Abundance Of Common Taxa Research Articles

Assessing the consequences of sea level rise: effects of changes in the slope of the substratum on sessile assemblages of rocky seashores

Sea level rise can affect the biodiversity of coastal areas through alterations of features of the substratum, including the geometry of shorelines. Field measurements on the rocky shore of Calafuria (NW Mediterranean Sea) showed that a rise in sea level in the range of 5 to 50 cm would increase the availability of steep substrata (>40°) for assemblages of algae and invertebrates by up to 58% compared to current conditions. Comparisons between assemblages on horizontal and vertical substrata revealed clear differences in the composition and abundance of common taxa. Clearing experiments replicated on vertical and gently sloping surfaces at 2 heights on the shore were per- formed to test hypotheses about the role of recruitment and post-recruitment processes in maintain- ing differences between substrata. Comparisons of recolonizing assemblages with unmanipulated controls allowed us to test the hypothesis that a change in slope of the substratum affected the abil- ity of assemblages to recover after a disturbance. Results indicated that differences in the structure of assemblages between vertical and horizontal substrata were maintained by a combination of recruit- ment and post-recruitment processes, although the relative importance of these processes differed among taxa. Nonetheless, patterns of recovery of assemblages were comparable between the 2 sub- strata. These findings suggested that sea level rise, by increasing the proportion of vertical substrata at the expense of horizontal surfaces, will lead to the expansion of assemblages dominated by encrusting coralline algae and grazing gastropods, and the reduction of abundance of filamentous forms and barnacles. The resilience of assemblages, however, appeared unaffected by the aspect of the substratum.

Seasonal patterns of benthic arthropods in a subtropical Andean basin

We studied the seasonal variation of arthropod assemblages and the response to high discharge events in a subtropical Andean basin. Using abundance of common taxa and taxon richness of each sampled site, we examined the temporal variation of arthropods and related these changes with 12 environmental variables. Seasonal patterns of benthic arthropods were confirmed by uni- and multivariate techniques. Benthic abundance and taxon richness peaked on autumn-winter months, and declined abruptly on the month of peak discharge (summer season). Distinct taxonomic groups were dominant in each season influenced by discharge and seven chemical variables. Assemblage resistance to disturbance by spates was low. Rapid recovery of benthic arthropods after floods reflected high resilience of the system. The dominance of groups of short life cycles such as Ephemeroptera played an important role in this process.

Short-term colonisation by macroinvertebrates of cobbles in main channel and inundated stream bank habitats

In small, forested catchments, short rainfall events cause relatively rapid increases in river discharge that inundate previously dry streamside habitats and last for several days. We examined the colonisation and use of such newly inundated habitats by macroinvertebrates in the riffles of a cobble-dominated river over a 4-day period. First we simulated inundation by adding dry river rocks to the main channel of the stream. Second, we observed colonisation of streamside rocks that were inundated following rainfall and subsequent natural spate. Macroinvertebrates colonised rocks in both habitats within 24 h, yet overall macroinvertebrate abundance did not change significantly over the subsequent 3 days. The abundances of common taxa were similar on mid-channel and stream bank rocks but each habitat contained a unique suite of uncommon taxa. Taxon richness was significantly greater on the stream bank rocks, which we attribute to the non-selective colonisation of fauna to avoid high flows in the main channel during the spate. Our study shows that fauna use the stream bank habitat as a haven from high flows in the main river channel, but the fate of the animals colonising this habitat is uncertain.

A field‐based microcosm method to assess the effects of polluted urban stream sediments on aquatic macroinvertebrates

A method using field-based microcosms was developed to determine the effects of contaminated sediments on aquatic macroinvertebrates. Fine sediments from nonpolluted, moderately polluted, and severely polluted bodies of water were placed in microcosms positioned within the littoral zone of a nonpolluted wetland near Melbourne (Victoria, Australia). In three experiments, 47 taxa, including 18 Chironomidae, 6 taxa from other Diptera families, and 7 Hemiptera taxa, colonized the microcosms, mostly via eggs deposited by flying adults. The effects of sediment type on the presence and abundance of common taxa were considered statistically. Pollution levels in sediments (indexed either by a principal components analysis or by the concentration of zinc, the predominant metal) resulted in reduced occurrence and abundance of eight taxa but had no effect on another five taxa. These findings were validated with an extensive field database for the distribution of macroinvertebrates and associated concentrations of zinc in sediments from streams and wetlands in the Melbourne region. The occurrence of eight taxa and the abundance of two taxa varied at similar zinc concentrations in sediments from both the microcosms and the field. Patterns for another two species did not match the microcosm results, but these groups contained multiple species with potentially diverse responses. The present results suggest that contaminant levels in sediments probably have a direct effect on the occurrence and abundance of macroinvertebrates in bodies of water in urban areas. The microcosm method can be used to gather information regarding the effects of sediment quality on macroinvertebrates in lentic habitats, particularly for indigenous species that cannot be easily reared or tested in laboratory conditions. Because almost all macroinvertebrates in microcosms develop from eggs, the most sensitive life stages (i.e., first and second instars) are exposed to polluted sediments.

Hyporheic macroinvertebrates in riffle and pool areas of temporary streams in south eastern Australia

The hyporheic zone is an important refuge for invertebrates as surface water recedes in temporary streams. In this study, the structure and functional organisation of hyporheic macroinvertebrate assemblages in pool and dry riffle bed habitats of two episodic streams were compared over summer and winter. Multivariate analyses revealed macroinvertebrate assemblages differed significantly between streams, habitats and seasons. While some seasonal differences were expected, the differences between streams were not, given the similarity and proximity of the catchments, and were due to shifts in the abundance of common taxa. Distinct differences between riffle and pool habitats were evident in both the taxonomic and functional feeding group composition of the assemblages. In particular, riffle habitats contained greater numbers of taxa and individuals and a greater proportion of filter-feeding animals compared to pool habitats. Summer samples also had greater numbers of taxa and individuals and greater proportions of collector-scrapers than winter samples. The relative abundance of functional groups was similar between streams in summer but was more variable in winter. Patterns observed in the taxonomic and functional feeding group structure of the macroinvertebrate assemblages were more characteristic of perennial than episodic streams, despite the absence of regular surface flows. This could be attributed to the relatively constant hyporheic flow in these streams. We suggest that classifications of stream flow should consider hyporheic discharge (not just surface flow) as this clearly influences the stream biota.

Managerial decisions as experiments: an opportunity to determine the ecological impact of boat-generated waves on macrobenthic infauna

A previous correlative study showed that assemblages of macrobenthic infauna and abundances of common taxa on intertidal mudflats differed between a no-wash zone, where ferries had to minimize production of wash, and places where the production of wash was unrestricted (wash zone). This suggested that boat-generated waves (wash) are important in determining the structure of these assemblages. Causality between wash and the observed patterns could not, however, be unambiguously established, due to the absence of ‘before’ data. Here, a managerial decision to stop ferry services on the upper Parramatta River, Sydney, Australia during the 2000 Olympics was used as the basis for a manipulative experiment to examine the effects of changes in the amount of wash on these fauna. It was hypothesized that if wash is important in structuring infaunal assemblages, assemblages in the wash zone would become more similar to those of the no-wash zone following removal of the disturbing force, i.e. the ferry service. Similarly, if the smaller abundances of capitellids, nereids and spionids in the wash zone are caused by this disturbance, abundances should increase in the wash zone during the stoppage and decrease following the return of services. As hypothesized, assemblages within the wash zone became more similar to those of the no-wash zone following the temporary removal of the ferry services. Following the return of ferries, assemblages changed back towards (although not reaching) their previous state. Abundances of the polychaete families Nereididae, Capitellidae and Spionidae also increased at some sites during the cessation, although this pattern was not found in all sites and there was no general response to the cessation of wash as had been predicted. These results indicate that wash is important in structuring assemblages of macrobenthic infauna, although responses of individual taxa are more idiosyncratic. More importantly, they show that manipulations, resulting from managerial decisions, can be treated as testable hypotheses and utilized by scientists as experiments to test for causal relationships.

Fish Trophic Guilds and the Stucture of a Tropical Stream: Weak Direct vs. Strong Indirect Effects

Freshwater fishes exhibit spectacular biodiversity in the tropics. In contrast to temperate streams, where insectivorous fishes generally predominate, detrital— and algalfeeding fishes are widespread components of tropical ichthyofaunas. I compared the direct and indirect effects of a variety of fishes in structuring a neotropical stream insect assemblage. An exclosure/enclosure experiment was preformed to examine the role of different fish trophic guilds and densities on patterns of patch colonization by invertebrates. Insectivorous characid fishes had relatively weak impacts on overall invertebrate abundance, although significant reductions were observed for selected taxa (the mayflies Baetis sp. 2 and Leptohyphes, and the caddisflies Smicridea and Alisotrichia). This contrasted sharply with the strong effects of the grazing armored catfish Chaetostoma milesi and the detritivore Prochilodus mariae, which caused significant reductions in abundance of common taxa throughout the invertebrate assemblage. In addition, grazers and detritivores substantially reduced the amount of sediments and associated detritus accumulated on stream bottom substrata. A second experiment was preformed to evaluate the mechanism by which grazing and detritivorous fishes influence insect abundance. Insect abundance was compared between a treatment where resource depression and bioturbation was simulated by removing sediments manually to fish exclosure/enclosure treatments. As in the previous experiment, grazers and detritivores significantly reduced the abundance of many common invertebrate taxa, as well as detrital and algal resources. However, taxon—specific differences in colonization of the sediment—removal treatment suggest that several factors are important in explaining responses to fishes by insects. Many common taxa displayed decreases in abundance both in sediment—removal and fish treatments, indicating that sediment—processing activities by epibenthic fishes are important in explaining patch colonization for many members of the invertebrate assemblage. However, sediment depletion is not sufficient to explain the response of the abundant invertebrate, Baetis sp. 1, which showed substantial reductions in cages containing fishes (open and grazer enclosures), but high densities in both the fish—exclusion and the sediment—removal treatments. These data suggest that intimidation by fish, as measured by the presence of grazers, results in low rates of patch use by this numerically dominant mayfly. These findings contrast with studies reporting relatively weak effects of fishes in temperate zone streams. Fishes may be strong interactors in some neotropical steams, not through direct lethal consumption of prey, but rather by modifying the distribution and abundance of resources important to stream insects. In some instances the simple intimidation associate with the presence of a diverse and abundant fauna of bottom—feeding fishes also may influence insect abundance and distribution.