GENERAL PROPERTIES OF POPULATIONS 



27: 



(&) Interspecies 



(1) Competition of two species for 

 a common niche 



(2) Competition of two species for 

 a common food supply 



(3) Predator-prey interaction 



Some of the organisms that have been 

 most used in such laboratory studies are 

 yeast; protozoa (especially Amoeba, Chilo- 

 tnonas, Euglena, Stylonychia, Oxytricha, 

 Paramecium, Colpoda, Colpidium, Didin- 

 ium); the eggs of various aquatic animals 

 (p. 356); insects (especially Tribolium and 

 other grain beetles; flour moths; Drosophila 

 melanogaster; chalcid-flies; bees and ants), 

 and, among the vertebrates, chickens, 

 wrens, mice, monkeys and apes, 



3. Human Populations. Before the time 

 of Malthus the student of human popula- 

 tions focussed his attention on overpopula- 

 tion and underpopulation in relation to the 

 economics of the state (see Duncan, 1929). 

 Malthus was concerned largely with over- 

 population, a reflection of conditions in 

 England during his Hfe. His famous "Essay 

 on the Principle of Population as It Affects 

 the Future Improvement of Society" sug- 

 gested that population growth is a function 

 of the food supply. The impact of Malthus 

 on ecology was reported in the historical 

 section. Modern students of human popula- 

 tions are descendants of Malthus to a cer- 

 tain degree, although they do not, of 

 course, accept wholeheartedly his theory. 

 The ramifying and complex human popula- 

 tion studies will not be reviewed here. We 

 can point out that the principal motiva- 

 tions in such studies are medical, econom- 

 ic, and sociologic, and that their techniques 

 are statistical methods. An unusually com- 

 prehensive outhne of research in this field 

 was pubhshed in 1934 by the Population 

 Association of America (in Human Biology, 

 6: 223-239). This outline suggested that 

 the two major subfields are "larithmics," or 

 "factors in the numerical growth of popula- 

 tion," and "eugenics and euthenics," or 

 "factors in qualitative determination of 

 population." The ecologist finds population 

 studies valuable for the high technical 

 standards they set in the quantitative 

 analysis of data and for the knowledge that 

 has emerged, particularly about reproduc- 

 tion, mortality, and dispersion. 



4. Epidemiological Studies. These cover 

 population aspects of host-parasite relation- 

 ships. They are interspecies as used here, 

 although some workers refer to "epidemics" 



of single species populations to suggest 

 unusual abundance, \lost of the studies are 

 statistical and medical and deal with such 

 diseases in epidemic form as malaria, sleep- 

 ing sickness, typhus, typhoid fever, septiv. 

 sore throat, acute anterior poUomyeUtis, in- 

 fluenza, the common cold, diphtheria, and 

 tuberculosis. In these cases Homo sapiens is 

 the host population. Then, too, there are 

 some recent and fascinating experimental 

 investigations, set up so that an artificial 

 epidemic is estabUshed in a controlled pop- 

 ulation. The course of the epidemic is stud- 

 ied and the causal factors are analyzed. 

 Some of these studies have rather direct 

 clinical application, Uke those of Majoi 

 Greenwood on mouse plagues. Others art 

 entomological, rather than medical, like 

 Salt's work on chalcid-fly parasites, H. S. 

 Smith's work on the pupal parasite Mor- 

 moniella of the housefly, and Varley's 

 (1947) excellent analysis of population 

 balance in the knapweed gall-fly, Urophora 

 jaceana. Thus epidemiology Ues within the 

 scope of modern ecology (p. 60). 



5, The Approach through Theory.'^ The- 

 oretical population ecology has not ad- 

 vanced to a great degree in terms of its im- 

 pact on ecological thinking. There are some 

 significant papers. But the major develop- 

 ments and applications are yet to come. 

 Workers have concentrated at three differ- 

 ent levels: (1) mathematical rationaliza- 

 tions; (2) the social origins problem and 

 social facilitation (see pp. 59 and 410); 

 and (3) synthesis of knowledge to build up 

 a concept of population integration. In the 

 first field important contributions center on 

 population growth curves (Pearl and 

 Reed); on interspecies interactions in a 

 "self-contained" system (Lotka and Vol- 

 terra); and on the concept of population 

 equilibrium, balance or the "steady-state" 

 (Nicholson and Thompson). These matters 

 receive attention in later pages. 



6. Experiment Based on Theory. The ex- 

 perimental approach based on theory has 

 two aspects: the testing of postulates estab- 

 lished by rationahzation, and the design of 

 experiments in the light of theoretical sug- 

 gestions. Experiments based on theory have 

 progressed only far enough to show that the 

 future holds bright promise. (For a perti- 

 nent illustration of this approach, see 

 Crombie, 1945, 1946.) 



" See the monographic summary by Um- 

 berto D'Ancona (1942). 



