The alpine vegetation just below the bare windswept summits of those mountains consists chiefly of mosses (Rhaccomitrium and Arctostaphylos) mixed with Cladina and other lichens, dwarf whinberries, and stunted heather. Among the heather will also be found dwarf shoots of blaeberry (Vaccinium Myrtillus) and Alchemilla alpina. Patches of alpine bearberry (Arctostaphylos uva-ursi) and other plants beaten down by the wind form a soft carpet affording a pleasant change to the hill climber who has been scrambling upon rough screes and boulders.
The lower reaches of the mountains are covered with peat. The Caithness peat hags have been gradually denuded by the action of wind and by drainage, but on the lower slopes of many hills there is still a heavy covering. The prevailing moorland plant is heather (Calluna vulgaris), which grows luxuriantly in sheltered places where the conditions are favourable, and shelters a varied undergrowth according to the nature of the soil and the supply of moisture. At one time nearly the whole of Caithness was covered with a layer of heather-clad peat, with an undergrowth of such plants as Hypnum and Hyocomium, where the drainage was good, and Scirpus and Sphagnums in the bogs and marshes. Dwarf birches rowans, alders, and willows grew in sheltered places, while crowberry and Scilla verna covered the grassy sward of the lower moors fringing the coast. As the climate became more temperate, and as artificial drainage was introduced, peat gradually receded from a great part of the lowland plains. Heather also gives way in places where the bracken fern nourishes or the whin (Ulex Europaea) is common, so that the undergrowth of such plants as Oxalis, Trientalis, and Luzula increases in vigour.
Along the banks of the streams and rivers the vegetation varies according to soil, exposure, the supply of moisture, and the amount of protection afforded by the river banks. Some plants prefer a loam, others a sandy or gravelly bed. Birches, though stunted, occur on the steep banks of various streams with an undergrowth of ferns and mosses. In Langwell and Berriedale valleys the trees attain a goodly growth, but the distribution of scrubwood along the watercourses of Caithness, as a whole, is not so wide as it once was, Hazel is common along some parts of the Dunbeath valley, while the alder and willow are more partial to the Berriedale and the Langwell, perhaps on account of the difference of the underlying rock formation. In the gravelly loams along the river beds there occur Cynosurus cristaius, Festuca, and Agrostis. The loams in the lower reaches of the Thurso and Wick rivers yield Eleocharis palustris, Equisetum, and Cares with sedges, reeds, and rushes.
Along the Reisgill burn there is a varied flora under scrubwood consisting of birch, aspen, hazel, willows, and juniper. The birch woods of the Langwell valley are contracting in area, perhaps owing to deer as well as to the ravages of fungus and the denuding effects of wind. Young birches are not met with so often as seedling rowans. This is explained by the fact that birds help to distribute the berries of the rowan while the seed of the birch is spread chiefly by the action of the wind.
On the rocks and cliffs of the coastal regions are many varieties of lichens and marine algae. The sandy beaches at Sinclair Bay, Freswick, Dunnet, Thurso, and Reay have sand dunes and hillocks with the usual arenaceous vegetation, such as Amphilla arenaria, Carex, and Tortula, which help to fix the dunes and stretches of sand converting them into "links"as a Keiss and Dunnet. There are reaches of shell-sand at John o' Groat's and Murkle, where the lime gives vigour to the grasses and to attractive flowering plants such as Primula scoica and Parnassia.
The only important woodland plantations in Caithness are in the Duke of Portland's policies at Langwell, containing not only pine, larch, and other conifers, but a goodly variety of hard woods and ornamental trees.
In the Keiss mounds and primeval rubbish heaps investigated by Samuel Laing traces were found of the reindeer, the beaver, the great auk, and the goat. Fragments of the bones of these animals have been found in brochs and in situations indicating that they were contemporary with primeval man. Mr Laing also found traces of the wild ox, the horse, the stag, and the dog. The Norsemen, according to the sagas, hunted the reindeer in the uplands of Caithness.
Among extinct animals which persisted into the historical period were the wolf and the wild boar, which survived in Caithness until the seventeenth century. In the eighteenth century the wild cat, the pole cat, the badger, and the marten were fairly common in the county though now verging upon extinction, if not already extinct. The bat, the hedgehog, and the squirrel arc extremely rare and yet they occur in Sutherland. The common seal (Phoca vitulina) and the grey seal (Halichaerus gryphus) breed in the caves along the coast. Various species of whales occur from time to time in the neighbouring waters, such as the Right whale (Balaena biscayensis), the Sperm whale (Physeter macrocephalus), and the Rorqual. By far the most common of the cetaceans in British waters are the Finner (Balaenoptera musculus) and the Sei (B. borealis).
The county is rich in bird life. Mr Harvie Brown mentions about 220 different species (including residents and occasional visitors), and there have been numerous additions to the list of Caithness birds since the publication of his book about a quarter of a century ago. Along the coast sea-birds abound in number and variety, while owing to the extension of young plantations at Langwell and Berriedale woodland birds have steadily increased. Among birds now becoming rare are the sea-eagle, the osprey, the raven, and the ptarmigan (sometimes driven from its mountain haunts by injudicious burning of heather). The starling and some species of gull are on the increase.
Of reptiles the adder and the lizard occur in moorland districts, but the slow-worm, common in Sutherland, is somewhat rare. Frogs are plentiful and the toad is common, but the newt is not found.
Fishes abound both in inland and marine waters. Owing to the introduction of mechanical power in seafishing many new species of marine fish have become known. Such varieties as witch-soles and megrims, now commercial fishes, were practically unknown before the days of trawling. The industrial shell-fish (lobster, crab, whelk, and mussel) occur in fair abundance, while John o' Groat's buckies (Cyprea Europaea) well known to shell collectors, occur on the shores of the Pentland Firth.