Yellow and grey lichens on supralittoral rock
Ecological and functional relationships
The supralittoral lies above high water level of spring tides and is influenced by wave wash, splash and sea spray. The mobile fauna vary with the tidal submergence and wave exposure with marine intertidal species further up the shore at high tide and mobile species of terrestrial origin foraging down the shore as the tide recedes only to return to the top of the shore as the tide returns (e.g. spiders). Other species of terrestrial origin, notably mites (acarids), and some spring tails (Collumbola) and bristle-tails (Thysanura) seek refuge is cracks and crevices at high tide.
Lichens are a symbiotic association of a fungus and microalgae and therefore provide primary productivity as well as refuge.
Lichens are fed on by fungivorous Cryptostigmata and other acarid mites e.g. Hyadesia fusca, Phauloppia lucorum, Mycolates parmetiae and Ameronothrus maculatus (Colloff, 1983; Pugh & King, 1988; Bücking, 1998; Gilbert 2000), and potentially by some lichen dwelling tardigrades (Gerson & Seaward, 1977) and the bristle tail Petrobius maritimus (Joosse, 1976), while rotifers have been reported to consume lichen ascospores (Gerson & Seaward, 1977).
Gastropods such as Littorina saxatilis and Melarhaphe neritoides graze littoral lichens such as Verrucaria maura and Lichina pygmaea and could potentially graze other lichens in the lower supralittoral.
Detritus may accumulate within lichens, in pits or crevices and is fed on by detritivores such as acarid mites, tardigrades, and nematodes (Roth & Brown, 1976; Gerson & Seaward, 1977; Pugh & King, 1988).
Predators include acarid mites, some tardigrades (e.g. Milnesium tardigradum), spring tails (Collumbola) such as Anurida maritima, centipedes of terrestrial or intertidal origin (e.g. Strigamia maritima that may take isopods, amphipods and periwinkles), anthocorid bugs (e.g. Temnostethus pusillus that feeds on mites), and terrestrial or maritime spiders and pseudo-scorpions (Roth & Brown, 1976; Pugh & King, 1985a, 1988; Gilbert, 2000).
Scavengers include the sea slater (Ligia oceanica) in the lower supralittoral (Nicholls, 1931) and acarid mites (Pugh & King, 1985a; 1988)
The invertebrate fauna probably provides a food source for predatory birds, such as the rock pipit (Anthus spinoletta), while other vertebrates such as rabbits, goats, and sheep may graze foliose or fructicose lichens directly (Fletcher, 1980; Dobson, 2000; Gilbert 2000).
Seasonal and longer term change
Fletcher (1980) suggested that cloudy days during the year or the winter climate were most conducive to growth, although it was noted that lichen growth rates varied widely between different locations, between different species and even between different thalli of the same species at the same site. The colour of some thalli may change with season, for example Xanthoria parietina
becomes greener in winter or shaded conditions (Fletcher, 1980). In the winter months, the lichens at the bottom of the supralittoral may be overgrown by ephemeral green algae (e.g. Ulva
sp., filamentous green algae such as Ulothrix
sp.). On areas subject to bird-manuring (e.g. bird perches) littoral fringe lichens may be overgrown in the winter months by Prasiola stipitata
. At the top of the supralittoral, lichens may be shaded by the growth of terrestrial plants in summer. Summer annuals probably have little effect on the lichens, however, on sheltered sites, shading by large plants favour a more terrestrial lichen flora (Fletcher, 1980).
Jones et al. (1974) suggested that lichens were slow growing and long lived with estimated ages of 100 years or more. Therefore, changes in lichen communities was slow unless caused by pollution (Jones et al., 1974). James & Syratt (1986) reported that changes in the lichen community or loss of lichens in Sullom Voe could be attributed to direct competition between lichens (overgrowth), gastropod grazing (a minor effect), physical abrasion from wind, trampling or animal rubbing, human interference (e.g. building) or bird manuring. However, the supralittoral lichen communities were relatively stable at most sites monitored between 1978 to 1986 (James & Syratt, 1986).
Habitat structure and complexity
Several zones and their characteristic flora and fauna have been identified within the lichen dominated supralittoral of rocky shores. The complexity of the habitat is primarily dependant on the presence of crevices, recesses, ledges and rock pools in the rocky substrata, together with the additional niches and refuges provided by the lichen flora itself.
The supralittoral zone occurs above the upper littoral fringe, characterized by Verrucaria maura
, Lichina pygmaea
and small littorinids, and below the terrestrial zone characterized by inland lichens such as species of Parmelia
and maritime plants (e.g. Armeria maritima
and Festuca rubra
). The following supralittoral zones have been identified (Fletcher, 1973a&b, 1980; Dobson, 2000).
Factors affecting zonation
- The mesic-supralittoral, lying above the littoral fringe, receiving heavy spray and occasional submergence of the lower part during extreme spring tides. The mesic-supralittoral is characterized by encrusting cracked or areolate (composed of islands of tissue) lichens, or lichens with lobose margins. For example, the orange crustose Caloplaca marina and black foliose Lichina confinis form the lower part of the zone with grey-white Lecanora sp. above, although the species composition varies and may be composed of several species of Caloplaca and Lecanora and other lichens.
- The mesic-supralittoral is dominated by orange Caloplaca marina and white Lecanora actophila on sunny shores, forming the characteristic 'orange belt', but on shaded shores Caloplaca marina cover is reduced, being replaced by sparse Caloplaca thallincola and more common white and grey Lecanora species so that the zone becomes more 'leaden-grey' in colour.
- A submesic-supralittoral zone occurs above the mesic-supralittoral and is characterized by the regular occurrence of Xanthoria parietina. This zone is not always present, being rare or absent on shaded shores and diffuse or sparse on sunny shores exposed to wave or wind action where the zone is restricted to crevices and shallow fissures. However, Xanthoria parietina may dominate the supralittoral in areas affected by heavy bird manuring.
- The xeric-supralittoral (the 'grey' zone) receiving light spray and exposed to a harsh regime of wetting and drying. The xeric-supralittoral is characterized by cracked crustose or narrow lobed, substrata hugging foliose or narrow lobed shrubby lichens including Ramalina siliquosa, Tephromela atra, Rhizocarpon constrictum, Parmelia prolixa, Anaptychia runicata (as fusca), Lecanora sp., Ochrolechia parella, and many other species overlapping with the mesic or submesic below or the terrestrial zones above (Fletcher, 1973a&b; 1980).
- The species composition of the xeric-supralittoral is variable, depending on local topography, patterns of seawater deposition, runoff and drought.
- Damp rock crevices and fissures allow lichens of lower zones (e.g. Verrucaria spp.) to penetrate higher on the shore, while heavy bird-manuring may allow submesic communities to dominate. Crevices and shaded fissures may support unique flora e.g. the Sclerophytetum circumscriptae association (James et al., 1977).
- The top of the supralittoral is delimited by a zone of halophilic and halophobic lichens of more terrestrial origin, that integrates into coastal or maritime vegetation, e.g. Armeria maritima dominated communities (Rodwell, 2000).
- High shore rockpools may occur at the bottom of the supralittoral and support distinct communities (e.g. LR.G). Ledges or large fissures that accumulate debris and soil may be colonized by plants and their associated fauna (see Rodwell, 2000). Rock pool and vascular plant communities are not considered further here.
The species composition, growth form and diversity of the lichen flora and hence the complexity of the habitat for lichens and other fauna varies between and within the above zones. The extent and height and of each zone is dependant on substratum texture and type (including crevices, fissures and ridges); emergence regime, wind and wave exposure and hence the extent of spray and salt particles up the shore or inland; aspect and hence duration and intensity of light and temperature change; salinity; nutrient levels, and wind, slope, drainage and hence humidity (see sensitivity) (Fletcher, 1973a&b, 1980). For example:
- The lichen flora may be depauperate on hard nutrient poor rocks but more species rich on rough rock surfaces. The lichen flora of calcareous rocks differs from that of siliceous rocks markedly, and the xeric and terrestrial zones may merge, probably due to the difference in water retention of porous calcareous rock and its different chemistry and nutrient levels.
- Rock fissures may support particular lichen communities of shade or drought loving species.
- Wind and wave action affect the deposition of wave splash, sea spray and hence humidity of the shore. Wind driven sea spray may affect inland areas. The lower supralittoral tends to be base-rich and saline while the top of the supralittoral receives acidic, humic, nutrient rich, fresh water runoff from the terrestrial zone. The middle of the supralittoral (the xeric-supralittoral) tends to be the driest.
- The vertical extent of the supralittoral increases with increasing wave and wind exposure and terrestrial species are pushed further up-shore. On wave and wind sheltered shores (low amounts of spray) the mesic-supralittoral communities may be poor, and have less rocky surface to colonize due to penetration of terrestrial plants down the shore.
- Increased wind exposure alone causes increased desiccation and physical abrasion, reducing the numbers of species present.
- On shaded shores the supralittoral lichen zones are narrower but species coverage and richness is increased, partly because the environment is less harsh and partly due to greater development of terrestrial flora.
Supralittoral lichens trap detritus and provide a habitat, refuges and food for a number of species of Protozoa, Nematoda, Rotifera, Oligochaeta, Tardigrada, Insecta, Acari, and Mollusca, and a hunting ground for Acari, Arachnida, Insecta and birds (Gerson & Seaward, 1977; Fletcher, 1980; Pugh & king, 1985a&b, 1988; Morgan & Lampard, 1986; Kinchin, 1994):
- Pugh & King (1988) noted that the acarid mite fauna was divided into: maritime species generally confined to the supralittoral; non-maritime mites of terrestrial origin that forage or seek refuge in the supralittoral, and nomadic species that migrate freely between the littoral and supralittoral. The diversity of acarid mites was correlated with lichen growth form. Low encrusting lichens (e.g. Ochrolechia parella and Tephromela atra) supported few acarids. More foliose or cracked lichens (e.g. Caloplaca marina and Xanthoria parietina form pockets of detritus and support numerous acarids, while the bushy, erect growth of Ramalina siliquosa trapped little detritus and hence harboured few acarids. The zonation of different species of acarid is also dependant on shore height, desiccation, temperature and inundation (for details see Pugh & King, 1985b, 1988).
- Different species of tardigrades (water-bears) were found to be associated with different lichens and/or different shore heights in the supralittoral and littoral zones (Morgan & Lampard, 1986; Kinchin, 1994).
- The underside of lichens and rock crevices provide refuge for other arthropods such as the bristletail Petrobius maritimus and spring-tails (Collumbola) (Joosse, 1976; Gerson & Seaward, 1977).
- Damp rock crevices provide refuges for the sea slater Ligia oceanica and the gastropod Melarhaphe neritoides in the lower supralittoral and littoral fringe.
Little information concerning the energetics and productivity of lichen communities was available (Gerson & Seaward, 1977). Terrestrial lichen communities support large numbers of invertebrates, the lichens provide primary production to lichenophagous species and trap or generate detritus for detrivores, which are in turn consumed by predators (see ecological relationships). The annual primary productivity of lichen heaths was estimated to be only 100g/m² (Gerson & Seaward, 1977) but supralittoral communities are probably less productive. Fletcher (1980) suggested that lichens had low food value and low biomass so that few organisms were able to digest them and that supralittoral were, therefore, of more importance due to the cover and niches they provided rather than their productivity.
Colonization by lichens on the rocky shore has not been studied (Fletcher, 1980). Fletcher (1980) suggested that the fungal ascospores germinate on suitable substrata to form a 'plaque' and then trap the required species and physiological strain of microalgae to form the symbiotic lichen. Most thalli probably develop from the germination of several ascospores. The required symbiotic microalgae are probably free-living and ubiquitous, e.g. Hyella caespitosa
is the symbiotic alga in the lichen Arthopyrenia
, and Calothrix scopulorum
is the symbiotic alga of Lichina
(Fletcher, 1980). Fletcher (1980) suggested that the substratum surface probably requires modification by weathering before lichens can establish, so that colonization and initiation of new thallus growth was thought to take several years after exposure of new substratum.
Lichen ascospores and asexual propagules (isidia or soredia) may be transported by the wind and water. Ascospores may be transported in the guts of rotifers or grazing molluscs that may not fully digest spores, which could potentially germinate in their faeces. Similarly, asexual propagules may be dispersed attached to mobile insect fauna (Gerson & Seaward, 1977).
Tardigrades and other microscopic fauna (e.g. rotifers and nematodes) are probably incapable of significant active dispersal. Tardigrades, rotifers and nematodes may be dispersed in water droplets or attached to more mobile species such as insects. In dry conditions, tardigrades are capable of dehydrating (anhydrobiosis) to form dormant 'tuns', which could potentially be dispersed by wind (Kinchin, 1994). Tardigrades reproduce either sexually or asexually depending on species and asexual species are often associated with rapid colonization of habitats (Kinchin, 1994).
The remaining fauna (e.g. acarid mites, and insects) are highly mobile and probably capable of rapid colonization of available habitats from either the surrounding littoral, supralittoral or terrestrial habitats.
Time for community to reach maturity
The asexual propagules and sexual ascospores of lichens, and their symbiotic microalgae are probably ubiquitous, so that colonization of suitable habitats would be relatively rapid, once the substrata was suitable modified by weathering (see above). Lichens are very slow growing, rarely exceeding 2-5mm/ year (Jones et al.
, 1974). For example: crustose lichens were reported to show radial increases of 0.1mm /month while foliose species grow at 0.4-0.7 mm/month (Fletcher, 1980); Lichina pygmaea
was reported to grow 3-6cm/year at one site but only 0.5mm/year at others (Fletcher, 1980), and Ramalina siliquosa
grew at 2-3mm/ year and Parmelia sulcata
at 8mm/ year while most crustose species grew between 0.5-1mm/year in Sullom Voe (James & Syratt, 1986). However, lichen growth rates varied widely between different locations, between different species and even between different thalli of the same species at the same site (Fletcher, 1980). Cullinane et al.
(1975) noted that many of the lichens lost due to an oil spill in Bantry Bay were probably 20-50 years old, based on their size, and life spans of lichens have been estimated to be 100 years or more (Jones et al.
On rocky shores, succession was not clear, and lichen colonization tended to depend on available suitable substrata, rather than a successional pattern (Fletcher, 1980). In some instances the dying centres of some lichen thalli may be colonized by other lichen species. Overgrowth of one lichen species by another does not appear to be a regular occurrence on supralittoral or littoral rocky shores (Fletcher, 1980).
Overall, it may take several years for lichens to colonize new substrata and several more years to grow enough to provide shelter or refuges for invertebrate fauna. The invertebrate fauna itself is likely to be able to colonize the available habitat quickly.
This review can be cited as follows:
Yellow and grey lichens on supralittoral rock.
Marine Life Information Network: Biology and Sensitivity Key Information Sub-programme [on-line].
Plymouth: Marine Biological Association of the United Kingdom.
Available from: <http://www.marlin.ac.uk/habitatecology.php?habitatid=96&code=2004>