380 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1958 
Let us say that environment abounds in niches or opportunities for 
particular organisms to carry on, and that the course of evolution has 
been a process of filling these niches, meanwhile creating new ones 
at every step. Thus the oak tree, once established in its own niche, 
possibly a sunny, well-drained slope facing southwest, affords suitable 
niches to woodpecker, squirrel, fungi, and insects that find on it food 
or shelter, or both. We ourselves create niches for dogs, poultry, 
rats, maize, and wheat, and an impressive list of smaller organisms 
whose number may vary inversely with our use of soap and water. 
Any species moving into a niche has an effect or reaction upon 
the situation. Let us call this its role. The effect of this role may vary 
widely. Thus the squirrel’s habit of burying acorns is a means of 
planting many oaks, while its fondness for buds in early spring must 
be a more or less efficient substitute for the self-pruning that we see 
in the cottonwood. The role of coyote in keeping a balance among 
rodents and thus protecting the grasses and other plants that sustain 
the whole native menagerie of the Yellowstone area I have observed 
myself. From years of studying such remnants of undisturbed nature 
as I could, I have concluded that, on the whole and in the long course 
of time, unless an organism in its niche performs a role that con- 
tributes to the balance of nature, it is likely to be eliminated. 
Every organism we know about, with one interesting exception to 
be discussed later, is confined to a limited geographical area known 
as its range. Within that range it is usually restricted to certain 
favorable habitats that afford it an appropriate niche. These pat- 
terns are set by the operation of what are called limiting factors. 
Thus the commercial growth of maize is limited in the north by cold, 
in the west by dryness, and to the east by a combination of topog- 
raphy, soil chemistry, and economics. Any plant or animal requires 
a certain constellation of favorable conditions to survive and does 
not thrive beyond the point where any one of these conditions becomes 
unfavorable or, as we say, limiting. 
Some of the most interesting cases are those in which the limiting 
factor is the presence or absence of another species. Orchids may 
require certain fungi, cowbirds and cattle egrets follow grazing 
herds, walnut roots poison tomato plants, and tsetse flies carry 
sleeping sickness, thus keeping people out of an area and favoring 
certain African mammals which otherwise man would exterminate. 
The hunter and naturalist have long known that plants and animals 
occur in characteristic communities or groups. To speak of jack 
rabbits or prairie dogs is to suggest grassland. We would expect to 
see squirrels in a woodland of oak and hickory, ptarmigan and conies 
in the beautiful flowered alpine meadows above timber line. Even in- 
sects and microscopic forms of life may be known by the company 
they keep. 
