March 1995, looking across the fence of a kangaroo rat removal plot. On the left, kangaroo rats have been continuously present.

 

At right, the large-seeded, yellow-flowered annual Lesquerella gordonii is dominate where kangaroo rats have been removed.

Vegetation at Portal

Vegetation at the study site varies from open grassy areas, to stands of widely spaced shrubs (Gutierrezia, Ephedra, Flourensia), to dense stands of little trees (Acacia and Prosopis); the latter along usually dry watercourses.

Two assemblages of annual plants exist, corresponding to bimodal patterns of yearly precipitation, winter (Novermber to March) and summer (July to August), and have virtually no overlap in pheonology and species composition. These plants spend most of their lives in the soil seed bank and germinate, grow and reproduce only during the brief periods when sufficient soil moisture is available.Annual and perennial plants are censused twice per year at the end of the winter (during the 1978-1989 period this was referred to as spring) and summer (during the 1978-1989 period was referred to as fall) growing seasons.

Large-seeded winter annuals (seed mass > approximately 1 mg) increased on plots where rodents had been removed. By the winter season of 1983-1984, two species of Erodium - E. cicutarium and E. texanum - were several thousand times more abundant on rodent removal plots than on plots where kangaroo rats were present (Brown et al. 1986; see also Samson et al. 1992). Rodents had virtually no detectable impact on summer annuals even though the rodents were active and foraging when the summer annuals are in fruit and dispersing their seeds (Guo and Brown 1996.)

Enormous differences in densities of E. cicutarium and also Lesquerella gordonii were apparent in the winter of 1994-1995, but we have not yet completed analyses of the maganitude of the response. Complementing the increases in large-seeded species, small-seeded winter annuals have decreased on plots where rodents have been removed (Brown, et al. 1986, Samson et al. 1992, Guo and Brown 1996).

Removal of kangaroo rats also had dramatic effects on the perennial vegetation. By 1988 there was much greater cover of perennial and annual grasses on plots where kangaroo rats had been removed. Grass cover increased significantly only on plots where kangaroo rats had been excluded since 1977.


Aerial photograph of the study site taken in 1979.

Aerial photograph of the study site taken in 1995.
The dark squares on right photo are plots where either all rodents or just kangaroo rats have been removed since 1977 and the dense growth of large-seeded annual plants is clearly visible. Plots where all rodents or just Kangaroo rats have been removed only since 1988 have less dense vegetation, and plots where rodents have been present continuously since 1977 have the least annual cover.

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