220 The Ruffed Grouse 



and half the birds that reach maturity— and that may, under certain 

 conditions, far exceed these rates of destruction— cannot be dis- 

 missed hghtly, even though a part of the losses be conditioned by 

 other factors. 



It may be well for us at this time to review the meaning of "Hmit- 

 ing factor." It is that element which ultimately prevents any more 

 of a given species from existing or surviving than actually do. To 

 clarify the matter let us take a couple of simple, and factual, exam- 

 ples. 



The Lancaster County region of southeastern Pennsylvania is an 

 intensively farmed area. The vegetation on the land is mainly crops 

 (com, wheat, oats, tobacco, hay) and pastures. The woodlands are 

 small, scattered, and lacking in conifers. This range contains no 

 grouse. Clearly the limiting factor is a lack of suitable habitat, shel- 

 ter first, and food also but probably to a lesser degree. It is obvious 

 that predation is not limiting the grouse here, since there are no 

 grouse for them to capture. 



There are cases in the Northeast where good grouse habitat exists 

 almost literally in the shadow of the tall buildings of large cities. 

 Unless this habitat is protected as a virtual sanctuary, it will ordi- 

 narily contain no grouse. Man's interference, as a hunter, or in seek- 

 ing recreation, or cutting the underbrush for a park, or in other 

 pursuits, eliminates gi-ouse from this potentially habitable range. 



These are two cases where a single element serves definitely and 

 continuously to act as a limiting factor upon grouse— in both cases 

 an excluding type of action. Under more normal circumstances, rec- 

 ognition of the limiting factor is more difficult. Moreover it may 

 change from one year to the next. It is very likely to change as the 

 grouse population itself fluctuates. There may be times and places 

 that predation will act as the ultimate determinant of the level of 

 grouse population, but these will not be general. They will most 

 commonly be where the number of birds is well below both carry- 

 ing capacity and saturation point. There will be times at almost any 

 density of grouse when weather conditions will act as the major 

 brake to increase. This may merely condition predation, as in severe 

 winters, or may act more directly, through chick mortality. When 

 populations are high, disease may limit the birds or condition an 

 increase in predation, or the natural intolerance of the birds for their 

 own kind (each requiring so much territory) may be the factor that 



