the triggering cue and thus, all are variations of density- 

 dependent population regulation. 



We could find no evidence to support the theory of self- 

 regulation by populations. Mule deer on our study area 

 employed a range of intrinsic behavioral and physiological 

 mechanisms to deal with the variability of extrinsic factors. 

 Variable emigration, food habits and habitat use, and movement 

 strategies were all examples of such mechanisms. However, 

 these intrinsic mechanisms by themselves, did not function to 

 regulate population growth. Rather, they dampened the effects 

 of variable extrinsic factors. Thus, population growth was 

 not as fast nor decline as severe as might have been the case 

 in their absence. 



It was also unnecessary to invoke intrinsic mechanisms to 

 explain major changes in the population. Behavior that 

 contributed toward slowing population growth (parturition 

 territoriality and emigration) occurred at all densities and 

 did not halt population growth. A condition cycle (another 

 possible intrinsic factor??) that explained some variations in 

 reproduction and mortality was influenced to a large extent by 

 variable weather (an extrinsic factor). Our conclusion is 

 that intrinsic mechanisms interacted with extrinsic factors to 

 moderate population growth and decline, but did not directly 

 regulate population growth. 



The role and importance of extrinsic factors in 

 regulating or limiting population growth has received the 

 greatest attention from population biologists . Various 

 extrinsic factors (e.g., weather, food, space, predators) have 

 been suggested to regulate populations in either a density- 

 dependent or density-independent manner, or both. Regulation 

 by resource shortages (especially food) through density- 

 dependent feedback (Nicholson 1933) is widely accepted. 

 However, limitation of population growth through density- 

 independent changes in weather (Andrewartha and Birch 1954) 

 also has its adherents. 



We found that weather affected the deer population in 

 various ways, both directly and through influence on other 

 extrinsic factors such as food, space, and predation. Our 

 data clearly showed that fawn recruitment was directly 

 influenced by forage conditions (see Chapter 5). Through a 

 delayed condition cycle, adult mortality was also influenced 

 by forage conditions. We found no evidence, however, that 

 forage production, availability, or conditions were 

 substantially influenced by deer density. Thus, we also found 

 no evidence for population regulation through a density- 

 dependent feedback loop. Rather, the primary influence on 

 forage conditions was density-independent changes in weather. 

 This finding is strongly emphasized because it is a major 



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