Variable effect of a large suspension-feeding bivalve on infauna: experimenting in a complex system

Abstract

In soft-sediment habitats there are many examples of species that modify their habitat and thus can be expected to have an important influence on macrobenthic community structure. The large, suspension-feeding pinnid bivalve Atrina zelandica adds complexity to soft-sediment habitats by protruding into the water column and altering boundary-flow conditions and by providing predation refuges and substrates for epifaunal settlement. To investigate effects of A. zelandica density on macrobenthic community composition, we conducted a density manipulation experiment in 4 different habitat types in and around Mahurangi Estuary, New Zealand. Our experiment incorporated a comparatively large spatial and temporal scale: each habitat was separated by at least 1.75 km, and was sampled 3 times over 16 mo. Based on previous work, we predicted that macrofaunal community responses would differ between sites and would be stronger at sandy sites than muddy sites, and that variability in site hydrodynamic and sediment characteristics would help explain differences in benthic community responses to the density manipulation. While these predictions were supported, there was considerable temporal variation in the response. We also made predictions of the response of different aggregate macrofaunal groups to the A. zelandica manipulation (i.e. total numbers of individuals and taxa, suspension feeders, deposit feeders, top 2 cm dwellers, and mobile, short and long-lived species). Whether these predictions were supported varied spatially as well as temporally. To be able to generalise results, larger scale experiments, conducted at more than 1 site and at more than 1 time, are generally considered preferable. Although our A. zelandica manipulation experiment has these attributes, the results have demonstrated that the influence of this large suspension feeder on the associated macrofaunal community is not simple (except perhaps in sandy, relatively non-tidal environments), and illustrates our limited success in 'reducing' the complexity of this system using a field experiment. However, we were able to demonstrate that interactions between A. zelandica, site hydrodynamic conditions and sediment characteristics were all important in influencing macrofauna, rather than there being a simple A. zelandica density-macrofauna relationship. Thus, where multi-species interactions, indirect effects, non-linear biotic/abiotic interactions and threshold effects play an important role, specific experiments may not always lead to generalisable results, simply because the system is too complex.

Integrating heterogeneity across spatial scales: interactions between Atrina zelandica and benthic macrofauna

“…This, together with the general agreement of results between this survey and Cummings et al (2001), and the provision of a mechanism for A. zelandica to influence macrofauna by Norkko et al (2001), suggests that it is unlikely that the observed relationships between the 2 are due to some unmeasured environmental factor.…”

“…A highly regionalised response of benthic macrofauna to A. zelandica was also observed in the companion A. zelandica-density-manipulation experiment carried out in areas of Mahurangi Harbour and Martins Bay , implying that interactions between A. zelandica and benthic macrofauna were mediated through local species pools. The results found by Cummings et al (2001) and Norkko et al (2001) suggest that inside-harbour relationships are unlikely to be weaker than outside due to increased variability as a result of increased sample size.…”

Impacts of two introduced suspension feeders in Port Phillip Bay, Australia

“…Whilst it would be inappropriate to make broad generalisations about the effects of Sabella on hard and soft substrates throughout Port Phillip Bay based on single site/time experiments (e.g. Ruiz et al 1999, Cummings et al 2001) the results of these 2 studies highlight the uncertainty of inferring an invader's impact on the basis of its density and a limited knowledge of its ecology. In terms of prioritising management efforts, quantitative assessment of real impacts is imperative to assess the risk of ecological damage from these species.…”

“…m -2 ) rather than low (7.5 ind. m -2 ) densities (Cummings et al 2001). Interestingly, O'Brien et al (2006) examined the effect of Sabella in localised dense patches (clumps of 10 or more ind.)…”

“…For instance, because they live and feed right at the sediment -water interface, they The predictions we made at the onset of this experiment were based on the results of previous studies on species capable of having effects on benthic assemblages via their biological activities, in this case, suspension feeding and/or their physical presence in the habitat (e.g. Woodin 1976, Hines et al 1989, Crooks 1998, Cummings et al 2001. While the results for Styela were not necessarily in line with our predictions, it was the lack of significant effects of Sabella spallanzanii on macrofauna that was surprising.…”