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.
SOILS
THE second important group of factors in upland habitats is the nature of the soil covering—or perhaps more strictly, of the surfaces available for plant growth. Geologically, as we have seen, these surfaces may be classified either as stable or unstable, depending on whether they are still subject to active erosion or not. As habitats for plants there is a more profound difference between these two classes. Most of the unstable surfaces are rocky or are covered by rock fragments in various stages of disintegration, and even their physical properties differ greatly from those of fertile lowland soils. They are, in fact, soils in the making, and it is one characteristic of upland areas that they exhibit in profusion all the varied stages of soil-formation. We see the native rock breaking down under the action of frost and other weathering agents to rock fragments, which become
