54 J. H. PRICE, I. TITTLEY & W. D. RICHARDSON 



the distribution in the British Isles and, perhaps, in adjacent areas. At least on British shores, the 

 changes seem largely to have involved peripheral withdrawal, which a comparison between past 

 authenticated reports and present detailed observations clearly indicates. By contrast, in Dorset 

 a local tendency to the opposite effect has been suggested by Burrows (see Dorset: Kimmeridge 

 Bay). Evidence for this as a general tendency is slight and any detected expansion may remain both 

 local and ephemeral. 



Changes such as these are neither easy to predict nor simple to elucidate, even though the 

 British coasts clearly represent part of the northern distributional limits of the species. An increase 

 in environmental pollution may be a factor contributory to the local changes in some areas, but it 

 is not likely to be the principal overall reason. Long-term natural fluctuations or other major 

 events both within and outside the alga need to be examined in critical detail. This is not often 

 possible; there is a lack of biological data on many aspects of the species. Where there is evidence 

 sufficient to indicate possible influence on the distribution, the effects have been examined. 

 Harvey (1849 : pi. 91) indicated Padina pavonica to be '. . .[1] abundant in the Tropical Ocean, and 

 reaching its northern limit on the southern shores of England, without exhibiting any depaupera- 

 tion from climate. The British specimens are fully as large as those from warmer latitudes, and as 

 well coloured. [2] This being the case, one would naturally expect that it may yet be discovered 

 further north . . .'. Comments [1] and [2] bear on biological events and on distribution, and are 

 worth examining in the light of all available data: [1]. The majority of specimens from the 

 Mediterranean infralittoral appear to be of greater size than most British plants, although there 

 are exceptions (see Dorset: Chapman's Pool). Liddle (1975) has shown that P. sanctae-crucis 

 (correctly P. jamaicensis (Collins) Papenf.) responds to the stress of growth in the intertidal by 

 comparable reproductive maturity and activity at a size on average smaller than plants in the 

 analogous state in the subtidal. Since reduction in size does not necessarily, except in its extreme, 

 reflect depauperation, Harvey's remarks are to that extent correct. [2]. Individual northerly reports 

 are considered under counties. As to the more general pattern, it is not clear if the peripheral 

 distribution in northern Europe is due to past and current changes that are phasic, or to entirely 

 random or irregular variations in populations from time to time. If the pattern includes advance 

 and retreat components, regular or irregular, of distributional change, the available data reliably 

 demonstrate for recorded time only the retreat component outside the foci of distribution, although 

 within the latter at population level both components may be occurring contemporaneously in 

 different locations. At the level of overall British distribution, available information therefore 

 suggests that gross loss and gain changes normally require very long periods of time to be fully 

 effective, although locally adverse conditions may rapidly affect the distribution of populations. 

 There is certainly no evidence of both loss and reappearance in any of the areas peripheral to the 

 foci, most of the facts pointing to loss and just a few, such as the Cork Harbour find, to recent 

 ephemeral gain. Very long periods of observation would be necessary to establish the overall 

 nature of the pattern with certainty; some such observations are in progress by one of us 

 (W. D. R.). The shores to commence with are clearly those for which previous authentic records 

 exist in the absence of current populations. The best summary of the revealed overall pattern is 

 that the British, French Channel, Belgian and, Dutch coasts form an area of which the marine 

 environment is in places (the warmer, shallow water 'cases') tolerable to Padina from time to time. 

 Since the alga as now known is quite specific in its habitat requirements, only areas away from 

 the direct effects of strong wave-action are ever likely to support populations. 



Origins of populations and distribution foci 



Speculation on the origin of Padina populations and distribution foci on southern British shores 

 really requires more information. The outlined contraction in distribution could be taken to 

 indicate that the present areas with long histories of records (Devon; Dorset; Isle of Wight) are 

 simply relict foci of a distribution previously much more widespread on all parts of the coasts of 

 the British Isles. In view of recent work on the sea-surface temperatures of the Ice-Age earth 

 (Mclntyre et al., 1976; Lamb, 1977), this seems rather unlikely. If the present distribution results 

 from gradual and fluctuating progressive spread into Britain from the south at some time in the 



