Dave Raffaelli

Consistent patterns and the idiosyncratic effects of biodiversity in marine ecosystems

Revealing the consequences of species extinctions for ecosystem function has been a chief research goal and has been accompanied by enthusiastic debate. Studies carried out predominantly in terrestrial grassland and soil ecosystems have demonstrated that as the number of species in assembled communities increases, so too do certain ecosystem processes, such as productivity, whereas others such as decomposition can remain unaffected. Diversity can influence aspects of ecosystem function, but questions remain as to how generic the patterns observed are, and whether they are the product of diversity, as such, or of the functional roles and traits that characterize species in ecological systems. Here we demonstrate variable diversity effects for species representative of marine coastal systems at both global and regional scales. We provide evidence for an increase in complementary resource use as diversity increases and show strong evidence for diversity effects in naturally assembled communities at a regional scale. The variability among individual species responses is consistent with a positive but idiosyncratic pattern of ecosystem function with increased diversity.

Sediment toxicity testing: a bioassay approach using the amphipod Corophium volutator and the polychaete Arenicola marina

In this study, the amphipod Corophium volutator and the polychaete Arenicola marina were evaluated as test organisms for use in sediment toxicity bioassays by adapting standard protocols developed by the US Environmental Protection Agency (EPA) and US Army Corps of Engineers (COE) (1991) and Thain et al. (1994). Although these species have begun to be used for the assessment of the toxicity of marine and estuarine sediments, the detailed ecotoxicologies of these species are not well documented. In particular, the effects of specific contaminants of known concentrations on these species in this bioassay context are not known. Here, we report experiments carried out on both species using clean intertidal sediment contaminated with copper, zinc and cadmium. The results show clear dose-dependant effects of each metal on the survival and behaviour of both species, demonstrating the bioassay has considerable potential. However, the toxicity of copper was quite different for the two species and it is recommended that several taxa should be employed in such bioassays.

Interactions between the amphipod Corophium Volutator and macroalgal mats on estuarine mudflats

One of the most obvious effects of eutrophication in sheltered coastal areas and estuaries is enhanced growth of opportunistic macroalgae, which may form extensive mats over intertidal mudflats during the spring and summer. In the Ythan estuary, densities of the amphipod Corophium volutator (Pallas) in the sediment underlying weed mats were significantly lower than those in weed-free sediments, and are dominated by species characteristic of organically enriched, low oxygen environments such as Capitella capitata. Long-term data sets on Corophium abundance in the Ythan suggest that this species has declined dramatically throughout those parts of the estuary affected by weed mats.

Effects of macroalgal mats on intertidal sandflats: an experimental study.

The growth of green macroalgal mats is becoming increasingly common in many marine intertidal habitats. While the ecological effects of such growth has previously been experimentally investigated on mudflats, such experiments have rarely been performed on intertidal sandflats. This study investigated the ecological effects of macroalgal cover on a moderately exposed intertidal sandflat, Drum Sands, Firth of Forth, Scotland. Artificially implanted Enteromorpha prolifera (Müller) caused marked changes in the macrobenthos, together with significant changes in all the measured sediment variables. After 6 weeks, the weed significantly increased the macrofaunal diversity. The numbers of Pygospio elegans (Claparède) were significantly reduced under weed mats, while those of Capitella capitata (Fabricius), oligochaetes and gammarids increased. Percent water, organics and silt/clay contents, medium phi and sorting coefficient significantly increased in the sediments under weed mats which also became significantly more reduced between 1 and 8 cm depth. After 20 weeks, a macrofaunal community numerically dominated by C. capitata, with a significantly reduced diversity, was present under weed mats, while sediment variables were no longer significantly different from controls. The negative effect of E. prolifera on P. elegans was mainly due to larval filtering, suggesting that weed is likely to have detrimental effects on population maintenance of most species which rely on planktonic larval recruitment. These results are broadly similar to those obtained from algal manipulation experiments performed in much more sheltered, muddier environments. We suggest that a predictable deterioration in environmental quality results from the growth of macroalgal mats in soft-bottom habitats. However, the longer term effects of such algal growth are less predictable and depend upon the spatial distributions of the most abundant infaunal species and the spatial heterogeneity of weed mat establishment.

Constraints on body size distributions: an experimental approach using a small-scale system

Abstractu2002Holling’s (1992) proposition that discontinuities in biotic and abiotic processes generate structure in ecological systems is examined experimentally by imposing size-specific perturbations on marine sediment assemblages. Two kinds of perturbations were applied: organic enrichment and predation, each at two levels. Perturbations significantly affected the densities and relative abundance of the main invertebrate taxa and these effects were consistent with the known effects of enrichment and predation. However, there was little evidence of significant treatment effects on the overall benthic biomass or abundance size spectrum, supporting the contention that the spectrum is conservative and is probably constrained by habitat architecture.

Nutrient enrichment and trophic organisation in an estuarine food web

Abstract This paper reviews several long-term (30xa0years) data sets relevant to eutrophication in the Ythan estuary, Aberdeenshire, Scotland. These data sets are land-use in the catchment, nutrients in the river and estuary, macro-algal cover and biomass, mudflat invertebrate abundance and shorebird counts and distributions. The links between the observed patterns of change in these parameters are explored and the evidence for causal relationships assessed, especially in relation to experimental tests of potentially competing hypotheses. A likely scenario is proposed involving shifts in agriculture towards more nitrogen-demanding crops and a higher rate of application of nitrogen to the land; a consequent increase in nitrogen levels in the river and the estuary associated with an increase in the biomass and distribution of macro-algal mats; reductions in invertebrate densities (especially Corophium volutator ) in the worst-affected areas of the estuary and increases in abundance in the upper reaches; an initial increase in the shorebird populations followed by a decline and a shift in shorebird distributions towards areas less affected by macro-algal mats. Important ecological processes for which data are limited or our understanding is poor are identified and the need for rigorous testing of hypotheses is highlighted.

The behaviour of the nematode/copepod ratio in organic pollution studies

Abstract This paper reviews the literature on surveys of meiofaunal abundance along organic pollution gradients and experimental mesocosm studies on meiofaunal communities in an attempt to account for the variable behaviour of the nematode-copepod (Ne/Co) ratio. It is concluded that differences in the habitat requirements of nematodes, mesobenthic and epi/endobenthic copepods affect the responses of these groups to organic pollution. The response is gradient dependent for all groups, but probably not monotonic, at least for nematodes. Examination of different sections of the pollution gradient by different authors has therefore produced quite different results. If these factors are taken into account the reported behaviour of the Ne/Co ratio is readily understood.

In situ swimming behaviour of the amphipod Corophium volutator (Pallas)

Abstract An important factor structuring and maintaining spatial heterogeneity is mobility. The intertidal, burrowing amphipod Corophium volutator , can only move rapidly over large distances (10s of m–km) by swimming. An investigation into the swimming behaviour of Corophium on the Ythan estuary, Aberdeenshire, using emergence, settlement and plankton traps indicated that on most high tides very few animals emerge, swim or settle, and that these individuals are mostly adult males. Such swimming is probably related to reproduction, and with such low frequency will have little impact on spatial heterogeneity. However, periodically during the progression from neap to spring tides high numbers of juveniles swim on nocturnal high tides. This is reflected by higher, juvenile dominated, settlement after these periods. This swimming is probably related to dispersal and the survival of juveniles after dispersal will determine the subsequent patterns of heterogeneity at large (km), and potentially also small (m–cm), scales.

The ecological importance of the sand goby Pomatoschistus minutus (Pallas)

The sand goby Pomatoschistus minutus (Pallas) occurs seasonally in high densities in shallow water and intertidal sediments, preying extensively on small macroinvertebrates such as Corophium volutator (Pallas). P. Minutus densities were experimentally manipulated inside cages maintained on intertidal flats in the Ythan estuary, Aberdeenshire, U.K., to test the hypothesis that these fish might have a significant impact on their prey populations and invertebrate-community structure. After 4 wk, during which time goby mortality was negligible and fish appeared to feed normally, no effects of predation could be detected on the densities of the most abundant prey or in the size-frequency structure of the main prey C. volutator, even at artificially high fish densities. It is concluded that P. minutus alone is not capable of regulating prey numbers in the habitat studied although the combined impact of the estuarys diverse predator assemblage might be considerable.

Trends in research on shallow water food webs

Making sense of the spider-web networks of interactions between species in food webs has been a major pre-occupation of ecologists over the last 60 years. This review describes the early attempts to reduce this complexity through the grouping of individual taxa into functional categories (such as trophic levels), through adopting the energy flow or systems approach as epitomised by the International Biological Programme, and most recently by the derivation of web statistics by food web theorists. The strengths and weaknesses of these approaches are discussed in relation to empirical field experiments for unravelling the processes responsible for organising communities and an assessment made of the representation of these approaches in the marine biological literature.