146 



ANALYSIS OF THE ENVIRONMENT 



in fifteen minutes and evaporates snow in 

 dramatic fashion. 



A somewhat related phenomenon is the 

 location of a "frostless belt" or "orchard 

 zone" near, but not at the foot of, a moun- 

 tain slope. The belt is located low enough 

 for the descending night air to be warmed 

 by condensation, and higher than the point 

 reached by nightly accumulations of cold, 

 dense valley air. The "frostless belt" is 

 wanner than are the adjacent higher or 

 lower levels, and this relationship is espe- 

 cially important during the clear cold 

 nights, when frosts occur, in spring or 

 autumn. Its significance in animal ecology 

 has not been analyzed adequately. 



Wind Storms of Great Violence 



The high transport value of tropical hur- 

 ricanes, called typhoons in the Far East, 

 and temperate tornadoes, together with 

 their effects on the distribution of land life, 

 calls for a brief mention of these powerful 

 storms (cf, Darlington, 1938a). Tropical 

 hurricanes arise near the doldrums, where 

 convection regularly carries much water 

 vapor aloft. Latent heat released by con- 

 densation of water, if great enough, induces 

 an increased inflow of moist air; the con- 

 densation of this new moisture releases still 

 more heat energy, and so the storm gets 

 the enormous energy that keeps it going. 

 The force of the earth's rotation spins the 

 hundred-mile wide disk of activity off on 

 a course that may run a few thousand 

 miles over the oceans before subsiding. 

 Speed of translocation of the whole revolv- 

 ing mass of air is ordinarily about 10 or 12 

 miles an hour. The section of the storm to 

 the right of the general track is the more 

 dangerous half, since to the speed of the 

 circular hurricane is added the speed of 

 translocation of the whole. On the left of 

 the hurricane track, speed of general move- 

 ment is subtracted from the circular wind 

 velocity, and this becomes the less dan- 

 gerous part of the storm, though it still has 

 potent force. On land, much of the damage 

 is done by the left half, especially near the 

 hurricane center, where there is a rapid 

 reversal of wind direction. Objects some- 

 what adjusted to stress from one side may 

 break when quickly exposed to reversed 

 stress. Hurricane winds frequently blow at 

 the rate of 150 miles per hour, and smaller 

 gusts within the larger mass may have a 

 velocity of a hundred miles more. The great 



hurricane belts of the world include the 

 West Indies and Florida, the Philippine 

 Islands, China Sea, and the southern mar 

 gin of Asia. Hurricanes do not persist long 

 over land. 



Since every animal is a member of an 

 ecological community, the effect of a hurri- 

 cane and accompanying rainstorms may be 

 complex. A colony of aphids is at the center 

 of a fairly simple biocoenose that includes 

 ladybird beetles, syrphid flies, and many 

 more forms. A hurricane in Florida reduced 

 a given aphid population by 80 per cent, 

 destroyed syrphus fly larvae, ladybird larvae 

 and all other aphid predators except adult 

 coccinellid beetles. The storm swept away 

 all aphids infected by the fungus, Empusa. 

 Within a fortnight after the catastrophe, 

 the local population of selected aphids in- 

 creased some two-and-a-half times and was 

 about half as numerous as it was before 

 the storm struck it (Thompson, 1928). 

 The disruption of ecological routine in lit- 

 toral and forest communities is often great, 

 and the ecological balance, normal for the 

 local situation even when occiipied by a 

 climatic climax, may be delayed for years. 



Hurricanes sometimes move out of their 

 usual storm tracks, as did the great New 

 England hurricane of September, 1938. 

 This stonn carried with it birds from three 

 ecological habitats: 



1. Sea birds, normally found off the coast 

 of North Atlantic states, including Leach's 

 petrel, red phalerope, and the parasitic jaeger. 



2. One species from tropical seas, the sooty 

 tern. 



3. Birds from the Carolina coasts, especially 

 the snowy egret, Wilson's plover, gull-billed 

 and royal terns, and the black skimmer. The 

 last mentioned species was brought into New 

 England in large numbers. On its way north, 

 the hurricane swung inside Cape Hatteras, 

 where black skimmers were abundant, and 

 carried off large numbers. 



The storm had two other marked effects. 

 It picked up migrating birds, carried them 

 back north, and caused much loss of life 

 among land birds, especially of Cape Cod 

 (Hill, 1945). Other examples of aerial 

 transport will be discussed within the next 

 few pages. 



Tornadoes are intense, smaller storms of 

 great power that typically arise in the mid- 

 latitudes in the warm southern sector of an 

 otherwise routine and moderate cyclone 

 They are especially abundant in the Nortb 



