Multispecies Assemblages Research Articles

Parasite-like associations in rocky intertidal assemblages: implications for escalated gastropod defenses

Escalated prey defenses occur within multi-species assemblages but are largely evalu- ated by considering only direct interactions between predator and prey. If additional species compro- mise prey defenses, they might indirectly facilitate predation. Since the introduction of green crabs Carcinus maenas in the northwest Atlantic, many intertidal mollusks including the dogwhelk Nucella lapillus have acquired shell traits that have been interpreted as escalated defenses against this shell- crushing predator. I examined whether 'parasite-like' spionid polychaetes (Polydora sp.), which bore into and reside in dogwhelk shells but feed externally, indirectly facilitated predation by crabs. Small collections of dogwhelks from 19 sites around the Gulf of Maine (USA) revealed Polydora infestations at 16 sites. Intensive collections at 1 site showed that Polydora disproportionately infested large dog- whelks that are normally most resistant to predation. Presenting paired non-infested and fully- infested (Polydora bore holes in shell spire and siphonal canal) dogwhelks to individual crabs re- vealed that fully-infested individuals had significantly shorter times until first observed handling and shorter survival durations. In trials where single dogwhelks survived, they were always non-infested and their shell size scaled positively with crab size. These results suggest that Polydora increased both the size-specific susceptibility of dogwhelks to predation and the critical size of snails vulnera- ble to a given sized crab. Although Polydora altered this predator-prey interaction, mechanical tests showed no significant effect of Polydora on shell strength, so mechanical tests alone are not a reliable way to evaluate defensive traits. Since prey defenses can be modified by additional species, evalua- tions of changing shell defenses must consider both direct and indirect interactions.

Networks of spatial genetic variation across species

Spatial patterns of genetic variation provide information central to many ecological, evolutionary, and conservation questions. This spatial variability has traditionally been analyzed through summary statistics between pairs of populations, therefore missing the simultaneous influence of all populations. More recently, a network approach has been advocated to overcome these limitations. This network approach has been applied to a few cases limited to a single species at a time. The question remains whether similar patterns of spatial genetic variation and similar functional roles for specific patches are obtained for different species. Here we study the networks of genetic variation of four Mediterranean woody plant species inhabiting the same habitat patches in a highly fragmented forest mosaic in Southern Spain. Three of the four species show a similar pattern of genetic variation with well-defined modules or groups of patches holding genetically similar populations. These modules can be thought of as the long-sought-after, evolutionarily significant units or management units. The importance of each patch for the cohesion of the entire network, though, is quite different across species. This variation creates a tremendous challenge for the prioritization of patches to conserve the genetic variation of multispecies assemblages.

Earthworm burrowing behaviour in 2D terraria with single- and multi-species assemblages

Experiments were performed in 2D terraria to investigate the burrowing behaviour of different earthworm species from various ecological categories in single- and multi-species assemblages. The burrowing behaviour was quantified using image analysis software during a 2-week period. Terraria were found to reveal realistic impressions of the burrowing behaviour of various species according to the ecological classification of Bouche into epigeic, endogeic and anecic species. Results of the study also permit the recommending of classifying various earthworms as intermediate species, e.g. Aporrectodea longa as endo-anecic and Lumbricus rubellus as epi-endogeic. Burrowing activity of endogeic species was significantly reduced in multi-species compositions compared to single-species treatments. Moreover, burrowing activity of Octolasion tyrtaeum was significantly reduced in the presence of Lumbricus terrestris compared to the specific single treatment. This endogeic species profited from the burrowing behaviour, e.g. bioturbation, of L. terrestris, and as such this is circumstantial evidence for commensalism (species interaction, in which one partner benefits while the other is unaffected) between anecic and endogeic earthworm species. Simultaneous burrowing activity of a combined assemblage of both endogeic species, Aporrectodea caliginosa and O. tyrtaeum, was also significantly reduced compared with the particular single treatments. Thus, this seems likely to be a response of interspecific competition and trophic niche separation between endogeic species.

Are insect pollinators more generalist than insect herbivores?

Recent community-level studies have acknowledged that generalist species are more widespread than previously thought and highlighted their preponderant impact on community functioning and evolution. It is suggested that the type of interaction, trophic versus mutualistic, should affect species generalization level; however, no direct comparison has been made yet. Here, we performed such a comparison using 44 plant-insect networks describing either pollination or herbivory communities. Our analysis shows that the type of interaction does indeed have an impact on various aspects of species generalism, from the distribution of generalism in the community to the phylogenetic diversity of the plants with which a given insect species interacts. However, the amplitude of the observed differences depends on the aspect of species generalism studied. While the non-quantitative and quantitative measures of generalism suggest that pollinators interact with more plant species and more evenly than herbivores, phylogenetic measures clearly show that herbivores interact with plant species far more closely related to each other than pollinators. This comparative approach offers a promising perspective to better understand the functioning and evolution of multispecies assemblages by pointing out some fundamental singularities of communities depending on the type of interaction considered.

Faunal zonation of large epibenthic invertebrates off North Carolina revisited

The dominant populations of large epibenthic megafauna off North Carolina were mapped in the 1960s from the upper continental slope out to the Hatteras Abyssal Plain using multi-shot sea-floor photography. The present re-analysis of the original data using contemporary computer-based methods and geo-referenced mapping (GIS) reveal that the overall patterns inferred initially can be substantiated. Individual species occurred in narrow depth bands that hugged the topography along the entire sampling area, but multi-species assemblages emerge with the modern, more formal quantitative methods.

Benthic community structure and spatiotemporal thermal regimes in two upwelling ecosystems: Comparisons between South Africa and Chile

On the basis of 16‐yr satellite time series (1987–2003) of sea surface temperature (SST) and using univariate and multivariate statistical techniques, we quantified patterns of fluctuation in oceanographic conditions, as well as their spatial structure along the coasts of South Africa and Chile between 29.0° and 34.5°S. Since variations in environmental conditions over multiple timescales are driven by oceanographic‐atmospheric processes modulated by coastal morphology, all of which affect in different ways coastal organisms, we hypothesized that dynamic properties of the environment contribute to characteristic patterns on rocky intertidal biomass structure across upwelling ecosystems. Although long‐term mean temperatures were similar between coasts, the relative importance of annual, semiannual, intraseasonal, and high‐frequency SST fluctuations differed; South Africa is more strongly pulsed over shorter timescales. Similar spatial discontinuities in thermal regimes were observed, with changes at about 32°S. Multivariate summaries of benthic community structure derived from functional group biomasses revealed striking differences between South Africa and Chile, as well as between regions north and south of 32°S within each ecosystem. Between‐continent and regional‐scale differences in community structure were largely unrelated to mean SST, but correlated with the different measures of temporal variance in SST. For all functional groups, as well as aggregate measures of benthic community composition, the weekly to annual SST fluctuations explained large fractions (>35%) of biological variability. Results highlight the importance of quantifying the temporal variance of regimes of hydrographic conditions since multispecies assemblages are expected to respond in different ways to the driving oceanographic processes behind these fluctuations.

Competition and Intraguild Predation Among Three Species of Coccinellids (Coleoptera: Coccinellidae)

Abstract The establishment of exotic coccinellid species may be related to the decline in native coccinellid species within the United States. Coccinellids may interact indirectly by competing for shared resources and directly through intraguild predation. Prior studies have examined pairwise interactions among species and although several coccinellid species typically co-exist within habitats, multi-species interactions have rarely been investigated. To examine how aphid prey levels influence interspecific interactions among a multispecies assemblage, the exotic coccinellids, Harmonia axyridis (Pallas) and Coccinella septempunctata (L.) (Coleoptera: Coccinellidae), and a native coccinellid species, Coleomegilla maculata (DeGeer) (Coleoptera: Coccinellidae), were paired or placed in groups containing three heterospecific larvae and fed 2.4, 12.0, or 24.0 mg of aphids per larva daily. Larval survival, developmental time, and adult weight of individuals were measured. Greater amounts of prey typically resulted in increased adult weights and shorter developmental times. When Coleomegilla maculata and Coccinella septempunctata were paired, Coleomegilla maculata had higher survival rates at low prey levels and Coccinella septempunctata had higher survival at mid- and high prey levels. When the three species were combined, Coccinella septempunctata and Coleomegilla maculata had similar survival rates (17%) at low prey levels; at mid- and high prey levels, Coccinella septempunctata survival increased (58–88%), but Coleomegilla maculata survival remained similar (17–21%). Survival of H. axyridis was not affected by competitors. Even though intraguild predation occurs among these species, the responses to competitors varied based on prey level, coccinellid species, and the number of interacting larvae.

The importance of environmental heterogeneity for species diversity and assemblage structure in Bornean stream frogs

1. Several hypotheses have been proposed to explain the structure of multi-species assemblages. Among these, abiotic environmental factors and biotic processes are often favoured. Several recent studies examining anuran communities identified environmental factors to be only of minor importance in the composition of leaf-litter and canopy assemblages in pristine forests. Instead, spatial effects and spatially structured environments were considered more important. 2. In this study, we investigated whether these findings could also be confirmed for very heterogeneous stream habitats in the primary rainforest of the Ulu Temburong National Park, Brunei Darussalam. We thus investigated anuran assemblage compositions on 50 stream sites with regard to environmental and spatial influences. 3. Cross-product correlations indicated that both factors (spatial and environmental parameters) determined assemblage composition of anurans. Environment itself may be spatially structured, yet this interrelation did not contribute to the explainable variation of frog community compositions within the study area. 4. Detailed analyses of the environmental parameters with nonmetric multidimensional scaling revealed that community structure was mostly affected by three major environmental characters: stream turbidity, river size and the density of understorey vegetation. Based on these habitat characteristics, we assigned species to three distinct habitat guilds. 5. The results underline the importance of riparian habitat heterogeneity in pristine forests in structuring anuran assemblages. We conclude that different anuran assemblages, that is, leaf litter, canopy and stream communities, follow different assemblage rules and thus are not directly comparable.

Preservation of spatial and environmental gradients by death assemblages

Although only a few studies have explicitly evaluated live-dead agreement of species and community responses to environmental and spatial gradients, paleoecological analyses implicitly assume that death assemblages capture these gradients accurately. We use nine data sets from modern, relatively undisturbed coastal study areas to evaluate how the response of living molluscan assemblages to environmental gradients (water depth and seafloor type; “environmental component” of a gradient) and geographic separation (“spatial component”) is captured by their death assemblages. We find that:1. Living assemblages vary in composition either in response to environmental gradients alone (consistent with a species-sorting model) or in response to a combination of environmental and spatial gradients (mass-effect model). None of the living assemblages support the neutral model (or the patch-dynamic model), in which variation in species abundance is related to the spatial configuration of stations alone. These findings also support assumptions that mollusk species consistently differ in responses to environmental gradients, and suggest that in the absence of postmortem bias, environmental gradients might be accurately captured by variation in species composition among death assemblages. Death assemblages do in fact respond uniquely to environmental gradients, and show a stronger response when abundances are square-root transformed to downplay the impact of numerically abundant species and increase the effect of rare species.2. Species' niche positions (position of maximum abundance) along bathymetric and sedimentary gradients in death assemblages show significantly positive rank correlations to species positions in living assemblages in seven of nine data sets (both square-root-transformed and presence-absence data).3. The proportion of compositional variation explained by environmental gradients in death assemblages is similar to that of counterpart living assemblages. Death assemblages thus show the same ability to capture environmental gradients as do living assemblages. In some instances compositional dissimilarities in death assemblages show higher rank correlation with spatial distances than with environmental gradients, but spatial structure in community composition is mainly driven by spatially structured environmental gradients.4. Death assemblages correctly identify the dominance of niche metacommunity models in mollusk communities, as revealed by counterpart living assemblages. This analysis of the environmental resolution of death assemblages thus supports fine-scale niche and paleoenvironmental analyses using molluscan fossil records. In spite of taphonomic processes and time-averaging effects that modify community composition, death assemblages largely capture the response of living communities to environmental gradients, partly because of redundancy in community structure that is inherently associated with multispecies assemblages. The molluscan data sets show some degree of redundancy as evidenced by the presence of at least two mutually exclusive subsets of species that replicate the community structure, and simple simulations show that between-sample relationships can be preserved and remain significant even when a large proportion of species is randomly removed from data sets.

Effects of cloning and root-tip size on observations of fungal ITS sequences from Picea glauca roots

To better understand the effects of cloning on observations of fungal ITS sequences from Picea glauca (white spruce) roots two techniques were compared: (i) direct sequencing of fungal ITS regions from individual root tips without cloning and (ii) cloning and sequencing of fungal ITS regions from individual root tips. Effect of root tip size was investigated by selecting 20 small root tips (SRT, 1.0–2.0 mm long) and 20 large root tips (LRT, 5.0–6.0 mm long). DNA was isolated from each tip and PCR-amplified with fungal-specific primers. PCR reactions were divided into two portions, one of which was sequenced directly and one of which was cloned first followed by sequencing of 12 random clones. With direct sequencing all 20 SRT produced an identifiable sequence, while only 13 of 20 LRT (65%) yielded an identifiable sequence. With cloning and sequencing all 40 tips produced identifiable fungal ITS sequences regardless of size. Failure of direct sequencing in LRT was associated with the presence of multispecies assemblages. Cloning identified 18 taxa overall while direct sequencing identified four. Cloning was not affected by tip size and identified more taxa relative to direct sequencing, although cost and probability of observing lab-based contaminants (e.g. airborne or reagent-based) were higher. We suggest that standardized controls be run whenever clones are sequenced from environmental samples, including positive controls derived from pure cultures and negative controls that cover the entire extraction, amplification and cloning process. Additional studies on larger root segments and bulked samples are needed to determine whether cloning can detect fungi accurately and cost-effectively in complex environmental samples.

Response of Phytoplankton Community to Low-Dose Atrazine Exposure Combined with Phosphorus Fluctuations

The effects of atrazine on a controlled phytoplankton community derived from a natural freshwater wetland exposed to low doses of this photosynthesis-inhibiting herbicide were examined. The community was exposed for 7 weeks to doses of 0.1, 1, and 10 microg L(-1) atrazine, combined with changes in nutrient concentration, and the photosynthetic activity, biomass, and community structure were noted during the experiment. Responses of the phytoplankton community were examined in terms of photosynthetic activity, biomass, and community structure. Significant effects of atrazine on the phytoplankton assemblage, in terms of primary production and community structure, were highlighted, even at doses as low as 1 and 0.1 microg L(-1), when associated with phosphorus fluctuations. The most abundant Chlorophyceae decreased in concentration with increasing atrazine dose, whereas cyanobacteria were more tolerant to atrazine, particularly with increased nutrient supply. The subinhibitory doses of atrazine used in the present study confirmed the higher sensitivity of long-term exposure of multispecies assemblages under resource competition. Our study supports the emerging hypothesis that the increasing prevalence of cyanobacterial blooms in European aquatic systems may result from a combination of unbalanced nutrient enrichment and selective pressures from multiple toxicants.

Temporal and spatial variation in the early recruitment of fucoid algae: the role of microhabitats and temporal scales

The recruitment of a multi-species assemblage of fucoid algae, Ascophyllum nodosum (L.) Le Jolis, Fucus vesiculosus L., and F. distichus L. ssp. edentatus (Pyl.) Powell, onto artificial sub- strata over different temporal and spatial scales was examined on a semi-exposed rocky shore in Quebec, Canada. Measurements were taken at daily (1 d), fortnightly (15 d), and monthly (30 d) peri- ods in 3 microhabitats (exposed rock surfaces, under adult fucoids, and in tide pools) from July through September in 3 consecutive years (1998 to 2000). Recruitment was spatially and temporally variable among months and years with much less recruitment in 1998. Recruitment was consistently lowest in September. The density of reproductive adults did not generally affect the rate of recruit- ment, but the rate of recruitment was negatively correlated with wind speed, a possible indicator of wave action. Among microhabitats, the recruitment was over 10 to 50 times greater under the canopy relative to exposed surfaces and tide pools. Mortality of outplanted embryos was only slightly higher outside the canopy, which suggests that it is limited dispersal and not differential survival that leads to high numbers of recruits under the canopy. There were, however, no differences in the numbers of recruits accumulating from 1 to 30 d, suggesting that overall post-settlement mortality rates are extremely high.

Roadkills of vertebrate species on two highways through the Atlantic Forest Biosphere Reserve, southern Brazil

We assessed the magnitude, composition, and spatial and temporal patterns of road mortality of native vertebrates on two highways in southern Brazil from 18 January 2003 to 26 January 2004. The highways cross remnants of the Atlantic Rainforest, a global biodiversity hotspot, and differ in vehicle traffic and surrounding landscape. We compared the road-kill magnitude and composition of birds, mammals, and reptiles between roads and seasons. We used a modified K statistic to depict the spatial patterns of roadkills of these groups and tested the association between vehicle traffic and road mortality through linear regression. We recorded 869 kills of 92 species. The two roads differed regarding the abundance and composition of roadkills. Reptile road mortality was higher in summer than winter, but all other groups did not show significant difference in the magnitude of mortality between seasons. The composition of killed assemblages differed significantly for some of the taxonomic groups among seasons. We found only one positive association between roadkills and vehicle traffic (reptiles on one of the roads), suggesting that vehicle flow does not explain the road-kill temporal variation on these roads. Total vertebrate, bird, and mammal roadkills showed significant spatial aggregations possibly due to variation in vehicle traffic, highway design, and local landscape condition and arrangement. With expected expansion of the road network, mitigation measures for multi-species assemblages should include habitat protection, soil use regulation, road crossing structures, speed reducers, and campaigns to raise people’s awareness about road impacts on wildlife.

Complexity and idiosyncrasy in the responses of algae to disturbance in mono- and multi-species assemblages

There is considerable debate about whether stability (e.g. inertia) of an assemblage, or of individuals in an assemblage, is positively associated with the number of species or whether there are idiosyncratic effects of particular species. We assessed the general model that the loss of an individual alga, caused by trampling, is greater in monospecific than in multi-species stands but that the responses of algae are idiosyncratic, depending on the morphology of the species. The experiment was done on conspicuous and dominant algae with different morphology on temperate Australian rocky shores: the fucalean algae Hormosira banksii and Sargassum sp. and the coralline alga Corallina officinalis. We assessed the relative and interactive effects of the extent of trampling (number of paths) and the localised intensity of trampling (number of travels per path) on the three algae. The number of paths trampled (the extent of disturbance) had more impact on each alga than the number of times paths were travelled (the intensity of disturbance). As predicted, H. banksii was most susceptible to trampling at each level than were the coarser algae Sargassum sp. and C. officinalis. There was a consistent trend for each alga to be more inert to trampling when in the presence of the other two species than when in monospecific stands, but this was only statistically significant (P < 0.05) for the softer alga H. banksii. The responses of H. banksii and Sargassum sp. to disturbance seemed, in many cases, to be due to the presence of C. officinalis rather than to "diversity" per se. The relationship between the number of species and stability is complex in intertidal habitats, depending on the species and the combinations of species with which it grows.

The role of geomorphology in substratum patch selection by freshwater mussels in the Hawkesbury–Nepean River (New South Wales) Australia

Abstract Microhabitat preferences of freshwater mussels and associated substrate characteristics were investigated across a range of geomorphic reaches in the Hawkesbury–Nepean River, Australia. The structure of substratum patches available was strongly influenced by geomorphic reach type. In each reach type, mussel distribution was most frequently correlated with coarse sand and a roughness element characteristic for that reach. Roughness elements such as boulders and cobbles create a flow refuge and were linked with mussel size. Small mussels tended to be associated with boulder‐stabilized habitats and medium sized mussels with cobble habitats. Large mussels rarely co‐occurred with any particular roughness element. Individual species were strongly linked to geomorphic reach type. This association may be due to species' differences in ability to colonize available microhabitat types. The highly tolerant Velesunio ambiguus dominated shale reaches, characterized by fine sediments and human impacts. In contrast, Hyridella depressa dominated in gorges, utilizing small flow refuges among boulders, while H. australis were present in low densities across a range of substrate conditions. The persistence of multispecies assemblages in mussel beds throughout the Hawkesbury–Nepean River implies similar niche utilization among species. Partitioning of habitats across species on the basis of size suggests some degree of habitat selection, or differential survival. At the local scale, microhabitat characteristics influenced the size distribution and densities of mussel assemblages. Continuing declines in mussel densities are likely to result from ongoing channel modification and increased siltation resulting from changes to riparian vegetation. Copyright © 2008 John Wiley &amp; Sons, Ltd.

Community and foodweb ecology of freshwater mussels

Freshwater mussel (Superfamily Unionoidea) communities are important components of food webs, and they link and influence multiple trophic levels. Mussels filter food from both the water column and sediment with ciliated gills. Differences in cilia structure and arrangement might allow mussel species to partition food resources. Mussels are omnivores that feed across trophic levels on bacteria, algae, detritus, zooplankton, and perhaps, dissolved organic matter. Living mussels and their spent shells provide or improve habitat for other organisms by providing physical structure, stabilizing and bioturbating sediments, and influencing food availability directly and indirectly through biodeposition of organic matter and nutrient excretion. Effects of mussel communities on nutrient translocation and cycling depend on mussel abundance, species composition, and environmental conditions. Nutrient-related mussel effects influence multiple trophic levels. Healthy mussel communities occur as multispecies assemblages in which species interactions are probably very important. Food limitation and competition among species have been documented, but so have positive species interactions, and rare species have been shown to benefit energetically from living in species-rich communities. Effects of mussel species on ecosystem services and food webs vary across spatial and temporal scales, and the relative importance of competition and facilitation might change at different scales.

Pest Vulnerability Matrix (PVM): A graphic model for assessing the interaction between tree species diversity and urban forest susceptibility to insects and diseases

The limited tree diversity in urban areas increases the likelihood of mass mortality from outbreaks of insects and disease. Although information is available on pest susceptibility of individual tree species, it is difficult to determine from such data the vulnerability of a multi-species assemblage, such as an urban forest, to insects and diseases, or to assess the effects of either changing the tree species composition or the arrival of new pests. Our model, the Pest Vulnerability Matrix (PVM), enables municipal arborists and urban foresters to evaluate the overall vulnerability of their urban forest, to display this information and communicate it to others, and to evaluate the potential effects of emerging pests and diseases. PVM is a Microsoft Excel spreadsheet that provides a means for rapid graphic display of the interaction between tree species diversity and the susceptibility of the urban forest to pests by displaying each tree–pest interaction as a colored cell in a table. PVM calculates the percentage of trees affected by each insect or disease, and enables the urban forester to quickly identify (1) the most important pests and (2) the most vulnerable tree species. The model is designed to be flexible and easily modified by the user, and includes several newly emerging pests to allow the exploration of future “worst-case” scenarios. Two case studies of Northern California cities are presented demonstrating two potential applications of PVM. We conclude with a brief overview of the diversity–stability debate in the context of urban forests.

Cryptic Blooms: Are Thin Layers the Missing Connection?

Harmful algal blooms (HABs) are common in Monterey Bay, CA, and have resulted in repeated closures of shellfish fisheries and the poisoning and death of marine mammals. In the majority of instances, HAB events in this region are first detected by the presence of sick or dying animals. The phrase “cryptic blooms” was adopted to denote the appearance of poisoning at higher trophic levels with no prior evidence of a large phytoplankton bloom. We hypothesize that the onset of many HAB events goes undetected because the bloom is initially concentrated in discrete thin subsurface layers in the water column that are easily missed by conventional sampling and monitoring methods. In this paper, we report on the detection and monitoring of a subsurface layer of phytoplankton in northern Monterey Bay, CA, using a high-resolution, autonomous profiler. This ‘thin layer,’ which measured from 10 cm to 3 m in thickness (85% < 2 m; 54% < 1 m), persisted over a 7-day period near the base of the pycnocline. The phytoplankton assemblage in the layer was primarily composed of a multi-species assemblage of Pseudo-nitzschia including the toxin-producing species Pseudo-nitzschia australis. Concentrations of toxic phytoplankton (P. australis), cyanobacteria, and bacteria in the layer were significantly higher than outside the layer (P < 0.05). Counts of total Pseudo-nitzschia spp. showed similar levels of enrichment in the layer compared to outside the layer. Our findings indicate that, when monitoring for HABs, it is critical to sample at scales appropriate to resolve thin layers. Thin layers have been identified as a common recurrent feature in a variety of coastal systems, suggesting that the use of autonomous high-resolution vertical profilers coupled with targeted sampling, could allow more timely detection of HABs in many coastal environments.

Exclusion of rare taxa affects performance of theO/Eindex in bioassessments

The contribution of rare taxa to bioassessments based on multispecies assemblages is the subject of continued debate. As a result, users of predictive models such as River InVertebrate Prediction and Classification System (RIVPACS) disagree on whether to exclude locally rare taxa from the O/E index, where O is the number of taxa observed in a sampled assemblage, and E is the number that would be expected if the site were in a minimally disturbed reference condition. We assessed how the bioassessment performance of O/E was affected by the exclusion of taxa with site-specific, model-predicted occurrence probabilities that did not exceed thresholds of PT = 0+, 0.1, 0.2, … , 0.7. We assessed O/E performance for each of 10 predictive models applied to a total of 5685 stream and lake samples from throughout the contiguous USA. For 5 of the 10 cases, the standard deviation (SD) of O/E across reference sites was reduced by at least 0.02 O/E units when locally rare and uncommon taxa were excluded (PT = 0.5) from O/E, as compared with all taxa being included (PT = 0+). These reductions in SD denote increases in precision of the O/E index. We also assessed the sensitivity of O/E, measured by the % of test sites (that is, sites independently assessed as not being in reference condition) that were declared to be outside the reference distribution of O/E scores. Five of our 10 cases showed increases in sensitivity of ≥10% at PT = 0.5 as compared with PT = 0+. All cases that did not show increases in sensitivity or precision also showed no decrease in either of these performance measures as PT increased. A comparison of observed occurrence frequencies of taxa at reference and test sites qualitatively explained the size of the O/E sensitivity response in all 10 cases. This result suggests that effects of rare-taxa exclusion are a direct consequence of these occurrence frequencies rather than of predictive-model structures. Thus, we predict that other assemblage-based bioassessment tools are likely to show improved sensitivity when rare taxa are excluded.

Genetic identity of interspecific neighbours mediates plant responses to competition and environmental variation in a species‐rich grassland

Summary 1 Although outbreeding populations of many grassland plants exhibit substantial genetic and phenotypic variation at fine spatial scales (< 100 m 2 ), the implications of local genetic diversity for community structure are poorly understood. Genetic diversity could contribute to local species diversity by mediating the effects of competition between species and by enhancing species persistence in the face of environmental variation. 2 We assayed the performance of three genotypes each of a dominant tussock grass ( Koeleria macrantha [Ledeb.] J.A. Schultes) and dominant sedge ( Carex caryophyllea Lat.) derived from a single 10 × 10 m quadrat within a limestone grassland in Derbyshire, UK. Genotypes were grown in monoculture and grass‐sedge mixtures of different genetic composition in two environments of contrasting fertility. Species mixtures also included one genotype of the subordinate forb Campanula rotundifolia L. 3 When grown without neighbours, intraspecific genotypes responded similarly to environmental treatments. One genotype of the sedge performed worse in both environments than the other two sedge genotypes. 4 When grown in species mixtures, genotype performance was significantly influenced by the genetic identity of the neighbouring species for both the sedge and the grass. At high fertility, differential genotype performance was not sufficient to alter the expectation of competitive exclusion of the sedge by the grass. However, at low fertility, the competitive dominant depended on the genetic identity of both the grass and the sedge. In addition, each genotype of the grass performed best next to a different genotype of the sedge, and the identity of the best genotype pairings switched with environment. 5 Performance of a single genotype of the subordinate Campanula was not predictable by fertility alone, but by how fertility interacted with different neighbouring genotypes of both the grass and the sedge. 6 Results support the hypothesis that the genetic identity of interspecific neighbours influences plant performance in multispecies assemblages and mediates species’ responses to environmental variation. Such interactions could be a key factor in the contribution of local intraspecific genetic diversity to species diversity.