INTRODUCTION 



11 



avoided, at least in part, by tlie evolution of 

 space and of time separations, or by some 

 combination of these. Important as compe- 

 tition may be, it can readily be overstressed; 

 Clements and Shelford (1939, p. 166) 

 help to correct this tendency when they 

 state that "It is desirable to stress again the 

 fact that competition comprises a relatively 

 small number of tlie countless coactions 

 among animals." So far as predation is con- 

 cerned, tliis conclusion is supported down 

 to the species level— or to different races 

 within the species— by the generalization 

 of Volterra (1931), elaborated by Cause 

 (1935) and illustrated by Lack (1946), 

 showing that competition is lessened until 

 it may become relatively unimportant as a 

 result of differences in habitats and habits 

 of predators even when they otherwise 

 show much similarity. Such a qualification 

 does not aflFect conclusions concerning 

 competition between individuals of the 

 same subspecies unless these, too, come to 

 develop some slight dissimilarity in individ- 

 ual habits. 



With all these reservations, competition 

 is a potent factor in animal life, and its re- 

 sults are not always disoperative. In fact, 

 there is evidence for what may be called 

 the biological necessity of predacious types 

 that eliminate surplus populations by killing 

 oJSF weaker animals, especially when these 

 occupy marginal habitats filled beyond their 

 year-round carrying capacity. 



The basic cooperative relations, particu- 

 larly the more obscure protocooperations, 

 or biological facilitations, often are difiicult 

 to demonstrate conclusively under labora- 

 tory conditions even when using selected 

 situations and favorable organisms. They 

 become still more elusive in the field, 

 especially at the community level, and par- 

 ticularly for students well grounded in 

 skepticism. Some of the more apparent 

 protocooperations under these conditions 

 include: 



1. The role of bacteria in the formation 

 of soil and in its yearly renewal of fertility. 



2. The similar role of bacteria in the 

 mineral nutrient cycles of the sea and of 

 fresh-water communities. 



3. The full range of subtle interactions 

 between soil organisms and the soil. 



4. The mass effects of organisms on the 

 toxicity of media. 



5. The "rain" of dead organisms from 

 the surface of the ocean that permits the 



development of life in the great fightless 

 depths of the sea. 



6. The protocooperations inherent in the 

 definition of a dominant organism in the 

 community as one that receives the full im- 

 pact of environment and so modifies it that 

 associated species can five in areas they 

 could not otherwise invade. 



The efiects produced by plants and ani- 

 mals on their physical, chemical, and biotic 

 environment that prepare the way for con- 

 tinuing the community development show 

 both disoperative and protocooperative as- 

 pects. The disoperation concerns those 

 present occupants of the habitat whose 

 activities are making their own continuance 

 impossible in that particular place. The 

 protocooperations come in the preparation 

 of conditions that will permit the whole 

 series to move on towards the chmax. 



In the community, as well as in its com- 

 ponent biocoenoses or smaller fragments, 

 the forces making for ecological facilitation 

 are, in the long run, generally somewhat 

 stronger and more widespread than those 

 tending towards disoperation. 



In our ambitious attempt to set forth 

 ecological principles, it is fitting to empha- 

 size the unknown elements remaining in the 

 field. The very existence of some of these 

 is just beginning to gain recognition. 

 Others, at the present time, can be outlined 

 in qualitative terms only; still others, doubt- 

 less, are as yet wholly unknown. Some few 

 relations can be given fairly exact mathe- 

 matical treatment. There is much room for 

 pure humility among ecologists who are 

 trying to cope with these loosely foiTnulated 

 relationships, most of which cannot be ex- 

 pressed in exact quantitative formulations. 



The relations of individuals to tempera- 

 ture, light, and gravity, and to other 

 environmental factors, can often be stated 

 with approximate precision. Population 

 ecology is quantitative with respect to 

 description, at least under certain controlled 

 laboratory conditions, but even students 

 of this phase of the subject edge away from 

 prediction except on the basis of statistical 

 probability based on accumulated data. For 

 some students this situation produces an 

 avoiding reaction; for many it constitutes a 

 challenge; for others of us, less well- 

 equipped for quantitative studies, it has a 

 strong primary attraction. We enjoy work- 

 ing under the necessity of making needed 

 reservations and keeping in mind the many 



