Use of a reciprocal transplant study to measure the rate of plant community change in a tidal marsh along a salinity gradient

The relationship between environmental factors and the spatial distribution of maintained and actively used burrows of the grapsid crab Helograpsus haswellianus was studied at three saltmarsh sites in southeast Queensland, Australia. The sites had been modified by runnelling for mosquito-control, a method that transports low-amplitude tides to areas of saltmarsh. The study investigated the relationship between burrow density, burrow aperture size, and runnelling, as well as the effect of flooding or non-flooding tides and distance from a tidal source. Responses differed at the three sites. The most consistent pattern across all sites was that active burrows were most numerous between 30 and 50 m from the saltmarsh / mangrove interface at the landward side of the tidal source. At particular sites, there were significant relationships between burrow aperture size, tidal period, and the presence of runnels. Generally, few small burrows occurred low on the shore, while larger burrows were distributed across the shore to 50 m. At naturally dry sites, more burrows occurred within 5 m of the runnel, whereas at naturally wet sites, fewer burrows were found close to the runnel. As runnels transport low-amplitude tides, moisture conditions required for burrowing may very between flooding and non-flooding tides. Overall, the influence of tides on the density of crab burrows and their aperture sizes was of more importance than the presence of runnels alone