Rocky shore communities are particularly susceptible to the effects of extremes in weather. Stormy weather can cause the disturbances described above but at the same time, heavy seas produce spray which reduces desiccation stress for high shore species. Wet weather reduces the salinity of rock pools. Cold weather can cause freezing stress and even ice scour at high latitudes. Calm weather reduces wave action, prolonging the period of emersion, which has been reported to increase the mortality of barnacle cyprids (Connell, 1961a,b). Warm, dry weather increases desiccation stress. It is unsurprising that the geographical range of many rocky shore species is determined by climatic conditions. Not only can unfavourable weather conditions cause mortality to susceptible species, they can also affect the outcome of competitive interactions between species. The competitive ability of a species will be limited by increases in a stress to which it is susceptible. The prevailing climate ultimately affects the sea temperature which is the major factor determining the breeding success and therefore the distribution of most marine organisms. Sea temperature and, therefore, the distribution of organisms can be indirectly affected by the climate of more distant areas, acting through ocean currents. Changes in temperature may also affect reproductive output directly. Southern species, such as the barnacle Chthamalus stellatus, may produce fewer offspring over a shorter breeding season if temperature drops (Burrows et al., 1992).

The UK straddles a major biogeographic boundary with many southern species reaching their limits in Ireland or southwest Britain (Lewis, 1964). At various points on the UK coastline, species associated with warm temperate regions reach the northern limits of their distributions and arctic and boreal species reach their southern limits. The exact distribution range of species is usually variable in response to changes in climate. This fact is well illustrated by the changes in rocky shore communities in southwest England recorded from 1950 to 1993 by workers at the Marine Biological Association (Southward et al., 1995). During this time, the annual average local sea temperature followed a rising trend until 1960, cooled from 1961, markedly so from 1970 and began to rise again after 1981. During periods of warming, warm water species increased in abundance and extended their range while cold-water species declined. The reverse was true during periods of cooling.

Changes in the relative abundance of the cold-water barnacle Semibalanus balanoides and its warm-water counterparts Chthamalus stellatus and montagui can be seen with both location and time. These changes show strong links with climatic conditions (Southward, 1991, Southward et al., 1995) and larval supply (Kendall et al., 1985).

On the shores of Great Cumbrae in the Clyde Sea, C. montagui is restricted to high shore levels (Connell, 1961a,b; 1972) with S. balanoides dominating the mid and low shore. Connell (1961a,b) showed that competition with the faster growing S. balanoides restricted C. montagui to upper shore levels/ In the Southwest of England, Chthamalus spp. and S. balanoides coexist on the mid shore, though the exact balance of numbers varies with climate (Southward et al., 1995). In the 1950s, Chthamalus spp. dominated on south-western shores. A cold winter in 1962/3 led to a rapid increase in S. balanoides numbers and this species remained abundant during the period of cooling in the 1960s and 1970s. Chthamalus spp. numbers have increased since the late 1980s but have not yet reached the levels seen in the 1950s. The ratio of Chthamalus spp. to all barnacles shows a particularly strong relationship with climate. Bay of Biscay sea surface temperatures provide a good predictor of this index with a time lag of two years (Southward et al., 1995). This time-lag approximates to the average interval between the beginning of reproduction in successive generations of Chthamalus spp. The relationship with offshore sea temperatures suggests the strong influence of the ocean on distribution patterns in the English Channel.

The interaction between Chthamalus spp. and Semibalanus balanoides is greatly affected by physical factors. Temperature appears to be among the most important of these, affecting fecundity, settlement, recruitment and the outcome of direct competition for space. Low temperatures reduce the breeding period, settlement and recruitment of Chthamalus spp. (Kendall et al., 1985). Survival and recruitment in Semibalanus are adversely affected by high temperatures late in the year (Kendall et al., 1985). The coexistence of these barnacles in the Southwest of England owes much to the availability of space on the shore due to occasional years of low recruitment (Burrows, 1988; Southward, 1991). The relative reproductive output of S. balanoides and Chthamalus spp. varies with temperature. Since both have refuge populations producing a supply of larvae, both can take advantage when conditions swing in their favour. In areas dominated by S. balanoides, the presence of Chthamalus spp. on the high shore may be explained by increased mortality of the dominant species due to desiccation or heat stress. Wethey (1984), working in New England, showed that S. balanoides were able to overgrow, crush or undercut Chthamalus spp. on the high shore, thus out-competing them for space, when provided with shade.

Patella vulgata is usually the most abundant limpet on UK shores. Its range extends from the Arctic to the southern tip of Portugal. P. depressa is a more southern, warm water species. Its range extends from the North Wales coast to North Africa. In the late 1950s, towards the end of a period of warming which began at least as far back as 1920, P. depressa was well represented in the limpet fauna of Southwest England. Its numbers declined throughout the 1960s and 1970s and began to increase again in the mid 1980s. During the 1980s, a rapid increase in P. depressa numbers was recorded on the North Cornwall coast coupled with a decline in P. vulgata. Limpet grazing has important consequences for the community, leading, for example, to the dynamic Fucus-barnacle mosaics observed on moderately exposed shores (Hartnoll and Hawkins, 1985, Hawkins and Hartnoll, 1983). Absolute changes in limpet numbers can therefore have community wide effects. On the west coast of England, however, the absolute number of limpets stays more or less the same but the species balance changes.

Long-term changes in temperature have apparently led to changes in the southern limit of the range of the cold water kelp Alaria esculenta (Widdowson, 1970). Similarly warm water species such as the topshell Monodonta lineata and the barnacle Balanus perforatus were commoner and more widespread on Southwest English coasts in the 1950s than in the late 1960s. As with the limpets described above, several complementary pairs of rocky shore species exist. Each pair consists of a northern and a southern species. Where their ranges overlap, changes in the ratio of the two species are seen, mediated by some climatic factor. A well studied pair is the northern barnacle species Semibalanus balanoides and its southern counterparts Chthamalus stellatus and Chthamalus montagui.

The climate affects every stage in the life history of rocky shore species. Duration of the breeding season can be temperature dependent, as in Chthamalus stellatus and Chthamalus montagui producing fewer broods near their northern limits (Burrows et al., 1992). The survival and settlement of larvae is affected by sea temperature and hydrographic transport processes. The level of stress experienced by settled individuals at each shore level is tightly linked to climatic conditions. Thus, changes in climate affect the reproductive fitness and, therefore, distribution and abundance of individual species. Changes in the numbers of important species are likely to have profound effects on community structure. Since there is well documented evidence of changes in the distribution and abundance of rocky shore species in response to climate change (Southward et al., 1995; Hawkins et al., 1993) and rocky shore communities are easily surveyed, rocky shores may provide a means for monitoring long term climatic change.

The composition of the shore community also changes with the seasons. For example, increasing daylength in the spring encourages the proliferation of ephemeral algae such as green Enteromorpha and Ulva and brown Pilayella and diatoms. Seasonal fluctuations in these algae are especially pronounced high on the shore. The barnacle population is depleted by the foraging activity of dogwhelks from spring to early winter. The population is replenished by settlements of S. balanoides in the spring and Chthamalus in the summer and autumn. Such seasonal changes are common to many communities and can exist in the absence of long term changes.

Next Section                     References