FIG. 27-6 A column of army ants 

 moving across the forest floor, 

 and, right, a bivouac of army ants, 

 Barro Colorado Island, Panama Can 

 Zone (courtesy T.C. Schneiria). 



into those niches to which it is best adapted. The 

 great number of tree and plant species provides a 

 variety of niches, but the availability of niches of each 

 kind is limited so that their animal inhabitants are 

 accordingly reduced in numbers. 



In contrast with temperate regions where animal 

 adaptations are so largely concerned with the physical 

 environment and getting food, there needs to be 

 little or no adaptation in the tropical rain forest to 

 winter cold, inclement weather, lack of food, or desic- 

 cation. Inter-species competition and struggle, how- 

 ever, is harsh and exacting, and evolutionary forces 

 tend to perfect specializations that enable organisms 

 to fit better into their niches or invade new ones and 



tluis avoid much of tlie conipetition and predation 

 (Allee 1931, Mertens 1948, Dobzhansky 1950). 



One specialization in this connection is the ability 

 to hang from trees — animals suspended in mid-air are 

 almost inaccessible to attack by predators. Many 

 birds, particularly flycatchers, orioles, and honey- 

 creepers build pendant nests, as do certain solitary 

 wasps. The cocoons of many moths as well as chalcid 

 wasps are suspended by thin threads. Spiders in their 

 webs suspended off the ground are immune from 

 attacks of army ants. 



An unusual number of birds nest in holes in 

 trees — trogons, motmots, parrots, toucans, wood- 

 peckers — but this may be to protect them from the 



346 Geographic distribution ot communities 



