Diversity,palaeoecology and systematics of a marine fossil assemblage from the Late Triassic Cassian Formation at Settsass Scharte,N Italy

A marine fossil assemblage from the Late Triassic (Early Carnian) Cassian Formation is reported. It was retrieved by bulk sampling, including wet sieving and quantitative picking, and by quantitative surface collection. The collection consists of c. 460 specimens (foraminiferans not included) representing 54 species. In terms of abundance and species richness, it is strongly dominated by molluscs, especially gastropods. 97 % of the individuals are molluscs. The most abundant species are the gastropods Goniospira armata, Schartia carinata and Helenostylina convexa, followed by the scaphopod Plagioglypta undulata and the bivalve Palaeonucula strigilata. Disarticulated echinoderm ossicles (mostly echinoids, crinoids, few holothurians) comprise almost all of the rest of the assemblage. The studied assemblage shows moderate diversity, similar to those of previously reported assemblages or associations from basinal settings. However, it differs considerably in taxonomic composition from previously described associations of the Cassian Formation. The abundance of small gastropods is a result of their primary abundance in these ancient living communities and of the sampling method (sieving at 0.5 mm), because most of the previous associations were obtained by performing surface collections, in which small gastropods are easily overlooked. The fauna is interpreted as an autochthonous soft bottom assemblage dominated by species that lived in low epifaunal to shallow infaunal habitats. Detritivory, deposit feeding and microcarnivory represent the main feeding types. Most of the species were fully motile but slow, and either infaunal (scaphopods, nuculids, the gastropod Domerionina) or epifaunal (most other gastropods, echinoids). The present assemblage underlines the pronounced heterogeneity of the Cassian biota. The low grade of lithification and diagenetic alteration facilitates bulk sampling and the investigation of small species. This minimizes possible sampling and preservation biases, so the studied assemblage reflects the alpha diversity of this ancient living community to an unusually high degree. The following gastropod taxa are new: Helenostylina convexa n. sp., Schartia carinata n. gen. n. sp., Schartiinae n. subfam.; Cassianastraea Bandel non Volz is replaced with Bandelastraea nom. nov.     

Fossil liberation: a model to explain high biodiversity in the Triassic Cassian Formation

With 1429 animal species, the Triassic Cassian Formation in the Dolomites, Southern Alps (Italy), yields the highest species richness reported from any spatially constrained pre-Quaternary formation known to science. The high preserved diversity is partly attributable to a high primary diversity governed by the tropical setting, increasing alpha diversity, and the breadth of habitats spurring beta diversity. More important is the excellent preservation of fossils and the ease with which they can be extracted from the poorly lithified sediments. We propose the term ‘liberation Lagerstätten’ to capture this preservational window. In contrast to conservation Lagerstätten, liberation Lagerstätten like the Cassian Formation originate from normal marine conditions but low-grade diagenesis. Molluscs contribute substantially to species richness, comprising 67% of all invertebrate species in the Cassian Formation. The gastropod dominance (39% of all species) is nearly as great as in Recent tropical settings, contradicting the concept of a substantial Cenozoic rise.     

Recovery of benthic marine communities from the end‐Permian mass extinction at the low latitudes of eastern Panthalassa

Based on the quantitative community analysis using species‐level identifications, we track the restoration of benthic ecosystems after the end‐Permian mass extinction throughout the Lower Triassic of the western USA. New data on the palaeoecology of the Thaynes Group and Sinbad Formation are provided, which fill a gap between the recently studied palaeoecology of the Griesbachian–Dienerian Dinwoody Formation and the Spathian Virgin Formation. In the Sinbad Formation and Thaynes Group, 17 species (12 genera) of bivalves, 7 species and genera of gastropods and 2 species and genera of brachiopods are recognized. The new bivalve genus Confusionella (Pteriidae) is described. A comprehensive review of the whole Lower Triassic succession of benthic ecosystems of the western USA indicates that mid‐ and inner shelf environments show incipient recovery signals around the Griesbachian–Dienerian transition, during the Smithian and, most profound, during the early Spathian. Ecological data from youngest strata of the Dinwoody Formation as well as stratigraphic ranges of species suggest that the late Dienerian was likely a time interval of environmental stress for benthic ecosystems. Despite some evidence for short‐term environmental disturbances (e.g. shift of dominant taxa, transient drop in alpha‐diversity) during the Smithian–Spathian transition, benthic ecosystems did not show any notable taxonomic turnover at that time, in contrast to the major crisis that affected ammonoids and conodonts. Whereas alpha‐diversity of benthic communities generally increased throughout the Early Triassic, beta‐diversity remained low, which reflects a persistently wide environmental range of benthic species. This observation is in accordance with a recently proposed model that predicts a time lag between increasing within‐habitat diversity (alpha‐diversity) and the onset of taxonomic differentiation between habitats (beta‐diversity) during biotic recoveries from mass extinction events. The observation that beta‐diversity had not significantly increased during the Early Triassic might also provide an explanation for the comparably sluggish increase in benthic diversity during that time, which has previously been attributed to persistent environmental stress.     

Facies of two important Early Triassic gastropod lagerstätten: implications for diversity patterns in the aftermath of the end-Permian mass extinction

Two important lagerstätten of Early Triassic gastropods, the Sinbad Limestone (Utah, USA) and the Gastropod Oolite (North Italy) yield about 40% of all described Early Triassic species. This great contribution to the global diversity and the exceptional good preservation render high information content, which characterizes fossil lagerstätten. The Smithian Sinbad Limestone contains the most diverse Early Triassic gastropod fauna. At the type locality, it occurs in single, probably storm-induced shell bed within a series of high energy deposits underlain by intertidal microbial mats and subtidal oolite/peloid shoals. The main shell bed contains about 40 invertebrate taxa. Gastropods, scaphopods, and bivalves are most abundant and form an assemblage, which is dominated by small neritaemorphs, the opisthobranch Cylindrobullina convexa and the scaphopod Plagioglypta (annulated tubes). This assemblage lived on shallow, subtidal soft-bottoms based on sedimentological and ecological characteristics. The Dienerian (to Smithian?) Gastropod Oolite Member (North Italy) has extremely abundant, probably salinity-controlled gastropod faunas with low species richness. Almost monospecific assemblages of Pseudomurchisonia kokeni as well as assemblages with about four species are present in the Gastropod Oolite. Modern hydrobiid mudsnail faunas which are adapted to strongly fluctuating salinity in intertidal to shallow subtidal coastal areas form probably a suitable model for the Gastropod Oolite biota. Gastropods from the Werfen- and Moenkopi-Formation lagerstätten are well preserved compared to other Early Triassic deposits. The high contribution to the global diversity of just two sites suggests very incomplete sampling and preservational bias. However, the low richness of the major faunas reflects depauperate Early Triassic faunas and slow recovery from the Permian/Triassic crisis.     

Niches in the Anthropocene: passerine assemblages show niche expansion from natural to urban habitats

Human‐mediated habitat transformation is increasingly evident around the world. Yet, how this transformation influences species’ niche width and overlap remains unclear. On the one hand, human‐mediated habitat transformation promotes increased species similarity through trait‐based filtering, and an increased prevalence of generalist species with broad niches, resulting in functional homogenization. On the other hand, species that colonize transformed habitats could use empty niches, resulting in decreased species similarity and an expansion of assemblage‐level niche space. Here we explore these two alternatives in eight highly diverse passerine assembles in natural, rural and urban habitats in south and southwest China, a rapidly developing region of the world. Based on stable isotopes, we found that species’ niche width increased from natural to human‐made habitats, but there were no differences in niche overlap among habitats. Therefore, we found evidence for niche expansion, with generalists appearing to use empty niches created by human habitat modification, and with assemblages being comprised of complementary species. Further research is needed to determine whether increased between‐ or within‐individual niche variation is the main driver of niche expansion in transformed habitats.     

Partitioning the diversity of riverine fish: the roles of habitat types and non-native species

1. Quantifying how biological diversity is distributed in the landscape is one of the central themes of conservation ecology. For this purpose, landscape classifications are being intensively used in conservation planning and biodiversity management, although there is still little information about their efficacy. 2. I used data from 158 running water sites in Hungary to examine the contribution of six a priori established habitat types to regional level diversity of fish assemblages. Three community measures [species richness, diversity (Shannon, Simpson indices), assemblage composition] were examined at two assemblage levels (entire assemblage, the native assemblage). The relative role of non‐native species was quantified to examine their contribution to patterns in diversity in this strongly human influenced landscape. 3. Additive diversity partitioning revealed the primary importance of beta diversity (i.e. among‐site factors) to patterns in species richness. Landscape‐scale patterns in species richness were best explained by between‐habitat type (beta₂: 41.2%), followed by within‐habitat type (beta₁: 37.7%) and finally within‐site (alpha: 21.1%) diversity. Diversity indices showed patterns different from species richness, indicating the importance of relative abundance distributions on the results. Exclusion of non‐natives from the analysis gave similar results to the entire‐assemblage level analysis. 4. Canonical analysis of principal coordinates, complemented with indicator species analysis justified the separation of fish assemblages among the habitat types, although classification error was high. Multivariate dispersion, a measure of compositional beta diversity, showed significant differences among the habitat types. Contrary to species diversity (i.e. richness, diversity indices), patterns in compositional diversity were strongly influenced by the exclusion of non‐natives from the analyses. 5. This study is the first to quantify how running water habitat types contribute to fish diversity at the landscape scale and how non‐native species influence this pattern. These results on riverine fish assemblages support the hypothesis that environmental variability (i.e. the diversity of habitat types) is an indication of biodiversity and can be used in large‐scale conservation designs. The study emphasises the joint application of additive diversity partitioning and multivariate statistics when exploring the contribution of landscape components to the overall biodiversity of the landscape mosaic.     

The coexistence of fish species in streams: relationships between assemblage attributes and trophic and environmental variables

Diet overlap and niche breadth are well-known species traits from trophic ecology that can assist in explaining how species interact and coexist as well as the ecological mechanisms that influence biodiversity. In the present study, we analyzed the relationships between these trophic variables and indicators of resource availability with some attributes of fish assemblages (species richness, Shannon diversity index, evenness, density and individual body size). The physical and chemical characteristics of the biotopes (topography, water quality and conservation of slopes) were examined to identify possible patterns. Monthly sampling using electrofishing was conducted in 2003 along five streams located in the Cuiabá River watershed. The relationships between environmental variables and attributes of fish assemblages were evaluated using Spearman correlation. Species richness and abundance varied among streams, with higher values (54 and 82 species) found in low-gradient streams that drained small swampy areas discharging in Cuiabá River. Diet overlap showed significant and negative correlations with species richness, the diversity index, abundance, variation in body size and food availability and positive correlations with evenness and niche breadth. Environments that had greater food availability had a reduced variety of food items (smaller amplitude) and distinct values in terms of dietary overlap. Nevertheless, to explain resource partitioning in a fish assemblage (overlap), it is necessary to consider food availability and niche breadth (degree of trophic specialization) beyond the interaction of these variables with species richness and density. In conclusion, high diversity and abundance values were strongly associated with a high degree of trophic specialization (low amplitude of trophic niche) and a small degree of overlap in the diet.     

Larval ecology and morphology in fossil gastropods

The shell of marine gastropods conserves and reflects early ontogeny, including embryonic and larval stages, to a high degree when compared with other marine invertebrates. Planktotrophic larval development is indicated by a small embryonic shell (size is also related to systematic placement) with little yolk followed by a multiwhorled shell formed by a free‐swimming veliger larva. Basal gastropod clades (e.g. Vetigastropoda) lack planktotrophic larval development. The great majority of Late Palaeozoic and Mesozoic ‘derived’ marine gastropods (Neritimorpha, Caenogastropoda and Heterobranchia) with known protoconch had planktotrophic larval development. Dimensions of internal moulds of protoconchs suggest that planktotrophic larval development was largely absent in the Cambrian and evolved at the Cambrian–Ordovician transition, mainly due to increasing benthic predation. The evolution of planktotrophic larval development offered advantages and opportunities such as more effective dispersal, enhanced gene flow between populations and prevention of inbreeding. Early gastropod larval shells were openly coiled and weakly sculptured. During the Mid‐ and Late Palaeozoic, modern tightly coiled larval shells (commonly with strong sculpture) evolved due to increasing predation pressure in the plankton. The presence of numerous Late Palaeozoic and Triassic gastropod species with planktotrophic larval development suggests sufficient primary production although direct evidence for phytoplankton is scarce in this period. Contrary to previous suggestions, it seems unlikely that the end‐Permian mass extinction selected against species with planktotrophic larval development. The molluscan classes with highest species diversity (Gastropoda and Bivalvia) are those which may have planktotrophic larval development. Extremely high diversity in such groups as Caenogastropoda or eulamellibranch bivalves is the result of high phylogenetic activity and is associated with the presence of planktotrophic veliger larvae in many members of these groups, although causality has not been shown yet. A new gastropod species and genus, Anachronistella peterwagneri, is described from the Late Triassic Cassian Formation; it is the first known Triassic gastropod with an openly coiled larval shell.     

Realised niche changes in a native herbivore assemblage associated with the presence of livestock

Habitat partitioning is a common ecological mechanism to avoid competition among coexisting species, and the introduction of new species into existing assemblages can increase competitive pressures. However, situations of species in allopatry and sympatry only differing in species presence but not in environmental conditions are scarce. Thus, discerning whether niche segregation arises from competition or from different habitat preferences is usually unfeasible. Here, we analyse species’ habitat niches in an assemblage of native and introduced herbivores in southern Patagonia. We test if niche overlap is higher between native and domestic herbivores than among natives as expected from the relatively short time of coexistence, and we evaluate the effect of intra‐ and interspecific competition on niche breadth. We use a probabilistic multidimensional approach and null models to evaluate overlap and changes in niche dimensions. Overlap among native species is low as expected for species coexisting in evolutionary time. In native‐domestic species pairs, niche overlap was higher than among natives, although showing some niche segregation indicating niche differentiation in ecological time. Moreover, the presence of domestic species was associated with niche narrowing of both native and introduced species, revealing interspecific density‐dependent effects on their habitat niche during resource shortage periods.     

Exploratory multiscale analysis of the fish assemblages and habitats of the lower St. Lawrence River,Québec,Canada

This work presents a multiscale analysis of the fish diversity of the lower St. Lawrence River which flows from the Laurentian Great Lakes of North America. A database of about 14,000 fish sampling stations from the lower St. Lawrence is linked to five different habitat maps of the study area: hydrographical units, water masses, depth, sediment type and wetland type. We hypothesize that species turnover (beta diversity) will be significant among these habitat maps. For each map, we calculate a UPGMA dendrogram based on the Jaccard coefficient of similarity in species occurrences as a first assemblage–habitat model. A randomization test is then used to identify the significant dendrograms from which we infer fish assemblages. We then show that many species are actually selecting or avoiding habitats. Finally, species selecting particular habitats are described by some selected ecological traits that are expected to occur frequently in those habitats. We found assemblages for the hydrographical units but not for the water masses. The fluvial section is selected by many stenohaline and some euryhaline species, while the freshwater estuary and the brackish estuary are only selected by euryhaline species. In the fluvial section at the hydrographical unit scale, many species associated with lentic habitats are limnophilic and more vegetation-dependent while many species selecting lotic habitats are rheophilic insectivorous species. Significant assemblages are defined for depth, sediment type and wetland type. Taken together, we found strong empirical evidence of a diverse actively selecting littoral assemblage of small, low mobility fishes opposing a channel assemblage of larger, more mobile fishes.     

Mammalian beta diversity in the Great Basin, western USA: palaeontological data suggest deep origin of modern macroecological structure

Aim Recent work indicates that desert assemblages have elevated beta (β) diversity (between‐locality turnover of species composition). This study compares β diversities between the Great Basin and the Great Plains of the western USA over the last 17 Myr. Today, the Great Basin is a topographically diverse desert scrubland to woodland and the Great Plains are low‐relief temperate grassland, but 17 Ma they were more similar in topography, climate and land cover. A georeferenced database of mammal occurrences, complied from several sources, is used to test two hypotheses for the elevation of Great Basin β diversity: (1) that tectonic change of the topography has driven increased habitat packing in high‐ and low‐elevation habitats, and (2) that climatic cycling in the Pleistocene has driven faunas from neighbouring provinces to overlap in the region. Location The Great Basin of the USA, centred on Nevada, and the central Great Plains of the USA, centred on Nebraska. Methods Mammalian faunal lists compiled from published records and the records of many museums, available online, were partitioned into time‐slices ranging from the recent to 17 Myr old. Beta diversity was calculated for each time‐slice in two ways: multiplicative β diversity using first‐order jackknife richness, and additive beta diversity using Simpson's evenness. Results Beta diversity is elevated in Nevada relative to Nebraska today. Beta diversity has been higher in the Great Basin since the Pleistocene and possibly since the late Early Hemphillian (c. 7 Ma). Beta diversity in the Late Barstovian (c. 13.5 Ma) of the Great Plains was higher even than β diversity in the Great Basin of today. Main conclusions The elevated β diversity in the Hemphillian supports the tectonic change hypothesis. The patterns of β diversity in the Recent, Pleistocene and Hemphillian all suggest that local‐scale processes are important. The β diversity of the Late Barstovian Great Plains supports other studies indicating increased primary productivity or species packing.     

Anisian floras from the NE Iberian Peninsula and Balearic Islands: A synthesis

This paper focuses on the diversity and palaeontological significance of the Anisian floral record in the Triassic of the northeastern Iberian Peninsula. A summary of published information is followed by new records from macrofloral localities at Rodanas (Aragonese Branch of the Iberian Range) and at Maya del Baztán (Basque-Cantabrian Domain, Pyrenees). Both of these new localities also yielded three Anisian microfloral assemblages. These records provide new evidence of Anisian (basal Middle Triassic) floras in Spain. They are found to be somewhat similar to the classical flora of the Grès à Voltzia Formation (Upper Buntsandstein) of the northern Vosges in north-eastern France, of whose age has been shown to be early Anisian.     

Stochastic losses of fire‐dependent endemic herbs revealed by a 65‐year chronosequence of dispersal‐limited woody plant encroachment

The factors responsible for maintaining diverse groundcover plant communities of high conservation value in frequently burned wet pine savannas are poorly understood. While most management involves manipulating extrinsic factors important in maintaining species diversity (e.g., fire regimes), most ecological theory (e.g., niche theory and neutral theory) examines how traits exhibited by the species promote species coexistence. Furthermore, although many ecologists focus on processes that maintain local species diversity, conservation biologists have argued that other indices (e.g., phylogenetic diversity) are better for evaluating assemblages in terms of their conservation value. I used a null model that employed beta‐diversity calculations based on Raup–Crick distances to test for deterministic herbaceous species losses associated with a 65‐year chronosequence of woody species encroachment within each of three localities. I quantified conservation value of assemblages by measuring taxonomic distinctness, endemism, and floristic quality of plots with and without woody encroachment. Reductions in herb species richness per plot attributable to woody encroachment were largely stochastic, as indicated by a lack of change in the mean or variance in beta‐diversity caused by woody encroachment in the savannas studied here. Taxonomic distinctness, endemism, and floristic quality (when summed across all species) were all greater in areas that had not experienced woody encroachment. However, when corrected for local species richness, only average endemism and floristic quality of assemblages inclusive of herbs and woody plants were greater in areas that had not experienced woody encroachment, due to the more restricted ranges and habitat requirements of herbs. Results suggest that frequent fires maintain diverse assemblages of fire‐dependent herb species endemic to the region. The stochastic loss of plant species, irrespective of their taxonomic distinctness, to woody encroachment suggests that the relevance of niche partitioning or phylogenetic diversity to the management of biodiversity in wet pine savannas is minimal.     

Diversity and community composition of butterflies and odonates in an ENSO-induced fire affected habitat mosaic: a case study from East Kalimantan, Indonesia

Little is known about the diversity of tropical animal communities in recently fire‐affected environments. Here we assessed species richness, evenness, and community similarity of butterflies and odonates in landscapes located in unburned isolates and burned areas in a habitat mosaic that was severely affected by the 1997/98 ENSO (El Niño Southern Oscillation) event in east Kalimantan, Indonesian Borneo. In addition related community similarity to variation in geographic distance between sampling sites and the habitat/vegetation structure Species richness and evenness differed significantly among landscapes but there was no congruence between both taxa. The species richness of butterflies was, for example, highest in sites located in a very large unburned isolate whereas odonate species richness was highest in sites located in a small unburned isolate and once‐burned forest. We also found substantial variation in the habitat/vegetation structure among landscapes but this was mainly due to variation between unburned and burned landscapes and variation among burned landscapes. Both distance and environment (habitat/vegetation) contributed substantially to explaining variation in the community similarity (beta diversity) of both taxa. The contribution of the environment was, however, mainly due to variation between unburned and burned landscapes, which contained very different assemblages of both taxa. Sites located in the burned forest contained assemblages that were intermediate between assemblages from sites in unburned forest and sites from a highly degraded slash‐and‐burn area indicating that the burned forest was probably recolonised by species from these disparate environments. We, furthermore, note that in contrast to species richness (alpha diversity) the patterns of community similarity (beta diversity) were highly congruent between both taxa. These results indicate that community‐wide multivariate measures of beta diversity are more consistent among taxa and more reliable indicators of disturbance, such as ENSO‐induced burning, than univariate measures.     

Global patterns of specialization and coexistence in bird assemblages

Aim Increased specialization has been hypothesized to facilitate local coexistence and thus high species richness, but empirical evaluations of the richness–specialization relationships have been relatively scant. Here, we provide a first assessment of this relationship for terrestrial bird assemblages at global extent and from fine to coarse grains. Location World‐wide. Methods We use two indices of specialization that describe species‐level resource use: diet and habitat specialization. The relationship between richness and mean assemblage‐level specialization was independently assessed at realm, biome‐realm, 12,100 km² equal‐area grid cells and fine‐grained scales. To identify assemblages that are diverse relative to environmental conditions we: (1) applied quantile regressions, (2) statistically accounted for other environmental variables which may constrain richness, and (3) parsed the data according to the residuals of a model relating species richness to the environmental variables. Results Assemblage species richness increases with both measures of specialization at all scales. Statistically, richness appears constrained by levels of specialization, with the highest richness values only found in specialized assemblages. Richness is positively associated with specialization even after accounting for gradients in resource availability. Net primary productivity and assemblage specialization have complementary statistical effects on assemblage species richness. Contrary to expectations based on niche partitioning of local resources, the relationship between specialization and richness is steep even at coarse scales. Main conclusions The results demonstrate that for an entire clade, totalling > 9000 species, specialization and species richness are related, at least for diverse assemblages. The strong patterns observed across scales suggest that this relationship does not solely originate from (1) limits on coexistence in present‐day assemblages, or (2) increased specialization in richer assemblages imposed by species’ abilities to partition ecological space. Instead, regional‐scale influences on the species pool may determine much of the observed relationship between richness and specialization. Although causal attribution is not straightforward, these findings support the idea that, for the scale of our analysis, specialization may be related to the past origination of high‐diversity assemblages, rather than their contemporary assembly.     

Early Triassic bivalves from the Feixianguan Formation in Xingyi,Guizhou and the Ximatang Formation in Qiubei,Yunnan (southern China)

Based on newly collected materials from the Lower Triassic Feixianguan Formation of Xiongwu section in Xingyi, Guizhou and the Ximatang Formation of Ximatang section in Qiubei, Yunnan, southern China, nine species belonging to seven genera are described. Two bivalve assemblages are recognized and regionally correlated in South China. The bivalve assemblage from the Feixianguan Formation of Xiongwu exhibits a higher diversity including seven species belonging to seven genera: Claraia griesbachi, Leptochondria virgalensis, Entolium (Entolium) microtis, Towapteria scythicum, Bakevillia exporrecta, Bositra sp., and Unionites? fassaensis. In contrast, the bivalve assemblage from the Ximatang Formation of Ximatang has a much lower diversity, consisting of only two species, i.e., Claraia griesbachi and Claraia radialis. Additionally, C. griesbachi and C. radialis, as the most common species of Claraia in the Early Triassic, are revised.     

Survival and recovery of calcareous foraminifera pursuant to the end-Permian mass extinction

Ninety-one percent of calcareous foraminiferal genera became extinct during the end-Permian mass extinction. The Early Triassic Epoch was a survival phase characterized by a short-lived proliferation of disaster forms and then a prolonged interval of low diversity. The orders Miliolida and Lagenida experienced limited taxonomic re-diversification in Early and Late Anisian time, respectively. All fusulinoidean fusulinides became extinct in Late Permian time, and only two non-fusulinoidean genera persisted into the Early Triassic. Triassic fusulinides diversified to just five genera before the order became entirely extinct in Late Triassic time. Involutinides originated in Olenekian time from an unknown ancestor. They did not significantly diversify until Late Triassic time. To cite this article: J.R. Groves, D. Altiner, C. R. Palevol 4 (2005).     

Are Assemblages of Aquatic‐Breeding Anurans (Amphibia) Niches Structured or Neutral?

Local niche‐based processes and dispersal are important determinants of assemblage composition and species diversity. However, there is no consensus about the relative importance of niche and spatial processes to explain the distribution of anuran species in tropical systems. In our study, we analyzed the niche and neutral effects on anuran assemblages and found that biotic interactions were a predictor of assemblage structure. The Eltonian concept of niche was the best predictor for the structure of aquatic‐breeding anuran assemblages, as species tended to co‐occur more often than would be expected by chance. We suggest that the lack of environmental effect could be explained by differences in the pattern of movement between arboreal and non‐arboreal anurans. Once there is a reduction in the number of arboreal anurans in open areas, the importance of habitat heterogeneity to explain assemblage composition should decrease. The lack of correlation between the spatial component in our model and species composition is evidence that spatial processes, such as migration, did not play a major role in structuring local assemblages. Anurans are generally assumed as having poor dispersal ability, yet this assumption is not true for all anuran species. We suggest that future studies should include key behavioral traits, such as site fidelity and homing behavior, as these traits can represent the dispersal abilities of anurans and dispersal ability seems to be important when we try to predict patterns of anuran distribution.     

Niche versus neutrality in structuring the beta diversity of damselfly assemblages

1. Differences among communities in taxonomic composition – beta diversity – are frequently expected to result from taxon‐specific responses to spatial variation in ecological conditions, through niche partitioning. Such process‐derived patterns are in sharp contrast to arguments from neutral theory, where taxa are ecologically equivalent and beta diversity results primarily from dispersal limitation. 2. Here, we compared beta diversity among assemblages of damselflies (Odonata: Zygoptera), for which previous experiments have shown that niche differences maintain genera within a community, but patterns of relative abundance for species within each genus are shaped primarily by neutral dynamics. 3. Using null‐model and ordination‐based methods, we find that both genera and (in contrast to neutral theory) species assemblage composition vary across the landscape in a deterministic fashion, shaped by environmental and spatial factors. 4. While the observed patterns in species composition conflict with theory, we suggest that this a result of weak ecological filters acting to produce spatial variation in assemblages of ecologically similar species undergoing ecological drift within communities. Such patterns are especially likely in systems of relatively weak dispersers like damselflies.