bris deposits become progressively more well developed over time (Webb et al. 1987; McAuliffe 1994; Parker 1995; Bowers 1997; Valiente-Banuet et al. 1995b). Older soils usually contain well developed argillic horizons and calcium carbonate (CaCO3) accumulation in the Bk horizon, both reducing water availability (Hennesy et al. 1983). Thus cactus populations show different structures and composition among different-aged soils, leading to major differences in the population dynamics. For example, soil evolution for Epringlei in Baja California Sur reduces seed production, seedling establishment, survival, and growth, and populations in younger-soil deposits have positive rates of increase (C. Silva-Pereyra, A. Valiente-Banuet, and J. Ortega, unpublished observations). This pattern has also been reported for other P pringlei populations and for Fouquieria columnaris (McAuliffe 1991). In all cases, soil ge-omorphic development in alluvial landscapes is a key to spatial heterogeneity in plant population dynamics of cacti, leading to a metapopulation approach (Fig. 6.3).
To understand the population dynamics of cacti, a hi-erarchial organization of biotic and abiotic factors should be considered (Fig. 6.3). Biotic interactions may be the main factors that affect cacti at a local population level, whereas dispersion among populations and the effects of soil types on recruitment become important at the metapopulation level. Finally, the geomorphic history of the localities in which populations of cacti occur should be analyzed to understand population dynamics at the landscape level (Fig. 6.3).
Cacti have great diversity in the Sonoran, Chihuahuan, and Tehuacan-Cuicatlan deserts. Due to the relative uniformity of deserts and the apparently stressful conditions imposed upon desert species, early studies that were mostly au-toecological have dominated the discussion about how diversity is maintained (Noy-Meir 1973). However, since 1990 different studies have progressed toward an under-
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