Academic journal article The American Midland Naturalist

Effects of Plains Pocket Gopher (Geomys Bursarius) Disturbances on Tallgrass-Prairie Plant Community Structure

Academic journal article The American Midland Naturalist

Effects of Plains Pocket Gopher (Geomys Bursarius) Disturbances on Tallgrass-Prairie Plant Community Structure

Article excerpt

ABSTRACT.-Our objective was to evaluate the effects of soil disturbances created by the plains pocket gopher (Geomys bursarius) on the structure of a tallgrass-prairie plant community. We predicted that soil mounds and burrows would provide sites for the establishment of subordinate plant species, thereby increasing regional plant community richness in this ecosystem that is highly dominated by perennial C^sub 4^ tallgrasses. Our results, however, revealed that plant species richness and biomass were temporarily decreased or unaffected in areas disturbed by gophers. Moreover, the species found locally on mounds and burrows were a subset of the dominant plants present in the undisturbed plant community and graminoids were more frequent on disturbances than forbs. Our results indicate that perennial graminoids predominate in the rapid recovery of vegetation on pocket gopher mounds and burrows. This preempts the establishment of less frequent forbs and, contrary to our predictions, decreases plant community richness.

INTRODUCTION

North American pocket gophers (Geomyidae) are fossorial rodents recognized as important agents of soil alteration in grasslands (eg., Grinnell, 1923; Mielke, 1977; Andersen, 1987; Huntly and Reichman, 1994). They build extensive belowground foraging burrows and deposit tailings on the soil surface as mounds (Andersen, 1987; Reichman and Smith, 1990; Benedix, 1993). Recently, pocket gophers have been labeled "ecosystem engineers" because their physical activities modify, maintain or create habitats and alter resource availability for other organisms (eg., Huntly and Inouye, 1988; Moloney et al, 1992; Jones et al., 1994). Local plant responses to the burrow and mound disturbances created by pocket gophers are well documented in some ecosystems (e.g., McDonough, 1974; Hobbs and Mooney, 1985; Spencer et al., 1985; Huntly and Inouye, 1988; Martinsen et al., 1990). Mounds are thought to influence plant species diversity by acting as seed catchments (Laycock, 1958; McDonough, 1974; Hobbs and Mooney, 1995) and providing space and resources for less competitive, colonizer species to become established (e.g., Schaal and Leverich, 1982; Hobbs and Hobbs, 1987; Goldberg and Gross, 1988; Reader and Buck, 1991; Davis et al., 1995). Mound disturbances can be primary sites of establishment for subdominant forbs and ruderal plants. Consequently, gopher mounds often undergo gap phase dynamics that differ from the surrounding plant community (Foster and Stubbendieck, 1980; Inouye et al., 1987; Martinsen et al., 1990; Stromberg and Griffin, 1996). By creating a mosaic of patches with various resource characteristics and successional ages, pocket gopher mounds frequently enhance resource heterogeneity and create distinctive spatial patterns in the plant community (eg., Hobbs and Mooney, 1985; Reichman et al., 1993; Moloney and Levin, 1996). This heterogeneity likely is responsible for the increases in overall community richness at larger scales attributed to pocket gopher activity (e.g., Tilman, 1983; Spencer et al., 1985; Inouye et al., 1987; Collins, 1989) despite plant species richness being increased (Andersen and MacMahon, 1985; Martinsen et al., 1990; Reader and Buck, 1991), decreased (Gibson, 1989) or unaffected (Umbanhowar, 1995) directly on a mound.

Because burrows are less conspicuous and more difficult to experimentally manipulate than mounds, their impact on overlying vegetation has received considerably less attention. The most significant effects observed over burrows result from the consumption of belowground plant parts by gophers (Andersen, 1987; Reichman and Smith, 1990; Benedix, 1993), although the effect of the altered soil environment surrounding a burrow per se can influence plant responses even in the absence of herbivory (Reichman and Smith, 1985; Reichman, 1988; Reichman et al., 1993). Plant biomass overlying abandoned burrows can remain lower than undisturbed areas for several years due to impeded root regrowth and lower nutrient and water availability (Reichman and Smith, 1985; Reichman, 1988). …

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