December 15, 1922] 



SCIENCE 



673 



of specific energy; some «an even stand what 

 may aptly be called eeologic punishment. 



In any one locality the field observer comes 

 to recognize a few or many rather intangible 

 units which he calls "ecological niches"- — sep- 

 arate cubby-holes or dwelling places or habi- 

 tats (in ithe narrowest sense), which differ in 

 essential respects from one another. If the 

 topography and vegetation be varied, there are 

 many of these niches; if more uniform, there 

 are few of them. Each niche is separately 

 occupied by a particular kind of bird, and the 

 locality supports just as many species of birds 

 as there are niches; furthermore, the numbers 

 of individuals of each bird are correlated 

 directly with the degree of prevalence or dom- 

 inance of the niche to which that particular 

 bird is adapted. In other words, — and here is 

 the crux of the idea, — both the number of the 

 species and the number of the individuals of 

 each species, in a locality, are directly de- 

 pendent upon the resources of the environment, 

 from an avian standpoint. The same notion 

 holds, of course, for all other animals, including 

 Homo. 



Bate of reproduction in any species has been 

 established down through past time so as to 

 supply the population needed to keep the ap- 

 propriate niche filled. This rate varies with 

 the natural prevalence of the niche, and with 

 the hazards to which the niche occupant is ex- 

 posed. Not only that, but a wide margin above 

 the normal need is provided to meet that ex- 

 treme emergency which may arise but once in 

 a thousand generations; in other words, there 

 is produced a large surplus — an apparent great 

 waste — of individuals over and above what is 

 needed to keep the appropriate territory fully 

 populated, in order to save the species from 

 extinction at some critical moment; for ani- 

 mate nature abhors a vacuum no less than does 

 inanimate natm-e. A recent writer in Science 

 (LV, May 12, 1922, pp. 497-505), Professor 

 A. F. Shull, has, in another connection, called 

 this fact of over-production the "factor of 

 safety." He says : "The entire struggle for 

 existence is based on the principle that security 

 and advancement are best assured through 

 wasteful over-production." The employment of 



the factor of safety, I would say, is a manifest 

 device on the part of nature ito insure con- 

 tinuity of species, and hence also to make evo- 

 lution possible. 



A British ornithologist, Mr. H. E. Howard, 

 has lately put out a book in which he elab- 

 orates exhaustively the idea of the importance 

 of territory to bird life. Kind and availability 

 of territory determine the kind and amount of 

 bird life. In final analysis, when a territory, 

 or, as I would express it, more explicitly, an 

 ecological niche, becomes full, and this in 

 normal times comes to pass very quickly, the 

 individuals within the species constitute each 

 other's worst enemies. Continued conflict for 

 space — for a piece of land, for an area of 

 meadow, for a section of tree-trunk, for a given 

 unit of volrmie of twiggery or foliage— is plain, 

 to be seen by any diligent observer of bird life. 

 The resulting pressure for territorial expan- 

 sion reminds one of the same pressure obtain- 

 ing among humans ; only, among birds, there is 

 no organized warfai'e. The process is one of 

 struggle as between individuals or pairs of 

 individuals, between neighbors, indirectly, per- 

 haps, as a rule; but also, often, direotly, by 

 personal action. The most fit to compete, 

 sometimes the most fortunate, will survive; the 

 less fit will be eliminated. The survival pros- 

 pects of each single individual are small. Vast 

 numbers of individuals are poured in. The 

 "safety factor" in numbers is there in order to 

 insure the persistence, and continued adaptive 

 improvement, of the species. 



Let us now return to more matter-of-fact 

 considerations. What have been some of the 

 effects of the settlement of California by the 

 white man, upon the environments of birds t 

 Have any ecological niches been effaced? Have 

 any niches been added? Have some been re- 

 duced in prevalence and others increased in 

 prevalence, relatively? What have been the 

 effects upon ithe niche-occupants? 



Perhaps the most conspicuous changes 

 wrought in the appearance of the landscape in 

 the southwest have resulted from irrigation. 

 In substantiation of this statement, many of 

 my readers can doubtless appeal to his own 

 memory. I, mj^self, recall traversing long 



