The roles of the cockle Cerastoderma edule L. on ecosystem functioning: cockle comings and goings

摘要

There is increasing interest regarding the impacts of human activities on the functioning of marine systems. A primary driver of change to marine systems is through the impacts of fishing. Biomass-dominant target species have the potential to mediate a number of ecosystem functions, either directly or indirectly, through the influences that taxa have on ecological processes and/or other biotic or abiotic components of the system. This thesis investigates the roles of the cockle, Cerastoderma edule on ecosystem functioning within intertidal sedimentary systems. A series of investigations revealed that cockles have the potential to mediate benthic primary productivity through their roles in the recycling of nutrients and effects of sediment structure and have impacts upon assemblage biomass and functional diversity. However, the roles of cockles on other aspects of ecological functioning were less apparent. An investigation also assessed the suitability of the use of assigning taxa to functional groups when assessing functional diversity. Taxa were shown to have the potential to change their feeding activity following disturbance, with evidence suggesting a change to benthic-pelagic coupling. This change however, would not have been observed with investigations of functional traits alone and thus supports the use of direct measures of functions to support functional trait diversity measures. It is imperative for ecological investigations to consider long-term changes to population dynamics. However, particularly in marine systems, such data are generally lacking. This thesis presents a novel approach, using information from sea fisheries reports to gain a semi-quantitative 30-year data set on cockle landings within Morecambe Bay, north-west England. This technique revealed evidence of temporal changes to the functional processes at a large scale. This thesis provides evidence for cockles acting as key contributors to ecological functioning. The functional role of cockles is tempered by the high degree of redundancy within these assemblages, highlighting a number of issues relating to the use of field studies and encouraging a move towards increased use of traditional ecology in future studies.