The fiddler crab (Uca pugnax), long regarded as a southern species, is on the move. With its northern range limit of Cape Cod (1989) expanded to Scituate, MA, 60 miles north, in 2003, there were signs that fiddlers might be taking advantage of new territory being warmed by climate change. In 2014, PIE Scientist David Johnson confirmed that fiddlers were indeed moving north when he discovered them in the Rowley River marshes, within the PIE-LTER site. He has now documented fiddler colonies as far north as Hampton, NH, 80 miles beyond the 2003 limit.
Given that adult fiddlers are able to overwinter in marshes north of their range by capping their burrows, it is the thermal tolerances of the planktonic larvae that sets their true northern distribution. Higher-than-average water temperatures in Massachusetts Bay and the Gulf of Maine during summers of 2012 and 2013, a time of larval dispersal and recruitment for U. pugnax, suggest that the move to new northern sites occurred during this period, and that warming ocean waters associated with climate change are the likely driver of this range expansion.
Female fiddler carrying eggs (photo A. Bulseco-McKim)
Species like fiddler crabs that burrow into the marsh creek banks are new to PIE marshes. So-called "ecosystem engineers", they can have significant effects, both positive and negative, on marsh structure and function by influencing soil strength, plant productivity, biogeochemical cycling, and carbon storage. They feed off the marsh surface, scooping mud into their mouths and filtering out the digestable particles, many of which are benthic microalgae. Thus we predict a strong effect on microalgae, leading to other food web impacts. In turn, fiddlers will provide a new food resourse for predators such as blue herons.
Male fiddler in burrow (photo J. Whitcomb)
Although densities are presently low, the vanguard populations of fiddler crabs at PIE may have stronger per-capita effects than individuals in fully mature colonies, and as densities increase over time, their impacts may increase. We will take advantage of LTER data from many ‘pre-fiddler’ years and compare that to ‘post-fiddler’ data to understand the whole- ecosystem implications of a new species on ecosystem function and structure. We will also be watching for other species, such as blue crab (Callinectes sapidus), already observed in the Gulf of Maine, that may be crossing the traditional north-south barrier of Cape Cod.
For the full story, see Johnson, 2014….
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