Multiple roles of brachyuran crabs in wetlands : their importance for bioturbation and central role in the food web

Abstract

Ecologically functional and resilient vegetated wetlands are crucial to cope with increased human impacts which accompany urbanization. Literature describes that species-rich ecosystems hold a higher functional redundancy where species that fulfil the same functional roles make the ecosystem more resilient. Within intertidal vegetated wetlands, like mangroves and salt marshes, brachyuran crabs are found in high numbers and fulfil multiple functional roles. Their bioturbation and feeding activities, for example, can have ecosystem wide effects. Bioturbation including burrow construction and maintenance can indirectly influence mangrove plant growth and increase CO2 release while their feeding activities contribute to nutrient dynamics and their place in the food web can contribute to trophic redundancy. Bioturbation was investigated by assessing the burrow morphology of different brachyuran crabs in Hong Kong SAR mangroves using 3D scanning techniques. Burrow surface area and volume together with burrow densities were used to quantify bioturbation potential, i.e. the potential of a species to bioturbate its environment. Next, the CO2 flux of burrows created by a salt marsh crab within Cape Cod, Massachusetts, US, was investigated across different burrow densities, organic matter content and marsh grass presence. Further, trophic redundancy of macrobenthic communities, crabs and gastropods, across a nitrogen gradient within Hong Kong mangroves was examined using trophic position estimations based on stable isotope analysis. One required factor to calculate the trophic position is the trophic discrimination factor. This factor quantifies a predictable shift in isotopic composition between consumer and food source. However, previous research has questioned the use of global factors and argue for the use of species-specific factors. Therefore, the applicability of global and species-specific factors to calculate the trophic positions of macrobenthic feeding guilds was investigated. Results showed a larger bioturbation potential for species that created more complex and larger burrows. This implied that species with a larger bioturbation potential have a different impact on ground water flow and soil oxygenation known to impact mangrove trees nutrient provision. These results illustrated different functional roles concerning bioturbation activities of brachyuran crabs. Within a salt marsh, crab burrows increased the CO2 release from the soil, which firstly illustrated another indirect effect of bioturbation, and secondly led to implications for established methods of carbon budget assessments where the presence of crab burrows is overlooked. Furthermore, species-specific trophic discrimination factors were recommended over global factors to calculate trophic positions of benthic macrofauna. However, results showed that a combination of different species-specific trophic discrimination factors should be applied for different benthic macrofaunal feeding guilds. Lastly, nutrient pollution across Hong Kong did not seem to affect the trophic redundancy of the macrobenthic faunal community. In fact, microhabitat availability and connectivity may play a more important role.. The yet unknown effects of the ever-increasing urbanization on the seemingly resilient Hong Kong mangroves, heed a warning and calls for continued conservation action. Overall these results highlight the important multifunctional roles of brachyuran crabs simply by their day-today burrowing and feeding behavior.