Global Shore Communities

7.4.1 Salt marshes and mudflats

Salt marshes and mudflats are soft shores and therefore vulnerable to both natural and human disturbance. Given a slow and steady rise in sea levels and an adequate input of sediment, it might be expected that mudflats and salt marshes will be able to grow apace with rising sea levels and provide a natural means of increasing coastal defences. If this were to happen it might offer some redress for the long-term reduction of salt marshes due to so-called 'land reclamation' and infill from dumping that has taken place worldwide on mudflats and salt marshes over many centuries. However, this would require a cessation of the ever-increasing construction of coastal defences, extensions to promenades, car parks, and other amenities which continue to destroy or else interfere with the natural growth of this particular type of soft shore.

Unfortunately, it is far from certain that salt marshes will be able to respond positively to rising sea levels. Throughout the British Isles the tendency during the past four decades has been for a reduction and fragmentation (Hughes et al., 2000). It is therefore probable that many already fragmented salt marshes will be even further reduced and may perhaps disappear entirely as sea levels rise (Fig. 7.29). Whether or not there is one specific cause for this habitat loss is at present far from certain. For some salt marshes in south-east England it has been suggested that the failure of the marshes to regenerate, even when encouraged by the provision of sediment, is due to the grazing activities of the invertebrate fauna, particularly the polychaetes Nereis diversicolor (the ragworm), and the mud shrimp Corophium volutator. Organic pollution and agricultural run-off have also been noted for a number of years as causing ecological problems for the natural flora and fauna of low-lying coastal habitats. In Scotland the most detailed study has been on the River Ythan where the total oxidized nitrogen input has increased four fold over the past 30 years (Gillibrand & Balls, 1998) causing blooms of opportunistic green macro-algae (Raffaelli, 2000).

In south-east England, studies on the disappearance of salt marshes began in the 1930s as a result of the loss of inter tidal eelgrass (Zostera marina). Recent studies have recorded rising levels of organic matter in the estuarine sediments which have been accompanied

Salt Marsh Climatic Climac Community
Fig. 7.29 Fractionated salt marsh at Taobh Tuath (Northton, Harris, Outer Hebrides) which may be liable to further erosion as sea levels rise.

by increasing abundance of a number of polychaete and copepod species (Hughes, 1999). In transplant experiments, ragworms have been noted to reduce the survival of Zostera noltii by grasping the leaves and pulling them into their burrows. From this and other studies it has been suggested that two conditions exist on the mudflats. In one state, the mudflats are dominated by plants, including algal mats and Zostera spp., which prevent colonization by the burrowing invertebrate fauna, as in the Ythan Estuary. The other possibility is that the invertebrate fauna inhibit plant colonization and therefore the estuary does not develop a significant plant cover. Consequently, attempts to restore mudflat and salt marsh communities will inevitably involve careful management of this contest (Hughes et al., 2000). Laboratory-based experiments have also demonstrated significant negative effects of N. diversicolor abundance on the survival of Spartina anglica seeds

(Emmerson, 2000). The extent of the area of unvege-tated shore is also crucial to the regeneration of salt marshes. Where areas of reclaimed agricultural land have become reflooded due to sea-wall failure, smaller, sheltered sites have been noted as re-establishing salt marsh more readily, while larger sites more commonly revert to unvegetated tidal flats (French et al., 2000).

7.4.2 Rising sea levels and mudflats

The environmental problems that surround mudflats are numerous. Rising sea levels, which in the past might have been generally beneficial in extending this habitat in sheltered estuaries and inlets, are now more likely to be disadvantageous as plant colonization will probably be hindered by pollution, human disturbance and interference with natural sedimentation processes. Remedial action, including planting with species that might raise the shore level, e.g. Spartina anglica, is unlikely to take place due to the belief that such artificial intervention could reduce the access of wading birds to mudflats. This is not necessarily the case for all mudflats. In Scotland the planting of Spartina has been discouraged even though the late flowering of the species makes it unlikely that it will prove as dangerous in this respect as further south (Fig. 7.30). Anxieties concerning erosion and landscape preservation demand instant action, which sadly usually means that public opinion has more faith in civil engineering enterprises than the power of vegetation to hold the landscape in place.

In North America there is considerable concern about the expansion of the common reed into areas which in the past supported salt marshes. The common reed (Phragmites australis), although it can be regarded positively in terms of its ability to stabilize estuary shores in brackish waters and provide shelter for birds and other animals, tends to produce monospecific stands and therefore reduce plant biodiversity. In recent years in the coastal marshes of the north-eastern United States P. australis appears to have acquired a competitive advantage over a broad range of habitats, from tidal salt marshes to freshwater wetlands. The change in the ecology of the common reed may be due to a variety of causes which it is suggested are all connected to some degree with human disturbance. It has been claimed that the North American population has been infiltrated with aggressive European genotypes (Burdick & Konisky, 2003). Any human disturbance that lowers salinity in brackish waters is likely to aid the

Fig. 7.30 Cord grass (Spartina anglica) in the estuary of the River Eden (Fife, east Scotland) where it was planted in 1948. The colony was successfully raising the level of the shore and would have provided protection against rising sea levels had it not been systematically eradicated during the 1990s in the probably mistaken belief that it might have interfered with the access of wading birds to the mudflats.

Fig. 7.30 Cord grass (Spartina anglica) in the estuary of the River Eden (Fife, east Scotland) where it was planted in 1948. The colony was successfully raising the level of the shore and would have provided protection against rising sea levels had it not been systematically eradicated during the 1990s in the probably mistaken belief that it might have interfered with the access of wading birds to the mudflats.

clonal spread of Phragmites australis as small rhizome fragments are established more readily under conditions of reduced salinity. Experimental studies (Chambers et al., 2003) have shown that salinity, sulphide, and prolonged flooding combine to constrain the invasion and spread of Phragmites in tidal wetlands through their physiological effects on ionic and carbon balance coupled with oxygen availability. Consequently, invasion takes place more readily in marshes occupying lower-salinity regions of estuaries as well in marshes that have been hydrologically altered. Climatically, it is noteworthy that periods of increased rainfall aid the growth of Phragmites australis. A study carried out during the 1997-98 El Niño event showed that soil pore-water salinities were negatively related to precipitation during the three years of the study, and that the growing season during the El Niño year was one of the wettest of the past century. These changes were associated with a 30% increase in shoot density, stems which were 25% taller, and an order of magnitude increase in inflorescences (Minchinton, 2002). It is therefore possible that should there be an increase in the frequency of El Niño years a spread of less salttolerant invasive species throughout brackish habitats such as salt marshes is to be expected.

7.5 HARD SHORES 7.5.1 Cliffs and caves

Irrespective of their height, it is inaccessibility that makes cliffs probably the least disturbed of all habitats. Provided cliffs are of sufficient size they are generally free from grazing, cutting, burning and detailed scientific investigation. Some exceptions are notable. Pickled rock samphire (Crithmum maritimum; Fig. 7.31) was once so popular and valuable that people risked their lives to collect it from precipitous rock faces. Shakespeare describes the collection of this much sought-after herb: 'How fearful ... half-way down hangs one that gathers samphire; dreadful trade!' (Shakespeare, King Lear). Apart from such exceptions and the occasional visits from adventurous goats and sheep, the cliff vegetation is one of the best examples of totally natural and understudied plant communities that remain in proximity to human habitation. In early summer the display of floral colour on the exposed tops and faces of coastal cliffs is spectacular. Cliff

Fig. 7.31 Rock samphire (Crithmum maritimum). The leaves were formerly much sought after for pickling. Common on cliffs and rocks in England and Wales but rare in Scotland.

vegetation can be highly variable. Such are the range of cliff environments in relation to geology, soil, exposure and moisture that they can provide suitable habitats for species associations with contrasting ecological requirements. Thus depending on location and climate, cliffs can shelter arctic-montane, maritime, calcicole and calcifuge plants and even woodland understorey species with and without an upper canopy of trees.

Cliffs can be subdivided into two contrasting types, namely hard cliffs and soft cliffs (Figs. 7.327.33). In the latter, variation in vegetation is maintained due to constant physical disturbance which prevents certain species becoming dominant and thus allows a variety of species to coexist (Cooper, 1997). Hard cliff faces, which are almost vertical or exposed to extreme conditions, usually lack any significant vegetation cover. Hard and soft cliffs denote merely contrasting ends of a range of possibilities. Depending on the hardness of the rock the degree of erosion will vary and the contrasting effects of instability destroying vegetation as opposed to erosion providing fresh sites and nutrients will have a decisive role in the development of the vegetation. Further variation in the nature of the cliff face vegetation comes from the relative deposition of sea spray, freedom from desiccation on north-facing sites, and access to water through seepage and variation in rock types. One feature in common with all cliffs is that as vegetation builds up with time, it eventually becomes unstable and erodes, thus renewing the succession cycle and preserving biodiversity.

Fig. 7.32 A soft Old Red Sandstone cliff on the Island of Hoy, Orkney. The vegetation clinging to the cliffs is dominated by the greater wood rush (Luzula sylvatica).

7.5.2 North Atlantic cliffs

Even in the cold and stormy climate of North Atlantic shores the vegetation clinging to cliffs can vary from dense forest-like stands of trees to tundra-like assemblages of dwarf-herbaceous and woody plants. Starting at the base of the cliffs and working upward, a succession of plant communities can often be observed. Lichens in particular the bright orange Xanthoria parietina, are found at the base of most sea cliffs. Above this most disturbed zone, crevices and ledges provide a habitat for halophytic species such as sea plantain (Plantago maritima), the buck's-horn plantain (Plantago coronopus), rose root (Sedum rosea) and sea aster (Aster tripolium).

Fig. 7.33 A hard Stromness flagstone cliff at Marwick, Orkney, where vegetation cover is minimal.

Cliff faces are the preferred habit for species that can tolerate sea spray and summer drought but would not survive inundation or competition. Thrift (Armeria maritima), and sea plantain (Plantago maritima) have an even wider distribution extending throughout the northern hemisphere, including survival on mountain tops. These species would have been widespread at the end of the last ice age but are now restricted to altitud-inally disjunct sites where they find habitats with minimal competition. Towards the upper areas of cliffs and extending onto the cliff top where there is no grazing, as on sea stacks, there can be found tall herb communities with wild angelica (Angelica sylvestris), red campion (Silene dioica), the common primrose (Primula vulgaris), foxglove (Digitalis purpurea) and sorrel (Rumex acetosa). Later in the summer there can also be found grass of Parnassus (Parnassia palustris).

Although coastal cliff plant communities escape the predations of grazing mammals they are subject to colonization by sea birds especially during the breeding season. Dense bird colonies cause major changes in cliff vegetation. Too much disturbance and excess guano can eliminate flowering plants entirely. However, the more normal consequence of large nesting colonies of auks and fulmars is to favour those species which respond to large inputs of nitrogen. Most striking among these is scurvy grass (Cochlearia officinalis). In areas at the cliff top, where colonies of greater black-backed gulls congregate, the habitat is marked by luxurious growth of grasses such as Yorkshire fog (Holcus lanatus) along with sorrel (Rumex acetosa) and related docks (R. obtusifolius, R. crispus) and often stinging nettle (Urtica dioica). Puffin colonies are often associated with colonies of mayweed (Triplospermum mar-itimum) and orache (Atriplex spp.).

Caves provide an opportunity to investigate the shade tolerance of cliff-inhabiting species. The depth of the Smoo Cave on the north coast of Scotland provides just such an opportunity. The last flowering plant to be found in proceeding to the rear of the cave (Figs. 7.34-7.35) is the opposite-leaved golden saxifrage

Isla Pascua Casas Los Nativos

Fig. 7.34 The Smoo Cave, north coast of Scotland (Sutherland), the largest opening of a limestone coastline cave in Britain. The depth of this cave makes it possible to observe the relative shade tolerance of cliff-inhabiting plants, the most tolerant flowering plant being the opposite-leaved golden saxifrage (Chrysosplenium oppositifolium; see Fig. 7.35).

Fig. 7.34 The Smoo Cave, north coast of Scotland (Sutherland), the largest opening of a limestone coastline cave in Britain. The depth of this cave makes it possible to observe the relative shade tolerance of cliff-inhabiting plants, the most tolerant flowering plant being the opposite-leaved golden saxifrage (Chrysosplenium oppositifolium; see Fig. 7.35).

(Chrysosplenium oppositifolium). As plants adapt to shade, the leaves generally become thinner and prone to sugar leakage which can bring about fatal fungal infections. In the case of C. oppositifolium, the principal soluble sugar is sedoheptulose, which being a 7-carbon sugar is not readily metabolized by fungi, which may be advantageous in extremely shaded habitats where solute leakage encourages fungal attack. A type of heath community specific to cliff tops is maritime sedge heath (Fig. 7.36). This usually develops a short distance back from the edge of the cliff where salt drenching is reduced, and consists of a community that is rich in various species of Carex together with dwarf heather (Calluna vulgaris) and crowberry (Empetrum nigrum).

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