Quantifying effects of pollution on biodiversity: a case study of highly diverse molluscan assemblages in the Mediterranean

Abstract

Structured sampling designs are important in the assessment of environmental impacts of variable ecological systems. Recent developments have provided a useful framework extending existing univariate techniques into a multivariate context. Measures of taxonomic relatedness have also been introduced, which complement existing measures of diversity of assemblages. In this study, the potential effects of sewage discharge on spatial patterns of highly diverse molluscan assemblages in a Mediterranean rocky subtidal habitat were investigated. Nine 20 cm•20 cm quadrats were taken from each of three sites (80 m-100 m apart) within a putatively impacted location near a sewage outfall (I) and at each of two control locations (Cs) by destructive sampling by SCUBA divers at a depth of 3 m-4 m. A total of 5507 specimens of 151 species were collected. The average and the variance in total abundance of molluscs were greater, on average, at I than at Cs. Higher abundances at the sewage outfall were largely driven by greater numbers of juvenile molluscs. The Shannon diversity of molluscs (H¢) was significantly lower at I, but no difference among locations was detected for the total number of species (S). In addition, the taxonomic distinctness (D*) of molluscs was greater at Cs, although it was more variable at I. Multivariate analyses showed that there was a significant difference in the structure of assemblages at I compared with Cs. The location near the outfall was characterized by greater abundances of several species, including especially the gastropods Pusillina philippi, Bittium latreilli, and Bittium reticulatum. There was also greater variability in the structure of assemblages among sites and among quadrats at control locations compared to those near the outfall. Using a suite of univariate and multivariate measures, including diversity indices, detailed information on taxonomic structure and analyses of variability at different spatial scales, provided useful insights into the effects of sewage impacts on these diverse assemblages. These results also highlighted the importance of analysing measures of variance, as well as mean in considering effects of stress in natural communities.

Measures of taxonomic distinctness do not reliably assess anthropogenic impacts on intertidal mollusc communities

“…Indices of taxonomic distinctness have previously been found to be more sensitive to human disturbance than other diversity indices (Warwick & Clarke 1995, Terlizzi et al 2005, Roberts & Connell 2008, but this was not supported in our study. Although none of the indices tested using 1-way ANOVAs discriminated significant differences between control and impacted groups, the behaviour of taxonomic distinctness indices in response to enrichment by sewage suggests that they are in fact less sensitive than conventional indices.…”

“…They found that phylogenetic diversity was sensitive to the experimental manipulation of both factors, whereas Shannon diversity was not. In the same way, Terlizzi et al (2005) found significant differences in AvTD values between control and impact locations for subtidal mollusc communities around a sewage outfall, but no significant difference in the mean number of species. Nevertheless, in other habitats (freshwater streams), taxonomic distinctness measures have been found unsuitable for detecting disturbance.…”

“…The master list in this study was comprised of 87 species distrib-uted among 16 orders. In contrast, the nematode list of consisted of 395 species distributed among 4 orders, and the molluscan list of Terlizzi et al (2005) consisted of 151 species from 19 orders. In a study describing the effects of disturbance on insect communities from freshwater rivers, Marchant (2007) proposed that lack of sensitivity in AvTD was a result of too little variation at mid-high taxonomic levels.…”

Effects of sewage pollution on the structure of rocky shore macroinvertebrate assemblages

“…Polyplacophora, like most gastropod species, seem to prefer non-polluted habitats. Earlier studies (Airoldi, 2003;Terlizzi et al, 2005a) have advanced the possible explanation that the increase of suspended solids near the sewage discharges could change the sedimentation rates, which may have negative effects on gastropods. Atalah & Crowe (2012) have also found consistent differences in the assemblages of molluscs related with nutrient enrichment, which suggests that those assemblages can be potential indicators of pollution in coastal areas.…”

“…Nevertheless there are several studies focusing on intertidal populations of polychaetes (Dauer & Conner, 1980;Elías et al, 2006;Jaubet et al, 2011), molluscs (Bishop et al, 2002;Terlizzi et al, 2005a;Vallarino and Elías, 2006;Atalah & Crowe, 2012) or crustaceans (Calcagno et al, 1998;De-la-Ossa-Carretero et al, 2010). But studies dealing with the effect on the entire intertidal benthic community are very rare (Littler & Murray, 1975;López-Gappa et al, 1990;Archambault et al, 2001, Klein & Zhai, 2002.…”