112 ANIMAL ECOLOGY 



the country for miles, making agriculture impossible, and in 

 some places forming a danger to lambs, since the latter get 

 caught inextricably on the thorns of the blackberry plants. 

 Finally, after a good many years there is often a natural dying 

 down of the plague. This is in most cases not due to the 

 direct efforts of mankind in killing off the pests, but appears 

 rather to be due to the animal striking a sort of balance with 

 its new surroundings, and acquiring a set of checks which act 

 fairly efficiently. Thus the rabbits in New Zealand now 

 apparently have periodic epidemics, which reduce the popula- 

 tion. A parallel case among plants is that of the Canadian water 

 weed (Elodea or Anacharis canadensis), which was introduced 

 into Europe by an enthusiastic botanist during the nineteenth 

 century, and subsequently spread for some years like wild- 

 fire, choking up rivers and lakes. After a certain time it 

 appeared to lose its great multiplying power, and has now 

 settled down to be a normal and innocuous member of the 

 flora. The fame of Anacharis is still so great, however, that a 

 certain town council in Britain, faced with a plague of '' water- 

 bloom " in one of their lakes (water-bloom being caused by 

 species of blue-green algae increasing abnormally in the water), 

 hopefully stocked the lake with swans in order to eat down the 

 Anacharis which was living there quite harmlessly. There are 

 now a great many swans there. 



1 8. We have seen that animals possess extremely high 

 powers of increase, which sometimes have a chance of being 

 realised with results which are often very remarkable. Such 

 high powers of increase do not merely reside in animals which 

 have very large broods or breed very fast : there is no animal 

 which could not (theoretically) increase enormously, given 

 sufficient time and opportunity. Not more than fifty years 

 at the maximum, or in most cases not more than two or three 

 years, are required to achieve this result. For instance, the 

 opossum (Trichosurus vulpecula), which was introduced from 

 Australia into New Zealand, has only one young one every 

 year, yet in some places it has increased alarmingly. 

 Thomson ^'^^ says that the black-tailed wallaby introduced on 

 to Kawau Island " ate out most of the vegetation, and starved 



