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How steady is the nested pattern in saline grassland true bug communities? Effects of sampling effort and data completeness on nestedness

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How steady is the nested pattern in saline grassland true bug communities? Effects of sampling effort and data completeness on nestedness

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Abstract

Nestedness is currently one of the most explored structural patterns of biological communities, especially since its major application in studies on ecological networks. Research on the nested pattern, as a spatial structural descriptor of communities, was mostly focused on theoretical background in the conservation of habitat islands. Here, we explore, to what extent nested true bug communities are present in saline grassland fragments of the Pannonian ecoregion and what are the reasons for their nestedness. In 2015 and 2017, we sampled true bugs from 17 saline pastures in northern Serbia. We applied three different sampling procedures (i.e. gradient in sampling effort) for collecting true bugs: along 20 m long transect, 3 × 30 m long parallel transects and total inventory. Analyses showed significant nestedness of the true bug communities and consistency of the pattern regardless of the data type (binary vs. weighted data) and applied metrics (NODF vs. spectral radius, WNODF vs. spectral radius). However, with the decrease of sampling effort and/or the exclusion of grass feeders, all metrics gradually detect less frequently significant nestedness. The only constant factor which was found to induce nestedness in true bug communities was true bug species richness. Plant cover features showed a weak influence on the nested pattern and landscape features did not take part at all. We believe nestedness of true bugs was the result of differences in habitat complexity among sampling sites, but detailed studies are needed to reveal which abiotic and/or biotic factors are responsible for it. For further studies, priority should be given to investigating the influence of management of saline grasslands on the nestedness in true bug communities.

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For adequate conservation planning, ecologists need to understand the driving factors of the species richness and composition patterns of arthropod assemblages. The present study scrutinizes how the vegetation, the surrounding matrix and the process of fragmentation affects the diversity and the composition of arthropod assemblages in dune slack meadow remnants of an agricultural landscape in southern Hungary. Spiders, orthopterans and true bugs were sampled with sweep netting in a total of 25 sand steppe, mesotrophic wet meadow and alkaline meadow patches. Our general findings showed that the structure and species composition of arthropod assemblages depended primarily on the vegetation type. From among the size and shape of the patches, their extent of isolation and the types of the surrounding landscapes only the latter one could be shown to have a significant effect, though this was also restricted to the group of spiders. Therefore, the structure and composition of spider assemblages may be used as indicators for the adjacent agricultural activities. Unlike its effect on species composition, the effect of vegetation type was not consistent on the species richness of the arthropod assemblages and functional groups. The influence of vegetation type was greater on the species richness of herbivores (specialist and generalist true bugs and grasshoppers) than on that of the carnivores (spiders, bush crickets and carnivorous true bugs). Based on the fact that neither patch size nor the extent of isolation, i.e. the measures of fragmentation, influenced the species richness of the majority of the arthropod groups significantly, we conclude that fragmentation was not the main threat for the studied arthropod fauna. Furthermore, the present study calls attention to the limited efficiency of Island Biogeography Theory in the case of relatively large, traditionally managed grasslands embedded in extensive agricultural landscapes, like the general landscape pattern of the Great Hungarian Plain.
Article
River basins are among the most threatened ecosystems. The species diversity of several European river basins decreased seriously during the last decade due to loss of habitats and increasing land use pressure on the remaining habitats. We studied true bug assemblages in various land use types of grassland fragments and dikes as linear grassland habitats in the agricultural landscape of the lower reach of the Tisza River Basin. We tested the effects of the recorded variables of habitat quality, surrounding landscape and land use type on the abundance, species richness and composition of true bugs. Altogether, 5,389 adult Heteroptera individuals representing 149 species in 13 families were collected. The factors which influenced significantly the species richness of different trophic levels (i.e. herbivors, predators) and degrees of food specialization (i.e. generalist and specialist herbivors) were concordant. Contrary to this, the factors which influenced the abundance of the different feeding groups varied strongly. We emphasise the vegetation and land use types as primarily influential factors for insects. Excluding the grass-feeding species, the number of both generalist, specialist herbivorous and predaceous species were lower in agricultural swards, i.e. hay-meadows and pastures than in old field and dike habitats and their number increased with increasing vegetation diversity. Due to the high species richness and abundance observed in dike and old field habitats compared to agricultural swards, we emphasise their importance for conservation of insect diversity and we stress the negative effects of agricultural intensification on the remaining grasslands of the lower reach of the Tisza River Basin.
Article
Ecological presence–absence matrices capture information of species occurrences among a number of sites. Statistical inference of matrix structure often used a fixed–fixed (FF) null model in which matrix entries are randomized, but the row and column total of each random matrix match those of the original matrix. However, in a stochastically assembled meta-community, row and column totals of a random assemblage might be expected to vary among matrices. Here we introduce a 4-step proportional–proportional (PP) algorithm that creates null matrices in which the row and column vary randomly, but the average row and column totals in a set of PP matrices are unbiased and match those of the original matrix. We tested the performance of the PP algorithm with 5 sets of artificial matrices and one large set of 288 published empirical matrices. Compared to the FF algorithm, the PP algorithm has better power to detect segregated and nested matrices, but it is vulnerable to Type I errors if row and column sums have small variances. The PP algorithm identified only 9% of empirical matrices as significantly segregated, compared with 30% identified by the traditional FF algorithm. The choice between whether to use the PP or the FF algorithm is similar to the distinction between random and fixed effects in a mixed-model ANOVA. For robust analysis, it may be desirable to use both the PP and the FF algorithms with the same data matrix.
Article
The biotas of archipelagos and fragmented habitats frequently show a 'nested' structure. That is, the species composition of a small island or fragment tends to be a subset of the next larger island or fragment, and the set of biotas as a whole forms a nested series. Several indices exist that allow 'nestedness' to be quantified and its statistical significance assessed. Like the species-area relationship, nested structures are observed in many types of habitat and in many taxa, and also like the species-area relationship, it has been suggested to have implications for biological conservation, particularly in connection with the 'single large or several small' (SLOSS) debate regarding nature reserve design. In a perfectly nested archipelago of habitats, a large, species-rich reserve would necessarily conserve more species than any combination of smaller ones because (by definition) these smaller biotas would contain only those species present in the larger reserve. If the structure deviates from perfect nestedness, however, the situation is less straightforward, and depends on the degree of nestedness and the processes generating nested structure. Nested structure is generally attributed to differential local extinction of species, which would result in smaller biotas converging on the same set of extinction-resistant species. Nested structure can also be produced by differential immigration, nested habitat distribution, and passive sampling. Computer simulations demonstrate that highly nested patterns can be produced by passive sampling, and that these tend to be 'outlier-rich'. In this case, large reserves have no advantage over aggregates of smaller reserves with equal total area. More work needs to be done on the underlying causes of nestedness before its full significance for biological conservation can be confidently assessed.
Article
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.
Article
Summary 1. Ecological networks have been shown to display a nested structure. To be nested, a network must consist of a core group of generalists all interacting with each other, and with extreme specialists interacting only with generalist species. 2. Studies on ecological networks are especially prone to sampling effects, as they involve entire species assemblages. However, we know of no study addressing to what extent nestedness depends on sampling effort, despite the numerous studies discussing the ecological and evolutionary implications of nested networks. 3. Here we manipulate sampling effort in time and space and show that nestedness is less sensitive to sampling effort than number of species and links within the network. 4. That a structural property of an ecological network appears less prone to sampling bias is encouraging for other studies of ecological networks. This is because it indicates that the sensitivity of ecological networks properties to effects of sampling effort might be smaller than previously expected.
Article
Summary • During the last few years a variety of methods have been applied in Switzerland to preserve and enhance biological diversity in agricultural systems. The purpose of this study was to evaluate grassland management techniques in respect of their effectiveness within a managed area and to examine how these areas contribute to species diversity at a landscape scale. • We examined insect diversity in grasslands subject to different management in a heterogeneous landscape in part of the Swiss Jura. Four study areas with varying landscape structure were selected and, in each area, meadows of two grassland management types were investigated. • The true bugs (Heteroptera) were chosen as an indicator group for insect diversity on the basis of previous work that had shown that the richness of the bug fauna correlates strongly with total insect diversity. • The variance of the heteropteran species data was partitioned into spatial (area) and management components. Area accounted for 35·4%, management for 29·7% and the interaction management × area for 7·2% of the species variance. The species diversity was greater in extensively managed meadows than in intensive ones; extensive sites had more individuals and showed a more even rank abundance distribution. • Individual species differed in their responses to management. Two species benefited from intensification whereas six species were affected negatively by intensive management. Two main groups of species did not appear to respond to management; these were mostly widespread species occurring in a variety of habitats, and polyphagous species which live in a wide range of grasslands but which show a certain affinity to managed meadows. • Our study indicates that extensive management of grasslands can enhance both local and regional insect diversity in agricultural landscapes. Extensively managed meadows were species-rich habitats that supported some rare and specialized species. In contrast, the bug community of intensive meadows was dominated by more widespread and less specialist species.
Article
1. Agricultural intensification is a major cause of biodiversity loss in European farmlands. Grasslands are particularly important habitats for the conservation of rich insect assemblages of Central and Eastern Europe. Although regular grazing or mowing of these grasslands is required to maintain diversity, there is no information about how such management and other factors influence Hemiptera assemblages. 2. We studied leafhopper and true bug assemblages in semi-natural grasslands in three regions of the Great Hungarian Plain. We investigated how local vegetation factors and landscape complexity influence assemblages and whether they interact with management effects. 3. Seven pairs of intensively (>1 cow/ha) versus extensively (∼0.5 cow/ha) grazed pastures were sampled in each region by sweep-netting. 4. Sward height was the most important explanatory factor for leafhoppers (84 species, 27264 individuals), as it increased both species number and abundance, and influenced assemblage composition. The extent of grassland surrounding the sample sites negatively affected leafhoppers, whereas extensive grazing decreased abundance and influenced composition. True bug assemblages (140 species, 6656 individuals) were positively affected only by mean sward height, whereas regional differences determined the community composition of both taxa. 5. We conclude that vegetation structure is the primary factor shaping Hemiptera communities and that the various types of grasslands studied are all important habitats for the taxon. Therefore, cattle grazing in its current form is beneficial for the rich Hemiptera fauna in lowland pastures of Hungary. However, in some cases, local and landscape factors and great regional differences may confound the effects of grazing, and this must be considered in conservation planning in the future.
Article
Question: How many vegetation plot observations (relevés) are available in electronic databases, how are they geographically distributed, what are their properties and how might they be discovered and located for research and application? Location: Global. Methods: We compiled the Global Index of Vegetation-Plot Databases (GIVD; http://www.givd.info), an Internet resource aimed at registering metadata on existing vegetation databases. For inclusion, databases need to (i) contain temporally and spatially explicit species co-occurrence data and (ii) be accessible to the scientific public. This paper summarizes structure and data quality of databases registered in GIVD as of 30 December 2010. Results: On the given date, 132 databases containing more than 2.4 million non-overlapping plots had been registered in GIVD. The majority of these data were in European databases (83 databases, 1.6 million plots), whereas other continents were represented by substantially less (North America 15, Asia 13, Africa nine, South America seven, Australasia two, multi-continental three). The oldest plot observation was 1864, but most plots were recorded after 1970. Most plots reported vegetation on areas of 1 to 1000 m2; some also stored time-series and nested-plot data. Apart from geographic reference (required for inclusion), most frequent information was on altitude (71%), slope aspect and inclination (58%) and land use (38%), but rarely soil properties (
Article
We set up two alternative hypotheses on how environmental variables could foster nestedness; one of “nested habitats” and another of “nested habitat quality”. The former hypothesis refers to situations where the nestedness of species depends on a nestedness of discrete habitats. The latter considers situations where all species in an assemblage increase in abundance along the same environmental gradient, but differ in specialisation or tolerance. We tested whether litter-dwelling land snails (terrestrial gastropods) in boreal riparian forest exhibited a nested community structure, whether such a pattern was related to differences in environmental variables among sites, and which of the two hypotheses that best could account for the found pattern. We sampled litter from 100 m2 plots in 29 mature riparian forest sites along small streams in the boreal zone of Sweden. The number of snail species varied between 3 and 14 per site. Ranking the species-by-site matrix by PCA scores of the first ordination axis revealed a similarly significant nested pattern as when the matrix was sorted by number of species, showing that the species composition in this meta-community can be properly described as nested. Several environmental variables, most notably pH index, were correlated with the first PCA axis. All but two species had positive eigenvectors in the PCA ordination and the abundance increased considerably along the gradient for most of the species implying that the hypothesis of “nested habitats” was rejected in favour of the “nested habitat quality” hypothesis. Analyses of nestedness have seldom been performed on equal sized plots, and our study shows the importance of understanding that variation in environmental variables among sites can result in nested communities. The conservation implications are different depending on which of our two hypotheses is supported; a conservation focus on species “hotspots” is more appropriate if the communities are nested because of “nested habitat quality”.
Article
Nestedness has been widely reported for both metacommunities and networks of interacting species. Even though the concept of this ecological pattern has been well-defined, there are several metrics by which it can be quantified. We noted that current metrics do not correctly quantify two major properties of nestedness: (1) whether marginal totals (i.e. fills) differ among columns and/or among rows, and (2) whether the presences (1's) in less-filled columns and rows coincide, respectively, with those found in the more-filled columns and rows. We propose a new metric directly based on these properties and compare its behavior with that of the most used metrics, using a set of model matrices ranging from highly-nested to alternative structures in which no nestedness should be detected. We also used an empirical dataset to explore possible biases generated by the metrics as well as to evaluate correlations between metrics. We found that nestedness has been quantified by metrics that inappropriately detect this pattern, even for matrices in which there is no nestedness. In addition, the most used metrics are prone to type I statistical errors while our new metric has better statistical properties and consistently rejects a nested pattern for different types of random matrices. The analysis of the empirical data showed that two nestedness metrics, matrix temperature and the discrepancy measure, tend to overestimate the degrees of nestedness in metacommunities. We emphasize and discuss some implications of these biases for the theoretical understanding of the processes shaping species interaction networks and metacommunity structure.
Article
Nestedness analysis has become increasingly popular in the study of biogeographic patterns of species occurrence. Nested patterns are those in which the species composition of small assemblages is a nested subset of larger assemblages. For species interaction networks such as plant–pollinator webs, nestedness analysis has also proven a valuable tool for revealing ecological and evolutionary constraints. Despite this popularity, there has been substantial controversy in the literature over the best methods to define and quantify nestedness, and how to test for patterns of nestedness against an appropriate statistical null hypothesis. Here we review this rapidly developing literature and provide suggestions and guidelines for proper analyses. We focus on the logic and the performance of different metrics and the proper choice of null models for statistical inference. We observe that traditional ‘gap-counting’ metrics are biased towards species loss among columns (occupied sites) and that many metrics are not invariant to basic matrix properties. The study of nestedness should be combined with an appropriate gradient analysis to infer possible causes of the observed presence–absence sequence. In our view, statistical inference should be based on a null model in which row and columns sums are fixed. Under this model, only a relatively small number of published empirical matrices are significantly nested. We call for a critical reassessment of previous studies that have used biased metrics and unconstrained null models for statistical inference.
Article
We examined nestedness and potential mechanisms causing that distributional pattern in resident butterfly communities of the Toiyabe Range, a mountain range in the central Great Basin of western North America. We tested whether life history characteristics, including habitat use and vagility, affected the relative degree of nestedness or mean species incidence. We also tested whether nestedness at the level of individual species was independent of life history. Relationships between distributional patterns and habitat use, particularly in ecologically sensitive riparian areas, are relevant to ongoing conservation planning in the Great Basin. The distributional pattern of the 68 resident butterfly species in 19 Toiyabe Range canyons was significantly nested, as was the distribution of all functional subgroups that we tested. Life history affected neither relative nestedness of species groups nor mean species incidence. More than 80% of the individual butterfly species that inhabit the Toiyabe Range had distributions that were more nested than expected. Colonization does not appear to have played an important role in determining the composition of butterfly communities in Toiyabe Range canyons. Likewise, selective dispersal has probably played a minor role in producing nested distributions of Toiyabe Range butterflies. Our results suggest either that impacts to riparian areas are not jeopardizing species viability, or that highly sensitive butterfly species have already been extirpated from the Toiyabe Range.
Article
Knowledge of the distribution of rare species is crucial for species conservation in fragmented habitats. Species communities often exhibit nestedness, i.e. species in species-poor sites comprise a subset of richer ones. Thus, rare species are confined to species-rich sites. We evaluate whether plant and fungal communities in 46 old-growth spruce forest patches (0.17–12 ha) exhibit nestedness. The question whether a single large patch or several small patches capture most species (i.e. the SLOSS-issue) is evaluated in combination with species saturation analyses. All species groups exhibited significant nestedness. Area was generally related to nestedness, i.e. rare species were over-represented in the largest patches. Species saturation analysis indicated that large patches accumulated more Red-list species in patch interiors than small patches. Thus, rare and Red-list species were best captured in large patches. However, nestedness also emerged in equal sized sample plots, i.e. rare species were over-represented in high quality habitats. Thus, small habitats of high quality should not be neglected in a conservation perspective.
Article
Habitat fragmentation of semi-natural habitats is a serious problem in nature conservation because it leads to a decrease in habitat area and an increase of the isolation of habitat islands. In this paper, we examined the effects of habitat fragmentation on species richness and assemblages of four taxonomic groups. First, we used species-area relationships to assess the effect of habitat area on species richness per habitat island. Second, cumulative species-area curves were used to assess the question whether large habitat islands contain more species than several small islands of the same total area. Third, we examined to what extent species assemblages of species-poor habitat islands are nested subsets of those of species-rich ones. We analysed presence-absence data of vascular plants and bryophytes in 36, and of day-active butterflies and grasshoppers in a subset of 23 habitat islands (montane calcareous fens). The species-area relationship of each group was positive but was highly significant only for vascular plants. The slopes of the relationship on the log-log-scale were rather low (z-values: 0.06-0.11). The cumulative species-area curves of all groups showed that a set of small habitat islands contained more species than a set of a few large habitat islands with the same total area. The results of our nestedness analyses depended on the underlying null models of the test statistics. Applying the method of Wright & Reeves (1992), all groups were significantly nested within the archipelago of habitat islands. Yet, using the method of Brualdi & Sanderson (1999), none of the groups showed non-random patterns. Our study showed that species richness increased with the area of habitat islands, but overlap among them was so low that even small habitat islands contributed to overall species richness. Therefore, a mosaic of several wetland islands, including small ones, is necessary to maintain species diversity at the landscape level.
Article
Nestedness has been one of the most reported patterns of species distribution in metacommunities as well as of species interactions in bipartite networks. We propose here a straightforward approach for quantifying nestedness using quantitative instead of presence–absence data. We named our estimator WNODF because it is a simple modification of the nestedness index called NODF. We also introduce the NODF-Program that calculates the above described nestedness metrics as well as metrics for idiosyncratic species and sites. Statistical inference is done through a null model approach, in which the user can choose among five null models commonly used for presence–absence matrices as well as three randomization algorithms for matrices that contain quantitative data. The program performs multiple analyses using many matrices. Finally, the NODF-Program provides four sorting options that, together with the null algorithms, cover a range of possibilities to test hypotheses on the possible mechanisms producing nested patterns. By using a set of model matrices, we showed that WNODF differentiates nested matrices with distinct structures and correctly identifies matrices with no nested pattern as having zero degree of nestedness.
Article
Many island communities exhibit a highly nonrandom pattern of species composition in which smaller biotas contain successive subsets of the species in richer ones. Here, we examine the composition of continental communities to see whether this 'nested subset' pattern holds and, if so, to determine its spatial scale and environmental correlates. Granivorous rodent assemblages at 202 sites in Great Basin, Mojave, Sonoran and Chihuahuan deserts were analysed collectively. Separate analyses were also conducted on sites within each desert, habitat (desert scrub, desert grassland, sand dunes, shrub-steppe), and on desert-habitat combinations. Nested subset patterns of species composition were found to characterize the entire assemblage of 202 sites, all Great Basin sites, all Sonoran sites, and various habitat groupings of sites within deserts. However, groupings of sites by habitats among deserts did not exhibit this structure. Results suggest that three conditions may be necessary for the development of ne
Article
Scientific interest in the impact of habitat fragmentation on biodiversity is increasing, but our understanding of fragmentation is clouded by a lack of appreciation for fundamental similarities and differences across studies representing a wide range of taxa and landscape types. In an effort to synthesize data describing ecological responses of animals to fragmentation across two classes of independent variables (taxonomic group and landscape), we sampled 148 studies of five major faunal groups from the primary literature and analyzed data on 13 variables extracted from those studies. We focused our analyses on three classes of dependent variables (effects of area and isolation on species richness, z values, and nestedness and species composition). Area ranged over more orders of magnitude than isolation and tended to explain more variation in species richness than isolation. There were few matrix or taxon effects on the patterns we investigated, although we did find that sky islands tended to manifest isolation effects on both species richness and nestedness more frequently than other patch types. Sky islands may offer insight into the future of habitat patches fragmented by contemporary habitat loss, and because they show a stronger effect of isolation than other patch types, we suggest that isolation will play an increasing role in the biology of habitat fragments. We use multiple lines of evidence to suggest that our understanding of the role of isolation on community assembly in fragmented landscapes is inadequate. Finally, our observation that consistent taxonomic differences in community patterns were minimal suggests that conservation actions intended to mitigate the negative effects of extinction may have far-reaching effects across taxonomic groups.
Indication of environmental heteromorphy and habitat fragmentation by invertebrate communities in grasslands
  • Gallé
Gallé, L., Györffy, Gy, Hornung, E., Körmöczi, L., 1988. Indication of environmental heteromorphy and habitat fragmentation by invertebrate communities in grasslands. In: Bohac, J., Ruzicka, V. (Eds.), Proc. Vth Int. Conf. Bioindicatores Deteriorisationis Regionis, Inst. Landscape Ecology CAS. Ceské Budéjovice, p. 167.
Priručnik Za Tipološko Istraživanje I Kartiranje Vegetacije
  • I Horvat
  • S Horvatić
  • M Gračanin
  • G Tomažić
  • H Em
  • B Maksić
Horvat, I., Horvatić, S., Gračanin, M., Tomažić, G., Em, H., Maksić, B., 1950. Priručnik Za Tipološko Istraživanje I Kartiranje Vegetacije. Nakladni zavod Hrvatske, Zagreb.
Chapter 13— nestedness and community assembly
  • Lindenmayer
Lindenmayer, D.B., Fischer, J., 2006. Chapter 13-nestedness and community assembly. In: Habitat Fragmentation and Landscape Change: an Ecological and Conservation Synthesis. Island Press, Washington, pp. 133-141.