The historical review of scientific work done in the Outer Hebrides precedes a scientific profile of the archipelago including geographical, geological, botanical, zoological and resource outlines. Effects of the use of natural resources by man on the natural environment are discussed. Management of land and water for the purposes of nature conservation are given by descriptions of the Loch Druidibeg and Monach Islands National Nature Reserves and other Sites of Special Scientific Interest. These exemplify existing case studies and procedures in nature conservation conducted by the Nature Conservancy Council and others. In conclusion, the Outer Hebrides are set in the context of the National Planning Guidelines of the Scottish Development Department which show resources of ‘national’ importance in agriculture and nature conservation.

The Outer Hebridean fauna is almost entirely the result of the chance characteristics of opportunist colonizers modified by subsequent adaptation and sometimes later immigrants. A few primitive relicts may persist (e.g. possibly, the moth Nyssia zonarid), but the fauna is certainly not largely composed of early post-glacial invaders. These, traditional theory argues, are likely to have been eliminated on the mainland by later arrivals but to have been able to persist on the islands by the timely breaking of hypothetical land-bridges.

The influence of the original island colonizers is discernible through the persistence of similar traits in both ancestral and descendant populations despite massive genetical differences produced by stochastic sampling, and often by an apparently random pattern of differentiation between islands (e.g. field mice, cats, men, wrens). Adaptation is difficult to detect in the presence of unmeasurable founder effects, but is shown by large-scale clines (such as bridling in guillemots or colour phase frequencies in arctic skuas), by the existence of similar local races in different species (notably by melanism in Lepidoptera, especially Triphaena comes; but also possibly in bumble-bees and dragonflies), and by evidence of local selection (e.g. Cochlicella acuta).

In the light of these many pressures, simple equilibrium theories of island biogeography are woefully inadequate; every species has to be considered on its merits. This can be done for terrestrial vertebrates, and virtually all but the Pygmy Shrew prove to have been introduced by man.

The unsettled cool-maritime climate of the Outer Hebrides represents an extreme modification of that of the British Isles generally; yet while it imposes strains, as an environment for man it has long been far from unproductive. Frequency of rain as showers, wind, high humidity and slow evaporation provide difficulties; but there is a relatively long, if slow, growing season ending in a mild autumn. Results of instrumental observations over 120 years are discussed; trends are compared, but closer analysis is hampered by changes in location of station. Some indication of extent of gain from increased shelter is given.

South of St Kilda, Atlantic water reaches the edge of the Continental Shelf west of the Hebrides from the eastern side of the Rockall Channel, whereas the main north-east going current from the western side of the channel probably lies close to the shelf-edge to the north of St Kilda. On the shelf, Atlantic water mixes with Clyde and Irish Sea water to the south-west of the Small Isles, and the mixed water so formed diverges to flow partly northward through the Little Minch and partly southward to Barra Head, then northward along the western coasts of the Outer Hebrides. On the outer shelf, dense winter-formed bottom water persists into the autumn, implying that advection is weak in this region in summer.

Data of temperature and salinity are sparse around the Outer Hebrides. Annual temperature cycles at depth on the shelf can be explained in terms of the cycle of wind mixing. A non-divergent Continental Shelf wave gives rise to strong diurnal components in the tidal streams upon the outer shelf and in the Sound of Harris, but causes no perceptible disturbance to the semi-diurnal rise and fall.

The major rock types which comprise the Outer Hebrides are described and the geological history of the region is discussed. The control exercised by the variations in structure and lithology on the topography is illustrated and an assessment made of the economic potential of the various rock units.

The results of measurements of contour areas at 1:63,360 scale allow a quantitative elaboration of the geomorphoiogical regions identified by Professor Alain Godard. Appropriate graphical techniques clearly indicate the sharp altitudinal breaks between the elements of these regions and show how they vary from island to island. Sample slope measurements are compared with clinographic estimates from the contour areas and suggestions made about relative rates of erosive processes in the different regions. It is hoped that some light is thrown on the relative importance of rock type, differential uplift and of Tertiary and glacial processes in the geomorphoiogical differentiation of the area.

Contrary to previous ideas, it is suggested that Harris and the major part of Lewis were covered by a large local ice cap during the last glacial phase. Local glaciers from central Lewis and the interior of North Lewis moving over the eastern fringe across the Eye peninsula and towards the north-west coast appear to have overridden and redistributed deposits from earlier glaciers of external origin. There is some evidence that the northern tip of Lewis and the Tolsta Head peninsula were also invaded by external ice during the final glacial phase. The application of a theoretical icesheet-surface profile substantiates the concept that Scottish mainland ice at its maximum advanced over northern parts of the island. According to the C-14 date of an organic layer beneath glacial till at Tolsta Head the last incidence of external and/or local ice in the island occurred after 27,300 years BP. Deglaciation phenomena at Uig, south-west Lewis, mark an earlier stage in the dissolution of the ice cap, while hummocky moraine in the mountain and submountain zone indicate the retention or renewed formation of valley glaciers during the late Devensian.

Several types of machair landforms are found in the Uists (Outer Hebrides) but the most distinctive are low altitude plains. Morphological and stratigraphical evidence suggests that both erosional and depositional processes, operating over a long period of time, have produced these sand plains and other landforms. A vast but relatively finite (partly shell-derived) sand supply was swept landwards across the gentle offshore rock platform assisted by a rising sea level to provide the initial pre-machair coastal accumulation. Phases of development can be dated using archaeological evidence, and 14C dating of offshore peat suggests that initiation probably took place before 5700 BP. Although the evidence is fragmentary, machair development appears to have consisted of long periods of stability, interrupted by episodes of major environmental disturbances. There is also evidence of coastal erosion and associated physiographic changes.

The distribution of soil parent materials in relation to glaciological features is dealt with; from the scoured bare rock of Harris products of Tertiary weathering have been transported northwards and deposited as coarse textured till which underlies the Lewis till plain. Clayey till in the north of Lewis would appear to be deposited by mainland ice which crossed the area. The dominant genetic soil found throughout the islands is a peaty podzol with iron pan which contains an intensely indurated horizon. Peat formation has developed on this soil as a result of the acidity of the parent material and anaerobic waterlogged conditions. Only on well sloping sites were peaty podzol soils with free drainage beneath the pan observed, otherwise the peaty podzol with very poor drainage beneath the pan is better described as a peaty gley soil. Brown earth soils were seen only on ultra-basic rocks on markedly sloping sites.

As a result of exposure and frequent high winds the calcareous dunes of the machair are unstable. Fresh sand is deposited on the dry back slopes with a diluent effect on soil fertility. Areas of the machair having imperfect rather than poor drainage provide some of the best arable land which would be rated as Class 3 in the Land Use Capability Classification of the Soil Survey.

The Outer Hebrides are oceanic in character, bathed by comparatively warm waters derived from the North Atlantic Drift. The spring tide range is 3·5 m (west coast) and 4 m (east coast).

The southern and western coasts include some of the most exposed shores in Britain and the steep rocky shores exhibit species and zonation patterns typical of north-west Britain. The sublittoral zone is mainly rocky and is dominated by beds of Laminaria hyperborea, which are very extensive west of the Uists.

The eastern sides of the larger islands are much more sheltered and have numerous sea lochs many of which have great lengths of very sheltered rocky shore, dominated by extremely vigorous growths of fucoid algae, particularly Ascophyllum nodosum. Some of the lochs have extensive ramifications, with localized tidal tidal rapids supporting a rich and luxuriant flora and fauna.

The paper reviews the literature on the ecology of the rocky shores, on the seaweeds in particular, and includes a full list of all seaweeds recorded in the islands with their distribution by island. (Two hundred and sixty-four species are recorded and this is about 38% of the British total.) Thus the seaweed flora is rich and diverse; the presence of selected species is discussed.

The large brown seaweeds grow very luxuriantly in the Outer Hebrides and have been used by man there for many centuries. The history of this utilization is briefly reviewed. The fucoids and Laminaria spp. were formerly used extensively as a source of alkalis (sodium and potassium carbonates) and iodine. Nowadays very large tonnages of Ascophyllum are regularly harvested for the production of alginates; quantities of cast up Laminaria hyperborea are also used for this purpose.

The abundance and composition of the macrofauna of Outer Hebridean beaches is determined mainly by the degree of exposure to wave action. Most of the beaches are on the western seaboard, many without any form of shelter, and thus receive the full force of the Atlantic swell.

Twenty principal sites in Lewis and Harris were sampled for intertidal macrofauna.

Beaches of open aspect are populated by amphipods and Eurydice pulchra with very low densities of polychaetes. The sands of the Minch coast and some of the western beaches which are sheltered by islands or headlands usually have a Tellina–Nephtys association, with a tendency for Tellina tenuis to be replaced by Donax vittatus with increasing shelter. Very sheltered inlets and the fords of the Uists have a well-developed Cerastoderma-Macoma association, while Traigh Mhor in Barra, dominated by Cerastoderma edule, has the richest fauna of any beach in the Outer Hebrides.

The history of previous molluscan work in the Outer Hebrides shows that most work has been done offshore, and that the littoral zone of Lewis and Harris has been neglected. In the present paper mollusc populations of different macro- and micro-habitats are examined empirically, and compared and contrasted with each other. Habitats have been chosen to exhibit as wide a variety of environment as possible, from the wave-lashed exposed rocks facing the Atlantic to the very sheltered sea lochs opening into the Minch. The richness and diversity of the fauna is noted.

In July 1976 six rocky littoral sites and eight rocky sublittoral sites in Lewis and Harris were sampled for polychaetes. The sites ranged from those on the exposed west and north coast to more sheltered east coast sites.

A total of 107 species were recorded during the survey and are presented in an annotated species list. Syllidae are the dominant family both in number of species and number of individuals present, although the Serpulidae, Spionidae and Sabellidae are also well represented. Similarity of habitats sampled at different sites makes it possible to relate differences in species composition to the conditions prevailing at these localities. Such comparisons are made for four habitats—Corallina officinalis, sediment under stones on the shore, Laminaria holdfasts and the upper shell valves of Pecten. Twenty polychaete species were found in the C. officinalis habitat, 24 species in sediment under stones on the shore, 61 species in laminarian holdfasts and 22 species on the upper shell valves of Pecten.

In general the sublittoral macroinvertebrate fauna, although lacking the Lusitanian elements found in the warmer waters of south-west England and Ireland, was found to have a high species diversity and to exist in an environment virtually free from man-made pollution.

The paper reviews four topics in relation to the vegetation of the Outer Hebrides: (a)history and flora, (b) environment and habitat, (c) geographical relationship, and (d) conservation. The preliminary flora edited by J. W. Heslop Harrison in 1941 remains the best published list, and there is need for a modern flora. A provisional checklist is presented, based upon recently published data, especially the Atlas of the Bristish Flora (Perring and Walters 1962) and the Critical Supplement (Perring 1968) Miss M. S. Campbell has been responsible for a great deal of recent recording. Geology and climate are critical factors, and soils are mainly peat, except in the extensive machair areas. Significant habitals are coastland, wetland, peatland, moorland and upland. woodland is scarce, though there are good amenity woodlands. Each habitat is reviewed in relation to the floristic lists in the Nature Conservation Review (Ratcliffe 1977b). There has been no phytosociological study of the vegetation overall, though particular localities such as St Kilda have been described. The geographical elements are listed, and related to those of Skye, Shetland and Faeroe. The flora is more restricted than that of the mainland. Important categories recognized are (a) species of localized distribution, (b) southern species reaching their northern limits and (c) ‘Atlantic’ species. The origin of the flora is discussed, and it is concluded that the facts have not yet been clearly demonstrated. Eight potential threats are identified as fire, changing agricultural practice including drainage, large-scale peat, sand and gravel extraction, the oil industry, wave-energy generation and tourism.

The distinctive ecological elements of the machair environment, calcareous sand and an oceanic climate, which combine to give a lime-rich but unstable habitat, in which machair vegetation develops, are discussed initially. A number of different studies employing both phytosociological and ecological approaches, and drawing data from machairs in the Uists, Monach Islands and South Harris, are summarized. Based upon these, the main characteristics of machair vegetation and its key ecological relationships are described. Finally a model of machair vegetation is advanced in which two distinct types of vegetation, ‘dune’ and ‘pasture’ are recognized, and patterns of vegetation development are explained by means of ‘phases’ and ‘sequences’, terms applied to describe relative location of types of vegetation in both time and space.

The vegetation of the peatland of Lewis is described. Several different types of mire are distinguished according to their morphological features. The gradual transition from minerotrophic conditions within valley mires to fully ombrotrophic conditions in the blanket mires of watersheds is demonstrated. Whilst most of the blanket mire of Lewis is affected by severe peat erosion, examples were chosen to demonstrate the nature of the flora and vegetation within those mire systems at present unaffected by erosion, as well as the more characteristic eroded peatland.

Fifty-eight epiphytic lichen species are listed from Lewis and Harris, and their geographical connections are discussed. Twenty-nine of them were collected on this island for the first time. Judging from the occurrence of Lecanactis homalotropa, Pyrenula laevigata and Ramalina obtusata (which are confined to very few places in the British Isles today and may be regarded as relics) on a small number of old and mostly dying sycamores near Rodel at the southern end of Harris, there had been an older forest of which these trees form the last remnants and from which the colonization of Stornoway Woods may have originated. Some of the lichens also occur on dwarf-shrubs which have certainly been present all the time, or on rocks or ground. Man may have played a part in introducing lichen-species along with timber. A few notes on epiphytic algae, fungi, mosses and ferns are added.

Centred on Loch Druidibeg, macrophyte vegetation was surveyed in and around 30 lochs in northern South Uist. Alkalinity ranges from 0·014 to 2·33 m.-equiv −1, conductivity (25°C) from 100 to 33,900 μs cm−1, chlorinity from 0·66 m.-equiv −1 upwards. Lochs are classed broadly as non-calcareous (gneissic), calcareous (machair) and brackish. This is an example of a Hebridean range not found elsewhere in the British Isles. Fen vegetation is highly modified by grazing, whilst tall reedswamp and floating-leaved plants are scarce on the mainly exposed and rocky shores, factors which contribute to the predominance of Fucoids in brackish water, of the open Littorella-Lobelia association in non-calcareous water. The Potamogeton filiformis-Chara association is confined to sand in calcareous machair lochs. Deep freshwater vegetation is typified by Isoetes, Potamogeton perfoliatus and P. praelongus.

The very wide ranges in alkalinity and conductivity are reflected in some unusual species' distributions. For example, in freshwater lochs (conductivity up to 660 μs cm−1) several species like Isoetes lacustris, confined elsewhere to oligotrophic water, also occur in South Uist at moderately high alkalinities (to 1·8 m. -equiv 1−1). Conductivity varies with chloride concentration in all but the calcareous (machair) lochs; brackish lochs range from 2500 μs cm −1), which floristically, apart from Fucus ceranoides, is fresh water, to Fucoid-dominated rocks, and Ruppia on silt, in conductivities up to 33,900 μs cm−1. Of other angiosperms, Potamogeton gramineus appears to tolerate conductivities of 22,000 μs cm −1. Causal distribution in general is discussed.