214 



ANALYSIS OF THE ENVIRONMENT 



are not closely stenokous; mere distance is 

 a separating factor, although there may be 

 an entire absence of other physical or biotic 

 barriers. Euryokous ecological units with 

 only relatively high vagiUty tend to be cos- 

 mopohtan, as shown by the wide distribu- 

 tion of many fresh-water organisms and 

 communities. One must avoid easy generali- 

 zations, and reef corals provide an interest- 

 ing test case. As an ecological community, 

 coral reefs are tropicopolitan. They are 

 stenokous, and their primary constituents, 

 the reef-building corals, have low vagiiity. 

 The active dissemules are tiny, ciliated, 

 weak-swimming larvae that have only a 

 brief interlude of activity, during which, 

 however, they may be carried by ocean 

 currents. 



The use of reef corals to test the opera- 

 tion of the principles under discussion has 

 an important handicap: the taxonomy of 

 corals presents difficulties. Differences be- 

 tween described species may result from 

 ecological rather than from hereditary in- 

 fluences (Hickson, 1906). Accepting the 

 species as described, Verrill's account 

 (1902) indicates that the corals of the 

 West Indies spread with good uniformity 

 from the Bahamas and Florida to Colon in 

 Panama. They can be traced down the 

 Lesser Antilles to Venezuela and are di- 

 rectly related to the corals of Brazil. Ber- 

 muda has an impoverished coral fauna de- 

 rived from the Atlantic region of tropical 

 America. Verrill found the absence of Ac- 

 ropora muricata from Bermuda especially 

 noteworthy, since this is among the most 

 important and abundant of West Indian 

 forms. He suggests that the larval period of 

 this and other missing species from Bermu- 

 dian waters is too brief to allow them to 

 make the journey of rather more than 700 

 miles even with the aid of the Gulf Stream. 

 Vaughan (1912) advanced a similar sugges- 

 tion to account for the scarcity of Acropora 

 and the absence of whole families of corals 

 from Hawaii. Although abundant in the 

 other coral reefs of the world, the acroporas 

 are entirely absent from the Pacific coast 

 of the Americas. A related genus, Montip- 

 ora, is found in tropical waters, except 

 those of the Atlantic. There are few coral 

 genera in the West Indies that do not oc- 

 cur in Indopacific waters, although, con- 

 versely, a number of important genera from 

 the latter region are unknown among West 

 Indian reefs. The fungiid corals are repre- 



sented by some five genera and some forty- 

 six species in the Indo- West-Pacific reefs. 

 The entire family is absent from Atlantic 

 coasts, and there is but one species {Fungia 

 elegans) on the west coast of North Amer- 

 ica (Ekman, 1935). The low vagiiity of the 

 coral larvae evidently limits the distribution 

 of many forms. 



Environmental conditions are seldom 

 static. In addition to the diurnal, seasonal, 

 and longer cycles, there are the great cli- 

 matic trends (p. 80) that have made a 

 marked impress on the distribution of 

 plants, animals, and communities. As condi- 

 tions change either in a short-run or more 

 enduring pattern, ecological units exposed 

 to them may meet the changed conditions 

 by dying off. This is the probable reaction 

 of stenokous organisms with weak vagiiity 

 if the environmental change is relatively 

 sudden and extreme. An unusually early 

 and heavy frost or a sudden flood takes a 

 heavy toll, as do the more unusual drastic 

 changes, such as tidal waves, tornadoes, or 

 volcanic eruptions. Slower changes often re- 

 sult in a local dying-out of many organisms 

 and communities. The more euryokous 

 forms may survive the changed conditions, 

 thanks to their greater toleration, and may 

 even increase in numbers as a result of 

 slackened competition. Given time, even 

 some of the less tolerant units may accli- 

 mate and survive. Frequent changes in con- 

 ditions often result in the selection of var- 

 ious escape mechanisms: encystment, bur- 

 rowing, movement into the burrows made 

 by others, and emigration sometimes on a 

 spectacular scale as with lemming. 



There are also physiological escapes, the 

 ecological aspects of whose evolution will 

 be discussed in a later section (p. 705). 

 Homoiothermy is such a partial escape that 

 has been achieved at the individual level by 

 birds and mammals only. Homoiothermal 

 animals are free from many of the limita- 

 tions imposed by temperature on poikilo- 

 thermal forms. A few other animals have 

 found partial freedom from temperature 

 restrictions, often by cumbersome methods. 

 Solitary wasps can excavate in sand that has 

 a wasp-numbing temperature by flying in 

 sunlight until thoroughly warmed and then 

 digging briefly in the cold earth before an- 

 other warming flight. Similarly, by appro- 

 priate alternations of digging and flying in 

 cooler air, bembioid wasps can dig through 

 a sandy surface that is hot enough to kill 



