Species Abundance Curve Research Articles

Different habitat condition proportions on farms affect the structure and diversity of dung beetle (Coleoptera, Scarabaeidae, Scarabaeinae) communities

The continuous changes and increase of land use into ranching and agriculture have caused shifts in the composition and structure of dung beetle communities due to the modification of vegetation structure. The impact of these changes can be approached using dung beetles as ecological indicators. Agricultural, ranching, and forestry are often integrated into the same farm in different proportions (landscape level), and their degree of integration with habitats changes over time and space. We assessed if different habitat proportions of crop, pasture, and forest land on three farms affect the structure and diversity of dung beetle (Coleoptera: Scarabaeinae) communities. Farms included from three to four habitat conditions each (plantations of timber species, banana-coffee plantation, living fences, pastures, secondary and riparian forest). Pitfall traps with dung and carrion as bait were placed across each habitat condition of each farm during three different sampling periods. Across all samplings, 1,198 dung beetle individuals belonging to 21 species were captured. Species diversity and composition vary according to the type of farm and the main factor that modifies this tendency is the proportion of pasture land composing the farm. Farms with a lower proportion of grazing land (11.96% for forestry farms and 32.19% for agricultural farms) and denser vegetation canopy, which cast more shadows, had greater beetle diversity. Conversely, farms having a greater proportion of grazing land had low diversity and a dominant tendency in the species abundance curve. Umbraphile species dominated the forest farm, indicating a strong affinity for shaded environments such as the one provided there, while species displaying no habitat preference exhibited higher abundance in the ranching farm. Additionally, a notable prevalence of small-sized species was observed in the ranching farm, contrasting with a relatively even distribution of sizes in the remaining landscapes. These tendencies suggest that shade positively influences biodiversity conservation. Riparian vegetation, living fences, and banana-coffee plantations are important connectivity elements in agricultural landscapes for shade-adapted dung beetle species.

Apple flower-visiting insects’ diversity and abundance in selected central Kenya orchards

Flower-visiting insects are essential in maintaining a healthy and productive agricultural landscape through ecosystem services such as pollination. Fruits are important sources of vitamins and micronutrients, and several fruit crops depend on animal pollination, which enhances their nutritional content. Besides honey bees, apple orchards can sustain a large number of arthropod species that also aid in pollination. In this present study, we assessed the diversity and abundance of insect flower visitors in six apple orchards in Nyeri and Laikipia East. Six plots, each with more than 20 apple trees, were purposefully selected. Data were collected through timed visual searches and sweep netting. Sampling was done from 0900 hours to 1700 hours, six days a week, for five months consecutively in 2019. Diversity indices were computed using the Shannon-Wiener diversity index, while abundance patterns were displayed using species abundance curves. Differences in species diversity and abundance between sites were compared using a one-way analysis of variance. A total of 1,221 insects belonging to 23 families, 82 species, and 4 orders (Hymenoptera, Lepidoptera, Coleoptera, and Diptera) were recorded. The order Hymenoptera (bees, wasps, and ants) was the most abundant, with 1,176 individuals, and had the most species-rich taxa, with 56 species in 9 families. The order Coleoptera (beetles) was second, with 26 individuals and 12 species in 6 families. Lepidoptera were the least abundant with 10 individuals and the least species-rich taxa with 7 species in 5 families, followed closely by Diptera with 7 individuals and 7 species in 3 families. There were significant differences in flower visitors’ composition among the six sites. The study provides important information on the status of key apple flower visitors, which can guide orchard management practises to increase apple yield through pollinator conservation. Conserving apple pollinators will enhance fruit production, promote the livelihoods of farmers, and contribute to the national economy.

Composition and diversity of fish assemblages along a shallow beach of a neotropical coastal lagoon

Coastal lagoons are inland water bodies connected to the sea and influenced by freshwater discharge, so these environments undergo constant changes in physical–chemical water characteristics. Transitional environments are important for promoting breeding, refuge, and food areas, besides serving as possible nursery habitats for fish species. In the present study, we analyzed the structure of the fish assemblages in a shallow marginal area of Mirim Lagoon, which belongs to the Laguna Estuarine System (LES), by seasonally evaluating the composition, structure, and diversity of the assemblages. Fieldwork was carried out between September 2017 and August 2018 with three monthly replicates using a beach seine net. Analyses were conducted to highlight the general patterns of the assemblages and to verify seasonal variations in composition, abundance, and diversity. The common species were defined using a species abundance curve, while the seasonal variation in diversity was evaluated using integrated rarefaction and extrapolation curves that correspond to the three orders of Hill numbers. The differences in the specific seasonal composition were indicated by dbRDA and the existence of statistical differences was tested using a PERMANOVA. In total, 3,556 individuals distributed in 16 families and 26 species were caught, with the predominance of Atherinella brasiliensis, Eucinostomus melanopterus, Coptodon rendalli, Eucinostomus argenteus, Mugi lliza, Anchoa tricolor, and Achirus lineatus that represented approximately 98% of the total. Regarding seasonality, the highest abundance was caught in the winter, while the lowest abundance was caught in autumn. The PERMANOVA indicated differences between spring and autumn and between autumn and winter. The main differences between these seasons are associated with the number of individuals caught, species abundance, and species composition. The presence of juveniles of several fish species, especially non-resident species, demonstrates the importance of the shallow area of Mirim Lagoon as a breeding, growth, and protection site that serves as a possible nursery habitat for these species.

Tree diversity and regeneration status in the different forest types of Kotla watershed (Uttarkashi, Uttarakhand, India)

The present study aimed to assess the diversity and regeneration status of tree species in different forest types viz., Pinus forest (PF), Pinus-Oak mixed forest (POF) and Deodar forest (DF) of Kotla watershed (Barkot, Uttakashi, Uttarakhand). The data were collected through quadrat method and analyzed quantitatively. A total of 28 tree species, belonging to 21 families were recorded in the sampling area (3 forests 10 plots in each plot size 400 m2). Fagaceae, Pinaceae, Ericaceae, Fabaceae and Juglandaceae were the major families (in terms of number of species). The species-area curves (SACs) of PF and DF reached an asymptote while it predicted more species (din not reach asymptote) in case of POF. The resulted values of different diversity indices (i.e., Dominance, Simpson, Shannon, Evenness, Margalef, and Equitability) revealed that the POF was most diverse (in tree diversity) followed by PF and DF. The ranked species abundance (RSA) curve of POF was log normal type but geometric series type for PF and DF. The density-diameter curves (d-d curves) was reverse J-shaped for POF while in case of PF and DF, the higher densities were observed for middle DBH classes in comparison to lowest and highest DBH classes. The overall regeneration status of the forests in the area was fair (25.8% tree, 18.6% saplings and 55.6% seedlings). The present study provides a deeper understanding of tree diversity pattern and regeneration status from a pocket of IHR.

A new species abundance distribution model including the hydrological niche differentiation in water-limited ecosystems

The formation and maintenance mechanism of biodiversity is a key research topic in community ecology. Establishing a model that can be used to explain and predict the changes of community biodiversity is of great significance in facilitating the analysis and protection of biodiversity. How hydrological niche differentiation determines species abundance distribution requires further exploration. In this paper, by analyzing the characteristics of evolutionary stability of plant communities, the evolutionary stability theory of community water resource allocation is studied in water-limited environments. The water consumption of an individual biomass growth is used to quantify the hydrological niche of the species. The arithmetic mean and harmonic mean ratio of the hydrological niche of a species in the community as a new diversity index is used to measure community hydrological niche differentiation. A new hydrological niche plant diversity model is established, and the species abundance curve and species area curve, including the community hydrological niche differentiation factor, are obtained. It was discovered that the distribution model of species abundance was close to the Fisher log series distribution, which unifies and expands the classical diversity theories. The quantitative relationship between the hydrological niche and plant diversity is given. It is proposed that the plant diversity of a community is determined by regionally available water resources and changes in the species abundance that are filtered by the local biological and abiotic environment and driven by the full utilization of water resources.

Local taxonomic spectra in plants, animals, fungi and terrestrial protists show common mathematical patterns

Taxonomic spectra, i.e. relations between supraspecific taxa by the number of included species, remain poorly understood in aspect of the mathematical properties. We studied taxonomic spectra of plants (Magnoliophyta, Bryophyta), animals (Coleoptera, Aves), fungi (Agaricomycetes) and terrestrial protists (Myxomycetes), found in the Homilsha Forests National Nature Park (North-East of Ukraine), and concluded that they correspond to the hollow-curve distribution at the level of genera, families and orders. The spectra of most taxa, as shown by the Akaike information criterion, are closely approximated by the log-series distribution model at all taxonomic levels. This type of distribution is typical for the species abundance curves, based on collections made from small areas. At the same time, in the genera–families–orders row the similarity to the lognormal distribution increases. The central values and variability vary considerably between different taxonomic groups and ranks, however, without affecting the type of distribution. The number of orders in all taxa except Bryophyta has reached the saturation and coincides with the curve of the estimated number of orders according to the Chao1 coefficient. For families and especially genera the correspondence with estimated number of species is much lower. Our results do not confirm the assumption that hollow-curve distributions of taxonomic spectra result from the artificial fragmentation of taxa. These distributions neither depend on the insufficient knowledge about the species composition at the locality, nor reflect the size of the studied area. The presence of such distributions in both local and global biota of different groups may be explained by the common features of their evolution, especially by the existence of relict orphan groups. The fact that in Homilsha Forests the kurtosis and skewness of distributions decreases in the genera–families–orders row can therefore be explained by the relatively low percentage of the high-rank orphan taxa in the local biota. This may be a common feature of communities studied at small geographical scale, since orphan taxa often demonstrate a high level of endemism. Comparative studies of local communities from different climate zones may help to understand how universal are the patterns, described herein.

A geometric approach to scaling individual distributions to macroecological patterns

Understanding macroecological patterns across scales is a central goal of ecology and a key need for conservation biology. Much research has focused on quantifying and understanding macroecological patterns such as the species-area relationship (SAR), the endemic-area relationship (EAR) and relative species abundance curve (RSA). Understanding how these aggregate patterns emerge from underlying spatial pattern at individual level, and how they relate to each other, has both basic and applied relevance. To address this challenge, we develop a novel spatially explicit geometric framework to understand multiple macroecological patterns, including the SAR, EAR, RSA, and their relationships. First, we provide a general theory that can be used to derive the asymptotic slopes of the SAR and EAR, and demonstrates the dependency of RSAs on the shape of the sampling region. Second, assuming specific shapes of the sampling region, species geographic ranges, and individual distribution patterns therein based on theory of stochastic point processes, we demonstrate various well-documented macroecological patterns can be recovered, including the tri-phasic SAR and various RSAs (e.g., Fisher’s logseries and the Poisson lognormal distribution). We also demonstrate that a single equation unifies RSAs across scales, and provide a new prediction of the EAR. Finally, to demonstrate the applicability of the proposed model to ecological questions, we provide how beta diversity changes with spatial extent and its grain over multiple scales. Emergent macroecological patterns are often attributed to ecological and evolutionary mechanisms, but our geometric approach still can recover many previously observed patterns based on simple assumptions about species geographic ranges and the spatial distribution of individuals, emphasizing the importance of geometric considerations in macroecological studies.

Multivariate Analysis of Rotifers Community from Safari Zoo Lake Lahore, Pakistan

Rotifers are zooplanktons that react more sharply to the environmental changes.
 Objective:To better understand the community composition of rotifers in Safari Zoo Lake, Lahore, PakistanMethods:We collected 16 water samples on monthly basis, dividing lake into four major sites: northern,southern, eastern, and western sites. Each of these was further subdivided into four locations, for a totalof 16 sampling sites. A total 23 rotifer species were recorded from the lake with Brachionus calyciorushaving maximum abundance. A species abundance curve was plotted between months and number ofspecies of rotifers indicating their lowest and highest abundance through the study period.Results:A Cluster Analysis yielded three main groups of closely related species including species likeBrachionus quadridentatus, Synchaeta stylata and Brachionus calyciorus etc. We used a PrincipalComponent Analysis (I and II) to study variation in the rotifer community on a seasonal basis and amongsampling sites. A biplot of Principal Component Analysis (PCA I) reected the relationship of rotifers withthe months. Some of the species showed a positive positive relationship, while others showed negativeone. PCA II was plotted between months and physico-chemical parameters showing their negative andpositive relations.Conclusion:We conclude that a lesser replacement of fresh water, increased density of waterfowl, reptilia andsolid waste left over during boating has resulted in eutrophication of lake leading to overall low diversity ofrotifers observed.

Fish assemblage in a semi-arid Neotropical reservoir: composition, structure and patterns of diversity and abundance.

The aim of this study was to analyse the composition, structure and spatial and temporal patterns of diversity and abundance of the ichthyofauna of the Santa Cruz Reservoir in semi-arid Brazil. Data were collected quarterly at eight sampling locations on the reservoir between February 2010 and November 2011 using gillnets from 12- to 70-mm mesh that were left in the water for 12h00min during the night. We evaluated the composition, structure and assemblage descriptors (Shannon-Wiener diversity index and equitability, respectively) and catch per unit effort by the number (CPUEn) and biomass (CPUEb) of the ichthyofauna. The 6,047 individuals (399,211.6 g) captured represented three orders, ten families and 20 species, of which four belonged to introduced species. The family Characidae was the most abundant with a total of 2,772 (45.8%) individuals captured. The species-abundance curve fit the log-normal model. In the spatial analysis of diversity, there were significant differences between sampling sites in the lacustrine and fluvial regions, and the highest values were found in the lacustrine region. In the temporal analysis of diversity, significant differences were also observed between the rainy and dry seasons, and the higher values were found during the dry season. Equitability followed the same spatiotemporal pattern as diversity. The Spearman correlation was significantly negative between diversity and rainfall. A cluster analysis spatially separated the ichthyofauna into two groups: one group formed by sampling sites in the fluvial region and another group formed by the remainder of the points in the lacustrine region. Both the CPUEn and CPUEb values were higher at point 8 (fluvial region) and during the rainy season. A two-way ANOVA showed that the CPUEn and CPUEb values were spatially and temporally significant. We conclude that the spatial and temporal trends of diversity in the Santa Cruz reservoir differ from those of other Brazilian reservoirs but that the fish community composition and spatiotemporal patterns of abundance were similar.

Differing assemblage composition and dynamics in T4‐like myophages of two neighbouring sub‐alpine lakes

Summary Bacteriophages play an important role in plankton population dynamics and biogeochemical cycling, but their community dynamics and diversity are still poorly known, especially in fresh waters. We conducted a 1‐year investigation of the T4‐like bacteriophages in the surface waters of two Western European peri‐alpine lakes (lakes Annecy and Bourget), using fingerprinting and cloning‐sequencing approaches targeting the major capsid protein g23 gene. Our results suggest that T4‐like bacteriophages are diverse (we found several new sequences) and differed substantially between the two lakes. Further, these phages displayed seasonal patterns with marked shifts in community composition. Examining the dynamics of some individual constituents (e.g. DGGE bands), we found that (i) the majority of these myoviruses were rare (mean relative abundance <1%) and only a few were abundant (mean relative abundance >5%); (ii) the dominance of the most abundant DGGE bands changed throughout the year and their dynamics were very different between the two lakes; (iii) only a few bands (e.g. <20%) were detectable throughout the year and their proportion was higher in the oligomesotrophic Lake Bourget. Our results suggest a highly dynamic T4‐like myoviral assemblage. T4‐like‐viruses seem to conform to a standard model of community organisation and a common type of species abundance curve, with mainly rare organisms that occasionally may become abundant (termed a ‘seed bank’ for virus assemblages).

Analysis of Structure and Diversity of the Kilengwe Forest in the Morogoro Region, Tanzania

This study investigates the structure, species composition, and diversity of a section of the Kilengwe Forest in Tanzania. In order to accomplish the proposed objectives, 18 plots of 20 m × 20 m were randomly established in the forest and the number of tree species in each plot was identified and counted. The most important families and species were determined using importance value indices at the respective taxonomic levels. Diversity was measured using the Shannon-Wiener and Fisher alpha diversity indices. A total of 276 stems/ha representing 93 species/ha within 26 families were documented from 0.72 ha. Fabaceae and Julbernadia globiflora were the dominant family and species, respectively. Seventy-eight percent of the total species were rare. The average basal area of the forest was 7.1 m2/ha. The Shannon-Wiener index (4.02) and Fisher’s alpha diversity (35.5) indicated high species diversity within the forest. The species-area and species-abundance curves revealed an escalating trend implying that more sampling efforts could result in a higher number of species existing in the forest. The size class distribution displayed a reverse J-shaped pattern; however, the larger size classes DBH >50 cm were not represented. The study suggests the necessity for anthropogenic disturbance control as this is the major source of forest degradation in the studied area.

Neutral-like abundance distributions in the presence of selection in a continuous fitness landscape

The neutral theory of biodiversity has successfully explained the observed composition of many ecological communities but it relies on strict demographic equivalence among species and provides no room for evolutionary processes such as selection and adaptation. Here we show how to embed the neutral theory within the Darwinian framework. In a continuous fitness landscape with a quadratic maximum, typical for quantitative traits, selection does restrict the extant species to have trait values close to optimal, but due to the continuous nature of the landscape, there are always many such species available, and so competition among them is effectively neutral. For sufficiently small mutational steps, the community structure fits perfectly to the Fisher log-series species abundance distribution. In general, the selection-mutation process, when superimposed on demographic and environmental noise, yields a small set of characteristic patterns for species abundance curves. A survey of these patterns is given.

Transient dominance in a central African rain forest

The large‐crowned emergent tree Microberlinia bisulcata dominates rain forest groves at Korup National Park, Cameroon, along with two codominants, Tetraberlinia bifoliolata and T. korupensis. M. bisulcata has a pronounced modal size frequency distribution around ∼110 cm stem diameter: its recruitment potential is very poor. It is a long‐lived light‐demanding species, one of many found in African forests. Tetraberlinia species lack modality, are more shade tolerant, and recruit better. All three species are ectomycorrhizal. M. bisulcata dominates grove basal area, even though it has similar numbers of trees (≥50 cm stem diameter) as each of the other two species. This situation presented a conundrum that prompted a long‐term study of grove dynamics. Enumerations of two plots (82.5 and 56.25 ha) between 1990 and 2010 showed mortality and recruitment of M. bisulcata to be very low (both rates ∼0.2% per year) compared with Tetraberlinia (2.4% and 0.8% per year), and M. bisulcata grows twice as fast as the Tetraberlinia. Ordinations indicated that these three species determined community structure by their strong negative associations while other species showed almost none. Ranked species abundance curves fitted the Zipf‐Mandelbrot model well and allowed “overdominance” of M. bisulcata to be estimated. Spatial analysis indicated strong repulsion by clusters of large (50 to <100 cm) and very large (≥100 cm) M. bisulcata of their own medium‐sized (10 to <50 cm) trees and all sizes of Tetraberlinia. This was interpreted as competition by M. bisulcata increasing its dominance, but also inhibition of its own replacement potential. Stem coring showed a modal age of ∼200 years for M. bisulcata, but with large size variation (50–150 cm). Fifty‐year model projections suggested little change in medium, decreases in large, and increases in very large trees of M. bisulcata, accompanied by overall decreases in medium and large trees of Tetraberlinia species. Realistically increasing very‐large‐tree mortality led to grove collapse without short‐term replacement. M. bisulcata most likely depends on climatic events to rebuild its stands: the ratio of disturbance interval to median species' longevity is important. A new theory of transient dominance explains how M. bisulcata may be cycling in abundance over time and displaying nonequilibrium dynamics.

Comparative evaluation of species diversity indices in the natural deciduous forest of Mt. Jeombong

Based upon the vegetation data of woody plants by plot sampling method in the natural deciduous forest of Mt. Jeombong, the study was carried out to examine importance value, rank abundance curve, and species abundance curve, and comparatively evaluate seven different species diversity indices for Log series, species richness, Margalef index, Shannon–Wiener index, Simpson index, McIntosh index, and Berger–Parker index, according to canopy layer and life form. The minimal area which meant only few more species were increased was 3.48 ha in total. The dominant species of overstory was Quercus mongolica, while Acer pseudo-sieboldianum was the dominant species of both midstory and understory. According to rank abundance curve and species abundance curve, species evenness was low. Log series and the Margalef index had a similar tendency. Also, the Shannon–Wiener index, Simpson index, and McIntosh index showed a similar tendency. However, the Berger–Parker index showed a similar trend with Log series and Margalef index. The dominant species of all growth forms were Quercus mongolica, Acer mandshuricum, and Syringa reticulata var. mandshurica. According to rank abundance curve and species abundance curve, species evenness was also low. Log series and the Margalef index had a similar trend. Also, the Shannon–Wiener index, Simpson index, McIntosh index, and Berger–Parker index showed a similar tendency.

Diversity of lignicolous and Ingoldian fungi on woody litter from the River Kali (Western Ghats, India)

Three hundred and sixty woody litter samples naturally deposited in a stream and dammed section of the River Kali in southwest India were assessed for the diversity of lignicolous fungi (ascomycetes and anamorphic fungi) via damp chamber incubation and aquatic hyphomycetes (Ingoldian fungi) via bubble chamber incubation. Fifty-six fungal taxa were recorded with the highest of 47 taxa in the stream and 34 taxa in the dam. Twenty-five taxa were common to both sites. The core group (≥10%) comprised one ascomycete (Massarina australiensis), two anamorphic taxa (Acrogenospora sphaerocephala and Sporoschisma saccardoi) and 11 Ingoldian fungi (Anguillospora crassa, A. longissima, Clavariana aquatica, Cylindrocarpon sp., Flagellospora curvula, F. penicillioides, Lunulospora curvula, L. cymbiformis, Lunulospora sp., Triscelophorus acuminatus, T. monosporus). The species richness, average taxa per wood sample, richness of core group fungi and fungal diversity were higher in the Kaiga stream than in the Kadra dam. Species abundance curves showed higher frequency of occurrence of fungi in the stream than in the dam woody litter. The rarefaction indices of fungal species against number of wood samples revealed similar pattern of ascomycetes in both locations, while the richness of anamorphic and Ingoldian taxa was higher in the stream than in the dam. The anamorph/teleomorph ratio ranged between 2.4:1 (Kadra dam) and 3.3:1 (Kaiga stream), which is higher than the ratios reported from other geographical locations (0.7:1−2:1). This study reconfirmed that the method of incubation influences the assessment of fungal communities in woody litter and that Ingoldian fungi also constitute an important component of woody litter.

Linking individual behaviour to community scale patterns in fungi

Abstract The fungi comprise a separate kingdom of life and epitomise the indeterminate growth form. Very little is known about the factors that influence the nature of fungal diversity and the link between individual behaviour and the structure and function of fungal communities is particularly poorly understood. Here, we present a theoretical framework that is capable of elucidating this link. An individual-based model for fungal community dynamics is introduced that has been developed from a physiologically based model for the fungal phenotype. The model is used to explore the role of individual interactions, the production of an external inhibitor field and the quality of the external environment on the structure and diversity of the resulting community. We show that traits relating to growth rate, autophagic behaviour and the production of inhibitors are key in influencing the success of a particular genotype in a community. The species richness increases with the amount of available resource. This is the first model of fungal community dynamics that introduces the concept of a biomass-based abundance distribution function that can be described by the log-normal form which typically corresponds to communities in equilibrium. The species abundance curve was stable to changes in the relative location of inocula, although the ranked abundance of the individuals was not. We present the first attempt to identify the traits that affect the form of that curve. Future studies should examine the role of environmental heterogeneity and spore dispersal.

The copepod communities of the north and south Pacific central gyres and the form of species-abundance distributions

Epiplanktonic copepods were sampled on 10 cruises in the Pacific central gyres, 7 in the north gyre and 3 in the south gyre, between 1964 and 1973. These gyres are the largest biomes, stable, ancient, down-welling, oligotrophic and with little temporal variation. The data from each cruise were standardized to numbers per 10 3 m 3 ; no data from the south gyre cruises has been published before. The structure of the communities was analysed with species-abundance curves and ordination. One hundred and eighty-two species were found in all, 118―158 per cruise, 73 in all cruises. Double-centred ordination of those 73 showed three distinct sets of cruises: south (Isaacs-Kidd net), north (Isaacs-Kidd net) and north (bongo net). The distance in species-space between the north and south gyres is the same but orthogonal to the distance between samples collected by the two nets! Sixteen species abundance distributions (SADs), from 10 cruises and 6 combinations of them, were used to test the hypotheses that such distributions are mildly platykurtic and increasingly left-skew with increasing sample size. All SADs were sigmoidal on rank abundance plots of the log abundance, and agreed with the hypotheses, clarifying the mathematical form. Such replicated, large sample, SADs are rare.

Response of earthworm communities to soil disturbance: Fractal dimension of soil and species’ rank-abundance curves

Soil structure degradation and its relationship with soil fauna communities is a crucial issue in soil management. The aim of this work is to analyze soil fractal dimensions and the earthworm community structure along a perturbation gradient in a Typic Argiudoll soil with different combinations of annual crops and pastures. Samples were taken in four sites: a natural grassland (NAT) without cultivation for 30 years, and three pastures of 1–3 years (P1, P2, P3). Five undisturbed soil blocks were taken per site in order to evaluate soil structural complexity. Two fractal dimensions were measured on images of polished sections of the blocks: the mass fractal dimension of pores ( Dmp), and the mass fractal dimension of soil solid complement ( Dms). Biomass, number and species composition of earthworm communities were assessed by hand-sorting 15 samples of 25 cm × 25 cm × 20 cm on each site. Species rank vs. abundance distribution were fitted to geometric series and broken stick models. Shannon index and expected species number by rarefaction ( E( S)) were calculated. Both soil solid and pore space evaluated may be considered as fractals in all sites analyzed. We found that the fractal dimension of pores was significantly ( p < 0.03) higher for NAT and P3 (1.63 ± 0.03 and 1.62 ± 0.02, respectively) than for P1 (1.43 ± 0.05). Species richness was P1: 3, P2: 7, P3: 7, NAT: 9. Species abundance vs. rank curves shows better fit to the geometric series model in all sites. We found the highest values of Dmp in sites with the higher diversity and species richness of earthworms, suggesting a trend between the observed biological activity and the fractal dimension of the habitat.

The invisible niche: Weakly density-dependent mortality and the coexistence of species

Weakly density-dependent effects, characterized by fractional scaling exponents close to one, are rarely studied in the ecological literature. Here, we consider the effect of an additional weakly density-dependent term on a simple competition model. Our investigation reveals that weak density-dependence opens up an “invisible niche”. This niche does not constitute a new mechanism for coexistence, but is a previously unexplored consequence of known mechanisms. In the invisible niche a weaker competitor can survive at very low density. Coexistence thus requires large habitat size. Such niches, if found in nature, would have a direct impact on species-area laws and species-abundance curves and should therefore receive more attention.

Species abundance models and patterns in dragonfly communities: effects of fish predators

We investigated if dragonfly larvae community composition and species abundance curves are sensitive to variation in predation intensity, and whether the fit to a particular niche partitioning model could be used to make inferences about mechanisms structuring communities. The approach taken was to compare communities in lakes either having or lacking fish predation. Dragonfly species classified as active, strongly dominated the dragonfly communities in fishless lakes, and low active species dominated fishless lakes. As activity level is known to correlate with susceptibility to fish predation this indicates that these communities are structured by fish predation. Fitting relative abundance data to five niche partitioning models showed that the same model fitted data from both types of habitats (fish/no fish). This means that the observed differences in relative abundances were substitutive, i.e. the relative abundance of a rank stayed constant, even though the identity of the species having this rank changed. The best fit to data from both types of lakes was found for the random assortment model, which is usually interpreted as an indication that the community is not structured by within‐guild interactions. This interpretation for fishless lakes did not seem to agree with other community measures (i.e. lowered diversity and evenness and no relationship between species richness and dragonfly biomass), which indicate that the community is structured by within‐guild interactions. Moreover, a detail in the fitting procedure, the number of species included in the analysis, affected which model that fitted data best. Thus, we question if fitting niche partitioning models to data can provide mechanistic understanding of how resources are partitioned in natural communities.