14 ANIMAL ECOLOGY 



means of staying in the same place and obtaining the animals 

 carried along in the air. In the tropics certain big spiders 

 are actually able to snare small birds in their webs. In the 

 sea an enormous number of animals sit still in one place and 

 practically have their food wafted into their mouths. Indeed, 

 food is probably not usually a limiting factor for such animals, 

 and competition is for space to sit on. Hence it is that we 

 find these animals behaving superficially like plants. Over 

 great areas of the tropical seas (dependent probably on certain 

 temperature conditions of the sea, or upon the plankton living 

 in such waters) corals almost completely replace seaweeds on 

 the seashore and shallow waters, where they feed like other 

 animals on plankton organisms or upon small organic particles 

 in the water. Corals on a reef usually form zones, each 

 dominated by one or more species, just as in plants. The 

 zonation is apparently determined by gradients in such factors 

 as surf-action, amount of silt in the water, etc. Amongst the 

 corals grow various calcareous algae which resemble them 

 very closely in outward appearance. As we go farther from 

 the equator, plants become relatively more and more abundant 

 on the shores of the oceans, but even in Arctic regions certain 

 groups of animals, e.g. hydroids, may form zones between 

 other zones composed of seaweeds. ^^^ 



12. There is another vertical gradient in conditions which 

 is clear-cut and of universal occurrence. This is the gradient 

 in salt content of the water from mountain regions down to 

 sea- level. Through the action of rain all manner of substances 

 are continually being washed out of the rocks and soil. These 

 pass into streams and rivers and accumulate temporarily in 

 lakes and ponds at various levels. But since the salts are 

 always being washed down, we find that on the whole the higher 

 we go the purer is the water. Exceptions must be made to 

 this rule in the case of places which have the higher parts of 

 their mountain ranges composed of very soluble rocks, or 

 in places like Central Asia, which have high plateaux on 

 which many salt lakes develop. But, on the whole, we can 

 distinguish an upland or alpine zone of waters containing few 

 salts and often slightly acid in reaction. Lower down, the 



