PE EIE[R-Rg RESEARCH ON - HJ Andrews Experimental Forest

with a birdseed-Crisco mixture . Traps were
run for 5 full days .
Two such grids were established to sampl e
the range of variability within the Thompso n
site, and each was trapped twice, once in summer
and once in winter, as follows : Grid I :
July 22 to 27, 1969, and March 22 to 27 ,
1970 ; Grid II : September 6 to 11, 1969, and
February 18 to 23, 1970 . The results (table 1 )
show several patterns . There is a difference
between summer and winter populations i n
almost all cases as well as a number of differences
between plots . From other work on
small mammal populations we would predict
that there would also be variations from year
to year .
Since this method assumes that all th e
mammals on the inner grid are caught, it i s
important that trapping be highly efficient .
Study of the trapping data (Miller 1970) suggests
that winter populations may not be as
trappable as summer populations, so winte r
estimates in table 1 may be low . We have
planned further work to increase trapping
efficiency and so improve the accuracy of
these estimates .
Meanwhile, it is possible with our presen t
data to obtain at least preliminary values for
small mammal biomass (table 2) . Note that
the differences between the two plots in tota l
biomass are rather small, and that the dro p
from summer to winter, in total biomass, is
Table 1 .-Small mammal density estimates from grid-trapping (number per hectare )
Species
Grid I Gri I I
July March September February
Sorex trowbridgei 16 .8 3.0 8.2 3 .0
S. vagrans 4 .3 2.6 1 .3 . 4
Neurotrichus gibbsi 3 .5 .9 3.6 1 . 3
Microtus oregoni 1 .3 .9 1 .7 1 . 7
Peromyscus maniculatus 1 .7 1.3 2.2 1 . 3
Total 27.6 8.7 17 .0 7 .7
Table 2.-Small mammal biomass (grams per hectare)
Species
Grid I
Gri d If
July March September February
Sorex trowbridgei 82 .3 18.5 41 .0 16 . 5
S. vagrans 27 .4 17 .8 6.7 2 . 7
Neurotrichus gibbsi 27 .9 7.7 51 .4 11 . 1
Microtus oregoni 19 .8 14 .1 28.6 27 . 9
Peromyscus maniculatus 27.7 22.0 36.6 22 . 2
Total 185 .1 80 .1 164 .3 80.4
20 1