84 Bulletin American Museum of Natural History. [Vol. XXXVI, 



THE LIFE ZONES OF THE COLOMBIAN ANDES. 



The ascent of a lofty mountain on which, faunally, several hundred feet 

 of altitude may represent several hundred miles of latitude, is a profoundly 

 impressive and fascinating experience for the zoogeographer. One is amazed 

 by the distinctness of the life-zones encountered and is led to speculate on 

 the origin of their strongly characterized floras and faunas. That the alti- 

 tudinal distribution of plants should conform closely to belts or zones, the 

 limits of which are determined primarily by temperature, is not surprising; 

 but that such mobile creatures as birds should be confined within certain 

 more or less definite boundaries by these invisible barriers is a convincing 

 evidence of their potency as well as of the sensitiveness of the organisms on 

 which they act. 



It does not follow that every species will occur in only one zone, it appar- 

 ently being a general law that wide latitudinal or faunal distribution implies 

 also great altitudinal or zonal range. Examples may be found among birds 

 as widely apart in relationships and habits as the Turkey Vulture and House 

 Wren. Both range at sea-level from the South Temperate to the North 

 Temperate Zone; both are found from the tropical lowlands to the temper- 

 ate uplands of the Colombian Andes. 



These birds, and a few others like them, are, however, marked exceptions 

 arid while a species may advance a small distance beyond its true zone, a 

 surprisingly large number of species are found in only one zone. The zones 

 themselves are not, of course, more sharply defined than the ranges of the 

 species which characterize them. 



No one can stand at the foot of a snow-crowned mountain in the tropics 

 without at once realizing that temperature, as it is influenced by altitude, 

 is obviously the dominant factor in producing the floras and faunas en- 

 countered between base and summit. Where humidity, and in certain 

 instances, character of the soil, add their influence, the boundary lines 

 between life zones are often very sharply defined. One may pass, for 

 example, from the upper border of the arid tropics on the eastern slope of 

 the Western Andes at San Antonio into the dense forests of the humid 

 subtropics on their crest in less than two minutes, and experience a complete 

 change in bird-life. But even where temperature alone is active, and there 

 is no marked difference in rainfall, the forest being continuous, an alti- 

 tudinal difference of 1000 feet may bring one into an essentially new avi- 

 fauna. Such a phenomenon we observed when traveling from El Piiion 

 (alt. 9600 ft.) to just above El Roble (alt. 8600 ft.), on the trail between 



