Volume 12 Number 4 



The Plant World 



A Magazine of General Botany 

 APRIL, 1909 



OVERLAPPING HABITATS.* 

 By Francis E. Lloyd. 

 During the past few years it has become increasingly evident 

 that very closely related species may occupy the same habitat 

 and geographical range. That they do not inhabit exactly the 

 same area is to be expected, and may be explained largely bv 

 the element of chance. But both in the case of closely allied 

 species and of widely different ones, the habitats may be widely 

 different, in part, and in part identical. That is, the habitats 

 overlap, and here are the same in their effect upon vegetation, 

 while different conditions prevail in the habitat areas which 

 are not coincident. It is the purpose of this paper to present a 

 case of this kind which came to my attention in N. Zacatecas, 

 Mexico. 



The topography of this region may be roughly described 

 as composed of mountain ridges separated by wide valleys. 

 The valleys, in turn, consist of alluvial flats, from which extend 

 gravelly plains of low gradient to the foothills of the ridges. 

 These wide, gentle slopes which are so characteristic of the 

 deserts of this and contiguous regions may well be called foot- 

 slopes. It will be understood that the footslope, as one nears 

 the sierra, becomes broken into low, rounded ridges, between 

 which lie arroyos, water-courses which are dry except during 

 rains. These low ridges merge into the foothills and these into 

 the main ridge of the sierra. 



It is quite apparent to the observer that the vegetations 

 of these three topographical features are different, but it is 

 pertinent to say that some elements of the sierra vegetation are 

 to be found through the entire extent of the hills, footslopes 

 and flats. Others are found only in the footslopes and hills, 



♦Presented at the meeting of the Botanical Society of America at Baltimore, Dec 

 ember, 1908. 



