Site Description


This view of the study site shows the cattle fence surrounding the site in the foreground. The long silvery fence toward the top of the photo is plot fence.

Established in 1977, the Portal LTREB was set up to study interspecific interactions between seed-eating rodents and ants, as well as community vegetation structure through time.

Located in an arid grassland, the site is desert shrubland ecotone, 7 km east of Portal, Arizona. A relatively consistant Chihuahuan desert scrub vegetation covers the area. Soil is a fairly homogeneous mixture of alluvial boulders mixed with and overlaid by finer particles. Terrain is relatively flat except where it is dissected by several temporary watercourses. Vegetation varies from open grassy areas, to stands of widely spaced shrubs (Gutierrezia, Ephedra, Flourensia), to dense stands of trees (Acacia and Prosopis).

The site comprises about 20 ha divided into 24 plots, each 50 x 50 meters in size and randomly assigned to different treatments: removal of some or all rodent or ant species, or addition of supplementary seeds. While plots initially assigned seed additions, and plots assigned some of the other treatments were changed to additional replicates of rodent and ant removal treatments in 1988, several plots have had all rodents, just kangaroo rats, or all ants removed contiuously while other plots have served as unmanipulated controls for the entire two-decade period.

Portal's luxurious
accommodations

Data Collection

Initial design and changes in treatments made in 1986 and 1988 are included in a table downloadable in the meta data.

We have data collection standardized, so that there are no changes in methodology to diminish the value of our long-term data sets.

In July 1977, plots were laid out in a 7 x 7 grid, with 6.25 m separating each of the conspicuous and permanently placed metal grid stakes.

Rodents are trapped during a 2- or 3-night period each month as near as possible to the new moon. Trapping is done by baiting a single Sherman live trap at each grid stake for one night. Each plot has 49 traps set.

Captured individuals are weighed, sex and reproductive status determined, and hind foot length is measured. They are marked, formerly with ear tags or toe clips and now with PIT (permanently implanted transponder) tags, then released.

Ants are censused during periods of maximum activity, each summer, by counting colonies of the large


Chaetodipus baileyi or Bailey's pocket mouse is weighed

Pogonomyrmex rugosus carrying a seed back to its nest.

Pogonomyrmex species on each plot and colony entrances of the other species within a circle of 2-m radius around 49 grid stakes.

Annual plants are censused each spring and fall, at the end of the winter and summer growing seasons, respectively, by counting individuals on 16 quadrats per plot, each 0.25 m2 in area and located 1 m south of alternate grid stakes.

Similarly standardized and spatially referenced methods have been used at less frequent intervals to census birds, quantify seed removal by rodents, birds and ants, measure grass and perennial shrub cover, and collect other relevant data.

In addition to standardizing data collection protocols, efforts are taken at minimizing human activities on the study site and its organisms and to ensure that the unavoidable impacts are equal across all plots and experimental treatments.

For example, we confine foot traffic to east-west running trails along each row of grid stakes and ensure all plots receive equal amounts of rodent trap bait and foot traffic.

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