August, 1955 
ple in one study known to the writer: 
the forest study by Vestal & Heermans 
(1945). 
Species-Area Curve.—A species-area 
curve can be constructed from data ob- 
tained from the species list for the various 
plot sizes. The maps of the central mil- 
acres, figs. 8 and 9, furnished the data for 
deriving average numbers of species in 
quadrats of 1/256-, 1/64-, and 1/16-mil- 
acre sizes. Species lists, compiled by 1/4- 
milacre units, for each of the 9-milacre 
squares furnished the data for computing 
the averages for quadrats of 1/4- and 1- 
milacre sizes. The total number of spe- 
cies found in the 9-, 25-, 40-, 50-, 65-, and 
75-milacre areas provided the figures for 
the larger plot sizes. “These data are 
shown in table 13. 
When the data are plotted on semiloga- 
rithmic paper, the resulting curves are 
S-shaped, fig. 13. From this type of curve, 
certain reference areas can be determined 
with the method described by Vestal 
(1949). These are (1) the smallest rep- 
resentative area, (2) the minimum area 
for assignment to type, after this referred 
to briefly as minimum area, and (3) the 
30 
25 
20 
NUMBER OF SPECIES 
a 
ie) 
0.003 0.01 0.03 0.1 0.3 
Evers: Hitt Prairies oF ILLINOIS 391 
Table 13—Numbers of plant species in 
plots of various sizes, Sampson and Phegley 
hill prairies. 
NuMBER OF SPECIES* 
Pior Size, Miracres 
SAMPSON | PHEGLEY 
PRAIRIE | PRAIRIE 
B/D Giardini eae ae: pe al\ 1.98 
Ld earns an ie MPI Poe 3.79 3.94 
LIV ELIT Rey eee 2 ene ae aR or 6.56 TAS 
Ade cera ae ee 133 eG) 
Ds See ee eres yy) 13.44 
he AN tee RR Pd get edt 18.0 26.0 
DOP ete ty eee 21.0 20.0 
LUD ess tart a Sie tees ei — 28.0 
0s et cos eth ay 20.0 --- 
(OD YIN airy res Rel Lt pa ene ae — 29.0 
TSO. OE etn pe me C 24.0 = 
_* The average number of species per quadrat for plot 
sizes of 1 milacre and smaller; total number of species in 
larger quadrats. 
area of fair-sized stand. The smallest rep- 
resentative area (Ar) is the smallest one- 
piece area having some claim to be rep- 
resentative. It is the effective plot-size 
of Vestal & Heermans (1945). The min- 
imum area (Am), a plot-size that is 
large enough to include all the important 
PHEGLEY—y 
! 3 10 30 100 
AREA (MILACRES) 
Fig. 13.—Species-area curves for Phegley (grazed) and Sampson (ungrazed) hill prairies. 
Ar, smallest representative area; Am, minimum area for assignment to type; Af, fair-sized stand. 
