THE ANIMAL AND ITS ENVIRONMENT 



103 



never actually does. There are always con- 

 trolling influences, such as competition for 

 food, the ravages of the environment, in- 

 fectious disease, and many others. Further- 

 more, a single species may fluctuate widely 

 from season to season. Grasshoppers may 

 be very numerous one year, devouring all 

 vegetation over large areas, whereas the 

 next year there may be very few. Barring 

 man's intervention, this may be caused 

 by unseasonal weather during the young 

 stages when the organism is sensitive to 

 adverse conditions. Ruffed grouse may in- 

 crease steadily for several years until they 

 reach great numbers, then suddenly de- 

 cline, much to the disgust of the hunter. 

 Indeed, this rise and fall in the population 

 of certain species is called rhythm, and it is 

 so constant that it can be predicted. Some- 

 times animals reach tremendous numbers, 

 then go into a decline from which they 

 never recover, and eventually become ex- 

 tinct. The passenger pigeon is a good exam- 

 ple. In other cases, like the American bison 

 and the whooping crane, an attempt is 

 being made to save them from extinction 

 by the animal who nearly caused it in the 

 first place, namely, man. 



We have seen that various controlling in- 

 fluences keep any one species of animal 

 from realizing its potentiality for spreading 

 its kind over the earth. However, if the so- 

 called reproductive pressure is allowed to 

 exert itself ever so little, the results are 

 often unfortunate. One example will suf- 

 fice. The English sparrow was first intro- 

 duced in Brooklyn, New York, in 1850 and 

 1852 for the purpose of controlling certain 

 insect pests that were destroying the shade 

 trees of the city. In England the bird was 

 desirable and because of its natiual ene- 

 mies existed in modest numbers. In Amer- 

 ica, however, it was free from its predators 

 and the full powers of its reproductive 

 capabilities came into play. Within a few 

 years it became a pest. Instead of eating the 

 insect pests, it fed on garden produce and 

 cereal grains, man's own food, and in addi- 



tion destroyed other insect-eating birds. 

 By 1886 it had spread to Salt Lake City and 

 today its distribution is continent-wide. One 

 interesting fact has been noted about the 

 English sparrow: it no longer populates the 

 streets of our cities as it did a veneration 

 or two ago when the horse and buggy was 

 a common means of transportation. Grass- 

 hoppers and other insects clinging to the 

 car radiator are a poor substitute for the 

 horse droppings that dotted the streets 

 some years past. 



Such mistakes as the one just described 

 have been made on numerous occasions by 

 man. Sometimes injurious animals are im- 

 ported into new regions because they have 

 escaped border inspections and have subse- 

 quently become established, later to be- 

 come serious pests. Great care is now taken 

 to prevent this from happening. Many 

 states have inspections on railroads and 

 highways to keep any injurious pests out. 

 Airplanes must be carefully inspected when 

 they fly from one region to another, par- 

 ticularly when the two are great distances 

 apart. The danger of introducing certain 

 disease-carrying insects, such as mosqui- 

 toes, into a new environment is obvious. 



Man has intentionally introduced some 

 animals to prey upon others with excellent 

 success. One good illustration is that of the 

 ladvbird beetle introduced from Australia 

 a few years ago to destroy the cottony 

 scale which was playing havoc with the 

 citrus crops of California. This required the 

 research efforts of an entomologist who 

 studied the enemies of the cottony scale in 

 its native Australia. The ladybird was fi- 

 nally decided upon and when brought to 

 this country proved very successful in par- 

 tially controlling the pest. It has not, as yet, 

 become a pest itself. 



Our economic zoologists are well aware 

 of the great precaution that must be exer- 

 cised in interrupting the delicate balance 

 of life in any environment. For example, 

 when the insecticide DDT was first avail- 

 able for public consumption, many enthusi- 



