xii INTRODUCTION 



The different approaches that have been adopted by the workers on 

 different groups, the different population parameters they try to measure, 

 and the different terminology they use, all tend to impede the fruitful 

 comparison of results obtained for various kinds of animals. These difficulties 

 of comparison will be apparent to any reader, and are themselves considered 

 in the discussions. For example, it is not easy to observe marine fish in their 

 natural habitat and sea fisheries deal with a bulk product largely assessed by 

 weight. It is not surprising that fishery biologists have tended to concentrate 

 their approach on the mass population dynamics of fish and their food. 

 Conversely it is easier to observe the behaviour of individual birds or land 

 mammals than to count them and obtain statistics on mortahty rates and the 

 like, so ornithologists and mammalogists have tended to build up their 

 information from individual field observations rather than the statistical 

 analysis of large samples. Insect ecologists find both approaches beset by 

 difficulties and have to contend with special problems of their own: for 

 example the absence of overlap in the generations of many insect species, 

 which makes their numbers tend to fluctuate rather violently. Environments 

 and populations alike differ in their stability, and the extent and consequences 

 of these differences are argued about in several of the discussions. 



Comparative population dynamics also suffers from a lack of the necessary 

 data, and some of this deficiency has arisen because biologists have not 

 always known what the essential parameters are. A detailed, comparative 

 analysis of population parameters within one group is one approach which 

 may lead to a clearer recognition of what data are vital, and hence to a 

 simplification of practical problems of observation. Holt attempts such an 

 analysis for fish. 



A resiUence to artificial population changes seems to be a prerequisite if a 

 species is to be exploited successfully. Although some resilience may be a 

 characteristic of all species, not all have been able to contribute a substantial 

 yield to man and yet show no long-term decline in abundance. The reactions 

 of populations to exploitation can, however, be used as evidence as to 

 whether or not all populations possess natural regulatory mechanisms, and 

 should throw light on how such mechanisms operate. Nikol'skii reviews 

 the features of fish populations which can be considered as adaptive and 

 compensatory, and several other papers contribute evidence on population 

 regulation. The relative importance of factors intrinsic to the population 

 and extrinsic environmental factors in the control of populations is still a 

 moot point, as is apparent from the discussions. 



Progress in the understanding of population dynamics in relation to 

 exploitation has been much assisted by the use of mathematical models. As 

 is discussed on p. 379, some form of model is essential for almost any advance, 



