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The Assembly of Hanging Gardens: Effects of Age, Area, and Location

The structure of plant communities, defined as the identities, abundances, and distributions of their species (Rejmanek 1977), is a result of, and may elucidate, ecological processes. The similarity of the vegetation may be one index of structure that can be used to measure ecosystem processes. Westman (1978) and Whittaker and Woodwell (1978) commented on the use of similarity indices to quantify community response to disturbance or stress. I expect community structure to depend on the physical environment, its history, and dispersal events (Levin 1976a, 1976b), i.e., a combination of genetic and physiological processes. Preston (1962) argued that structure will be similar to the extent that these processes are similar. Dissimilar structure should indicate differences in the processes. Pickett (1976) noted that changing structure also depends on evolutionary history. Diamond (1975) proposed assembly rules for communities which relate similarity of distribution to habitat, historical, and dispersal factors. Connor and Simberloff (1979) criticized Diamond's (1975) assembly rules, explaining that while the interactions may take place, the rules are either circular or not properly tested. Here I examine some of the properties that might be the basis for some of Diamond's (1975) assembly rules with tests that address a null hypothesis. I examine both association between pairs of species among sites and the similarity of sites. As a case study I use hanging gardens found in the Narrows, Zion National Park, United States. These are small isolates, 1-100 m2, located at seeps on the vertical walls of a narrow canyon. I presented details of the sampling design and other results elsewhere (Malanson 1980; Malanson and Kay 1980). To summarize: species were censused in 29 hanging gardens, not randomly selected because of difficult access. The habitat within the gardens is fine grained for the species tolerances and requirements. Nineteen hanging gardens were classified as floodprone with an expected return frequency for flash floods less than 7.5 yr, and seven I classified as flood-safe, with an expected return frequency greater than 15 yr. Three gardens could not be classified unequivocally. The two groups allow for some error without overlap. Of the 48 species recorded, four rare species could not be identified in the field and were not collected; these and two species found only in the three unclassified gardens are not used in this analysis. Twenty-eight species were found in the 19 flood-prone gardens, five exclusively. The flood-safe gardens contained 37 species, 21 exclusively. The flood-safe gardens are richer in species than the flood-prone gardens (mean = 12.0 vs. 5.8; table 1). The mode of dispersal differed significantly between flood-prone and flood-safe gardens, with spores being more common in the former and seed types in the latter. This difference may also be related to the larger area of the flood-safe gardens (mean