Measures of precision for dissimilarity‐based multivariate analysis of ecological communities

Ecological studies require key decisions regarding the appropriate size and number of sampling units. No methods currently exist to measure precision for multivariate assemblage data when dissimilarity‐based analyses are intended to follow. Here, we propose a pseudo multivariate dissimilarity‐based standard error (MultSE) as a useful quantity for assessing sample‐size adequacy in studies of ecological communities. Based on sums of squared dissimilarities, MultSE measures variability in the position of the centroid in the space of a chosen dissimilarity measure under repeated sampling for a given sample size. We describe a novel double resampling method to quantify uncertainty in MultSE values with increasing sample size. For more complex designs, values of MultSE can be calculated from the pseudo residual mean square of a permanova model, with the double resampling done within appropriate cells in the design. R code functions for implementing these techniques, along with ecological examples, are provided.     

Bergmann's Rule – what's in a name?

Despite the great interest it generates, the definition of Bergmann's Rule is vague and often contested. Debate focuses on whether the rule should be described in terms of pattern or process, what taxa it should apply to and what taxonomic level it should be associated with. Here I review the historical development of studies of Bergmann's Rule. I suggest that Bergmann thought that his rule should be strongest at the intra‐specific level, rather than between closely related species as is usually thought. I argue that the rule is a pattern that can be studied regardless of mechanism in any taxon and at any taxonomic level.     

Grassland Leafhoppers (Hemiptera: Auchenorrhyncha) as Indicators of Habitat Condition – A Comparison of between-site and between-year differences in Assemblage Composition

The Auchenorrhyncha (leafhoppers) show great potential as indicators of grassland habitat quality, which would make them useful as a conservation tool. However, they are known to have labile populations. The relative importance of site identity and the year of sampling in the composition of leafhopper assemblages on chalk grassland are assessed for two sets of sites sampled twice. The study included a total of 95 sites (one set of 54, the other of 41), and demonstrated that for both sets the vegetation community and geographical location had high explanatory value, while the influence of year was small. The conclusion is that, notwithstanding population fluctuations, the leafhopper assemblages are a good indicator of habitat quality, and represent a potentially valuable tool in grassland conservation and restoration.     

Identification of zoonotic Giardia genotypes in fish

Apart from a single record in a shark, there have been no published studies conducted on Giardia genotypes in fish. The present study investigated the prevalence of Giardia in cultured fingerlings (n = 227), wild freshwater (n = 227) and wild marine/estuarine species (n = 255) of fish in Western Australia by PCR amplification at the 18S rRNA, glutamate dehydrogenase (gdh), triose phosphate isomerase (tpi) and beta-giardin (bg) loci. Results revealed a low prevalence of Giardia, 3.8% (27/709), in fish hosts. The zoonotic Giardia species, Giardia duodenalis assemblages A, B as well as G. duodenalis assemblage E and Giardia microti were detected. The identification of zoonotic species of Giardia highlights the public health importance of investigating parasites within fish host species.     

Assessing ant assemblages: pitfall trapping versus nest counting (Hymenoptera, Formicidae)

Pitfall trapping and nest counting are the most common census methods for ant assemblages. We examined the concordance between pitfall catches and nest counts on dry grassland. Spearman rank correlations and non-metric multidimensional scaling of the Bray Curtis similarity index revealed moderate concordance between the data collated by the two methods, but overall method-related differences were considerable. The dissimilarity was influenced by the type of land management, but not by trapping period or plot shape. Trapping success depended on nest density, ground vegetation cover and species-specific traits (inhabited stratum, colony size, foraging distance). Even when these factors were taken into account, the convertibility of pitfall trap and nest density values was unsatisfactory: the census method proved to be crucial in designing ant-ecological studies and interpreting literature data. Received 11 November 2005; revised 9 February 2006; accepted 1 March 2006.     

Representing species in reserves from patterns of assemblage diversity

Aim A positive relationship between assemblage diversity (AD) – equivalent to the biotic version of the environment diversity, ED, method – and species diversity has been reported. This being true, reserve networks with many different assemblages would be expected to represent more species than reserve networks including fewer and less different assemblages. This idea was tested using European species atlas distributions of terrestrial vertebrates and plants. It is asked whether: (1) maximizing AD within one group would represent species diversity of this group better than expected by chance; and (2) maximizing AD within one group would represent species diversity of other groups better than expected by chance. Location Europe. Methods Three ordination techniques (non‐metric multidimensional scaling, detrended correspondence analysis and correspondence analysis) are used to summarize patterns of compositional turnover within assemblages. p‐Median location‐allocation models are then calculated from ordination space to measure the degree of expected species representation within the group being sampled as well as the expected representation within other groups. Results are compared with near‐optimal solutions obtained with complementarity‐based algorithms and to a null model obtained by simulating selection of areas at random. Matrix correlation analysis was also performed to investigate broad patterns of covariation in compositional turnover of assemblages of species belonging to different taxonomic groups and these values were compared with correlation in species richness scores between groups. Results The AD model did not always represent more species of the group being sampled than expected by chance (P < 0.05). Results were independent of the method and taxonomic group considered. Effectiveness of AD within one group to represent species of other groups varied, but in most cases it was worse than using complementarity‐based algorithms as a surrogate strategy. Even when correlations indicated high coincidence between assemblages, taxonomic‐based surrogates did not always recover more species than expected by chance (P < 0.05). Main conclusions Results are discussed in the light of two possible explanations: (1) the AD model is based on unrealistic assumptions, namely that species have equal probability of having the centre of their distributions anywhere in ordination space and that species display unimodal, symmetrical, bell‐shaped response curves to gradients; (2) particular implementation of methods may be inadequate to summarize useful complementarity among assemblages, especially for restricted‐range species. We conclude that both arguments are likely to play a role in explaining results, but that opportunities exist to improve performance of existing surrogate strategies.     

Application of trophic transfer efficiency and age structure in the trophic analysis of fossil assemblages

We evaluate onshore-offshore trends in age-frequency distributions and trophic transfer efficiencies using 11 modern death assemblages off the Texas coast. Trophic transfer efficiencies within trophic levels offer little insight over that achieved by a size-frequency distribution. Production/biomass ratios will always be 1 in the fossil record. Within trophic-level estimates of paleogrowth efficiency, the ratio of paleoproduction to paleoingestion (Pⁱ_glt/Iⁱ_lt where i indicates the i^th trophic level and lt indicates the time-averaged value) follow the expected ecological trend precisely in that paleogrowth efficiency is consistently higher in primary consumers than in predators in all 11 death assemblages. Paleoutilization efficiency, the ratio of predator paleoingestion to prey paleoproduction, I²_lt^°/P¹_glt^°, may provide information on the degree of bias in the preservation of primary (1 °) and secondary (2 °) consumer trophic groups. I²_lt^°/P¹_glt^° fell below 0.1 in most cold-seep and bay assemblages, indicating a large surplus of primary consumers. In sharp contrast, I²_lt^°/P¹_glt^°     

Are biological classifications of headwater streams concordant across multiple taxonomic groups?

1. Studies assessing human impacts on freshwater ecosystems are typically based on a single taxonomic group, often macroinvertebrates or fish. Unfortunately, the degree to which such macroinvertebrate or fish‐based surveys can be generalised across other taxonomic groups remains largely unknown. A prerequisite for useful generalisations is that different taxonomic groups exhibit concordant patterns of community structure across sites. 2. We examined the concordance among fish, benthic macroinvertebrates and bryophytes in 32 streams in a boreal catchment in Finland. Our goal was to test how consistently different taxonomic groups classify stream sites; for example, can site groupings based on macroinvertebrates be used as a surrogate for bryophyte or fish assemblage classification? 3. Our results show that community classifications in headwater streams are not concordant across taxonomic groups, at least not at the within‐river system scale. The lack of concordance reflected the fact that all three groups responded to different environmental factors. Macroinvertebrate community structure was mainly correlated with stream size and pH, whereas bryophytes were related to water colour, nutrient content and in‐stream habitat variability. Fish community structure was best described by stream depth, substrate size and water oxygen concentration. 4. Our results suggest that great care should be taken when typologies based on benthic macroinvertebrates, or any other taxonomic group, are extrapolated to other groups in creating typologies of lotic environments.     

Resource quantity, not resource heterogeneity, maintains plant diversity

Resource heterogeneity has often been proposed to explain the maintenance of plant species diversity and patterns of species diversity along productivity gradients. Resource heterogeneity should maintain biodiversity by preventing competitive exclusion because different species are superior competitors in different parts of a heterogeneous environment. In natural systems, however, resource heterogeneity covaries with average resource supply rate, making the effect of heterogeneity difficult to isolate. Using a novel experimental approach, we tested the independent effects of resource heterogeneity and average supply rate on plant species diversity. We show that the average supply rate of the most limiting resource controlled species diversity, whereas heterogeneity of this resource had virtually no effect. These findings also suggest that biodiversity declines with increasing productivity because at high enough levels of productivity one resource may always be driven to sufficiently short supply to exclude many species.     

Stable isotope proxies for evaluating biodiversity in stream biota

Differences in δ¹³C and δ¹⁵N values in stream biota are caused by several environmental conditions. Variations in abundance, species richness and the assemblage structure of stream biota are also caused by several environmental conditions. Hence, abundance, species richness and the assemblage structure of stream biota are expected to be strongly correlated with the differences in value of stable isotopes. In this study, the gaps in δ¹³C and δ¹⁵N between periphyton and charr are discussed in terms of the abundance, genus richness, and assemblage of benthic invertebrates at each site. Gaps in δ¹³C between periphyton and charr were strongly correlated with some aspects of mountainous area and the genus richness of benthic invertebrates at each site. The gaps in δ¹⁵N between periphyton and charr were strongly correlated with the abundance and assemblage structure of benthic invertebrates at the location tested. The δ¹³C values of predators were correlated with some aspects of mountainous area and the assemblage structure of the benthic invertebrates. The δ¹⁵N values of predators were correlated with genus richness and the assemblage structure of the benthic invertebrates. These results suggest that the value gaps in δ¹³C and δ¹⁵N can be used to assess biodiversity and could provide indices for estimating the biodiversity in a stream.