R.M. Warwick

A new method for detecting pollution effects on marine macrobenthic communities

A method is described by which the pollution status of a marine macrobenthic community may be assessed without reference to a temporal or spatial series of control samples. Theoretical considerations suggest that the distribution of numbers of individuals among species should behave differently from the distribution of biomass among species when influenced by pollution-induced disturbance. Combined k-dominance plots for species biomass and numbers take three possible forms representing unpolluted, moderately polluted and grossly polluted conditions, one curve acting as an “internal control” against which the other can be compared. Field data from unpolluted communities and from a well documented temporal pollution gradient support the model, but further empirical testing is required.

Increased variability as a symptom of stress in marine communities

Abstract An increase in variability between samples collected from impacted vs. control areas is described for four different types of marine communities: meiobcnthos subjected to organic enrichment, macrobenthos in the vicinity of the Ekofisk oil-field, reef-corals following the 1982–3 El Nino and fish on coral reefs which are subjected to mining. In each case there is a clear log-log relationship between the variance and the mean abundance for all species in a particular treatment group. The standard deviation for a given mean increases with increased perturbation in all cases, but is most marked for the meiobenthos and macrobenthos examples. Variability in species diversity (H′) tends to increase with increasing levels of perturbation, but this increase is only significant for the macrobenthos. In all cases a pronounced increase in variability among replicate samples from perturbed treatments was revealed by multivariate analysis (non-metric Multi-Dimensional Scaling ordination). A comparative Index of Multivariate Dispersion (IMD) is suggested as a measure of this increased variability.

The level of taxonomic discrimination required to detect pollution effects on marine benthic communities

Five data sets (three for macrobenthos and two for meiobenthos), describing the species composition of benthic assemblages in relation to pollution gradients, have been subjected to multivariate and univariate analysis, using various hierarchical levels of taxonomic aggregation of the species data. In no case was there any substantial loss of information at the family level. Identification of the fauna to this level would obviate most of the time-consuming problems associated with the analysis of benthic samples for the purposes of assessing pollution effects. Sample groupings in the multivariate analyses of the macrofauna data related more closely to the pollution gradient at the level of phyla than of species in two of the three examples, and the relationship was no worse for phyla in the remaining example. It is tentatively suggested that anthropogenic effects modify community composition at a higher taxonomic level than natural environmental variables, which influence the fauna more by species replacement. This differential response could be potentially useful in separating pollution effects from those of natural environmental variables such as water depth and sediment type.

Similarity-based testing for community pattern: the two-way layout with no replication

The large, sparse arrays of species counts arising in both field and experimental community studies do not lend themselves to standard statistical tests based on multivariate normality. Instead, a valid and more revealing approach uses informal display methods, such as clustering or multi-dimensional scaling ordination (MDS), based on a biologically-motivated definition of pairwise similarity of samples in terms of species composition. Formal testing methods are still required, however, to establish that real assemblage differences exist between sites, times, experimental treatments, pollution states, etc. Earlier work has described a range of Manteltype permutation or randomisation procedures, making no distributional assumptions, which are termed ANOSIM tests because of their dependence only on (rank) similarities and the analogy to one and two-way ANOVA. This paper extends these tests to cover an important practical case, previously unconsidered, that of a two-way layoutwithout replication. Such cases arise for single samples (or pseudo-replicates) taken in a baseline monitoring survey of several sites over time, or a mesocosm experiment in which “treatments” are replicated only once within each experimental “block”. Significance tests are given for the overall presence of a treatment (or time) effect, based on a measure of concordance between rank similarities of samples within each block (or site); the role of the two factors can be reversed to obtain a test for block effects. As in the analogous univariate ANOVA test, the method relies on absence or relative weakness of treatment x block “interactions”. Its scope is illustrated with data from two experimental and two field studies, involving meiofaunal communities from soft-sediment and macro-algal habitats. It is seen also to accommodate a modest derree of missing data. Whilst the failure to replicate adequately is not encouraged—a richer inference is available with genuine replication—the paper does provide a limited way forward for hypothesis testing in the absence of replicates.

A comparison of some methods for analysing changes in benthic community structure

Statistical methods for analysing changes in community structure fall under the three general headings of univariate, graphical/distributional and multivariate. These methods are applied to a variety of benthic community data (macrobenthos, meiobenthos, corals, demersal fish), from a variety of localities (intertidal/subtidal, temperate/tropical) and over both spatial and temporal scales. Four general conclusions emerge from this comparative study: (1) The similarity between sites or times based on their univariate or graphical/distributional properties is usually different from their clustering in multivariate analyses. (2) Species dependent (multivariate) methods are much more sensitive than species independent (univariate and graphical/distributional) methods in discriminating between sites or times. (3) In examples where more than one component of the fauna has been studied, univariate and graphical/distributional methods may give different results for different components, whereas multivariate methods tend to give the same results. (4) By matching multivariate ordinations from subsets of environmental data to an ordination of faunistic data, the key environmental variables responsible for community change may be identified.

Analysis of macrobenthic and meiobenthic community structure in relation to pollution and disturbance in Hamilton Harbour, Bermuda

A comparison of the community structure of the macrobenthos and meiobenthos at six stations in Hamilton Harbour, Bermuda showed that the two components were affected differently by environmental disturbance. Univariate statistical analysis of the macrofauna species data gave clear indications of disturbance at two stations, which did not relate to levels of contaminants. The meiofauna were apparently undisturbed at all localities investigated. The explanation for this differential response was most likely to be physical disturbance of the sediment by the passage of large cruise liners. Multivariate statistical analyses indicated that differences in faunal composition between stations for both macrobenthos and meiobenthos were not determined by differences in water depth or sediment type. However, the faunal composition did correlate with certain anthropogenic variables, particularly the tributyl tin concentrations in the water column. Thus, multivariate analyses detected differences in community composition which could be related to the pollution gradient at contaminant concentrations below those at which univariate measures could detect any stress-response. For macrobenthos, taxonomic aggregation of the species data to family level resulted in little loss of information both in univariate and multivariate analyses but for the nematode component of the meiofauna aggregation from genus to family level resulted in a substantial loss of information.

Quantifying structural redundancy in ecological communities

Abstract In multivariate analyses of the effects of both natural and anthropogenic environmental variability on community composition, many species are interchangeable in the way that they characterise the samples, giving rise to the concept of structural redundancy in community composition. Here, we develop a method of quantifying the extent of this redundancy by extracting a series of subsets of species, the multivariate response pattern of each of which closely matches that for the whole community. Structural redundancy is then reflected in the number of such subsets, which we term “response units”, that can be extracted without replacement. We have applied this technique to the effects of the Amoco-Cadiz oil-spill on marine macrobenthos in the Bay of Morlaix, France, and to the natural interannual variability of macrobenthos at two stations off the coast of Northumberland, England. Structural redundancy is shown to be remarkably high, with the number and sizes of subsets being comparable in all three examples. Taxonomic/functional groupings of species within the differing response units change in abundance in the same way over time. The response units are shown to possess a wide taxonomic spread and, using two different types of randomisation test, demonstrated to have a taxonomically and functionally coherent structure. The level of structural redundancy may therefore be an indirect measure of the resilience or compensation potential within an assemblage.

Species size distributions in marine benthic communities

SummarySpecies body size distributions from eight temperate benthic communities show a highly conservative pattern with two separate lognormal distributions, corresponding to the traditional categories of meiofauna and macrofauna. The meiofaunal mode occurs at a dry body weight of 0.64 μg and the macrofaunal mode at 3.2 mg, with a trough between them at 45 μg. It is suggested that there is a particular body size at which meiofaunal life-history and feeding traits can be optimised, and another for macrofaunal traits. As size departs in either direction (larger or smaller) from these optima, fewer species of the same size are able to co-exist. The split occurs at 45 μg because many life history and feeding characteristics switch more or less abruptly at about this body size, compromise traits being either non-viable or disadvantageous. Meiofauna and macrofauna therefore comprise two separate evolutionary units each with an internally coherent set of biological characteristics.The expression of this conservative pattern is modified by water depth: the proportion of macrofauna species increases from intertidal situations to deeper water, and it is suggested that mechanisms of resource partitioning and diversity maintainence in the meiofauna and macrofauna are affected differentially by sediment disturbance. Salinity does not affect this proportionality, and so does not differentially affect mechanisms for maintaining species diversity in any particular size category of animals. Meiofauna species size distributions may be modified in sandy sediments because of physical impositions on interstitial or burrowing lifestyles.Brief discussion of some implications of these observations includes speculations on the larval ecology of macrofauna, on gigantism in Antarctic invertebrates, and on the benthic Sheldon spectrum.

Relearning the ABC: taxonomic changes and abundance/biomass relationships in disturbed benthic communities

For marine macrobenthic communities, a shift from higher biomass dominance with increasing levels of disturbance can be determined by the abundance/biomass comparison (ABC) method. This response results from (i) a shift in the proportions of different phyla present in communities, some phyla having larger-bodied species than others, and (ii) a shift in the relative distributions of abundance and biomass among species within the Annelida (specifically Polychaeta) but not within any of the other major phyla (Mollusca, Crustacea, Echinodermata). The shift within polychaetes reflects the substitution of largerbodied by smaller-bodied species, and not a change in the average size of individuals within a species. In most instances the phyletic changes reinforce the trend in species substitutions within the polychaetes, to produce the overall ABC response, but in some cases they may work against each other. Indications of pollution or disturbance detected by this method should be viewed with caution if the species responsible for the polluted configurations are not polychaetes. These observations provide an aid to interpretation of the ABC plots, especially in some situations where they have been deemed to give a false impression of the disturbance status of a community.

Macrofauna Production in an Estuarine Mud-Flat,

The importance of the extensive intertidal mud-flats to the energy budget of estuaries has frequently been stressed, but seldom quantified. Smidt (1951) has discussed the production of both macro- and meiofauna from the Danish Waddens in general terms but, because of the wide coverage, his information is not detailed enough to quantify annual production in terms of g/m2. The present study was designed to compare the productive importance of macrofauna and meiofauna in detail at a single site. This paper deals only with the first phase of this study, the macrofauna, which are retained on a 0.5 mm sieve for the major part of their productive life.