Local Population

Interactions between species such as nurse plant phenomenon, predation, seed dispersal, pollination, and competition, among others.

Figure 6.3. Hierarchical organization of biotic and abiotic factors affecting the population dynamics of cacti.

standing about the role of biotic interactions on species coexistence in addition to the analysis of abiotic conditions.

Cactus-dominated communities support a surprisingly diverse fauna and flora that include hundreds of species (Table 6.5). The classification systems of the vegetation of Mexico (Miranda and Hernández 1963; Rzedowski 1978) recognize different plant associations dominated by cacti: Nopaleras, mostly in the Chihuahuan and the Sonoran deserts, and "sarcocaulescent shrublands" including Cardonales, Tetecheras, and "Stenocerales" dominated by giant columnar cacti in the Tehuacán-Cuicatlán Valley, the southern part of the Chihuahuan Desert in Querétaro and Hidalgo, and the Sonoran Desert. Plant associations dominated by Carnegiea gigantea and Pachycereuspringlei occur in the Sonoran Desert (Turner et al. 1995). Indeed, P pringlei is the most widely distributed dominant species in Baja California. In the Tehuacán-Cuicatlán Valley, a total of nine vegetation types are dominated by giant columnar cacti (Osorio et al. 1996). In these ecosystems, trees and shrubs, while modifying the microenvironmental conditions beneath their canopies, are controlling the availability of resources to other organisms by creating special habitats. Thus nurse-plant associations significantly contribute to the establishment of nearly all cactus species, acting as "ecosystem engineers" (Jones et al. 1997) and playing a major role in determining the structure and functioning of desert ecosystems (Valiente-Banuet and Ezcurra 1991).

Species can concentrate beneath the canopies of nurse plants—in some communities 60% of the species are found there (Valiente-Banuet 1991). This affects intra-specific competition, as for Carnegiea gigantea (McAuliffe and Janzen 1986), and interspecific competition (McAuliffe 1984; Valiente-Banuet et al. 1991b). Once a columnar cactus grows to more than 1 m in height, it may outcompete its nurse plant, defining a cyclical relationship in which the association starts as a facilitation process and

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