ANIMAL POPULATION

Dominion, Volume 23, Issue 22, 21 October 1929, Page 18

ANIMAL POPULATION

ARE VARIATIONS CAUSED BY CLIMATE? SCIENTIFIC VALUE OF SOLUTION Amongst the papers presented to the Agricultural Section of the Conference of Empire Meteorologists, held in London in August last, was an interesting one on “Variations in Animal Populations” by Mr. Charles Elton, M.A, of the Department of Zoology and Comparative Anatomy, University Museum, Oxford. “We are all aware,’! said Mr. Elton, “that plagues of mice, various kinds of • insects, and other animals, • occasionally harry the land, but recent research has shown that marked .variations in animal populations are the rule .rather than the exception. Not only beetles and mice, but also hippopotami and elephants show them. Numbers seldom remain constant from one year to the next, though in some cases the fluctuations are not so pronounced as in others. It is found that with some animals these variations pass through cycles of astounding regularity. For instance, the cockchafer beetle usually appears in large numbers every three or four years in Europe. , The details of this cycle vary in. different parts of Europe, but in certain parts the cockchafer outbursts have occurred exactly every three years for the last CO years. “The question of interest was the extent to which climate is concerned in these fluctuations. Since their effects would be so widespread and all pervading, it is natural to expect that variations in climate would be closely related with the fluctuations in population. In order 1 , to investigate this question it is desirable to choose, first, species of animals in which the phenomena are clear cut, long series • of records are available and the biological factors at work are understood. Among these, the most noteworthy are, perhaps, lemmings and mice. “In all countries of the world in which the subject has been investigated, wild mice and voles are subject to very violent and sometimes astonishing fluctuations in numbers. In the northern and alpine regions of - the Northern Hemisphere the mice and voles are replaced by certain species of lemmings.- Much has been written about the migrations of these animals in Norway, where they sometimes reach spectacular proportions, and long series of records regarding them are available. These show that migrations occur every three or four years. Related animals such as voles and mice, whatever their habitats, are subject to the same periodicity. “Furthermore, willow grouse in Norway go through similar cycles, and there is some evidence that the cycles are parallel with those of the lemmings. It might be imagined that epidemics might be responsible for these periodic fluctuations, but it is difficult to conceive how such widely differing species could be attacked at the same time, when it is known- that the epidemics to which they are subject are caused by entirely different organisms. There is some evidence that the cycles are related to the amount of the snow covering on the Norwegian mountains.” Mouse and Lemming Cycles. “But more remarkable than the synchrony of the fluctuations in Norway is the fact that corresponding fluctuations amongst the lemmings of Labrador and - Northern Canada agree closely with them in phase. Roughly speaking, it can be shown that all the region from Northern Labrador, and Baffin Land, to Scandinavia and the British Isles have mouse or lemming cycles whieh tend to come in the same years. This region is that covered by the area of low air pressure of which Iceland is the centre. One would, therefore, naturally look for the cause of the variations in some climatic cycle which affects this low-pressure area as a whole, possibly influencing rainfall or snowfall.” “The snowshoe rabbit or varying hare of Canada has a ten-year cycle in its numbers which has been going on for the past 90 and probably for the past 120 years, according to the records of the Hudson Bay Company. The economic importance of these cycles is considerable. The lemming, for instance, is the main food of the fox, and when lemmings are scarce, foxes, and therefore furs, are scarce also. The yield of fur will fluctuate with the number of lemmings. Again, the varying hare is food for fox, lynx and other fur-bearing animals. “The ten-year cycle has been observed in the numbers of other animals such as the wolf, the mink, the fisher (a large weasel) and the pine marten. Particularly good statistics arc available for the fisher, and these show that its cycle has run parallel to that of the varying hare but in an opposite sense. “Here again, in the ten-year cycle, it is difficult to imagine any other controlling cause than that of climatic fluctuations.” The Scientific Aspect. “The elucidation of these cycles,” Mr. Elton states, "happens to be of great importance economically, as well as from a scientific point of view. For instance, the mouse cycle is one - of the worst problems of forestry,' since voles eat or destroy seedlings of trees, and even young trees themselves. In Labrador, the mouse-cycle affects almost every living thing, and is one of the keys to the native life. In Europe, mouse-plagues are frequent and costly, and so far little controlled. The rabbit-lynx-fox cycle affects the fur trade, and rabbits are an important factor in destruction of planted seedlings of forest trees. In South Africa, and in the Volga region in Russia, the rodent cycle is the key to the problem of plague, for every few year? there are outbreaks of endemic plague caused by the periodic epidemics of wild rodents. It may be mentioned, also, that there are important fluctuations in the numbers of edible fish.” “The biologist needs the help of the meteorologist in the work of elucidation, but the meteorologist, also, has something to gain. These marked cycles in animal numbers, Mr. Elton, thinks, . should suggest to the meteorologist periodicities in climate and weather for which he should search. They may also help him to find more satisfactory means of indicating climate than the statistics he at present employs.