ANIMAL POPULATION

Hokitika Guardian, 24 October 1929, Page 8

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., ol the Department of Zoology and Comparative Anatomy, University Museum, Oxford. 4

“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 rathei than the exception. Not only beetles and mice, hut 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. l'oi 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 chockchafer outbursts have occurred. exactly every three years for the last 60 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 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 migration 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 these 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 which tend to come in the same years. This region is that covered by the area of low air pressure of which Iceland istfie centre. One would, therefore#!) naturftjjy look for the cause of the variations in some ■climatic cycle which affwts this lowpressure 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, as the main food of the fox, and when lemmings are scarce, foxes, and therefore furs, are senree 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 are available for the fisher, and these show that its cycle has run parallel to that of the varying hare hut 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. “ Tlie elucidation of these cycles,” Mr Elton states, “ happens to he 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 kevs to the native life. Tn Europe, mouse-plagues are frequent and costly, and so far little controlled. The rab-

bit-lynx-fox cycle affects the fur trade, and rabbits are an important factor in destruction of planted seedlings of forest trees. 111 South Africa, and in the Y T olga region in Russia, the rodent cycle is the key to the robleni of plague, for every few years there are outbreaks of 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.