A "comprehensive" school of population regulation also 

 developed (Thompson 1929, Andrewartha and Birch 1954). 

 Theories developed therein held that at some time, all factors 

 were involved in population control. In contrast to the 

 "balance" emphasized by Nicholson, these authors emphasized 

 the constant fluctuations of populations and indicated that 

 populations were controlled by the total environment. The 

 importance of weather or climate as a major regulating factor 

 is usually apparent in the models of this "school" (Birch 

 1957). Milne (1957: p. 260) summarized the view of Thompson 

 as follows: "where and when the organism meets favorable 

 conditions its numbers increase, but these conditions never 

 endure long enough for unlimited increase; nor do unfavorable 

 conditions endure long enough for a decrease of numbers to 

 zero— except in places that are in any case unsuitable for the 

 organism." This view was essentially the same as that of 

 Andrewartha and Birch (1954) in that the most important way in 

 which animal populations are limited is "by shortage of time 

 when the rate of increase, r, is positive." Milne's (1957: p. 

 265) own theory generally agreed with the comprehensive 

 school, though he revived density related terms: "For the 

 most part, control of increase is due to the combined action 

 of (a) density-independent and (b) imperfectly density- 

 dependent environmental factors." Milne believed that only 

 rarely did the perfectly density-dependent factor of 

 intraspecif ic competition play a role in population control. 

 He also believed that a balance occurred in the environment, 

 not between populations and the environment; that is, over 

 time and space, favorable environmental conditions were 

 balanced by unfavorable environmental conditions . A more 

 recent comprehensive view (Huf faker and Messenger 1964) held 

 that density-dependent processes control populations in 

 favorable environments and density-independent factors control 

 populations in unstable or marginal habitats. 



These earliest theories centered on the role of extrinsic 

 factors in population regulation: food, predators, weather, 

 disease, shelter, etc. ( Krebs 1972). Intrinsic factors and 

 individual differences in animals were largely ignored. They 

 came to the fore, however in a new school of thought, 

 developed in the mid- to late- 1950s, which emphasized 

 individual differences in animals and proposed that intrinsic 

 changes in the population could lead to self -regulation 

 (Chitty 1955, 1960) . 



Proponents of self regulation believed that uncontrolled 

 competition for resources could ultimately lead to destruction 

 of the food base and extinction. Because most populations do 

 not decline to extinction, it was argued that some form of 

 self regulation must hold animal population levels below those 

 where resource destruction occurs. 



