Читать книгу Mountains and Moorlands - W. Pearsall H. - Страница 12

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 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, perhaps, distributed in the wet wool of sheep, for, as far as is known, no mammals eat the inflorescences although snow-buntings habitually eat the dry fruits in winter and so may help to disperse seeds. The rush is commonest on sheep-infested mountains, and although it occurs to at least 3,700 ft., I have looked for it in vain on the high and grassy Scotch summits where deer habitually graze.

However, it seems certain that the effects of altitude are differential, affecting the seed-production most, flower-production less and vegetative growth least. The analysis of these effects shows that they vary little as between districts receiving great differences in rainfall, and they can thus be attributed mainly to the diminution of mean temperature with increasing altitude. Thus temperature, though it actually operates by controlling the relative rates of development, affects the distribution mechanism.

It is interesting to carry this problem a little further by considering how these things affect a little rush-moth, Coleophora caespititiella (see Pl. 30), that lives in association with the moor-rush and also with the common rush. Its life-history is not very well known, but moths are mature and the eggs are apparently laid in June–July, on or near the flowers of the rush. The larvae then feed on the growing seeds inside the developing fruit. By about the end of August, the infection of a fruit capsule becomes noticeable because of the presence of the larval case, a small cylindrical and white papery object in which the larva may live (see Pl. XI). The larvae, possibly usually with the case, leave the rush-heads in late autumn and hide in the surrounding vegetation until the following summer. With certain obvious precautions, the presence or absence of the white larval cases can be used to study in an approximate way the extent to which the population of heath-rush is infected by the moth. The data also give a picture of the altitudinal distribution of the moth. This is much more restricted than is that of the rush on which it lives. In the central Lake District, in 1942, the frequency of the larvae decreased rapidly from a maximum infection of about 40 per cent of the capsules at 700 ft. and no signs of the moth were seen above 1,800 ft., although in that district the moor-rush goes up to 3,000 ft. Now at first it was thought that the larval cases might become more frequent at a higher level later in the year. In fact larval cases were never seen above this level except in the abnormal summer of 1947, when some were found at 2,000 ft. on the south-facing slopes of Saddleback.

It seemed obvious at first that at higher altitudes the lower temperatures would retard the development both of rush-flowers and of the moth growth-cycle, for both last a year. When no infection was found above 1,800 ft. it was thought that the lower average temperatures might so retard the development of the larvae from the egg to the case stages, that the cases were not produced at higher levels even although there was infection. In this case the larvae might fail to over-winter or the whole growth-cycle might take two seasons. However, no evidence of a later infection at higher levels could be found.

A possible alternative explanation was that, as suggested earlier, the whole growth-cycle of the moth might get “out of step” with that of the rush, so that mature moths and “infectable” rush-flowers (i.e. in the young stage when they are infected) might not coincide in time.

This does, in fact, happen, though not quite in the manner expected. It was found, in samples from the higher levels, that only the early maturing fruits were infected by Coleophora. It followed that there was normally no infection above 1,800 ft. because no rush-flowers were normally open in July above that altitude (1944 and 1945). Even in the abnormal summer of 1947, no sign of infection was seen above 2,000 ft. (and this on a south slope) in the Lake District, and in the Eastern Highlands (Ben Wyvis and Rothiemurchus district) none was noted above 1,400 ft. On the whole, then, it seems as though the main population of mature Coleophora individuals comes out at one time, about June–July. It may then infect any rush-flowers which are then open. This severely limits its altitudinal range, for as we have already seen, the high-level flowers are not mature at these early dates. One difficulty about these findings is that there seems no reason why the cycle of development of the moth should not be retarded somewhat at the higher levels just as that of the rush-flowers is. If this were the case, a small number of late-maturing individuals should appear at higher levels. No individuals of this type have been seen, nor has it been possible to find signs of rushes which might have been infected in this manner. It seems to be only possible to explain this apparent absence of the mature moths at higher levels by assuming also a temperature bar to their development such as we have already encountered in the flatworm Planaria alpina.

There are many further observations that could usefully be made on this matter. It appears that Coleophora is generally confined to lower altitudes on the Eastern Highlands as compared with the Lake District, and, at first, it seemed that lower mean temperatures might explain this. However, I have not seen this moth at all in the Western Highlands or Islands—perhaps because I have generally been too early or too late for the moth and too early for the larval cases. Certainly the creature seems to be much less common in this area of high rainfall, a result that could not be easily explained on the grounds of temperature alone.

The brief summary of upland climates and the analysis of their possible effects on animals and plants suggests that temperature has much to do with the zonation of plants and animals we observe on ascending a mountain. It controls the distribution of some organisms because they are not able to live in the higher average temperatures of the lowlands. In other cases, it seems that the low montane temperatures so lengthen the life-cycle that it cannot be completed in the short mountain summer. Perhaps more often low temperature retards some part of the developmental cycle, so that we get short-winged insects (see here), or plants unable to produce flowers and fruit. For these reasons, some zonation of organisms is inevitable as altitude rises and temperature falls.

In practice, the most widespread influence of altitude is the change in the character of the prevailing plant communities, with all that it implies in its effect on animal habitats. Most noticeable is the disappearance of woodlands and trees with their varied faunas and ground floras. As this commonly takes place at about 2,000 ft. and as the restricted montane species appear above that level, we may take it as a convenient altitudinal separation of montane and sub-montane zones.

Within the limits thus defined by temperature other factors must play their part. Every naturalist knows that shelter from wind is often vitally important, so that here and there among the mountains there are oases in which the frequency of plant and animal life is altogether different from that found on the exposed and wind-swept faces. Within the limits imposed by temperature, humidity also exerts its restrictions, not only by presenting a range of habitats running from pool or rivulet to desiccated rock, but by influencing the character of the soil. It is to the consideration of these soil conditions that we must now turn.