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Effect of Seed Size on
Removal by Rodents1
William G. Standley2
Seeding is commonly used for restoring depleted vegetation. Many seeding projects fail because rodents eat
the seeds (Bramble and Sharp 1949,
Spencer 1954, Nelson et al. 1970). A
variety of techniques for reducing
the impact of rodents have been
tested, but few have been successful.
Most often resource managers poison
rodent populations before seeding,
but this method is largely unsuccessful because of rapid immigration of
new individuals (Sullivan 1979, Sullivan and Sullivan 1984).New methods of biological management could
be developed that use information
gained from diet and behavior studies to reduce destruction of seeds by
rodents. Many studies show that certain rodents prefer particular species
or sizes of seeds (Reynolds and Haskell1949, Reynolds 1950, Abbott
1962, Gashwiler 1967, Smith 1970,
Lockard and Lockard 1971, Srnigel
and Rosenzweig 1974, Everett et al.
1978, Price 1983).Thus, whenever
alternative plant species are available
that both meet the resource manager's objectives and have seeds not
preferentially foraged by local seed'Paper presented at symposium, Management of Amphibians, Reptiles, and
Small Mammals in North America. (Flagstaff, AZ,July 19-2 1, 1988.)
2WilliamG. Standley, formerly a graduate student, University of Arizona, Arizona
Cooperative Fish and Wildlife Research
Unit, is currently Animal Ecologist, EG&G
Energy Measurements, lnc., c/o NPR- 1, P. 0.
Box 127, Tupman, CA, 93276.
Abstract.-Plots located in southeastern Ariiona
were seeded with small and large grass seeds. After
3 days, virtually all large seeds were removed by rodents, while small seeds were still present 36 days after planting. Thus, managers may increase seed survival in this area, without removing rodents, b y seeding with small seeds rather than large seeds.
eating rodents, seeding could be successful even with rodents present.
In southwestern deserts of North
America, where range managers are
attempting to restore rangelands depleted by overgrazing (Cox et al.
19821, kangaroo rats (Dipodornys sp.)
and pocket mice (Perognathus sp. and
Chaetognathus sp.) are some of the
primary seed eaters (Brown et al.
1979b). As early as 1950, Reynolds
suggested that the influence of Merriam's kangaroo rats (Dipodornys rnerriami) on seeding success depends on
the size of seeds used. Brown et al.
(1979b), Inouye et al. (1980), and
Price (1983) all found that heteromyids preyed selectively on large
seeds. In this study, I investigated
the prediction that fewer small seeds
than large seeds would be removed
by rodents in a seeded area in southeastern Arizona.
Study Area and Methods
The study area was on the USDA
Forest Service Santa Rita Experimental Range located 45 km south of
Tucson, Pima County AZ, which is
thoroughly described by Martin and
Reynolds (1973). The vegetation was
typical Sonoran desert-scrub, dominated by mesquite (Prosopis juliflora),
burroweed (Haplopappus tenuisectus)
and cholla (Opuntia spp.). Annual
precipitation averages 36 to 43 cm
and is bimodal, with peaks in winter
and summer. Plots were seeded fol-
lowing all recommended procedures
(Jordan 19811, using large and small
seeds, both separately and together.
Seeded plots were located within a
slightly sloped I-hectare area with a
Comoro soil type, at an elevation of
1300 m. I compared the number of
seeds surviving on 4 experimental
plots to the number of seeds surviving on a control plot which was protected from rodents.
The study area was prepared by
removing large shrubs by hand and
plowing small plants with a disk. The
control plot was protected from rodents with a rodent-proof fence similar to that used by Brown et al.
(1979a). All rodents within the exclosure were removed by trapping before seeding. Each of the 5-15 x 17 m
plots was seeded with 3 evenly
placed pairs of 15 m rows, one pair
for each of 3 treatments which were:
(1)small seeds planted at a rate of
175/m (0.15 g/m), (2) large seeds
planted at a rate of 100/m (4.7 g/m),
and (3) 5 small and large seeds
planted together at 88/m and 50/m
(0.07g/m and 2.35 g/m), respectively. Seeding rates were chosen according to recommended rates for
similar sized seeds (Jordan 1981).
The treatment assigned to each pair
of rows was randomly selected. The
small seeds were blue panicgrass
(Panicurn antidotale) which weighed
an average of 0.85 mg each. The large
seeds were barley (Hordeurn vulgare)
which weighed an average of 47.0
mg each. All seeds were planted with
a cone seeder at a depth of 1 to 2 cm
on 21 June 1984, just before expected
summer rains. Because blue panicgrass seeds are very small and difficult to recover from the soil, they
were dyed with water soluble green
food coloring before planting. Barley
seeds were also dyed to avoid a possible bias.
The species of rodents on the experimental plots were monitored by
placing 100 live traps at 10 m intervals on and around the plots on the
5th and 6th nights after planting.
Traps were baited with a mixture of
both sizes of seeds and checked at
midnight and sunrise.
The number of seeds surviving on
plots was monitored by collecting
soil samples from the rows immediately after planting and at 3,9,18,
and 36 days after planting. One random sample was taken from each
quarter of every row each time.
Samples were not taken from the
outer meter of any row because the
cone seeder applied seeds at a more
variable rate at the beginning and
end of each row.
Soil samples, 2.5 to 3.5 cm deep
and 15 x 25 cm in area, were taken
lengthwise along each row with the
aid of a two-sided, fixed-area sampler and a trowel. The samples were
placed in paper bags, and oven-dried
at 50 C for 24 hours. Seeds were recovered by shaking soil samples
through a series of Tyler sieves
(#5,#10,#14,#18,#20,and #25) for 3
minutes. The number of seeds remaining were counted by examining
the contents of each sieve, both dry
and immersed in a salt water solution, through a 10X viewing scope.
The average number of seeds recovered in the soil samples taken
from the 4 experimental plots divided by the total number found in
the control plot times 100 was used
as a seed survival index (SSI). This
dimensionless index permits comparison of the removal of different
sized seeds by rodents even though
they were planted at different rates.
It also standardizes for the experi-
mental error contributed by the difficulty of recovering small seeds. The
granivorous arthropods and birds
present on the study area had equal
access to control and experimental
plots, so should not have biased SSIs.
Results
Eleven of 17 individual rodents captured on or around the plots were
heteromyids: 9 were Merriam's kangaroo rats, and 2 were bannertail
Seeds sown separately
m
3
9
18
SMALL SEEDS
LARGE SEEDS
36
DAYS AFTER PLANTING
Seeds sown together
SMALL SEEOS
0
3
9
18
36
DAYS AFTER PLANTING
Figure 1 .-Seed survival index (average number of seeds recovered in 4 experimental pbts
divided by total number of seeds recovered in the control plot times 100) for small and large
seeds. (A) Seeds sown separately and (B) Seeds sown together.
kangaroo rats (D.spectabilis). Two
white-throated woodrats (Neotoma
albigula), 2 southern grasshopper
mice (Onychomys torridus), 1 deer
mouse (Peromyscus rnaniculatus) and
1 cotton rat (Sigmodon hispidus) were
also captured.
Whether large and small seeds
were planted separately or together,
the SSIs were higher for small seeds
than for large seeds starting with 3
days after planting (fig. 1). After 36
days, the large seeds planted either
separately or with small seeds were
virtually gone from experimental
plots (SSI = 0.4 and 2.1, respectively).
The SSI for small seeds planted separately was 76.5 after 36 days, while
the small seeds planted with large
seeds had an SSI of 43.6. The SSIs of
large seeds planted separately decreased at a faster rate than the SSIs
of large seeds sown with small seeds.
The SSIs of small seeds planted separately decreased at a slower rate than
the SSIs of small seeds sown with
large seeds, however. Complete data
are presented in Standley (1985).
Discussion
I do not present inferential statistics
to test for significant differences between large and small seed survival
because the experimental plots were
actually sub-plots rather than true
replicates (Hurlbert 1984). For this
study site, however, striking differences between the SSIs of large and
small seeds whether planted separately or together are certainly evidence that smaller seeds have a
much higher survival rate than large
seeds due to differential predation by
rodents.
The higher rate of removal of large
seeds planted separately compared
to large seeds planted with small
seeds may have occurred because the
lower density of large seeds in the
mixed rows made them less attractive to rodents. The relatively higher
rate of removal of small seeds
planted with large seeds, compared
to small seeds planted separately,
likely occurred because large seeds
attracted rodents to the rows, where
the rodents then ate both sizes of
seeds. Sullivan and Sullivan (1982)
observed the opposite effect when
seeding lodgepole pine (Pinus contorta). Lodgepole seed consumption
by rodents was reduced by planting
the relatively small lodgepole seeds
with sunflower seeds, which were
larger and more preferred by granivorous rodents present. The opposing results may be due to differences
in method of seeding (Sullivan and
Sullivan broadcast their seeds) or the
size of plots (Sullivan and Sullivan's
plots were larger). Another possibility is that the main granivorous rodents in their study area, deer mice,
are more selective than the heteromyids present in this study. Nine
days after planting there was a lower
SSI for small seeds planted separately (fig. la) than on 18 or 36 days,
which can only be attributed to variability in seeding rate and sampling
error.
It is possible that not all seeds removed by rodents, small or large,
were destroyed. Reynolds and Glendening (1949) found that the seed
caching behavior of Merriam's kangaroo rats actually increased spread
of some plant species.
Factors other than seed size affect
selection by rodents for particular
seed species, such as percent soluble
carbohydrates (Kelrick and MacMahon 1985, Kelrick et al. 1986; but also
see Jenkins 1988), moisture content
(Frank 1988a), and moldiness (Frank
1988b). For most seeds, however, resource managers have only the information on size available. This study
only compared the effect of size by
using grass seeds of similar composition that differ most in their linear
dimensions. The results of this study
support other studies which showed
that heteromyid rodents selected
large seeds and reduced standing
stocks of large seeds in the soil to a
greater extent than small seeds
(Brown et al. 1979a, Inouye et al.
1980, Price 1983).Therefore, when
site conditions and management
needs allow a choice of which species
to seed, resource managers should
consider the size of seeds when planning seeding in areas inhabited by
heteromyids.
Acknowledgments
Appreciation is extended to Drs. J. H.
Brown, H. L. Morton, and N. S.
Smith for guidance, and to S. Collins,
Dr. J. Cox, S. Horton, M. Podborny,
B. Travis, and D. Youkey for field
assistance.
The research was funded by the
Arid Land Ecosystems Improvement
Unit of the USDA, Agricultural Research Service.
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