Researchers often encounter taxonomic problems when analysing data on changes in macrobenthic assemblages due to pollutlon. In response, some authors have suggested that data at famlly and even phylum level may be sufficient to detect such changes. This hypothesis is commonly tested by visual comparisons of ordinabon plots generated by multidimensional scaling (MDS). Our study assessed this approach using a dataset previously shown to contain patterns in species composition that were related to both habitat type and heavy metal pollution. These data allow us to assess the ability of analyses at reduced taxonomic resolutions to detect 'signals' associated with pollution gradients among the 'noise' associated with habitat gradients. Such situations will arise in most studies of the impacts of pollutlon on inshore macrobenthic assemblages. The patterns associated with both habitat and pollution gradients were visible in ordlnations based on data pooled to hlgher taxonomic levels. The similarity among ordinations based on different sets of pooled data was not fully supported by more detailed analyses. Changing the number of dimensions in ordinations, the taxonomic resolution, the attribute:object ratio and the non-zeros ratio of datasets all affected the matrlx of association measures and the results of ordinations and Procrustes rotations. The effects of these changes were complex. Further work is needed to determine the likely effect of using data with reduced taxonomic resolution affect to detect impacts.
Herbivory is a key ecological process that often determines the composition and abundance of plants. Estimates of herbivory in seagrass meadows are typically lower than those in other vegetated coastal ecosystems, but herbivory can be intense when large herbivorous vertebrates are abundant. We surveyed rates of herbivory on 2 species of tropical seagrasses (Thalassia hemprichii and Enhalus acoroides), the abundance of herbivorous vertebrates, and the diet of 2 abundant herbivorous vertebrates (the green turtle Chelonia mydas and the rabbitfish Siganus lineatus) in lagoons adjacent to remote islands off northwestern Australia. Rates of herbivory in some deployments of tethered seagrass were more than 1000 times higher than rates of production and were among the highest recorded. Consumption exceeded production in half the deployments (9 of 18). Remote underwater video revealed that S. lineatus was the most abundant herbivore. Stomachs of S. lineatus contained mostly seagrass, and models based on stable isotopes indicated that seagrass was the primary source of nutrition. Stomach contents of C. mydas were more variable, containing seagrass and macroalgae (although the sample size was low), but models based on stable isotopes indicated that seagrass was likely the primary source of nutrition. Multiple lines of evidence suggest that the high rates of herbivory on the seagrasses T. hemprichii and E. acoroides are mainly due to direct consumption by the abundant S. lineatus, and perhaps also C. mydas. Seagrass is the primary contributor to the nutrition of both species.
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