By the end of the 21st century models of climate change predict that the air temperature over most of the British Isles will increase by between 2 and 3 °C and sea-level will rise by 40–50 cm. Over that period it will become windier and mean wave height will increase, as will the frequency of storms. These changes in climate and weather will impact the intertidal zone of the UK and will cause distribution changes in many of the common invertebrate species that live there. Where these changes are severe they may well impact on patterns of distribution of ducks and wading birds. In the British Isles a number of organisms live close to their geographical limits of distribution. Some of these species might be expected to extend their range as climatic restraints are relaxed. Species currently limited by cool summers or winter cold will move northwards. In most cases the effects on the distribution of waterbirds will be small. For example, the replacement of the Northern Limpet Patella vulgata by the Southern Limpet P. depressa is unlikely to adversely affect Eurasian Oystercatchers Haematopus ostralegus. Of wider concern is the possibility that as climate warms the abundance and productivity of brown algae will decrease. This is likely to have two significant effects for waders. First, it would represent a loss of potentially rich feeding grounds for species such as Ruddy Turnstone Arenaria interpres that feed on small easily desiccated invertebrates living on or below the seaweed. Secondly, as algae die or are broken away the resulting debris is exported to sediment habitats where it considerably boosts the in situ production of bacteria at the base of the food web. An increase in sea-level will only have a major impact on the extent of rocky shore invertebrate communities where shore topography prevents the upward migration of the biota. Where a seawall limits shores, for example, biological production will be curtailed as the area available for colonization decreases. Increases in the size of waves and the frequency of storms will mimic increasing exposure and there will be a significant reduction in algal production in areas that are affected.
The majority of scientists agree that the Earth's climate is getting warmer. Analysis of global air temperatures from 1856 to the present show the 1990s to be the warmest decade and 1997 and 1998 as the warmest years (Jones et al. 1999). Ocean temperatures have increased in parallel with those of the atmosphere. Long-term records from the English Channel (Fig. 1) show that from the early 1980s sea temperature increased slightly until 1990. During the following decade there was an increase of almost 1 °C. This was far greater than any change in the previous 100 years. In its warm phase, annual mean sea surface temperature off Plymouth is 11.5–12.2 °C whereas during its cold phase the corresponding temperature is 10.0–11.5 °C. As computer models forecast that by 2080 UK mean air temperatures will rise by between 1.5 and 3.2 °C, the trend in inshore sea temperature is likely to continue. As global temperature rises, the volume of water in the oceans will expand and the polar ice caps will reduce in volume. This will cause sea-levels to rise by a predicted world average of 13 cm by 2020, rising to 40 cm by 2080. In some parts of the UK sea-level rises may be partially offset by coastal sinking (Hulme et al. 2002).
Given that there will be substantial changes in climate in the coming decades it is necessary to make predictions concerning the magnitude and nature of the changes to coastal biodiversity that will also occur. For some coastal habitats this might be difficult, but predictions for changes in the biota of rocky shores are substantially aided by long running archive data from a number of UK and European sources as well as by a rich literature on the biology of the component species. The Marine Biodiversity and Climate Project (MarClim) looks at the long-term records of variations in the abundance of rocky shore species, analyses them simultaneously and makes models for the future biota of the UK. This project began in mid 2001 and as yet full analyses are unavailable. However, preliminary predictions for shallow water life around the coasts of Scotland have already been made (Hiscock et al. 2001). Should a warming of the sea influence the abundance/quality of the prey of waterbirds or change the biota of intertidal habitats in which they forage, local or even national patterns of bird distribution will be affected. The qualitative nature of such changes is predictable and will exacerbate the altered patterns of distribution that are already recorded as a response to milder winters between the mid 1980s and the late 1990s (Rehfisch et al. 2004). Changes at overwintering sites will be over and above any impact to Arctic-nesting species that might result from a compression of breeding habitat (Rehfisch & Crick 2003). Here we discuss the probable nature of changes to the invertebrate fauna of the rocky seashore together with predictions of the response of some waterbirds.
EFFECTS OF CLIMATE ON THE DISTRIBUTION OF ROCKY SHORE SPECIES
The geographical distribution of the dominant British and Irish intertidal rock species was mapped in the 1950s (Southward & Crisp 1956, Crisp & Southward 1958). The fauna includes a number of common species that are either at or close to their northern or southern geographical limits of distribution. There has been little subsequent research to contradict the conclusions of Hutchins (1947) and Lewis (1964) that species with north–south patterns of distribution have their geographical limits set by temperature. Lewis (1976) proposed that species close to their northern distributional limits in the UK were limited by the severity of winter conditions, whereas those approaching their southern limit of distribution were limited by summer conditions. Crisp (1964) demonstrated that in the particularly severe winter of 1962/63 the range of a number of southern species was reduced. Such extreme conditions undoubtedly serve as a check on the expansion of the range of a species but it is highly unlikely that winter conditions alone set distributional limits.
Lewis and his co-workers (Lewis et al. 1982, Kendall & Lewis 1986, Lewis 1986, 1996, 1999, Kendall & Bedford 1987) investigated the way in which climate affects a suite of dominant rocky shore species including the southern barnacle Chthamalus montagui and the top-shells Osilinus (Monodonta) lineata and Gibbula umbilicalis. Studies on the trochids suggested that as species approached their northern limit of distribution they spawned and recruited over a far shorter season than in the centre of their range. Nevertheless, populations close to their northern limits do produce gonads and spawn annually (Kendall & Lewis 1986) and animals transplanted beyond their normal range can develop and spawn (Lewis 1986). Northern species at the southern edge of their range are unlikely to be limited by the failure of gonad development, although their reproductive season may well switch to an earlier time of the year (Bhaud 1982). In the case of the northern barnacle Semibalanus balanoides, at its northern limit in the Arctic it spawns in mid summer (Feyling-Hansen 1953), in the centre of its range in April–May (Kendall et al. 1985) and close to its southern limit in February/March (Barnes & Barnes 1976). The distribution of this species on southern shores (Barnes & Barnes 1976), observations of settlement success (Kendall et al. 1987, Jenkins et al. 2000) and experimental manipulations (Wethey 1984, Jenkins et al. 1999) all suggest that desiccation of individuals recently settled from the plankton limits its southerly extension.
C. montagui is a southern species that reaches its northern limit of distribution in northwest Scotland. Unlike S. balanoides, C. montagui has the potential to spawn successive broods of larvae whenever food and warmth permit. M.P. Myares (unpubl. data) recorded that in Spain larval settlement onto the seashore takes place over much of the year. By contrast, Kendall and Bedford (1987) showed that in west Wales there is only a single settlement period. In southwest England, Burrows et al. (1992) recorded as many as four broods of larvae produced annually.
PREDICTING THE FUTURE FAUNA OF ROCKY SHORES IN THE BRITISH ISLES
The MarClim Project will make predictions about the future composition of the biota of rocky shores under a range of climate change scenarios. For many elements of the marine biota this would be a daunting task, but the fauna of the rocky shore is comparatively easy to quantify and as a result there is a rich literature. For similar reasons, many schemes for monitoring the impact of humans on the environment have included the study of rocky shores and these have provided, and in some cases continue to provide, a wealth of data. Both the information in the literature and that in databases accrued by monitoring organizations will be combined by MarClim to provide a powerful basis for the prediction of the future composition of rocky shore communities. The data resources available to the MarClim project are summarized in Table 1. Importantly, many of the data sets that will be considered cover the last decade of rising sea temperatures; unfortunately, however, many of the other principal data sets were suspended in the late 1980s as a result of changes in national science policy. Although regrettable, if new surveys are undertaken within the next few years using comparable methodology it will still be possible to assess the extent of change during the 1990s. MarClim will re-survey as many as possible of the principal sites used in earlier surveys and will re-map the distribution of the main rocky shore species of the British Isles.
Table 1. Principal datasets to be used within the MarClim programme.
Years in record
SW England, Isle of Man
UK, France and Portugal
UK, northern France
Rocky Shore Surveillance Group (J.R. Lewis)
Coastal Surveillance Unit (E. Jones)
Shetland Oil Terminal Advisory Group
Orkney Marine Biology Unit
Inshore fish (power stations)
Hydrography, plankton & fish
D.J. Slinn, J.R. Allen, T. Shammon
The distribution of the biota of rocky shores in the British Isles is complex and is set by the interplay of broad-scale factors, principally climate and oceanic circulation patterns, and local-scale processes such as biological interactions or variables relating to coastal topography (e.g. aspect, rock type exposure) and local hydrography (e.g. current strength, turbidity, tidal range). To predict the effects of climate change on the distribution of individual species it will be necessary to extract broad-scale signals from the noise caused by local-scale factors. Once the relationship between the abundance of a species and climate has been characterized, it will be possible to create predictive models that will simulate patterns of distribution under the various climate change scenarios. This stage of the project lies in the future but data already analysed indicate clearly the way in which climate change will have an impact on the flora and invertebrate fauna of rocky seashores.
Many rocky shores in the north of the British Isles are covered by the barnacle S. balanoides, a northern species seldom found in any abundance beyond Southern England. As Semibalanus declines it is generally replaced by C. montagui and C. stellatus. The relative dynamics of these barnacles in southwest England have been the subject of long-term study, the results of which are summarized in Figure 1(b) (from Hawkins et al. 2002) and should be viewed in conjunction with Figure 1(a). Figure 1(b) demonstrates that as Semibalanus populations decline, the Chthamalus species increase and vice versa. This relationship is mediated by climate via its effects on inshore sea temperature. Figure 1(b) is based on a data set that ceased in the late 1980s, but monitoring of barnacle populations has continued at a reduced number of sites. The most recent data show that since 1985 the warm-water barnacle species increased in abundance (Southward 1991, S.J. Hawkins & A.J. Southward unpubl. data). It should be noted that although sea temperatures have now reached levels observed in 1949–51, recent observations show that S. balanoides remains fairly common in the Plymouth area, although recruitment may be failing. Such information on the relative population dynamics of intertidal barnacles in relation to inshore sea temperature can be used as the basis for prediction of the effects of the warming of inshore waters.
Climate-driven species-level change also seems to have taken place in numerous other intertidal organisms during the study period, including the Southern Limpet P. depressa and the warm-water top-shells, O. (Monodonta) lineata (Southward et al. 1995) and G. umbilicalis (N. Mieszkowska unpubl. data). Figure 2 shows that P. depressa declined in abundance and range between the warm period of the 1950s and the cooler 1980s (S.J. Hawkins & A.J. Southward unpubl. data). A number of other common species, which are expected to respond to the warming of inshore waters, are listed in Table 2. A more complete list is given by Hiscock et al. (2001).
Table 2. Some common rocky shore species that are expected to respond to climate change in UK waters.
Widely recorded on west coast of Britain & Orkney; absent from east coast
Southern species recorded at a few locations in SW Scotland & Shetland
A southern species, present in the Channel as far east as Brighton
A southern species; reaches SW Scotland
A southern species as far as NW Scotland and Shetland
Southern species common on north and west coasts of Scotland, present in Orkney and Shetland; occasionally in N. North Sea
A northern species reaching its southern limit in SW Britain
Only recorded in SW Britain as far north as S Wales in British Isles
Southern species. Northern limit Channel Islands
Southern species found as far north as Channel Islands
A northern species commonly recorded throughout Scotland but reaching its southern limit in the Irish Sea
A northern species reaches its southern limit in Portugal
A southern species which reaches its northern limits at Anglesey in the British Isles
Commonly recorded on the west coast of Britain and present in Orkney; as far south as Filey in North Sea
A southern species commonly recorded on the west coast of Britain. N limit on north coast of Scotland
Southern species found as far north as Channel Islands
Osilinus (Monodonta) lineata
A southern species which reaches its northern limits in Northern Ireland and Anglesey
A southern species commonly recorded throughout Britain
Southern species; reaches Cornwall
A southern species abundant in parts of SW Ireland but only sporadically recorded occurrences in SW England and at a few locations on the west coast of Scotland
A northern species confined to the east coast of the British Isles; probably occurs on all North Sea coasts of Britain
Fucoid algae are characteristic of all but the most exposed intertidal areas of the northern European rocky seashores; in the infra-littoral zone they are replaced by larger laminarian species. Brown algae have a substantial effect on the composition of the community in which they live and on that of nearby assemblages. They have a direct role as a food source for grazing species, particularly gastropod snails, and as structural support for such animals as hydroids, ascidians and bryozoans. Fucoids also give shelter from heat and desiccation by keeping the rock beneath them moist and cool during the low water period. This makes it possible for animals and plants that are sensitive to desiccation to survive in an otherwise hostile environment. In this way, small crustaceans, sea anemones, hydroids, bryozoa and polychaete worms may be maintained by the presence of fucoid algae. Dead algae also sustain the biota on nearby sediment beaches where low in situ production is substantially boosted by the energy that comes from their decomposition. The strand-line community is also totally dependent on algal production.
Fucoid algae are a feature of northern shores (Ballantine 1961) and will be influenced by climate change. As sea and air temperatures increase it is probable that fucoid cover on moderately exposed shores will decline and as a consequence the biodiversity of rocky shores and the productivity of adjacent sediment assemblages will decline. The loss of fucoid algae, broadly related to temperature increase, may well be exacerbated by the effects of increases in storminess that are predicted as a consequence of global warming. Depending on their aspect relative to wind-driven seas, some rocky shores will become more exposed and as a consequence their biological properties will change. In addition to the loss of algae, increased storminess will widen both barnacle and lichen zones in the high shore, increase the shore height reached by red algal assemblages and replace barnacle/limpet associations by those dominated by mussels.
The predicted sea-level rise is unlikely to have a considerable effect on most macrotidal rocky shores. In the majority of cases the strong slope of the seashore continues well above the level of high water. As sea-levels rise so the physical zones occupied by each species will move up the shore, but as there is free space available at the top of the shore no species will be displaced. The major exceptions to this prediction will be in places where there is a wave-cut platform beneath cliffs or where the natural seashore has either been topped by an artificial sea wall or gives way to sediment. In both cases, rising sea-level will compress existing patterns of zonation.
EFFECTS ON INVERTEBRATE PREY OF BIRDS
Although a great variety of birds may feed on rocky seashores, very few are specialists relying exclusively on that habitat. At Robin Hoods Bay (North Yorkshire, UK) only nine bird species regularly fed on the rocky shore. Of these nine species there was a single passerine, the Rock Pipit Anthus petrosus, the remainder being either gulls or waders (Feare & Summers 1985). The Ruddy Turnstone Arenaria interpres and the Purple Sandpiper Calidris maritima are species particularly closely associated with intertidal rocky shores, where both species feed on small molluscs and crustaceans that live under stones, beneath macro algae or in cracks and crevices. Eurasian Oystercatchers Haematopus ostralegus are more specialized, being predators of limpets and mussels (Coleman et al. 1999). Although Dunlin Calidris alpina and Common Redshank Tringa totanus are often seen on rocky shores, they are largely transients that specialize in feeding on sand or mud. The Common Eider Somateria mollissima is characteristic of rocky habitats in northern Britain, where it feeds largely on mussels Mytilus edulis. In light of the observations on the effects of climate on the population dynamics of the more important rocky shore invertebrate species, it is possible to predict the impact such changes might have on waterbirds. The following predictions can be made:
1Climate change is unlikely to have a significant effect on the amount of biomass available for bird predation. Where large, more conspicuous species are concerned the major effect will be to change the balance between competing species. Thus in the south of England P. depressa will become more abundant than the Northern Limpet P. vulgata and C. montagui and C. stellatus will replace S. balanoides. It is likely that in species where reproductive success increases, the mean size of adult animals will decrease. This might have an impact on the efficiency with which prey are handled.
2There will be a decline in the abundance of fucoid algae and as a consequence small soft-bodied invertebrates that the algae protect from heat and desiccation will be less common. This might have an impact on species such as the Ruddy Turnstone.
3The decline in fucoid algae will have an adverse impact on the productivity of both sediments and strand line. Both habitats have their own avifauna, which might suffer as a consequence.
4Increased storminess will cause a limited increase in exposure and as a consequence an increased area of seashore will be dominated by mussels
EFFECTS OF CHANGES IN BIRD NUMBERS ON ROCKY SHORE INVERTEBRATES
In the previous section of this paper the potential effects of changes in the invertebrate fauna of littoral rock on waterbirds were outlined. However, the interactions between birds and invertebrates are not all necessarily ‘bottom-up’. Changes in bird populations are likely to affect invertebrates. Should the abundance of Arctic-nesting shore birds be reduced as a result of tundra being replaced by forest (Zöckler & Lysenko 2000), a significant source of predation on small molluscs and crustaceans will be lost and as a result some populations might expand. Under warmer conditions, Common Eider might retreat northwards and, as a consequence, their predation on mussel beds might be reduced and hence their spatial coverage might extend.