Intraspecific and interspecific competition
Nurse plants Seedling predation Soil moisture Soil nutrients Light (PPF)
Figure 6.1. Biotic and abiotic factors affecting the life cycle of cacti.
egories for both species vary between 5 and 10% (Fig. 6.2). Although survivorship of N. tetetzo and P pringlei seedlings does not have a high elasticity value (5%), this life cycle stage plays a significant role in the population dynamics because their survival probabilities are low and highly variable (6 x io-4 in N. tetetzo after 2 years, Godínez-Alvarez et al. 1999; 0.01 in P pringlei after 1 year, C. Silva-Pereyra, A. Valiente-Banuet, L. Valiente, P. Dávila, and J. Ortega, unpublished observations). The main mortality factor for seeds is predation by birds, rodents, and ants; for seedlings, desiccation by direct solar irradiation is the principal mortality factor. The survivorship pattern of these cacti indicates a high mortality for seeds and seedlings and greater survivorship for older individuals (a type III curve; Steenbergh and Lowe 1969; Valiente-Banuet and Ezcurra i99i). This survivorship pattern has also been found for other species of cacti, such as Ferocactus acanthodes (Jordan and Nobel 1981), F. cylindraceus (Bowers 1997), Lophocereus schottii (Parker 1989), Mammillaria gaumeri (Leirana-Alcocer and Parra-Tabla 1999), and Stenocereus thurberi (Parker i987). Because the survivorship of seeds and seedlings depends on the arrival of seeds to sites under perennial plants (Fig. 6.1), the association of cacti with nurse plants is an important interaction in the population dynamics of cacti. The protection provided by nurse plants decreases the probability of mortality of seeds and seedlings, thereby increasing the probability that seedlings grow to maturity (Turner et al. 1966; Steenbergh and Lowe 1969, 1977; Valiente-Banuet and Ezcurra 1991).
Elasticity analyses of N. tetetzo and P pringlei also indicate that seed dispersal is an important interaction in the population dynamics of cacti (Fig. 6.2). Frugivorous birds and bats consume the fleshy fruits of different species of cacti, such as C. gigantea (Steenbergh and Lowe 1977; Olin et al. 1989) and N. tetetzo (Valiente-Banuet et al. 1996). These fruit-eating animals can transport seeds directly to the canopies of nurse plants, affecting the population dynamics of these cacti. Bats are actually better dispersers than birds for N. tetetzo, because they transport intact and viable seeds directly to the nurse plants, thereby maintaning a continual flux of seeds to sites for seedling establishment.
Although demographic models indicate that biotic interactions are more significant than abiotic factors, soil evolution plays a major role in the population dynamics of Ppringlei (McAuliffe 1991; Valiente-Banuet et al. 1995a). In alluvial landscapes, cacti have their densest populations in young, coarse-textured soils (Parker 1989, 1995; McAuliffe 1991), with marginal populations in older alluvial soils that have higher per capita mortality than those in younger deposits. These observations suggest declines from what originally were denser populations (McAuliffe 1991). In alluvial regions, soils derive from the same source and usually share the same climatic conditions, but pedogenetic processes occur at different times, e.g., soils of alluvial fans and de-
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