Productivity and Populations 299 



"carrying capacity is a property of a unit of range" rather than a 

 property of a species. Errington and Hamerstrom (1936) define 

 carrying capacity for bobwhite quail as "the upper limit of survival 

 possible in a given covey territory as it exists under the most favor- 

 able conditions." They then restate it "as the level beyond which 

 simple predation upon adult birds, their o\\ti territorial intolerances, 

 and their tendencies to depart from coverts overcrowded with their 

 own or some otlier species do not permit continued maintenance of 

 population." In a later paper, Errington (1941) again summarizes 

 it thus: ". . . carrying capacity functions as a threshold of security 

 below which the numbers of wintering birds can rarely be forced 

 very far through attacks of wild predators alone." While we repeat^ 

 edly refer to the carrying capacity of the environment or range, 

 these definitions clearly imply that it may not be a property of the 

 range alone but also partly a species property. This is commonly 

 the case with the rulfed grouse. 



Any unit of grouse range unquestionably has a fixed upper limit 

 of winter survival, but this limit is conditioned primarily by environ- 

 mental attributes, such as the strategic location of cover components, 

 predation, and weather plus innate species characteristics, rather 

 than by limitations of food and shelter. Just as carrying capacity for 

 bobwhite quail is more a function of species properties than is the 

 case of the hoofed mammals, so carrying capacity for ruffed grouse 

 is even less dependent upon the food and shelter conditions of the 

 range than in the case of the quail. 



The food and shelter conditions in normal grouse range have a 

 potential carrying capacity far in excess of the actual. Predation, 

 either direct or conditioned, usually reduces the winter population 

 well below the ability of the range to sustain and shelter. Even more 

 important, the species itself will not tolerate crowding beyond a 

 definite point. This brings in the concept of "saturation point," a 

 purely species characteristic. When the potential carrying capacity 

 of the range is a density that exceeds the saturation point, then satu- 

 ration point becomes a component in determining the actual carry- 

 ing capacity. This seems to be the case on good grouse range. On 

 poorer or marginal range, the potential carrying capacity of the en- 

 vironment is more likely to be lower than the saturation point, thus 

 not allowing the latter to be a factor. In this case, the carrying capac- 

 ity may actually be an environmental attribute exclusively. But even 



