ANIMAL ECOLOGY OF JOHNSON COUNTY = 27 
latter, those found along the large streams, usually owe their 
present condition to the removal by man of a large number of 
trees which were once present. These are without exception 
closely pastured and so present the same life strata as do the 
scattered oak groves. The species of animals found in these sub- 
habitats, however, will differ from those in the oak, inasmuch as 
the species of plants are different. 
Elm timber without underbrush. This formation which should 
perhaps be ealled the alluvial plain forest, consists of that mix- 
ture of elm with soft maple, etc., as already indicated. Here the 
trees have usually attained considerable size and present a well 
developed tree-crown stratum, tree-trunk stratum, soil stratum, 
forest fungus stratum, and decaying wood stratum, with, for the 
most part, an absence of the undergrowth stratum due, as in the 
ease of the oak, to pasturing. These various life strata will 
yield to a similar treatment to that found in the oak formations. 
It is worthy of note, however, that the increased humidity will 
cause an increased number of fungi, and consequently an in- 
crease in the number of fungus-feeding forms. Certain mam- 
mals are found here which cannot be assigned to a particular 
stratum save that they nest in the tree trunks; the most notable 
of these are the raccoon and the fox squirrel. I have observed 
signs of raccoons in such formations at numerous places along 
the Iowa River and along the larger tributaries of that stream, 
for example, Clear Creek west of Tiffin. 
Elm forest with undergrowth. There are a few areas which 
are still covered by unpastured alluvial forest; one of these is 
found in Fremont township, Section 12. This formation differs 
from the preceding only in the presence of an abundant tangle 
of undergrowth and consequently has all strata of life present. 
These areas, while few in number, offer what are at present the 
most nearly original conditions of any animal habitat in the 
county and present a paradise to the collector of invertebrate 
forms. 
(4) Willow. The willow formations of the county can hardly 
be dignified by the name forest. They have been given chiefly 
because they occur as definite formations at intervals along the 
banks of the larger streams and are regularly found in a thin 
