Habitat Selection And Ecosystem Engineering Effects Of Dominant Burrowing Crabs In Salt Marshes In The Yangtze River Estuary, China

Coastal salt marshes are one of the most productive ecosystems but are fragile and sensitive to environmental changes. The salt marshes have a great diversity of burrowing crabs. As physical ecosystem engineers, crabs play important roles in controlling material cycles and energy flows in salt marsh ecosystems. Through a series of experimental methods, including field investigations, field controlled experiments and laboratory experiments, this study aimed to evaluate vertical movements of crabs' burrowing processes on soil, carbon and nitrogen in salt marsh, and analyze the effects of exotic plant invasions on crab distributions, burrow morphologies and ecological functions. The major findings are summarized as follows:Abundance and biomass of Sesarma dehaani in Spartina marsh were significantly greater than those in Phragmites marsh and mudflats. Soil water content and plant community characteristics of Spartina marsh also significantly differed from those of Phragmites marsh and mudflats. Moreover, analysis of feeding preference showed that S. dehaani consumed Spartina more than twice as much Phragmites. Therefore, Spartina provided compatible habitats for native crab S. dehaani through offering suitable food source and moderate environmental conditions.Plant litter had significant effects on some of crab community parameters. Spartina litter affected male S. dehaani biomass, and Spartina marsh with removal of litter had the highest crab density. In spring, body weight of female Helice tridens tientsinensis was the lowest in Spartina marsh with medium litter density; and it was lower in Spartina marsh with standing litter than with lodging litter. However, plant litter had no effects body weight of females in other seasons. Except for sex ratio, population parameters of H. tientsinensis were not significantly different between P. australis and S. alterniflora marshes, but those of S. dehaani had great defference between the marshes.Sediment properties showed a gradual transition from hydrophytic to terrestrial environments on creek sections. On the slope, the burrows were smaller in size and burrow density was higher, while at the edge and on the flat, the burrows were larger in size, and the burrow density was lower. The density of small crab burrows was greater, but the density of large burrows and total burrow opening area were lower in tidal creeks than in non-creek habitats. The environmental heterogeneities on creek sections met the requirements of different crab species, and hence the creeks of tidal salt marshes provided important ecotones for crabs.Plant communities significantly changed crab burrow morphologies through changing root distribution pattern, soil properties and other environmental conditions during the course of plant growth, plant invasion and plant colonization of mudflats in salt marshes. Burrows in Phragmites and Spartina marshes had higher branch number and sinuosity degree, but burrow depth, and volume compared to mudflats. Burrows in Spartina marshes had lower burrow depth and burrow volume, but higher opening diameter, sinuosity degree compared to Phragmites marshes. Burrows in young Spartina marshes had higher burrow depth and volume, but lower sinuosity degree than those in old marshes.Crab burrowing increased soil water content, carbon and nitrogen and decreased soil bulk density; and the turnover of soil and nutrients by crabs increased with burrow size. Plants (Phragmites and Spartina) significantly enhanced soil nutrient concentrations, but obstructed the vertical movement of materials through crab burrowing, limiting nutrient cycling and reuse. The burrowing behavior of crabs led to a net transport of soil to the surface. Burrowing crabs are clearly important ecosystem engineers that can entirely mix surface and deeper soils over a period of one-four years and accelerate litter decomposition and efficient reuse of nutrients by plants.