RELATIONS WITHIN A SPECIES 



347 



divisions. Such races probably vary because of 

 unlike environments, mutations, and selections. 

 However, unlike environments alone might favor 

 unique compositions of age classes, and social be- 

 havior can modify the influence of any other factor. 



DISTRIBUTION 



The compositions and activities of populations 

 are factors of distribution, but distribution itself is 

 analyzed best in terms of geographic range and local 

 occurrence of species. Geographic range is the po- 

 litical or over-all segment of the world that en- 

 compasses the distribution of a species, and includes 

 many sites not actually containing the species. 

 For this reason, local occurrence, any area in the geo- 

 graphic range that actually is occupied by the species, 

 is recognized. The distinction between range and 

 occurrence is important because the range of a species 

 might cover a large political segment of the world, 

 whereas local occurrences might be only islands 

 within the geographic range. 



GEOGRAPHIC RANGE 



The limitations upon the range of any species are 

 brought about by ecological factors. It is well known 

 that environmental factors vary geographically and 

 that organisms have definite environmental require- 

 ments. Therefore, depending upon the requirements, 

 there are definite limits (barriers) to the extent and 

 number of places in the world a species can inhabit. 



Although geographic restrictions exist, an entire 

 species often inhabits sites that many single, localized 

 populations could not occupy. This is possible be- 

 cause local populations are not exactly the same, 

 population composition altering geographically in the 

 form of unlike g^ne combinations (gene pools). This 

 local variation is within the framework of individual 

 gene combination fluctuations and spatial relation- 

 ships; however, each population deviates about par- 

 ticular average conditions, which are what change 

 spatially. These geographic variations within a 

 species are of many types. There might be local 

 modifications of reproductive and mortality rates, 

 hence spatial differences in population fluctuations. 

 The proportions of the separate age classes also 

 might conform to latitudinal or altitudinal environ- 

 mental gradients. Finally, and probably of greatest 

 significance, is the fact that local populations can be 



unlike structurally and functionally; this is the basis 

 for calling each a geographic race or, if these races are 

 sufficiently unique, a subspecies or variety of their 

 species. 



LOCAL OCCURRENCE 



The basic spatial units of a species can be described 

 in terms of density and spacing. Density is the num- 

 ber of individuals per unit area. Spacing considers 

 whether the individuals form closely knit groups but 

 with distinct isolation among groups {clumping); 

 maintain maximum separation from one another 

 {even spacing); or occur according to chance alone 

 {random distribution). Although density and spacing 

 when combined present a good picture of restricted 

 ranges, these phenomena are subject to so much 

 variation that they rarely can be measured. Fre- 

 quently, different age classes and sexes have inde- 

 pendent modifications in local spacing and density 

 and in seasonal and other periodic fluctuations in 

 numbers, spacing, and density. 



Density and spacing are reflections of environ- 

 mental conditions and species behavior. Different 

 habitats place restrictions on species numbers per 

 unit area and also cause clumping or even spacing 

 rather than random patterns. Random patterns 

 probably do not exist in nature. Nonrandom dis- 

 tributions are the rule because the environments 

 tend toward one or the other of the organized pat- 

 terns. In fact, a species that is locally random will 

 seem to be in clumps if local distributions occur as 

 widely separated habitat islands, or seem to be even 

 spaced if their habitat islands are of uniform size and 

 distance from one another. 



Although species behavior must act within en- 

 vironmental restrictions, behavior also has a distinct 

 influence upon density and spacing. The most im- 

 portant phases of behavior are means of offspring 

 distribution and social relationships. The offspring 

 of plants and animals may be very close to their 

 parents or some distance away. In certain species 

 an entire local area is composed of a single family 

 group; in others, the young are farther from the 

 parents and family groups do not form; spacing 

 varies from still fairly close to the parents, or at least 

 the female parent, to almost random over a wide 

 geographic area. In essence, the distribution of 

 many young may be a mechanical thing rather than a 

 true behavior pattern. In most plants this is the case, 



