British Isles.
Very good examples of considerable local variations in climate which can thus be detected are to be found in the eastern Pennines—particularly in the Teesdale-Baldersdale-Stainmoor district just south of Mickle Fell. Stainmoor itself is a well-known bog area (see here) which has a rainfall near to 55 in.; but this rainfall decreases very rapidly towards Lune Forest and Baldersdale on the north and east respectively, where other very different types of moorland vegetation hold sway. Very striking is the frequency of cloud-cover or showers over the Stainmoor bogs in contrast to the clearer skies of the drier and more easterly areas.
On a far grander scale, similar contrasts may very often be seen in the central Scottish Highlands. The eastern mountains, and perhaps especially the Cairngorms, may stand out cloudless or with small fair-weather clouds when the big western Bens are sunk in mist or dwarfed by rain-clouds. The contrast seems to become noticeable about a line drawn north and south through Loch Ericht or Dalwhinnie.
ALTITUDE AND ORGANISM
The influence of climate on upland organisms has so far only been considered in the most general way. We have observed that there is a correlation in distribution between certain types of soil condition and certain types of climate. Thus we assume that the bogs of the Western Highlands are associated with the wet climate. In a similar manner we may observe that there are some plants and animals found only at high levels, the special montane species, and we assume that they are there because they are in some way more suited to the severe climate existing at high altitudes. We have little evidence as to how the climatic factors are effective and it will be useful accordingly to discuss this matter a little more fully.
The distribution of plants is obviously a very important factor in animal distribution, not only for grazing mammals but also for the insects which live on and in plants. In such cases the influence of altitude may be indirect, and there are, as we shall see, instances of the distribution of the animal following that of the plant. If we are to consider plants, the influence of the soil needs to be taken into account, and we have already seen reason to believe that the wet climate may be effective through its influence on soil conditions. But climatic humidity varies greatly in different parts of the country—being high in the west and lower in the east. If this were the effective montane influence then we should expect to find a richer montane fauna and flora in the west. It is well known that on the whole there are on the eastern mountains more of the species restricted to high mountain life; so that in one aspect at least humidity cannot determine the altitudinal zonation. However, the fauna and flora of upland country as a whole is very different from that of the lowlands, in proportionate representation if not always in the individual species, and a large part of this upland fauna and flora is associated with the ill-drained and wet soils. What humidity does do is to give great areas dominated by a limited fauna and flora of this type, which is upland rather than montane and which is evidently related to the soil conditions induced by humidity.
The more common view and one which has been referred to and used already in this chapter, is that temperature largely controls the altitudinal zonation, and we may look at this problem as something which would repay attention from naturalists and as a subject which requires little in the way of special equipment.
The principal biological effect of temperature is that it greatly affects the rate of biological processes. Thus a lowering of temperature such as would be experienced at a higher level would retard growth and development so that there would be less likelihood of a given developmental process being completed within the shorter period available in a montane summer. Some upland organisms do in fact appear to take longer over a given process of development. A well-known case is that of a moth, the northern eggar (Lasiocampa callunae), which spends two years in the larval stage instead of the one characteristic of the original woodland race, the oak eggar (L. quercus). It is unlikely, however, that the difference is due to the lower temperature of the upland habitat. To double the period of development, or to halve the rate of development, would require a reduction of temperature of about 7·5° C. or 14° F., equivalent to an increase in altitude of about 4,500 ft.! The lengthened larval period may be just too long to fit into one growing season, but it seems more likely that the change in the length of the life cycle is either genetical or mainly due to nutritional differences imposed by the moorland habitat.
There are, of course, other ways in which lower temperatures may affect distribution. Where two organisms are dependent on one another for success, but possess life-cycles of different duration, an alteration in temperature may put the two life-cycles “out of step” with one another, as it were. A case which might involve something of this nature is one in which an insect mined or fed on a plant organ at some particular stage in development, as in an example discussed later in this chapter.
Lastly, of course, alterations in temperature may produce qualitative effects on plant and animal metabolism (in the widest sense), and it is perhaps in this direction that we have to seek an explanation of the tendency of certain insects to be represented by short-winged races at higher altitudes (see here). In plants, the effects of temperatures approaching the freezing point are often to induce the conversion of insoluble food-reserves like starch to soluble sugars. To this type of change has been ascribed the immunity of some evergreen plants from frost injury, which is attributed to the difficulty of freezing cells containing a high sugar-concentration. Undoubtedly the presence of these sugar solutions does confer on plant tissues a certain immunity from frost injury and the effect may easily help to account for the over-wintering of arctic and montane plants, just as it would undoubtedly be advantageous in helping to promote the rapid growth and early flowering observed in arctic climates. Dr. Scott Russell has verified the existence of high sugar-concentrations in spring in arctic plants collected on Jan Mayen Island and in the Karakorum mountains.
The only clear effect of this general type I know of in animal tissues is the very characteristic production of orange-coloured and fat-soluble pigments in certain aquatic copepods during the winter months and commonly also in cold, high-level tarns.
When one goes on to consider the ecological effects of these factors in nature, it is generally difficult to dissociate the effects of temperature and humidity. Thus the presence on mountain-tops of certain spiders usually found in damp cellars might plausibly be attributed either to high humidity or low temperature. A clearer example of the influence of temperature on animal distribution is that of the alpine flatworm, Planaria alpina, for this lives in water and is not therefore subject to the great variations in humidity which may effect mountain-top habitats. Planaria alpina is a small creature about a quarter of an inch long, resembling a somewhat flattened grey slug. It is a carrion feeder, living under stones in the margins of streams and in mountain runnels. In this country, these little water-courses usually contain a second, much darker species of flatworm, Polycelis nigra. The two species are always distributed in the same way, P. alpina at the higher levels, certainly at least to 2,000 ft., and P. nigra in the lower reaches of the water-course. This distribution is mainly a matter of temperature. Numerous observations in Britain and on the Continent have shown that P. alpina is never found in nature where the temperature exceeds 14° C., while P. nigra may be found where the water reaches as much as 20° G. Further, prolonged observations on the animals under controlled conditions by Mr. R. S. A. Beauchamp have shown that P. alpina cannot long survive temperatures exceeding 12° C. Thus in nature it occupies the high-level runnels and cold springs, occurring at high levels in mountain districts. There are reasons for believing that other animals confined to high-level streams and soils owe their distribution to similar effects, particularly perhaps certain insect larvae.
It is less easy to point to instances in which similar effects are produced on plant distribution, though they doubtless exist. Plants are not able to change their positions readily, and most of the high-level species are perennials, which means that the effects of the environment if not immediately lethal are likely to be the integration of the prolonged effects of the given habitat factor or factors. In some cases, perhaps especially in grasses, a given species is represented in the montane zone by separate races, often it may be not very distinct in form, but possessing