80 ECOLOGY AND LIFE HISTORY OF THE COMMON FROG 



predecessors is that, provided the conditions remain the same, the 

 population density of an animal is kept approximately constant by the 

 operation of "density-dependent factors": that is, factors that cause 

 the death rate to rise as the population rises, and to fall as it sinks. 

 This, of course is the principle of many automatic regulators, such as 

 thermostats. It is stated, moreover, that this is the only type of control 

 that can ensure a balance. It is important to note that one of the 

 premises on v^^hich the argument rests is that the numbers of an animal 

 vary only within quite small hmits, and that, even when some dis- 

 turbance occurs, such as a hard winter, the original numbers are 

 regained in a few seasons at the most. Lack also considers that the 

 usual idea that reproductive rates are adapted to death rates is erroneous : 

 it is the other way round, for he gives evidence to show that, in some 

 birds, the reproductive rate is fixed by natural selection so that it is 

 the most efficient rate. If, for example, more eggs than the average 

 are laid, the number of young birds reared is not more, but less, because 

 deaths from starvation are more numerous. It is my opinion, which I 

 shall support with examples, that in the frog these premises are not 

 always true. Regulating factors of the type described do occur, and 

 may be very important, but instability to the point of extinction is 

 characteristic of frog populations, and the disturbances are not short- 

 Hved, but may last for many years. Factors exist, I think, not dependent 

 on density at all, and they may even be inversely density-dependent, 

 so that they bear most severely when the population is smallest. 

 Catastrophes, in a special as well as a general sense, are of common 

 occurrence. It follows from this that I think that, under some conditions, 

 reproductive rates have become adapted to death rates. It is quite 

 possible that some of these differences are a matter of scale. Solomon 

 (1949) has pointed out that whether a person takes the view that a 

 population is stable or not depends very much on the time scale used 

 by that person. Viewed over a century, a population may appear 

 stable, but from one year to another, it appears unstable. Much the 

 same can be said in terms of space. The habitats of an animal are 

 arranged as a mosaic. The population of frogs in south-east England, 

 or in Hertfordshire, may appear stable from one year to another, but 

 the population of a pond within this area may rise from nothing to a 

 very large number and then decline again, and so appear violently 

 unstable to anyone observing it. It is necessary to be quite detailed in 

 defmitions. 



