The role of marine mammal carrion in the ecology of coastal systems

Carrion represents an important energy resource in the natural world, yet its ecological significance has often been overlooked. It also plays a crucial role facilitating energy transfer across trophic levels and between ecosystems. The aim of this thesis was to evaluate the role of marine mammal carrion in the ecology of coastal systems, investigating one of the most commonly occurring sources of marine mammal carrion in the UK, the grey seal (Halichoerus grypus). This was addressed by providing first a detailed documentation of the fate of a grey seal pup carcass with insights on the scavenging ecology existing in both terrestrial and marine ecosystems. On the shore, great black-backed gulls (Larus marinus), juvenile gulls and ravens (Corvus corax) fed on the carcass showing a distinct temporal succession using the food resource. The underwater carcass was initially dominated by Echinodermata (starfish), whose abundance dropped lately, while Malacostraca (crabs) were present in similar number during the whole monitoring. Bacterial activity was evident in both experiments. Predictability of seal carrion was then defined during the pupping season at one of the largest colonies in the UK, the Isle of May (Scotland). Data collected by aerial survey (11 years) and ground visual census (3 years) were used to estimate the inter-annual variability of carrion, from placentae and dead seals, according to its timing, biomass and energy released and spatial distribution on the island. For all measures considered, variability was below 34%, similarly to other resources, such as salmon runs, which appear to be predicted by consumers. Twenty one percent of the total biomass from dead seals was consumed at the end of the pupping season suggesting a clear response from the scavenging community to the presence of the resource carrion. The predictable nature of seal carrion was then tested exploring the spatial and temporal distribution of scavenging gulls at three geographical scales (regional, local and patch scales). The great black-backed gull was affected by carrion availability occurring on the Isle of May, while the herring gull (Larus argentatus) was not. In particular, the number of adult and juvenile great black-backed gulls feeding was directly correlated with carrion abundance, while searching behaviour was greatest after the mean seal pupping date and at the peak in mortality. The behavioural dynamics of scavengers were finally explored monitoring the feeding activity on pup carcasses and placentae of adult and juvenile great black-backed gulls. It was predicted that under conditions of predictable and abundant carrion an equal scavenging effort would be found for the two age classes when consuming carcasses. Hierarchical dominance was, instead, expected during scavenging activity on placenta as it represents a preferred energy-rich food item. Temporal trends of scavenging activity and time spent feeding on carcasses were in fact similar between the two, while young individuals spent more time feeding on placenta, highlighting the importance of this food source for juvenile gulls during winter. The house mouse was also found to scavenge on seal carrion, which until now has been undocumented. This study demonstrates the importance of marine mammal carrion as a resource for multiple facultative scavenger species in both the marine and terrestrial environment