Rodents

The Portal LTREB has used the same protocol, since 1977, to evaluate changes in the density, biomass, and population structure of rodent species in response to the experimental manipulations.

Trapping is conducted during the new moon, rendering approximately monthly trapping periods.

On all 24 experimental plots, and during a period of two or three successive days, Sherman live traps (23 x 8 x 9 cm) are placed at each of the 49 stakes that define the primary grid. Traps are baited with mixed bird seed or millet and each



Peromyscus mariculatus
Deer Mouse

plot is trapped for a single night, set as close to dusk and picked up as close to sunrise as possible. Standard procedure is to close the gates through the fences on the night of trapping to insure that only individuals residing on the plots are captured.

For each individual rodent captured, the following information is recorded: trapping date (month, day, year), trapping period, lunar period, plot, stake, trapping conditions, genus, species, sex, reproductive condition of males and females, hind foot length, weight, identification number for each individual, and nest location.

 

The most important manipulations involve removing selected species or all species of rodents and ants. Four categories of rodent removal are accomplished by cutting 16 holes of a specific size at ground level in the fending surrounding a plot and selectively trapping out the designated species: (1) all rodents removed, no holes; (2) all three kangaroo rat species (D. meriami, D. ordii, and D. spectabilis) removed, 1.9 X 1.9 cm holes; (3) banner-tailed kangaroo rats (D. spectabilis, the largest granivorous rodent species) only removed, 2.6 X 3.0 cm holes; and (4) no rodents removed (called equal-access plots), 3.7 X 5.7 cm holes.

The fencing uniformly excludes cottontail rabbits (Sylvilagus auduboni) from all plots, but does not restrict access by jackrabbits (Lepus californicus), which regularly jump over the fences. Eight plots are assigned the Dipodomys spp. (D. spectabilis, D. merriami, and D. ordii) removal treatment.


A small hole in the fence located above and to the right of the pocket knife allows kangaroo rats (genus Dipodomys) to pass through.

A combination of small gates (1.9 x 1.9 cm) and trapping/removal of Dipodomys spp. from these plots prevents colonization by these rodents but not by small Perognathus and Peromyscus granivorous (Perognathus penicillatus, Pg. flavus, Peromyscus eremicus, P. maniculatus, and Reithrodontomys megalotis), as well as two insectivorous (Onychomys leucogaster and O. torridus) rodents.

Two plots are assigned the D. spectabilis removal treatment: gates were placed as described above, except that the gates were large enough (2.6 x 3.0 cm) to allow the free passage of D. merriami and D. ordii as well as the small rodent species

A number of indirect interactions as a consequence of our experimental manipulations have been documented at Portal. There appear to be hierarchies of competitive interactions among rodent species, so that removal of the dominant species affects a second species, which in turn affects a third. The cascading effects of such hierarchical competition were seen in shifts of small-scale spatial distributions of the subordinate species.

Bannertailed kangaroo rats aggressively defend open microhabitats within their home ranges, especially the areas around their large, conspicuous burrows, against other rodents, especially other kangaroo rats (Frye 1983). Bowers et al. (1987) found that when D. spectabilis was removed, the sammler kangaroo rat (D. merriami) shifted its microhabitat use from under shrubs to more open areas, and several even maller rodent species then shifted to forage under the shrub cover.

The population of bannertail kangaroo rats declined precipitously in the mid-1980s (Valone et al. 1995), making the effects of D. spectabilis removal problematic. Bannertails went extinct on the entire study site in 1994.

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