A basis for classifying species is exclusiveness, 

 fidelity to the community. A species is exclusive 

 when it occurs only in a single area, habitat, or com- 

 munity ; characteristic (selective, preferential) when 

 it is abundant in one area or community but also oc- 

 curs in small numbers elsewhere ; and ubiquitous 

 (indifferent) when it is found more or less equably 

 distributed in a wide variety of communities. The 

 terms given in parentheses are synonyms used by 

 plant ecologists (Braun-Blanquet 1932). Exclusive 

 species are often rare and of little importance in the 

 dynamics of a community, but when they are con- 

 spicuous they often make useful indicator species for 

 identifying and recognizing community units. 



The recognition of characteristic species presents 

 special difficulties, since one must decide how much 

 more abundant a species needs to be in one com- 

 munity to be sure a definite preference over another 

 is indicated. In a distributional study of breeding 

 bird populations in Ontario (Martin 1960), a species 

 was considered characteristic of one type of vegeta- 

 tion if the species was at least three times more abun- 

 dant in it than in any other type of vegetation. This 

 was at population levels of from 1 to 9 pairs per 40 

 hectares (100 acres). For species reaching popula- 

 tion densities of from 10 to 100 pairs per 40 hectares, 

 preference was considered demonstrated if the species 

 were twice as abundant in one type of vegetation as 

 in any other. For populations greater than 100 pairs 

 per 40 hectares, differences of 50 per cent are prob- 

 ably significant. It seems logical that a stricter test 

 should be applied to small populations, for errors in 

 measuring the size of populations and random popu- 

 lation fluctuations attributable to factors other than 

 choice produce a relatively greater disturbance in the 

 data. An experimental study in measuring foliage 

 insect populations also indicated that populations 

 differing by a ratio of 3:1 could be accepted as sta- 

 tistically significant (Graves 1953). When the bot- 

 tom fauna of two ponds were sampled, true differ- 

 ences could be detected at minimum ratios between 

 their populations of 1.9 (Hayne and Ball 1956). A 

 species, to be termed characteristic, should also be 

 well distributed through a community, this to be in- 

 dicated by its occurrence in at least 50 per cent of all 

 samples taken (Thorson in Hedgpeth 1957). 



Another criterion for evaluating species is by 

 numbers of individuals present. Other things being 

 equal, a species in time of high population affects 

 other organisms to a much greater extent than it does 

 at times of low population. A species that is perma- 

 nently more abundant than another will consume 

 more food, occupy more nest-sites, and demand more 

 space ; hence its influence will be greater. Predomi- 

 nants are the more numerous constituents of a com- 

 munity, in contrast to members, which are species 

 of lesser importance. The dividing line between these 

 two categories is an arbitrary one. 



The time and duration of occurrence of a species 

 in a community affects the amount of influence it ex- 

 erts. Generally, the longer the yearly period during 

 which a species is active, the more important its role 

 becomes. Species may be classified on a temporal 

 basis into perennials, those which are active in a 

 community throughout the year, year after year ; 

 scasonals. which are present or active only part of the 

 year : and cyclics, which may be very important some 

 years but of negligible importance other years, as 

 evidenced by their wide fluctuations in numbers. Even 

 though present, a species is usually considered inac- 

 tive when it is hibernating or dormant or when it is 

 represented only by eggs, spores, or encysted stages 

 of its life cycle. 



The effect produced on the community by indi- 

 viduals and species may be modified by the way they 

 form secondary groupings within the community. 

 These minor aggregations of plants and animals are 

 called societies and are of various sorts (Shelford 

 1932). Layer societies occupy different strata, such 

 as the subterranean, ground, herb, shrub, and tree 

 societies in a forest ; local societies are usually parts 

 of layer societies but are more confined in area, as 

 groups of animals occupying an ant hill, a rotting 

 log or stump, or a restricted but distinctive area of 

 ground ; and seasonal societies include all the organ- 

 isms at particular times of the year. 



Other factors that affect the influence of a species 

 in the community are the size of individuals, their 

 metabolism, food habits, and general behavior. A 

 moose consumes more food than a mouse, and a 

 warm-blooded mouse more than a cold-blooded sala- 

 mander of the same size. A carnivore at the top of 

 several food-chains affects the lives of more different 

 species in the community web of life than a herbivore 

 feeding on plants. Burrowing rodents react on the 

 habitat more than do most birds. One factor may 

 cancel another. An individual of a perennial species 

 of carnivorous mammal certainly eats more than an 

 individual cold-blooded herbivorous insect of small 

 size and active only during the warm season. Yet 

 there may be 1000 insects to one mammal, so that in 

 the aggregate a single species of insect may actually 

 produce more disturbance than a single species of 

 mammal. The difficulty of evaluating the relative 

 effects of species is partly alleviated by calculating 

 their respective biomasses and energy requirements. 



The biomass of a species is the average weight 

 or volume of an average individual, multiplied by the 

 total number of individuals present. The computa- 

 tion of the biomass of each species thus corrects for 

 differences in size and number of individuals between 

 species. Because of differences between species in 

 body moisture and amount of inert substances such as 

 endoskeleton, chitin, shell, and the like, biomass is 

 expressed with greater accuracy in terms of dry 

 weight than wet weight or volume, and is even more 



20 Background 



