A long-term study (1949–2005) of experimental introductions to an island; freshwater amphipods (Crustacea) in the Isle of Man (British Isles)

Aim Using data from field introduction experiments with Gammarus spp. conducted in the rivers of a small island, commencing in 1949, with resampling in the 60s, 70s, 80s, 90s and finally in 2005, we aimed to examine the long‐term interaction of the native freshwater amphipod Gammarus duebeni celticus with the introduced G. pulex. Using physico‐chemical data from a 2005 island‐wide survey, we also aimed to find what environmental factors could influence the distribution of the two species. Location The Isle of Man, British Isles. Methods We used species distribution data from 10 river sites from a field introduction study in 1949–69, resampling of these sites in 1974/5, 1988, 1995 and 2005. We sampled a further 82 sites in 2005, where physico‐chemical characteristics of each site were obtained. ANOVA was used to ascertain significant differences in environmental variables between native and introduced amphipod assemblages. Multiple discriminant analysis was used to relate Gammarus distribution to environmental gradients in rivers. Results Field introductions of G. pulex into previously G. d. celticus dominated sites failed, despite the large propagule pressure in terms of numbers of individuals released and repeated introductions. The island‐wide survey revealed that G. d. celticus was found in areas of higher water quality than G. pulex. Sites where the two species co‐occurred also tended to be of lower water quality than native only sites. Continuing metal pollution from past mining activity may be a major factor excluding all amphipods from many rivers. The North American amphipod Crangonyx pseudogracilis, which was first discovered on the Isle of Man in one river site in 1995, was found in a further five sites. Main conclusions This ecological experiment, through long‐term monitoring, thus continues to elucidate factors influencing the distributions of introduced and native species. Under some physicochemical regimes, G. d. celticus is able to withstand its usual displacement by G. pulex, whereas increased river pollution could promote replacement of the native by the introduced species.     

Potential effects of the invasive ‘killer shrimp’ (Dikerogammarus villosus) on macroinvertebrate assemblages and biomonitoring indices

1. Water quality monitoring data from 10 watercourses and laboratory mesocosm studies were used to assess the potential impacts of the crustacean amphipod invader Dikerogammarus villosus on resident macroinvertebrate assemblage structure in Central European fresh waters. 2. The presence of D. villosus was associated with a decline in the prevalence of many native species, pollution sensitive as well as pollution tolerant, and changes in biotic indices, despite the trends of improved water quality coinciding with the invasion period. A general increase in the prevalence of other invaders was also noted. The potential impacts of D. villosus were substratum dependent, differing between stone, concrete and sand‐dominated sites. 3. Mean Multimetric Macroinvertebrate Index Flanders (MMIF) values were marginally lower when D. villosus was present (P < 0.06), as opposed to when other amphipod species or no amphipods were present, despite the improved water quality. Mesocosm studies showed that several macroinvertebrate taxa were completely eliminated in treatments with D. villosus, oligochaete worms, Caenidae mayfly, chironomids and tipulids being particularly vulnerable to D. villosus predation. Biological Monitoring Working Party (BMWP) scores were lower in mesocosms with D. villosus as opposed to the native Gammarus pulex or no amphipods at all. 4. We predict that resident macroinvertebrate assemblages in both Central Europe and Britain will come under increasing pressure as D. villosus invasions progress. Consequently, macroinvertebrate biotic indices, such as the MMIF or BMWP, may need to be revised to account for changes in taxa sensitivities to water quality as well as increased predation and competition.     

An acanthocephalan parasite mediates intraguild predation between invasive and native freshwater amphipods (Crustacea)

1. The balance of predation between closely related invasive and native species can be an important determinant of the success or failure of biological invasions. In Irish freshwaters, the introduced amphipod Gammarus pulex has replaced the native G. duebeni celticus, possibly through differential mutual intraguild predation (IGP). Theoretically, parasitism could mediate such predation and hence the invasion outcome. However, this idea remains poorly studied. 2. In a field survey, we show that the acanthocephalan parasite Echinorynchus truttae is present in more G. pulex populations than G. d. celticus populations. In addition, within parasitised populations, E. truttae is more prevalent in the invader than in the native. 3. We show for the first time that an acanthocephalan parasite mediates predation between its intermediate macroinvertebrate hosts. In a field experiment, E. truttae parasitism of the invader lowered IGP upon the unparasitised native. In laboratory experiments, parasitism of G. pulex significantly reduced their predatory impact on recently moulted female G. d. celticus. Parasitism also appeared to cause reduction in predatory behaviour, such as attacks per contact on precopula guarded female natives. 4. We conclude that higher parasite prevalence in invaders as compared with natives, by mediation of interspecific interactions, could promote species coexistence, or at least slow species replacements, in this particular biological invasion.     

An appraisal of a biocontamination assessment method for freshwater macroinvertebrate assemblages; a practical way to measure a significant biological pressure?

Freshwater invasive or alien species (IAS) can have a major impact on benthic macroinvertebrate assemblage structure and diversity. This has implications for accurate biological monitoring, the assessment of the ecological quality status of rivers and achievement of Water Framework Directive (WFD) objectives. Although IAS constitutes a major biological pressure to WFD objectives, current approaches to ecological status assessment tend to ignore their presence. This problem is compounded as biotic indices such as the Biological Monitoring Working Party (BMWP) score do not distinguish between native and IAS, when IAS tend to be more tolerant of organic pollution than the natives they replace. Biocontamination is the presence of an IAS in a system, and we tested a new method of biocontamination assessment, designed to be used alongside current routine water quality monitoring techniques, by applying it to biological monitoring data from the river monitoring programme of a small Island, The Isle of Man. Although 54% of monitoring sites exhibited no biocontamination, 19% showed low or moderate biocontamination and 27% high or severe biocontamination. Richness contamination was low (only two contaminated families being recorded), but abundance contamination was high in some sites (87% of individuals being IAS). Sites with a greater relative abundance of IAS individuals exhibited lower BMWP water quality. Within invaded sites BMWP monitoring was not responsive to changing chemical water quality, whereas within uninvaded sites it was. In invaded sites, the relative abundance of IAS increased as ammonia and BOD₅ increased. Our study shows current monitoring approaches mask the presence of AIS within assemblages, with some highly biocontaminated sites registering high BMWP biological quality. This new index represents a simple way to integrate the IAS biological pressure into established WFD monitoring programmes, to produce more comprehensive estimates of ecological quality status than are currently being realised.     

The enemy of my enemy is my friend: intraguild predation between invaders and natives facilitates coexistence with shared invasive prey

Understanding and predicting the outcomes of biological invasions is challenging where multiple invader and native species interact. We hypothesize that antagonistic interactions between invaders and natives could divert their impact on subsequent invasive species, thus facilitating coexistence. From field data, we found that, when existing together in freshwater sites, the native amphipod Gammarus duebeni celticus and a previous invader G. pulex appear to facilitate the establishment of a second invader, their shared prey Crangonyx pseudogracilis. Indeed, the latter species was rarely found at sites where each Gammarus species was present on its own. Experiments indicated that this may be the result of G. d. celticus and G. pulex engaging in more intraguild predation (IGP) than cannibalism; when the ‘enemy’ of either Gammarus species was present, that is, the other Gammarus species, C. pseudogracilis significantly more often escaped predation. Thus, the presence of mutual enemies and the stronger inter- than intraspecific interactions they engage in can facilitate other invaders. With some invasive species such as C. pseudogracilis having no known detrimental effects on native species, and indeed having some positive ecological effects, we also conclude that some invasions could promote biodiversity and ecosystem functioning.     

Environmental mediation of intraguild predation between the freshwater invader Gammarus pulex and the native G. duebeni celticus

Invasive species and environmental change often occur simultaneously across a habitat and therefore our understanding of their relative roles in the decline of native species is often poor. Here, the environmental mediation of a critical interspecific interaction, intraguild predation (IGP), was examined between invasive (Gammarus pulex) and native (G. d. celticus) freshwater amphipods. In the laboratory, IGP asymmetries (males preying on congeneric females) were examined in river water sourced from zones where: (1) the invader has completely displaced the native; (2) the two species currently co-exist, and (3) the native currently persists uninvaded. The invader was always a more effective IG predator, but this asymmetry was significantly weaker moving from ‘invader-only water’ through ‘co-existence water’ to ‘native-only water’. The constituent of the water that drives this mediation of IGP was not identified. However, balancing the rigour of laboratory experiments with field derived ‘environment’ has advanced understanding of known patterns in a native species decline, and its co-existence and persistence in the face of an invader.     

Sampling variation in macroinvertebrate data and implications for river quality indices

SUMMARY 1. The EC Water Framework Directive (2000/60/EC) recognises the need for biological monitoring. Indices derived from standard samples of macroinvertebrates are frequently used for the appraisal of the ecological quality of rivers. However, information on the errors or chance variation that can influence the value of an index is also important. 2. This paper describes a study to quantify the observed sampling variation in three ecological indices based on the Biological Monitoring Working Party (BMWP) score system across a wide range of river types and qualities. The indices are number of BMWP taxa, BMWP score and Average Score Per Taxon (ASPT). 3. The study sites were selected to encompass the four major groups within the River InVertebrate Prediction And Classification System (RIVPACS) site classification for Britain. Within each group, four sites which differed in ecological quality grade were chosen (total of 16 sites). At each site three standard RIVPACS samples were taken in each of spring, summer and autumn by trained staff. In each season, two samples were taken by one biologist and the third by a different individual to allow for within and between‐operator variation. 4. The effects of sampling variation within a season on the number of taxa, BMWP score and ASPT across all sites, irrespective of operator, could be represented by some simple parameters. We found that the sampling SD of the square root of the number of taxa, square root of BMWP score and the untransformed ASPT were roughly constant in each case, irrespective of site type or quality. For each index, SD for two and three seasons combined samples were smaller than for single season samples. 5. Inter‐operator influences on sample values were negligible (4–12% of total sampling SD) in this study. This underlines the importance of adequate training for all staff involved in extensive monitoring programmes which use standard procedures from one year to the next, but may involve different staff. 6. Indices for number of taxa, BMWP score and ASPT were all estimated with greater precision from combined season samples than from the averages of two or three seasons' samples. 7. This study enables us to estimate confidence intervals for the values of the number of taxa, BMWP score and ASPT based on single season, two or three season combined samples collected using standard RIVPACS procedures for any river site in Britain. The results can also be used in simulation models which incorporate the effects of sampling variation into assessments of the ecological quality of river sites based on the ratio of observed to RIVPACS expected values of these BMWP indices.     

Critical influence of seasonal sampling on the ecological quality assessment of small headwater streams

Variability in the ecological quality assessment of reference sites was tested on small headwater streams in Ireland. Although headwater streams constitute a large portion of the river channel network, they are not routinely monitored for water quality. Various metrics were used including the Irish Q-value and the newly developed Small Streams Risk Score (SSRS), and metrics applied elsewhere in the Atlantic biogeographic region in Europe, including the Biological Monitoring Working Party score (BMWP), the Average Score per Taxon (ASPT), the Ephemeroptera, Plecoptera and Trichoptera taxa (EPT), the Belgium Biotic Index (BBI) and the Danish Stream Fauna Index (DSFI). The AQEM (version 2.5a) assessment software was used to apply some of these metrics. The spring and summer datasets are used to test the performance of biotic metrics with respect to season, and the applicability of their use to assess the ecological quality of wadeable streams. The quality status of most sites assigned by the various metrics was high using the spring invertebrate data, and an apparent considerable deviation in quality status occurred when the summer data was applied. Seasonal differences were noted using all the biotic indices and are attributed to the absence of pollution-sensitive groups in summer. Seasonal variability in the water quality status was particularly evident in acidic streams draining non-calcareous geologies with peaty soils that had relatively lower numbers of taxa. Some indices applied reflect a greater seasonal difference in the quality category assigned. The least amount of variability between seasons was obtained using the ASPT and the SSRS risk assessment system. Results suggest that reference status is reliably reflected in spring when more pollution-sensitive taxa were present, and that a new ecological quality assessment tool is required for application in summer when impacts may be most severe. This highly heterogeneous freshwater habitat seems to have too few taxa present in the summer to reliably determine the ecological quality of the stream using the available indices. Handling editor: R. Bailey     

Direct comparison of assessment methods using benthic macroinvertebrates: a contribution to the EU Water Framework Directive intercalibration exercise

The aim of the intercalibration exercise presently performed by the EU is to identify and resolve significant inconsistencies between the ecological quality classifications of EU Member States and the normative definitions of the EU Water Framework Directive. Based on benthic macroinvertebrate data of two European stream types (small siliceous mountain streams and medium-sized lowland streams in Central and Western Europe) we correlated the indices of 10 river quality assessment methods (ASPT, BMWP, DSFI, German Multimetric Index, Saprobic Indices) applied in Austria, Czech Republic, Denmark, Germany, Poland, Slovak Republic, Sweden and United Kingdom. National class boundaries were compared via regression analysis. Assessment methods of the same type (Saprobic Indices, BMWP/ASPT scores) showed best correlation results (R²>0.7). The good quality status boundaries of the national methods deviated up to 25%; thus indicating the necessity to harmonize the national classification schemes. Prerequisites of the presented intercalibration approach are (1) a sufficiently large and consistent dataset representative of the respective common intercalibration types and (2) agreement on common type specific reference conditions.     

Differential physico-chemical tolerances of amphipod species revealed by field transplantations

Physico-chemical regimes of river systems are major determinants of the distributions and relative abundances of macroinvertebrate taxa. Other factors, however, such as biotic interactions, may co-vary with changes in physico-chemistry and concomitant changes in community composition. Thus, direct cause and effect relationships may not always be established from field surveys. Equally, however, laboratory studies may suffer from lack of realism in extrapolation to the field. Here, we use balanced field transplantation experiments to elucidate the role of physico-chemical regime in determining the generally mutually exclusive distributions of two amphipod taxa, Gammarus (two species) and Crangonyx pseudogracilis. Within two river systems in Ireland, the former species dominate stretches of well oxygenated, high-quality water, whereas the latter dominates stretches of poorly oxygenated, low-quality water. G. pulex and G. duebeni celticus did not survive in bioassay tubes in areas dominated by C. pseudogracilis, which itself survived in tubes in such areas. However, both C. pseudogracilis and Gammarus spp. survived equally well in tubes in areas dominated by Gammarus spp. Physico-chemical regime thus limits the movement of Gammarus spp. into C. pseudogracilis areas, but some other factor excludes C. pseudogracilis from Gammarus spp. areas. Since previous laboratory experiments showed high predation rates of Gammarus spp. on C. pseudogracilis, we propose that predation by the former causes exclusion of the latter. Hence, presumed effects of physico-chemical regime on macroinvertebrate presence/abundance may often require experimental field testing and appreciation of alternative explanations. Received: 14 June 1999 / Accepted: 20 January 2000     

The influence of biotope availability on macroinvertebrate assemblages in South African rivers: implications for aquatic bioassessment

1. Macroinvertebrate biotope preferences and the influence of differences in the availability of biotopes on individual taxa, macroinvertebrate assemblages and a biotic index, the South African Scoring System (SASS), were investigated in two regions of South Africa. 2. Among biotope differences in individual taxa and macroinvertebrate assemblages resulted in differences in SASS scores, with differences in assemblages being greater among biotopes than between sites. 3. All three metrics studied (SASS score, number of taxa and average score per taxon, ASPT), differed significantly among biotopes, with highest scores consistently recorded in the stones biotope, while lowest SASS scores and fewest taxa were recorded in the sand biotope. 4. SASS score and number of taxa were positively, while ASPT was negatively correlated with number of biotopes sampled. 5. The observed biotope differences highlight the importance of sampling and comparing data from sites separately for each biotope.     

Invader abundance and contraction of niche breadth during replacement of a native gammarid amphipod

The introduction of non‐native species to new locations is a growing global phenomenon with major negative effects on native species and biodiversity. Such introductions potentially bring competitors into contact leading to partial or total species replacements. This creates an opportunity to study novel species interactions as they occur, with the potential to address the strength of inter‐ and intraspecific interactions, most notably competition. Such potential has often not been realized, however, due to the difficulties inherent in detecting rapid and spatially expansive species interactions under natural field conditions. The invasive amphipod crustacean Gammarus pulex has replaced a native species, Gammarus duebeni celticus, in river and lake systems across Europe. This replacement process is at least partially driven by differential parasitism, cannibalism, and intraguild predation, but the role of interspecific competition has yet to be resolved. Here, we examine how abundance of an invasive species may affect spatial niche breadth of a native congeneric species. We base our analyses of niche breadth on ordination and factor analysis of biological community and physical parameters, respectively, constituting a summative, multidimensional approach to niche breadth along environmental gradients. Results derived from biological and environmental niche criteria were consistent, although interspecific effects were stronger using the biological niche approach. We show that the niche breadth of the native species is constrained as abundance of the invader increases, but the converse effect does not occur. We conclude that the interaction between invasive G. pulex and native G. d. celticus under natural conditions is consistent with strong interspecific competition whereby a native, weaker competitor is replaced by a superior invasive competitor. This study indicates a strong role of interspecific competition, alongside other known interactions such as differential intraguild predation, in rapid and expansive species replacements following biological invasions.     

Habitat heterogeneity promotes the coexistence of exotic seaweeds

Despite the progressive accumulation of exotic species in natural communities, little effort has been devoted to elucidating the mechanisms underpinning the coexistence of invaders in environmentally and biologically heterogeneous systems. The exotic seaweeds, Asparagopsis taxiformis and Caulerpa racemosa, exhibit a segregated distribution on Mediterranean rocky reefs. A. taxiformis dominates assemblages in topographically complex habitats, but is virtually absent on homogenous platforms. In contrast, C. racemosa achieves extensive cover in both types of habitat. We assessed whether differences in their distribution were generated by biotic interactions (between invaders and/or between invaders and natives) or by environmental constraints. Three models were proposed to explain seaweed distribution patterns: (1) invaders inhibit one another; (2) native assemblages, differing between complex and simple habitats, prevent the establishment/spread of one invader, but not that of the other; and (3) environmental conditions regulate the establishment/persistence of the seaweeds in different habitats. We removed the dominant invader and resident assemblages in each type of habitat. Moreover, A. taxiformis thalli were transplanted into the habitat dominated by C. racemosa to establish whether its failure to colonize the simple habitat was due to the lack of propagules or post-recruitment mortality. C. racemosa spread in the complex habitat was not influenced by the removal of resident assemblages, but it was slightly enhanced by A. taxiformis removal. Neither C. racemosa removal nor that of resident assemblages promoted A. taxiformis colonization and survival in simple habitats. Our results suggest that heterogeneity in environmental conditions can promote invader coexistence by mitigating the effects of negative biotic interactions. Therefore, the accumulation of introduced species in native communities does not necessarily imply established invaders fostering further invasion.     

The implications of an invasive species on the reliability of macroinvertebrate biomonitoring tools used in freshwater ecological assessments

Invasive species represent one of greatest threats to aquatic biodiversity globally and are widely acknowledged to be instrumental in modifying native community structure. Despite this, little is known about how the increasing range expansion of invasive taxa may affect routine biomonitoring tools widely employed to measure or quantify environmental quality in lotic systems. This study examined the impact of an invasive freshwater crayfish on commonly employed riverine macroinvertebrate biomonitoring tools (scores and indices) designed to respond to a range of stressors. Data from long-term monitoring sites on both ‘control’ and invaded rivers in England were examined to assess changes to biomonitoring scores following invasion by signal crayfish (Pacifastacus leniusculus). Results indicate that routine biomonitoring tools used to quantify potential ecological stressors which are weighted by abundance, such as the Lotic-invertebrate Index for Flow Evaluation (LIFE) score and Proportion of Sediment-sensitive Invertebrates (PSI), were subject to significant inflation following invasion. In contrast, indices based simply on the presence of taxa, such as the Average Score Per-Taxon (ASPT – a derivative of BMWP), displayed no changes compared to control rivers; or in the case of the Biological Monitoring Working Party Score (BMWP), NTAXA and EPT richness, no consistent pattern following invasion. Season had a significant effect on the interaction of crayfish and LIFE and PSI scores. Autumn samples were subject to statistical inflation following crayfish invasion whilst spring samples exhibited no significant change. The results suggest that care should be taken when interpreting routine macroinvertebrate biomonitoring data where non-native crayfish are present, or in instances where their presence is suspected.     

Effects of coexistence on habitat use and trophic ecology of interacting native and invasive amphipods

1. Invasive species in aquatic systems are major drivers of changes in biodiversity. Amphipods are key species in freshwaters, with invasive amphipods either replacing or coexisting with native species and often damaging local biodiversity. However, the consequences of interactions among native and invasive amphipods for their habitat use and feeding ecology and ecosystem function are not yet well understood. 2. We examined a number of streams in Brittany and Northern Ireland, with native and invasive amphipods, to evaluate the consequences of species interactions for both habitat use and diet. Our field studies centred on testing two proposed models: a cohabitation model without competition between two native species (Gammarus pulex vs Echinogammarus berilloni), and a competition model between an invasive and a native species (Gammarus pulex vs Gammarus duebeni celticus). For these three species, alone and in combination, we assessed their habitat use and feeding patterns, the latter through gut contents and stable C and N isotope analyses of their tissues. 3. When existing as single‐species populations, all three species used stream habitats broadly similarly, although G. pulex was more strongly associated with leaf litter and vegetation compared to pebble substrata than the other species. When G. pulex coexisted with either E. berilloni or G. d. celticus, the latter two changed to using all habitats equally, whereas the former retained its habitat preferences. 4. Similarly, all three species when alone had similar gut contents, with inorganic material predominating, followed by leaf and woody material and more rarely algae and invertebrates. When G. pulex coexisted with E. berilloni, the diet of the latter did not change; however, the frequency of inorganic matter, leaves and wood declined in the gut contents of G. pulex. When G. pulex coexisted with G. d. celticus, the pattern of gut contents did not change in either species. 5. When existing as single‐species populations, G. pulex had a broader range of isotopic signatures, both for δ¹³C and for δ¹⁵N, than the two other species, indicating a more variable diet among individuals. When G. pulex coexisted with either E. berilloni or G. d. celticus, the latter two had similar ranges of δ¹³C and δ¹⁵N, whereas for G. pulex the range was much less for δ¹³C and δ¹⁵N, suggesting a less diverse diet. 6. Our results infer two different modes of coexistence between native and non‐native amphipods. We have shown that the native species, which coexist stably, appear to show interference competition, leading to spatial habitat segregation, whereas competition for food and possible intraguild predation by G. pulex on G. d. celticus would explain why the distribution and density of the latter is affected by G. pulex. However, since all the species have a similar diet and feeding habit, we expect no great overall effect on ecosystem processes as a consequence of species interactions and displacements.     

Longitudinal and seasonal variation of the benthic macroinvertebrate community and biotic indices in an undisturbed Pyrenean river

The present work aims to analyze the spatio-temporal variability in benthic macroinvertebrate assemblages and biotic indices in an undisturbed and unpolluted Pyrenean river. Samples were collected seasonally over 2 year-cycles (2001–2002) at fifteen sampling sites along the Erro River (Ebro River Basin, Spain) during a exhaustive biomonitoring program following the IBMWP–IASPT scoring system protocol routinely applied in Iberia. Despite absolute values of the biotic indices showed high spatio-temporal variation, the IBMWP–IASPT scoring system proved useful because water quality classes were consistent throughout seasons and years as well as along-river. The original macroinvertebrate families’ presence/absence data matrix was reduced in a number of ways to conduct different statistical procedures in order to detect and separate the underlying near-natural spatial and temporal gradients of the assemblage composition in the Erro River. Along-river, spatial variation of the macroinvertebrate community composition was well assessed by similarity analysis, which clearly detected physical features on the river (drought-affected reach, gorge, towns and flow gauging weirs). Categorical principal component analysis (CATPCA) synthesized and jointly ordered macroinvertebrate samples in a spatio-temporal gradient in the factorial map defined by the first two principal components providing a parsimonious way to assess the assemblages’ variation. These two variation gradients throughout the macroinvertebrate families’ occurrence data were subsequently confirmed separately by several correspondence analyses and revealed additional information, as the representative families for each sampling site group and season could be identified. Furthermore, these spatio-temporal gradients were discussed and put in relation with changes in the aquatic habitat (water temperature, conductivity, total dissolved solids, water velocity, channel width, canopy cover, etc.). The near-natural functioning of the Erro River promoted us to emphasize that conservation efforts should aim to maintain the free-flowing as a permanent source of variability.     

Quantifying the responses of biological indices to rare macroinvertebrate taxa exclusion: Does excluding more rare taxa cause more error?

Including or excluding rare taxa in bioassessment is a controversial topic, which essentially affects the reliability and accuracy of the result. In the present paper, we hypothesize that biological indices such as Shannon–Wiener index, Simpson's index, Margalef index, evenness, BMWP (biological monitoring working party), and ASPT (Average Score Per Taxon) respond differently to rare taxa exclusion. To test this hypothesis, a benthic macroinvertebrate data set based on recent fifteen‐year studies in China was built for suppositional plot analyses. A field research was conducted in the Nansi Lake to perform related analyses. The results of suppositional plot simulations showed that Simpson's index placed more weight on common taxa than any other studied indices, followed by Shannon–Wiener index which remained a high value with the exclusion of rare taxa. The results indicated that there was not much of effect on Simpson's index and Shannon–Wiener index when rare taxa were excluded. Rare taxa played an important role in Margalef index and BMWP than in other indices. Evenness showed an increase trend, while ASPT varied inconsistently with the exclusion of rare taxa. Results of the field study also indicated that rare taxa had few impacts on the Shannon–Wiener index. By examining the relationships between the rare taxa and biological indices in our study, it is suggested that including the rare taxa when using BMWP and excluding them in the proposed way (e.g., fixed‐count subsampling) to calculate Shannon–Wiener index and Simpson's index could raise the efficiency and reduce the biases in the bioassessment of freshwater ecosystems.     

Native species richness buffers invader impact in undisturbed but not disturbed grassland assemblages

Many systems are prone to both exotic plant invasion and frequent natural disturbances. Native species richness can buffer the effects of invasion or disturbance when imposed in isolation, but it is largely unknown whether richness provides substantial resistance against invader impact in the face of disturbance. We experimentally examined how disturbance (drought/burning) influenced the impact of three exotic invaders (Centaurea stoebe, Linaria dalmatica, or Potentilla recta) on native abundance across a gradient of species richness, using previously constructed grassland assemblages. We found that invaders had higher cover in experimentally disturbed plots than in undisturbed plots across all levels of native species richness. Although exotic species varied in cover, all three invaders had significant impacts on native cover in disturbed plots. Regardless of disturbance, however, invader cover diminished with increasing richness. Invader impacts on native cover also diminished at higher richness levels, but only in undisturbed plots. In disturbed plots, invaders strongly impacted native cover across all richness levels, as disturbance favoured invaders over native species. By examining these ecological processes concurrently, we found that disturbance exacerbated invader impacts on native abundance. Although diversity provided a buffering effect against invader impact without disturbance, the combination of invasion and disturbance markedly depressed native abundance, even in high richness assemblages.     

Stream macroinvertebrate response to catchment urbanisation (Georgia, U.S.A.)

SUMMARY 1. The effects of catchment urbanisation on water quality were examined for 30 streams (stratified into 15, 50 and 100 km² ± 25% catchments) in the Etowah River basin, Georgia, U.S.A. We examined relationships between land cover (implying cover and use) in these catchments (e.g. urban, forest and agriculture) and macroinvertebrate assemblage attributes using several previously published indices to summarise macroinvertebrate response. Based on a priori predictions as to mechanisms of biotic impairment under changing land cover, additional measurements were made to assess geomorphology, hydrology and chemistry in each stream. 2. We found strong relationships between catchment land cover and stream biota. Taxon richness and other biotic indices that reflected good water quality were negatively related to urban land cover and positively related to forest land cover. Urban land cover alone explained 29–38% of the variation in some macroinvertebrate indices. Reduced water quality was detectable at c. >15% urban land cover. 3. Urban land cover correlated with a number of geomorphic variables such as stream bed sediment size (–) and total suspended solids (+) as well as a number of water chemistry variables including nitrogen and phosphorus concentrations (+), specific conductance (+) and turbidity (+). Biotic indices were better predicted by these reach scale variables than single, catchment scale land cover variables. Multiple regression models explained 69% of variation in total taxon richness and 78% of the variation in the Invertebrate Community Index (ICI) using phi variability, specific conductance and depth, and riffle phi, specific conductance and phi variability, respectively. 4. Indirect ordination analysis was used to describe assemblage and functional group changes among sites and corroborate which environmental variables were most important in driving differences in macroinvertebrate assemblages. The first axis in a non‐metric multidimensional scaling ordination was highly related to environmental variables (slope, specific conductance, phi variability; adj. R²=0.83) that were also important in our multiple regression models. 5. Catchment urbanisation resulted in less diverse and more tolerant stream macroinvertebrate assemblages via increased sediment transport, reduced stream bed sediment size and increased solutes. The biotic indices that were most sensitive to environmental variation were taxon richness, EPT richness and the ICI. Our results were largely consistent over the range in basin size we tested.     

Extended leaf phenology may drive plant invasion through direct and apparent competition

Invasive plants can inflict great harm, yet drivers of successful invasion remain unclear. Many invaders of North American deciduous forests exhibit extended leaf phenology (ELP), or longer growing season relative to natives. ELP may grant invaders competitive advantages, but we argue that ELP more potently drives invasion in the presence of herbivores. ELP invaders can support herbivores by lessening starvation during winter; consequently, native plants may suffer when attacked later through apparent competition. As modeled here, even short ELP can promote competitive success of invaders, and apparent competition sharply enhances ELP invader dominance. In ‘partial enemy escape’ scenarios, a less palatable ELP invader nearly excludes a preferred native where an invader without ELP could not. Together, ELP and apparent competition enhance invasion even when biotic resistance should suppress it, i.e. when the invader competes weakly or provides preferred forage. Thus, ELP‐apparent competition interactions grant invaders considerable success while challenging core tenets of invasion ecology.