NOTE 
The Species Commonality Index: A Method for Comparing Habitats 
Ecologists have used a variety of methods for 
comparing habitats in order to point up their 
similarity and to predict the efficacy of trans- 
plants. Some methods are based upon what are 
believed to be outstanding physical differences 
and similarities between the habitats, while 
others are based upon faunal and floral similari- 
ties and differences. The following is a method 
based upon faunistic similarity. 
For 3 years the writer has used a simple 
technique involving the number of species in 
common between two habitats in order to ar- 
rive at a single value for comparison. This num- 
ber, the Species Commonality Index (SCI), is 
arrived at by dividing the number of species 
common to both habitats by the total number 
of species present, as shown in the following 
example. 
HABITAT I HABITAT II 
Species Present 
Species Present 
A 
A 
B 
B 
C 
C 
L 
L 
M 
N 
O 
P 
R 
Q 
S 
u 
T 
V 
w 
Species in Common = 4 
Total Number of Species =13 
Species Commonality Index = 4 = .27 
15 
In order to make even semivalid compari- 
sons using this technique, the collections should 
be thorough and reasonably concurrent. The 
writer has found that a large group of interested 
students (for example a group of young biol- 
ogy majors), make very comprehensive collec- 
tions. The following data were compiled by 30 
students in an elementary ecology class at West- 
ern Washington College of Education during 
the months of March and April, I960, and 
show to what purpose the SCI might be used. 
From inspection of the data shown in Table 
1, it is apparent that Whatcom Rapids and Bad 
Pond are least similar. One would expect a 
rapids area to bear little faunistic similarity to 
a pond habitat, and thus it is seen that when- 
ever Whatcom Rapids is compared to a pond, 
the SCI is relatively quite low, except when 
compared with Mud Lake, and here is seen the 
third highest index. From this, it might be con- 
cluded that, ecologically, Whatcom Rapids is 
more similar to Mud Lake than to any other 
habitat shown, and thus, if no other informa- 
tion were available, it could be assumed that 
a transplant from the Rapids to Mud Lake 
might stand a fair chance of surviving. 
Further inspection of the data shows that 
Lake Fragrance and Good Pond are ecologically 
most similar. This is surprising in view of the 
fact that the two habitats are separated by more 
than 50 mi. and lie at elevations differing by 
more than 1,000 ft. Good Pond and Bad Pond, 
however, lie less than 100 ft. apart, but, accord- 
ing to the SCI, they seem to be relatively quite 
different. The writer had assumed that the two 
bodies were connected until subsequent investi- 
gation spurred by the low SCI revealed com- 
plete separation. Lacking other information, the 
writer would stake more on a transplant being 
successful between Lake Fragrance and Good 
Pond than between Good Pond and Bad Pond 
— even though the latter two are adjacent. This 
opinion is supported somewhat by knowledge 
307 
