242 



ANALYSIS OF THE ENVIRONMENT 



essentially a bacterial process that the dis- 

 tinction is perhaps not a valid one. The 

 succession of animal populations, in which 

 insects appear to predominate, that reduces 

 a fallen tree trunk from living tissue to soil 

 is well set forth by Savely (1939). The 

 transition from the freshly fallen leaves to 

 forest soil is accomplished by a quite diflFer- 

 ent series of populations, in which earth- 

 worms may be dominant. The decay of 

 dead vegetation in grassland seems to be 

 overshadowed, so far as transition to soil is 

 concerned, by its transformation in the 

 metabolism of larger herbivores, especially 

 mammals, many of which may subsist as 

 well for long periods on dry grass (hay) as 

 on fresh vegetation. 



Among air-breathing animals something 

 corresponding to filter-feeding in the sea 

 may be seen in the smaller bats and the 

 nighthawks and swifts, which depend on 

 aerial insects for food. Though their cruis- 

 ing of the air is by no means entirely at 

 random, such forms cannot be specialists 

 beyond the requirement of a specific size- 

 range of their food and its presence in the 

 air. The webs of spiders are likewise in 

 some respects a sieve-feeding device, strain- 

 ing insect food from the air as the net of 

 Oikopleura strains nannoplankton from sea 

 water. 



Specialization in relation to the nature of 

 animal food does not ordinarily extend to 

 special parts of the animal structures being 

 eaten. Exceptions to this rule are found 

 among certain parasites and blood-suckinp 

 animals. Bloodsuckers include vampire bats, 

 leaches, numerous adult insects, and mites 

 and ticks; within this series the special 

 adaptations for securing blood and finding 

 suitable prey are extremely diverse. 



The organization of any animal com- 

 munity exhibits much specialization to size- 

 ranges of food, as reflected by the common 

 terms "insectivore" and "carnivore." In this 

 relation the smaller members are the more 

 strictly limited, and the effects on evolution 

 are produced by a preponderance of a spe- 

 cial type of food rather than by exclusive 

 food relations. Foxes prey proverbially upon 

 chickens, but do not scorn meadow-mice or 

 even grasshoppers, and the prey of bears 

 ranges from large herbivores to ants. Powers 

 of rapid locomotion in carnivores, with 

 structural mechanisms modified in the same 

 direction as are those of their principal prey, 

 do not produce an exclusive food relation 



(cheetah and antelope, red wolf and deer;. 

 No more does the correlation of cryptic 

 coloration in the predator with that of the 

 animals preyed upon imply any great de- 

 gree of food specialization. 



Animal food can lead to further speciali- 

 zation, beyond the hmitations set by size- 

 grades, only when certain herbivores exist in 

 such numbers as to constitute a constantly 

 available prey, and strict monophagy de- 

 velops only among insect predators and 

 parasitoids, and among parasites in general 

 (see pp. 258 and 613). Some of the most 

 conspicuous food speciahzations of air- 

 breathing animals are found in those that 

 return to the sea or to fresh waters for their 

 food (mammals, birds, insects, and so on). 

 Specialization to taxonomic groups among 

 predaceous animals seems to be mainly in 

 correlation with availabihty. Such a relation 

 may be thought of in the bison-wolf rela- 

 tion of the Great Plains in the days of the 

 great bison herds, or in the feeding of birds 

 on a seasonally abundant species of insect. 

 The possibility of the final step toward 

 monophagy appears to be constantly open 

 through food speciahzation that reflects 

 originally merely availabihty. 



The scavengers that make use of animal 

 wastes and decaying animal bodies exhibit 

 numerous and remarkable specializations to 

 specific food materials and thus to specific 

 food environments. These food environ- 

 ments are at least as much biotic as are 

 those provided by the decay of plant mate- 

 rials. Familiar examples are seen in the life 

 histories of the dung beetles and in the elab- 

 orate three-year succession from vultures 

 and flesh flies to tenebrionid beetles in a 

 sizable animal cadaver (Doflein, 1914, pp. 

 249-257). In this succession specific adap- 

 tation to the stages of decay and to special 

 chemicals— i.e., lipoids, proteins, tendinous 

 tissue, and keratinoids— chemical adapta- 

 tions for finding the food, and modifications 

 of life history to make use of it and for dis- 

 persal, are evident. These specializations ex- 

 hibit the general trend toward fractionation 

 in adaptive evolution. The only evident ex- 

 planation for such a trend lies in the more 

 effective exploitation of food materials, the 

 tapping of potential surpluses as they de- 

 velop. Similarly elaborate specialization for 

 the utilization of animal wastes and remains 

 may be seen in the community of the sea 

 beach, in fresh-water lakes and rivers, and 



