Dragonflies include the migrant hawker, southern hawker and ruddy darter. On exposed parts of the coast, the muds and sands are scoured by the tides, and have no vegetation except possibly algae or eelgrass, but where the shoreline is more protected, internationally important salt marshes can form, with several uncommon species. The salt marsh contains glassworts and annual seablite in the most exposed regions, with a succession of plants following on as the marsh becomes more established: first sea aster, then mainly sea lavender, with sea purslane in the creeks and smaller areas of sea plantain and other common marsh plants. Scrubby sea-blite and matted sea lavender are characteristic plants of the drier upper salt marsh here, although they are uncommon in the UK away from the Norfolk coast. Grasses such as sea couch grass and sea poa grass are important in the driest areas of the marshes, and on the coastal dunes, where marram grass, sand couch-grass, lyme-grass and red fescue help to bind the sand. Sea holly and sand sedge are other specialists of this arid habitat, and petalwort is a nationally rare bryophyte found on damper dunes. Bird's-foot trefoil, pyramidal orchid, bee orchid, lesser centaury and carline thistle flower on the more stable dunes, where the rare Jersey cudweed and grey hair-grass are also found. The narrow 5-km (3-mi) belt of pines shelters creeping lady's tresses and yellow bird's nest. A 2005 survey at Holkham and five other North Norfolk coastal sites found that 39 per cent of visitors gave birdwatching as the main purpose of their visit. The 7.7 million day visitors and 5.5 million who made overnight stays in the area in 1999 are estimated to have spent £122 million, and created the equivalent of 2,325 full-time jobs. Holkham NNR is one of three sites within the SSSI that attract 100,000 or more visitors annually, the others being Titchwell Marsh and Cley Marshes. The large number of visitors at coastal sites sometimes has negative effects. Wildlife may be disturbed, a frequent difficulty for species that breed in exposed areas such as Ringed Plovers and Little Terns, and also for wintering geese. Plants can be trampled, which is a particular problem in sensitive habitats such as sand dunes and vegetated shingle.