Vol. IX] HALL & GRINNELL— LIFE-ZONE INDICATORS 43 



RECOMMENDED PRECAUTIONS 



It is perhaps needful here to warn those who make use of 

 our Hsts of indicators that Hfe-zones are not often abruptly 

 defined one against the other, but that belts of some width 

 may mark their boundaries, where overlapping or mixing of 

 ingredients occurs. If the locality to be tested happens to 

 be situated in such an indifferent position, trouble will nat- 

 urally be encountered and the true state of affairs will not 

 be discovered without a floral and faunal reconnoissance 

 radially in different directions; if on a steep slope, a few 

 rods may suffice; if on more level ground, some miles may 

 need to be traversed. 



Another thing to keep in mind is that only a few of the 

 species here listed for a given zone will be likely to occur in 

 any one locality. Various faunal and associational divisions 

 of life-zones exist (Grinnell, 1914, p. 64), so that the critical 

 species are usually limited in their distribution to but a 

 portion of their life-zone. But every such subdivision is 

 represented in our lists by two or more species. 



Then it i"s possible that some of our "indicators" have been 

 selected as such through an incomplete knowledge of their 

 distribution; in other words, in some cases where a species 

 is at first supposed to be an "indicator" (that is, a species 

 not occurring in two or more zones, but in only one), as its 

 distribution becomes better known the less closely may it be 

 found restricted within the limits of one zone, and therefore 

 the poorer "indicator" it becomes. However, a majority of 

 the species of both plants and animals we have selected are 

 conspicuous and well-known species, easily detected. In 

 some of the cases there is more or less "spilling over" locally 

 from the critical zone, in one direction or the other or in 

 both directions, though not involving an entire adjacent zone; 

 and with such species letters are affixed by which the zone or 

 zones in which it also occurs are designated. With each of 

 these species, its metropolis is so emphatically within the zone 

 for which it is listed, that its value as an indicator is para- 

 mount, especially when the observer takes pains to verify 

 the presence of two or more indicators — the more the better. 

 With migratory birds the zonal position indicated is, of 

 course, that of the breeding area. 



