COLONIZATION AND ESTABLISHMENT OF MISSOURI PRAIRIE PLANTS ON ARTIFICIAL SOIL DISTURBANCES. III. SPECIES ABUNDANCE DISTRIBUTIONS, SURVIVORSHIP, AND RARITY

We examined the process by which a pool of colonists was reduced to a pool of survivors in 160 small microsites on undisturbed tall-grass prairie in Missouri. Specifically, did the degree of species dominance shift as mortality proceeded and did rare and common species differ in their survivorship? Over 4 yr, 25 taxa of seedlings appeared and 12 survived until the end of the experiment; these pools contained 550 and 79 identified individuals, respectively. For vegetative shoots, 36 taxa appeared and 20 survived, with 1,750 and 387 identified individuals, respectively. Mean survivorship for seedlings (1 19 + 87 days) was briefer than for shoots (165 + 107 days). While species richness was lower among survivors than colonizers, it was not lower than one would expect based on random mortality. Rare species were not disproportionately lost given their population sizes in the microsites. Taxa that were common in the source pool remained common as survivors. The survivors showed more equitable species abundance distributions than the colonists, due to higher mortality among a few of the more common species. For the entire pool of colonists, there was no relationship between abundance and survivorship. In general, rare species did not have higher mortality than common species. These results do not support the notion of regeneration as a competitive "sorting out" of successful dominant from unsuccessful rarer species. Rather, they portray waves of turnover of tiny plants, with species largely acting independently of one another. THE TWO PREVIOUS analyses portray colonization and early establishment of prairie plants on disturbed and undisturbed microsites (Rabinowitz and Rapp, 1985; Rapp and Rabinowitz, 1985). This third paper examines for separate taxa the patterns of abundance and diversity and investigates several questions concerning rarity and commonness (Rabinowitz,1981): 1) Does the degree of"dominance" shift as establishment proceeds? Is the filter of mortality (Harper,1977) a "rlch get rscher, poor get poorer" process dunng which better competitors come to predominate? 2) Do common and rare species in the pool of seedlings and shoots differ in their sllrvivorships? Does regeneration appear to be the stage where limitation of abundance occurs for rare species (Grubb, 1977)? METHODS-Tucker Prairie Research Area, in Callaway County, MO, and its climate are t Received for publication 4 February 1985; revision accepted 28 August 1985. Funding for this study was provided by the U.S. National Science Foundation (DEB 78-1 1 179, DEB 81-03503, DEB 82-91500, and BSR 84-06126). The University of Missouri provided hospitality and logistical support. The paper profited from discussions with Daniel Simberloff, Nickolas Gotelli, and Montgomery Slatkin. described in Rapp and Rabinowitz (1985). In 1977, 80 small cylindrical sod plugs (5.8 cm diam and 12.0 cm deep) were removed and replaced by sterilized prairie soil. Eighty paired undisturbed controls were established 10 cm from each disturbance/resource. These sites were censused on 18 occasions over 4 yr, and the plants were marked with plastic bird-rings. At each census, the number of previous colonists remaining alive and the number of new arrivals were counted for graminoids and forbs categorized as vegetative shoots or seedlings. Any individual present at the previous census but no longer alive was recorded as dead since the last observation. Over the study, 14,779 observations were made on 3,661 individuals. The methods are described in greater detail in Rapp and Rabinowitz (1985). We identified seedlings and shoots to species whenever possible, generally at the threeor four-leaf stage, but many seedlings and some shoots died before they could be identified. Because there was little response to the provision of an unoccupied resource (Rapp and Rabinowitz 1985), we combined data from disturbance/resource and control sites to provide greater sample sizes. The survivors at the final census (Search 18 in late September 1980) were compared with all individuals that arose over the course of the experiment (Searches 1 to 17 from June 1977