518 



THE COMMUNITY 



(migratory birds). As a general rule, a 

 species population tends to occupy the same 

 food niche during the same stage of its hfe 

 cycle, but there are variations to this. 

 For example, the brown bear feeds upon 

 salmon when these fish are migrating to 

 their spawning grounds in the spring, and 

 eats berries in the autumn. 



In the third place, the food niche deter- 

 mines the meshes upon which a species 

 feeds in a given community. The food 

 niche, however, involves at least two ante- 

 cedents, namely, heredity and environ- 

 mental conditioning. The sum total of the 

 adjustments mentioned, therefore, deter- 

 mines what kind of food organisms can be 

 eaten. The nature and extent of such feed- 

 ing adjustments have been examined (pp. 

 236-262). Their variability is as great as 

 the diversity of foods in a community. The 

 food niche, then, becomes an ecologic ex- 

 tension of the heredity of a population. 

 Since different species have different he- 

 redities, they have different specific require- 

 ments, both within the same food niche in 

 ecologically similar species and between 

 different food niches in ecologically dis- 

 similar species. 



The total of these inherited adjustments 

 automatically places a species in a particular 

 food niche and, consequently, in a particu- 

 lar place in the food web. This selection by 

 the food web renders a particular species 

 subject to a dual role, that of a food and 

 a feeder. 



Within the inherited frame of these re- 

 quirements we have the entire gamut of 

 feeding habits, for example, from the 

 specific food-feeder relationship of the 

 yucca moth (Tegeticida alba)*" and the 

 yucca (Yucca jihmentosa) , to such omni- 

 vores as man, domesticated hogs, and 

 pitcher plants. 



Environmental conditioning (p. 352) is a 

 second antecedent affecting the food niche. 

 It has been discussed (p. 615) with refer- 

 ence to Hopkins' host selection principle. 

 This factor may not influence the general 

 character of the niche. With respect to the 

 food web, exposure of immature animals by 

 the parent or parents to a specific food or 

 foods reenforces the selectivity of the food 

 niche, as the ovipositing of species of bu- 



" Widely known in the literature by another 

 name, Pronuba yuccasella (cf. Riley, 1892; 

 Comstock, 1933). 



prestid and cerambycid beetles in a single 

 kind of tree or in a few species of trees 

 (Felt, 1905, 1906), and the nest-provision- 

 ing habits of many solitary wasps (Peck- 

 ham and Peckham, 1898) may serve to 

 maintain the quaUtative and quantitative 

 aspects of the food web. This, in turn, 

 would aid in autoregulation and self-main- 

 tenance of the community at the operational 

 level. 



Finally, there is the confusing factor of 

 availability. All the meshes of a food web, 

 if known, would be shown on a diagram of 

 the complete web. Some of these meshes 

 will hold species of low frequency of oc- 

 currence (rare species for this particular 

 community) ; many meshes will have 

 species of moderate frequency; and a few 

 meshes will be crowded with relatively 

 large populations (common species for this 

 particular community). Such complete in- 

 formation for a community never has been 

 accumulated. Important or influential 

 meshes may be unknown in a given sample 

 (Allee, 1930), or the presence or absence 

 of a mesh may be a consequence of the 

 secretive or aggregative tendencies of its 

 occupants. If examination of the stomach 

 of an animal shows it to be crowded with 

 specimens of a single species, at least 

 two explanations occur. First, the animal 

 in question may be able to tolerate physio- 

 logically and/or capture a single specific 

 food; that is, it may normally feed on a 

 single mesh. Second, the animal may have a 

 udde range of physiological tolerance and 

 be able to capture and feed upon a great 

 many meshes, only one of which was 

 available. 



Frequency of the meshes in a series of 

 related food webs or the relative frequency 

 of individuals of a given mesh, can be 

 analyzed by various methods. Such analysis 

 of interspecies and intraspecies population 

 ecology is essential for a full understanding 

 of the community; frequency spectra 

 (Raunkiaer, 1934) give another dimension 

 to the food web concept. 



In any community there is a relation 

 between the life of one trophic level of the 

 food web and that of any other level. 



Each mesh of the food web has an 

 average annual reproductive potential in the 

 sense of Chapman (1928). The sum of 

 these mesh potentials within a trophic level 

 equals the reproductive potential for that 



