232 



ANALYSIS OF THE ENVIRONMENT 



five-hundredths part of incident light 

 reaches ground level. 



Water 



Vegetation exerts well-known climatic 

 effects on humidity, on precipitation, and 

 wind (Geiger, 1927); it modifies the veloc- 

 ity of winds and of water currents; it 

 causes other changes even in lakes, large 

 rivers, and many arms of the sea. Coral reefs 

 also have a major biological influence and 



4000 ^ 



10 II 12 I 



TIME OF DAV 



Fig. 61. The effect of forest cover on light 

 intensity in Panama in February (Graphs 1, 3, 

 and 5) and on an Indiana beech-maple forest 

 in September (Graphs 2 and 4). A line con- 

 necting the lower points in Graph 1 would 

 summarize hght intensities during cloudy pe- 

 riods, and one connecting the upper points 

 would do the same for periods of brightest 

 light. ( Redrawn from Allee. ) 



produce strong physiographic effects. At a 

 much different level, subtle biotic condi- 

 tioning is important for many plant and 

 animal populations (p. 398). These mat- 

 ters will not be discussed here. There re- 

 main the ecological effects associated with 

 the small amounts of water enclosed by 

 pitcher plants, caught at the bases of leaf 

 whorls, as in bromehads, or held by moss 

 or in tree holes, to name but a few. Such 



waters constitute a series of small, distinct 

 environmental niches. We shall comment 

 further only on the hquid found in pitcher 

 plants, studies of which are summarized by 

 Lloyd (1942). 



Sarracenia, the widely distributed genus 

 of pitcher plants in eastern North America, 

 contains bacteriologically sterile hquid in 

 the young unopened pitchers. The open 

 pitchers, with captured prey, contain bac- 

 teria, but the leading part in the digestion 

 of the captured insects is taken by the pro- 

 tease of the pitcher liquor. In most cases 

 this enzyme acts best in an alkaUne medi- 

 um, but it is also active, in some instances, 

 when the Hquid is acid. Water is absorbed 

 by the plant from its pitchers, but not so 

 rapidly as is the nitrogenous material 

 formed by proteolysis of insect bodies in 

 the hquor. Phosphates are also absorbed. A 

 variety of digestive enzymes occiu: even in 

 the fluid of closed pitchers. In Nepenthes, 

 the pitcher plant of the Oriental tropics, 

 proteinase is secreted by the pitchers, and 

 there is little doubt that tryptic digestion 

 occurs over and above that carried on by 

 bacteria. 



Some organisms, plants as well as ani- 

 mals, are able to live within the modified 

 water; in fact, this speciahzed biocoenosis 

 includes animals that five only in pitchers 

 and are not found elsewhere. Thienemann 

 (1932) called these later organisms nepen- 

 thebionts in contrast with (a) those that are 

 occasionally found in pitchers, but usually 

 five elsewhere, and (b) those that regularly 

 pass their aquatic phase in Nepenthes, but 

 otherwise five elsewhere. Sarracenia also 

 has a number of closely adapted species of 

 animal associates not known apart from the 

 pitchers. The biocoenosis includes mosquito 

 larvae, a small tree toad, and a small 

 iguanid lizard (Anolis) that are not obligate 

 inhabitants of this niche. 



Habitat Niche 



Habitat niches may have a distinctively 

 biotic environment within the major com- 

 munities of which they form a part. 



Major parts of the plant formation in gen- 

 eral may be occupied as a shelter environ- 

 ment by animals; the root-mat of forest or 

 grassland, the moss-cushion of the tundra, 

 the leaf canopy of forest, or the grass-stem 

 tangle of the prairie have characteristic ani- 

 mal assemblages. Such large scale "biotic 

 habitat niches" differ profoundly in their 



