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 some ability to live under the especial montane conditions. The common upland grass, the sheep’s fescue (Festuca ovina) is thus represented in the montane zone by an allied highland species (F. vivipara) which has the ability to produce young plants in place of the floral structures. This feature is much accelerated by, if not wholly dependent on, the existence of humid conditions, and this is probably the reason why the viviparous form of this plant is found at low altitudes along the seaward margins of Western Britain.
It seems that in order to get some idea of how climatic factors affected upland plants one would have to consider the influence of whole seasons upon the growth of a chosen plant. After making observations upon a number of plants it became clear that there were good practical reasons for using a relatively common plant like the moor-rush (Juncus squarrosus) as material for estimating these effects of altitude. This plant has certain practical advantages for work of this type. It occurs at almost every altitude in Britain and it prefers the wet and base-deficient peaty soils which predominate in the uplands.
The plant consists of a rosette of rather fibrous leaves just above ground level with a long flower-stalk bearing an upper group of brownish flowers or fruits Pl. IX. The latter contain numerous small seeds. At ground level there is a woody stem having numerous roots. The flower-stalks are numerous, they are tough and so can be collected rapidly and transported for subsequent measurement. The fruits, small brown capsules about a sixth of an inch long, are also tough and numerous enough to give suitable numerical measurements. The inflorescence is laid down as part of a bud in summer. It develops the following year, and its length may be taken as a partial expression of the conditions favourable to growth in the preceding summer and
FIG. 15.—Effect of altitude on moor-rush, Juncus squarrosus: L, Length of flower-stalk; N, Number of flowers produced; R, Number of mature capsules.
also in the summer in which it has developed. These conditions affect reproduction in addition by controlling the number of flowers and, later, of fruits and seeds. The only method by which the plant is distributed is by the numerous small seeds.
If one studies the performance of such a common moorland plant at different altitudes, it is apparent that the amount of growth and the production of flowers, or better still, of fruits and seeds, both diminish as the altitude increases (see Fig. 15). But fruit production is affected far more than growth in length, so that a point is reached, generally about an altitude of 2,500 ft. to 2,700 ft., above which fertile fruits are not usually produced, although the plants may form inflorescences of considerable size and in other ways be capable of making satisfactory vegetative growth.
This effect is evidently due mainly to the retardation of the development of the flowers and fruits. Thus in the Lake District in 1942, flowering was completed during June at 700 ft., but it had not begun at the end of July at 2,000 ft., and, at 2,500 ft. to 3,000 ft., it was not complete by the end of August. Thus at these highest levels there was little or no chance of most of the fruits becoming mature and they did not in fact do so. Again, in late September, 1943, only one mature capsule per 20 plants was found on the summit of Ingleborough (2,373 ft.). These and similar facts thus suggest that viable seeds are not usually formed above about 2,500 ft. to 2,700 ft., although large and healthy plants can be found up to at least a thousand feet higher. Until 1947, viable seeds had not been collected from above 2,700 ft., but the exceptionally long and warm summer of that year led to very abundant seed production—so much so that viable seeds were obtained from 3,400 ft., on Ben Wyvis.
In view of the infrequency with which such seeds are formed at high levels, the presence of moor-rush plants at 2,700 ft. and upwards is interesting. They are certainly very long-lived (twenty years or more) and possibly originally due mainly to transported seeds. It is noticeable on some mountains that the plants are not only sporadic but also are often collected in colonies, suggesting a group of individuals centred round a parent plant which has fruited only at rare intervals. The fruits are,
