested subsets in species composition among ecological communities. We assembled presence-absence data from a broad range of taxa, geographic regions, and spatial scales; and subjected this collection of datasets to common analyses, including a variety of metrics for measuring nestedness and null hypotheses against which to evaluate them. Here we identify ecological patterns in the prevalence and strength of nested subset structure, and assess differences and biases among the available methodologies. In all, we compiled 279 presence-absence matrices, of which 163 do not overlap in their coverage of species and sites. The survey includes studies on vertebrates, arthropods, mollusks, plants, and other taxa; from north temperate, tropical, and south temperate latitudes. Our results were as follows. Statistically significant nestedness was common. Assemblages from landbridge archipelagos were strongly nested, and immigration experiments were least nested. This adds further empirical support to the hypothesis that extinction plays a major role in producing nested structure. Nestedness was positively correlated with the ratio of the areas of the largest and smallest sites, suggesting that the range in area of sites affects nestedness. Taxonomic differences in nestedness were weak. Higher taxonomic levels showed stronger nesting than their constituent lower taxa. We observed no effect of distance of isolation on nestedness; nor any effects of latitude. With regard to methodology, the metrics Nc and Ut yielded similar results, although Nc proved slightly more flexible in use, and deals differently with tied sites. Similarities also exist in the behavior of N0 (“N”) and Up, and between N1 and Ua. Standardized nestedness metrics were mostly insensitive to matrix size, and were useful in comparative analyses among presence-absence matrices. Most metrics were affected by the proportion of presences in the matrix. All analyses of nestedness, therefore, should test for bias due to matrix fill. We suggest that the factors controlling nested subset structure can be thought of as four filters that species pass to occur at a site: a sampling filter, a distance filter, a habitat filter, and an area filter – and three constraints on community homogeneity: evolutionary history, recent history, and spatial variation in the environment. The scale of examination can also have important effects on the degree of nestedness observed. Received: 13 September 1996 / Accepted: 16 September 1997     

Spatio-temporal nested patterns in macroinvertebrate assemblages across a pond network with a wide hydroperiod range

Nestedness has been widely used to measure the structure of biological communities and occurs when species-poor sites contain subsets of species-rich ones. Here, we examine nested patterns across the macroinvertebrate assemblages of 91 ponds in Doñana National Park, Spain, and explore temporal variation of nestedness and species richness in 19 temporary ponds over 2 years with differing rainfall. Macroinvertebrate assemblages were significantly nested; both pond spatial arrangement and environmental variation being important in driving nested patterns. Despite the nested structure observed, a number of taxa and ponds deviate from this pattern (termed idiosyncratic), by occurring more frequently than expected in species-poor sites, or having assemblages dominated by species largely absent from species-rich sites. Aquatic adults of winged insects, capable of dispersal, were more highly nested than non-dispersing taxa and life-history stages. Idiosyncratic taxa were found in ponds spanning a wide range of hydroperiods, although nestedness was higher in more permanent waterbodies. Monthly sampling demonstrated a gradual increase of species richness and nestedness from pond filling to April–May, when the most temporary ponds started to dry. Although the degree of nestedness of individual pond assemblages varied from month to month, the overall degree of nestedness in the two study years was practically identical despite marked differences in hydroperiod. Our results suggest that differential colonization and environmental variation are key processes driving the nested structure of Doñana ponds, that macroinvertebrate assemblages change in a predictable manner each year in response to cycles of pond wetting and drying, and that connectivity and environmental variability maintain biodiversity in pond networks.     

The restoration of tropical seed dispersal networks

Human activities have led to the loss of habitats and biodiversity in the Atlantic Rain Forest in Brazil. Ecological restoration aims to rebuild this biome and should include not only the reinstatement of species but also the reestablishment of complex ecological interactions and the ecological functions that they provide. One such function is seed dispersal, which is provided by the interactions between animal frugivores and plants. We studied seed dispersal networks in 3 different tropical forest sites restored 15, 25, and 57 years ago; temporal scales rarely observed in restoration studies. We investigated changes in network structure (nestedness, modularity, and network specialization) in these communities over restoration time. Although network size and the number of interactions increased with time since restoration, the networks were composed of generalist birds, and the large frugivores remained absent. Contrary to our expectations though, species richness was highest in the 25‐year‐old site, maybe due to the higher number of species used in the planting. Nestedness values were low in all 3 networks, but the highest nestedness was observed in the intermediate‐aged site. However, the oldest network was significantly modular and showed higher complementary specialization. These results suggest that 57 years after restoration, the complexity of mutualistic interactions in seed dispersal networks has increased, this enhancing ecosystem function in the Atlantic forest.     

Nestedness, biogeographic theory, and the design of nature reserves

I examine the relationship between nested distributional patterns and the degree to which several small reserves will contain more species than would a single reserve of equal total area (SLOSS). Nestedness is a common property of species distributions on real and habitat islands. However, there is considerable variation in nestedness among species distributions, some of which is related to the physical and biological background of the archipelagoes. Nestedness does not vary according to the taxonomic group examined (with the exception of aquatic invertebrates). Nestedness does vary between real and habitat islands (with aquatic invertebrates excluded), but not between oceanic and land-bridge islands. The more a biota is nested, the more likely it is that a single large reserve would preserve more species. However, nestedness is a rather poor predictor of SLOSS, as the vast majority of archipelagoes support a strategy of several small reserves, even though almost all of them are significantly nested. Nestedness says little about optimal reserve design and management, and appears to be a weak conservation tool. Received: 30 May 1995 / Accepted: 1 April 1997     

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.     

From sampling stations to archipelagos: investigating aspects of the assemblage of insular biota

Aim To investigate the formation of nestedness and species co‐occurrence patterns at the local (sampling station), the intermediate (island group), and the archipelago scale. Location The study used data on the distribution of terrestrial isopods on 20 islands of the central Aegean (Greece). These islands are assigned to two distinct subgroups (Kyklades and Eastern islands). Methods The Nestedness Temperature Calculator was used to obtain nestedness values and maximally nested matrices, the EcoSim7 software and a modified version of Sanderson (2000 ) method were used for the analysis of species co‐occurrences. Idiosyncratic temperatures of species and the order of species placement in the maximally nested matrices were used for further comparisons among spatial scales. The relationships of nestedness values with beta‐diversity, habitat diversity and a number of ecological factors recorded for each sampling station were also investigated. Results Significant nestedness was found at all spatial scales. Levels of nestedness were not related to beta‐diversity or habitat diversity. Nestedness values were similar among spatial scales, but they were affected by matrix size. The species that contributed most to the nested patterns within single islands were not the same as those that produce nestedness at the archipelago scale. There was significant variation in the frequency of species occurrence among islands and among spatial scales. There was no direct effect of ecological factors on the shaping of patterns of nestedness within individual islands, but habitat heterogeneity was crucial for the existence of such patterns. Positive associations among species prevailed at all scales when species per station were considered, while negative associations prevailed in the species per island matrices. All associations resulted from the habitat structure of sampling stations and from particularities of geographical distributions. Conclusions There was no clear‐cut distinction between nestedness patterns among spatial scales, even though different species, and partially different factors, contributed to the formation of these patterns in each case. There was a core of species that contributed to the formation of nested patterns at all spatial scales, while the patterns of species associations suggested that biotic interactions are not an important causal factor. The results of this study suggest that locally rare species cannot be widespread at a higher spatial scale, while locally common species can have a restricted distribution.     

The frugivory network properties of a simplified ecosystem: Birds and plants in a Neotropical periurban park

Frugivory networks exhibit a set of properties characterized by a number of network theory‐derived metrics. Their structures often form deterministic patterns that can be explained by the functional roles of interacting species. Although we know lots about how these networks are organized when ecosystems are in a complete, functional condition, we know much less about how incomplete and simplified networks (such as those found in urban and periurban parks) are organized, which features are maintained, which ones are not, and why. In this paper, we examine the properties of a network between frugivorous birds and plants in a small Neotropical periurban park. We found a frugivory network composed of 29 species of birds and 23 of plants. The main roles in this network are played by four species of generalist birds (three resident, one migratory: Myiozetetes similis, Turdus grayi, Chlorospingus flavopectus, and Dumetella carolinensis) and three species of plants (one exotic, two early successional: Phoenix canariensis, Phoradendron sp., and Witheringia stramoniifolia). When compared to reference data from other locations in the Neotropics, species richness is low, one important network‐level metric is maintained (modularity) whereas another one is not (nestedness). Nestedness, a metric associated with network specialists, is a feature this network lacks. Species‐level metrics such as degree, species strength, and module roles, are not maintained. Our work supports modularity as the most pervasive network‐level metric of altered habitats. From a successional point of view, our results suggest that properties revealed by species‐level indices may be developed at a later time, lagging the acquisition of structural elements.     

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

Seed 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.     

嵌套性:研究方法、形成机制及其对生物保护的意义

岛屿或者“生境岛”中的生物区系常常显示出一种嵌套结构 ,即物种较贫乏的岛屿中的物种是物种较丰富的岛屿中的物种的一个适当的子集 ,如果将各个岛屿中的生物区系排列起来就形成一个嵌套的序列。与种 面积关系一样 ,嵌套结构在很多生境类型和生物类群中也都存在。嵌套性对生物保护也有一定的意义 ,特别是与SLOSS争论 (是单个大的还是几个小的保护区能保护更多的物种 )有一定关系。在过去的十几年中 ,已经提出了一些方法 ,可以对嵌套性进行定量刻画和统计检验。同时 ,对嵌套性的形成机制也进行了大量的研究 ,其中选择性的迁移和选择性的灭绝是两个主要的原因。由于嵌套性分析只需要物种的存在 /不存在数据 ,使得很多调查数据都能够利用起来 ,因此 ,这是一个值得深入研究的领域     

On the meaning and measurement of nestedness of species assemblages

Nestedness of species assemblages occurs when thebiotas of sites with lower numbers of species tend to be subsets of the biotas at richer sites. We develop new quantitative and statistical techniques for measuring, testing, and comparing nestedness, and apply these methods to data from the literature. Significantly nonrandom nestedness was present in all 27 assemblages examined, and tended to be stronger in systems dominated by extinction, such as landbridge islands. Sets of assemblages that were very strongly nested were more likely to have greater species richness on one or a few large sites than on several smaller sites of equivalent total area — that is, to fall toward the single large side of the Single Large Or Several Small (SLOSS) continuum. Our analysis indicates that nestedness, when quantified as a single number for a presence-absence matrix, measures community-wide differences in incidence (the frequency of occurrence or distribution of species). Factors that lead to consistent differences among species in immigration or extinction rates cause strong patterns of nestedness of species assemblages. Nestedness is negatively related to beta diversity: nestedness is low when beta diversity is high, and vice versa. Conservation managers will thus seek to minimize nestedness and the development of nested structure in systems of nature reserves.