COMBINATIONS OF ENVIRONMENTAL FACTORS 



213 



have been worked out; we know, for exam- 

 ple, that the determination oi the length of 

 the frostless season at 1.5 meters above the 

 ground does not necessarily approximate 

 the frostless season for small seedlings or 

 for insects. Meteorological data accumulated 

 at standard weather bureau stations can be 

 applied to the plant climate in a given lo- 

 caUty only with caution, and to the climate 

 of crawling insects with still less cer- 

 tainty. 



ECOLOGICAL OPTIMA AND RELATED 

 MATTERS 



In the discussion of ecological optima 

 and related phenomena and principles, it 

 is possible to make much the same points 

 either by considering the ecological rela- 

 tions of some process or set of processes, 

 such as those concerned with development 

 or respiration, or by deahng with the 

 ecology of a recognizable ecological unit. 

 Ecological units, whether individuals, popu- 

 lations or communities, can be treated as 

 entities, since each is suflBciently integrated 

 to react in a more or less unitary fashion. 

 Populations and communities wiU be dis- 

 cussed at some length in Sections III and 

 IV. We are primarily concerned just now 

 with ecological relations of individuals. 



Most environmental factors normally pre- 

 sent a graded series of influences. The 

 minimum concentration (p. 198) constitutes 

 the ecological threshold for the ecological 

 unit or process; below this there is an eco- 

 logical zero so far as the given factor has a 

 final limiting efi^ect on toleration. A maxi- 

 mum concentration exists at the other ex- 

 treme, with an optimum at some interme- 

 diate point. An absolute pessim,um, can be 

 recognized as representing the existing con- 

 dition furthest from the optimum. These 

 values shift with different stages in the life 

 history, with individual or population dif- 

 ferences within the species, and with 

 species or serai difference within the com- 

 munity. The whole gamut of relationships 

 may be diagrammatically developed from 

 below the ecological minimum through the 

 optimum to the maximum and above, as 

 with temperature, or the series may be open 

 at either end. The absence of a solid sub- 

 stratum makes no difference to many fishes, 

 although, at the other extreme, its presence 

 may mark a sharply defined limit to distri- 

 bution. Water can scarcely be too abundant 



for many organisms; beyond some limit, a 

 further increase in the amount present does 

 not directly increase the ease of living foi 

 the individual, though it may increase the 

 area available for occupancy by the popu- 

 lation. 



Minimum, maximum, optimum, and pes- 

 simum apply also to the whole environ- 

 mental complex, except that the interrela- 

 tions are much more complex since all 

 forces in the effective environment must be 

 evaluated. The general ecological optimum 

 for the physical environment, considered as 

 a unit, may not coincide with that for any 

 single factor and may be far from the opti- 

 mum of one or more of them. Yet the opti- 

 mum may be a reality and provide the gen- 

 eral conditions of radiant energy, moistxire, 

 medium, and substrate, in short, of the 

 chemical and physical conditions at which 

 life is most successful for the ecological 

 unit under consideration. To be final, biotic 

 factors must also be considered; these usu- 

 ally complicate still more an already highly 

 complex situation. Moist land in the middle 

 latitudes appears to provide the nearest 

 approach we know to the ecological opti- 

 mum for temperate deciduous forests and 

 for the white man. Contrariwise, the humid 

 tropics give the ecological optimum for 

 tropical rain forests and for many monkeys. 



We are familiar with the fact that some 

 ecological units can Uve in a wide range of 

 temperature and are, therefore, eujythermal 

 rather than stenothermal. Similarly, we 

 know of wide and of narrow tolerances for 

 many phases of the environmental complex. 

 The same concepts can be applied to the 

 environment as a whole, and we recognize 

 ecological units that are euryokous, with 

 wide tolerances for many factors, and 

 others that are stenokous. The moss-dwell- 

 ing bryocoles show highly developed 

 euroky, and coral reef communities are 

 stenokous. 



Euroky and stenoky may or may not be 

 closely associated with the ability of an 

 ecological unit to cross barriers, that is, with 

 its vagility. Forms carried by air usually 

 have high vagility, whether they are flying 

 birds or passively transported spores. On 

 the other hand, Hawaiian achatinellid 

 snails, with special subspecies in each 

 mountain valley, are conspicuous examples 

 of low vagility. Local taxonomic races are 

 likely to be evolved in species with low 

 vagility even when the forms concerned 



