Seabird mediated link between marine and terrestrial ecosystems

Abstract

The meta-ecosystem concept describes spatial coupling of ecosystems through flows of energy material and organisms across ecosystem boundaries. Mobile species can act as biotic vectors to facilitate the movement of these items between ecosystems and can be essential to maintaining the meta-ecosystem. Global biodiversity and species movement are under increasing pressures of anthropogenic activity. A greater understanding of how mobile species influence the functioning of the meta-ecosystem will provide a better insight to how changes in one ecosystem may influence coupled ecosystems across landscapes. Throughout this thesis I use the marine-terrestrial meta-ecosystem connected by the seabird great skua as a model system to investigate this. I use a combination of observational and manipulation experiments to determine the effects on plant nutrient assimilation, soil processes, plant community assemblage and dynamics and invertebrate community assemblage. I show that great skuas transport large quantities of essential plant nutrients into the terrestrial ecosystem through faeces and pellets. This feeds into plant nutrition regardless of species and life history traits and potentially reduces the reliance of heather and tormentil on mycorrhizae symbionts for nutrient acquisition. This nutrient deposition has little influence over the functioning of the soil biota: the increase in soil nutrient pools varied, mineralization was unaffected by great skua abundance and the rate of decomposition and nutrient release from litter was largely determined by the litter quality rather than where the litter was buried. This also indicates that plants are accessing nutrients rapidly and directly from faeces through abiotic pathways and the excess is lost through surface run-off or leached through the soil. Above ground I demonstrated that this increased nutrient availability and disturbance both play a role in rapidly shifting the plant community from dwarf shrub and tall acid grassland communities to short, ruderal, graminoid dominated communities. In addition, I demonstrate that in areas of elevated great skua abundance invertebrate communities are characterised by a greater abundance of detritivorous orders, a greater abundance of small predatory ground beetles and a lower abundance of predatory orders such as spiders and large immobile ground beetle relative to areas of low great skua abundance. I use joint species modelling to demonstrate that increases in great skua disturbance is associated with lower abundance of less mobile species, whist reductions in vegetation structure are associated with lower abundance or large mobile species, but greater abundance of small fast bodied species. Interestingly my work highlighted a temporal coupling of the marine and terrestrial ecosystem. Exposing plant communities to a lower abundance of great skuas caused a decline in leaf nitrogen concentration across all species, but phosphorus concentration did not change and increased in some species. In addition, plant communities did not change substantially over three years. This work demonstrates how great skuas influence the key processes and functions in the terrestrial ecosystem. This is particularly relevant in light of recent changes to fisheries management in the EU, which is predicted to influence scavenging seabirds such as great skuas and negatively impact their ability to function as link species between the marine and terrestrial ecosystems. My results suggest that this will cause a reduced availability of inorganic nutrients in the soil. The plant community composition is will be slow to respond to this, and grass and ruderal species will remain dominant. Whilst there will be corresponding declines in nitrogen cycling in the terrestrial ecosystem, it is anticipated that phosphorus cycling will remain high. In addition, is predicted that the abundance of invertebrate species that were strongly influenced by direct disturbance will likely increase. However, those that are more influenced by vegetation structure such as spiders only recolonise once plant community composition recovers.