|Abstract or Summary
- I used field experiments to study how plants in two grassland communities colonized
soil mounds made by the Camas pocket gopher, Thomomys bulbivorus (Richardson). I
identified potential mound colonizers in each source of colonization (buried propagule bank,
seed rain, and established vegetation) and then measured species specific rates of
colonization on mounds built by T. bulbivorus. By selectively eliminating different avenues
of colonization on artificial mounds, I estimated the relative and combined effects of
colonization from (1) germination and growth of buried viable seeds and growth of root
fragments in the soil; (2) germination of seeds raining onto the mounds; (3) emergence of
buried vegetation and, (4) encroachment and establishment of adjacent vegetation. Artificial
mounds were good mimics of mounds built by T. bulbivorus judged by their similarity in
colonization rates and composition of colonizing species. I repeated the investigation in
adjacent pasture and prairie communities differing in species composition and abundances
to compare the effects of these differences on the colonization process.
Composition and abundance of species in the expressed and potential vegetation varied
considerably between pasture and prairie as did the two communities' response to identical
gopher disturbances. Percent cover of vegetation on mounds increased 3 times faster in the
Composition and abundance of species in the expressed and potential vegetation
varied considerably between pasture and prairie as did the two communities'
response to identical gopher disturbances. Percent cover of vegetation on mounds
increased 3 times faster in the pasture than the prairie; and vegetation on and off
mounds in the pasture was more alike (71% Similarity) than vegetation on and off
mounds in the prairie (50% Similarity).
Despite these differences, the relative contribution of each source of
colonization was strikingly similar in the two communities. Vegetative
encroachment and emergence contributed more to overall colonization rates (76%
in the pasture; 75% in the prairie) than did establishment from seeds or buried
root fragments. Emergence from underneath the mounds was favored by the
shallow depth of mounds, minimal alteration of the substrate associated with mound
building, and dominance of perennial species with erect growth forms. The small
area and high perimeter to surface area ratio resulted in a high percent
colonization from encroachment of surrounding vegetation. Colonization from the
rain and bank contributed less to mound closure and may have been limited by a
low abundance of propagules in those two sources.
Successful colonists differed in their patterns of colonization. Festuca rubra,
Agoseris heterophylla, Plantago lanceolata and Prune lla vulgaris colonized almost
exclusively via emergence. Fragaria virginiana colonized by the extension of stolons
both onto (encroachment) and up through mounds (emergence). Colonization from
the seed rain was important in many annual species, such as Ranunculus
occidentalis, Clarkia quadrivulnera, and Sherardia arvensis and the biennial species,
Hypericum perforatum. One annual species, Cynosurus echinatus colonized to
some degree from several modes of colonization. Mound disturbances had greater
forb and annual species cover in both communities than was represented in the
background vegetation, although the difference was much greater in the prairie.
Results of this and other studies of gopher disturbance suggest that the
relative abundance of perennials and annuals, evenness of species abundance and
competitive relationships can help to predict patterns of colonization and effects of
gopher mounds on community diversity.