The introduction of the Manila clam into British coastal waters in the 1980s was contested by conservation agencies. While recognizing the value of the clam for aquaculture, the government decided that it posed no invasive risk, as British sea temperatures would prevent naturalization. This proved incorrect. Here we establish the pattern of introduction and spread of the species over the first 30 years of its presence in Britain. We report archival research on the sequence of licensed introductions and examine their relationship in time and space to the appearance of wild populations as revealed in the literature and by field surveys. By 2010 the species had naturalized in at least 11 estuaries in southern England. These included estuaries with no history of licensed introduction. In these cases activities such as storage of catch before market or deliberate unlicensed introduction represent the probable mechanisms of dispersal. In any event naturalization is not an inevitable consequence of introduction and the chances of establishment over the period in question were finely balanced. Consequently in Britain the species is not currently aggressively invasive and appears not to present significant risk to indigenous diversity or ecosystem function. However it is likely to gradually continue its spread should sea surface temperatures rise as predicted.
Comprehensive and transparent protocols for calculating Minimum Number of Individuals (MNI) for archaeological faunal assemblages are critical to data quality, comparability, and replicability. MNI values for archaeological molluscan assemblages are routinely calculated by counting a select range of Non-Repetitive Elements (NREs). Most commonly, only the frequency of the spire of gastropods and the umbo or hinge of bivalves are recorded. Calculating MNI based only on the frequency of these NREs can underestimate the relative abundance of particular molluscan shell forms. Using archaeological mollusc assemblages from two sites in the Marshall Islands as a case study, we outline a new protocol (tMNI) that incorporates a wider range of NRE and calculates MNI based on the most frequently occurring NRE for each taxon. The principles that underlie the tMNI method can be modified to be regionally or assemblage specific, rather than being a universally applicable range of NRE for the calculation of MNI. For the Marshall Islands assemblages, the inclusion of additional NRE in quantification measures led to (1) a 167% increase in relative abundance of gastropods and 3% increase in bivalves (2) changes to rank order abundance, and (3) alterations to measures of taxonomic richness and evenness. Given these results for the Marshall Islands assemblages, tMNI provides more accurate taxonomic abundance measures for these and other archaeological molluscan assemblages with similar taxa. These results have implications for the quality of zooarchaeological data increasingly utilised by conservation biologists, historical ecologists and policy makers.
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