S. repandus



Tropical, southern Peru

Weberbauerocereus werberbaueri

Hummingbirds and bats

Sahley (1996)

All breeding systems are monoecious, except trioecious for P pringlei and andredioecious for N. mezcalaensis.

All breeding systems are monoecious, except trioecious for P pringlei and andredioecious for N. mezcalaensis.

North America are self-incompatible and produce fruits only in presence of nectar-feeding bats (Table 6.3), such as Choeronycteris mexicana, Leptonycteris curasoae, and L. nivalis (Valiente-Banuet et al. 1996, i997a,b).

An apparent dichotomy occurs within and outside the tropics among columnar cacti with "bat-pollinated" flowers (Valiente-Banuet et al. i997a,b), in which C. gigantea, P. pringlei, and S. thurberi at latitudes above 29° N are pollinated by a wide spectrum of animals, including birds, bats, and bees (Fleming et al. 1996). Fruit set for pollinators that are diurnal (birds and bees) versus nocturnal (bats) for C. gigantea is 68% versus 40%. In contrast, species in the tropics are self-incompatible and are pollinated exclusively by bats (Sosa and Soriano 1992; Valiente-

Banuet et al. 1996, i997a,b; Nassar et al. 1997; Casas et al. 1999). This predictability in pollinator availability is lower in extratropical areas, where nectar-feeding bats are seasonal migrants from Arizona and northern Sonora to the tropical deciduous forests of Sonora and Sinaloa (Rojas-Martínez et al. 1999). In contrast, Leptonycteris spp. have resident populations within the tropics, e.g., in the Tehuacán Valley (Rojas-Martínez and Valiente-Banuet 1996; Rojas-Martínez et al. 1999), as explained by the predictability of pollinators throughout the year (Valiente-Banuet et al. i997a,b; Rojas-Martínez et al. 1999). Anthesis for most columnar cacti lasts about 12 hours in the intertropical deserts of Venezuela, 13 to 15 hours in Mexico, and even longer (19-23 hours) for C. gigantea, P. pringlei, and S. thurberi in the Sonoran Desert (Alcorn et al. 1961; McGregor et al. 1959, 1962; Fleming et al. 1996; Valiente-Banuet et al. 1996). An obligate pollination mutualism occurs for Lophocereus schottii and the moth Upiga virescens in the Sonoran Desert (Fleming and Holland 1998).

The cactus family is ideal for studying breeding-system evolution, because trioecy occurs for P pringlei with males, seed-producing females, and hermaphrodites (Fleming et al. 1994), and androdioecy occurs for Neobuxbaumia mez-calaensis with male (female sterile) and hermaphrodite individual plants (Table 6.3; Valiente-Banuet et al. 1997a). Trioecy and androdioecy are uncommon sexual systems; androdioecy is uncommon because of the difficulty for male (female sterile) plants to invade hermaphrodite populations (Charlesworth and Charlesworth 1978). Other cactus species have dioecious or subdioecious breeding systems (i.e., with male or female sterile and hermaphrodite plants). Four species of Opuntia (O. glausecens, O. grandis, O. robusta, and O. stenopetala) are dioecious, two species of Mammillaria are dioecious (M. dioica and M. neopalmeri), and Selenicereus innesii is gynodioecious (Parfitt 1985; Del Castillo 1986; Hoffman 1992).

Seed Dispersal

Seed dispersal is an important stage in the life cycle of cacti; it can favor the success of seeds under the canopies of nurse plants (Valiente-Banuet and Ezcurra 1991; Godínez-Alvarez et al. 1999). The successful dispersal of seeds to these sites can increase the area of distribution of cacti and affects gene flow among populations (Howe and Smallwood 1982). Information about the mechanisms by which cactus seeds are dispersed under natural conditions is scarce. In this regard, Bregman (1988), analyzing the structural characteristics of fruits and seeds of more than 100 species of cacti, suggested that these plants can be dispersed by wind (anemochory), water (hydrochory), and animals (zoochory).

Of possible mechanisms of seed dispersal, anemochory and hydrochory are less studied. Anemochory has been suggested for individuals of the genus Pterocactus, which present dry fruits with winged seeds that are exposed when the fruit dehisce at maturity (Bregman 1988). Hydrochory has been suggested for seeds of the genera Astrophytum, Discocactus, Frailea, Gymnocalycium, Matucana, and Thrix-anthocereus. These cacti produce dry fruit containing relatively large seeds with a large hilum, a thin seed coat, and a small embryo, which presumably favor the dispersal by water (Bregman 1988).

Zoochory is the most common mode of transportation of seeds reported for different species of cacti (Table 6.4). Bregman (1988) suggested that seed dispersal by animals occurred in three ways: (1) seeds could be transported passively on the outside of animals, epizoochory; (2) seeds could be transported externally by an animal, synzoochory; or (3) seeds could be consumed by animals, endozoochory. Epizoochory is found only in epiphytic cacti of the genus Rhipsalis, whose fruits contain sticky seeds that adhere to the bill of birds feeding on the fruits. Synzoochory has been suggested for cacti of the genera Opuntia, Parodia, Blossfeldia, Krainzia, Strombocactus, and Aztekium, among others. Seeds of these cacti are generally predated upon by harvester ants (Pogonomyrmex spp., Messor spp.); however, dispersal can also occur when the ants accidentally lose seeds during transportation to their nests (Vargas-Mendoza and González-Espinosa 1992).

Endozoochory occurs for species of cacti that produce fleshy fruits. This kind of fruit is found in most species in the genera Opuntia, Epiphyllum, Hylocereus, Pachycereus, Ferocactus, Melocactus, Carnegiea, Neobuxbaumia, Myrtillo-cactus, Stenocereus, Cephalocereus, Subpilocereus, and Piloso-cereus, among others, and serves as an attractant to different groups of animals, such as reptiles, birds, and mammals that consume the pulp and seeds (Steenbergh and Lowe 1977; Silva 1988; Wendelken and Martin 1988; León de la Luz and Cadena 1991; Soriano et al. 1991; Vargas-Mendoza and González-Espinosa 1992; Cortes Figueira et al. 1994; Silvius 1995; Valiente-Banuet et al. 1996). Indeed, Opuntia fruit were probably consumed by extinct megafauna about 10,000 years ago (Janzen 1986).

In some cases, passage of seeds through vertebrate guts increases germination, such as for Melocactus violaceus (Cortes Figueira et al. 1994) and Stenocereus gummosus (León de la Luz and Cadena 1991), whereas for others, seed germination decreases or there is no effect, as for C. gigantea (Steenbergh and Lowe 1977), Neobuxbaumia tetet-zo (H. Godínez-Alvarez, A. Roj as-Martínez, and A. Valiente-Banuet, unpublished observations), Opuntia rastrera (Mandujano et al. 1997), and Stenocereus griseus (Silvius 1995). The central questions for seed dispersal by animals are to determine if animal vectors are effective dispersers that transport seeds to safe sites beneath nurse plants and to establish whether seed dispersal has consequences on the maintenance of cactus populations under natural conditions. Vargas-Mendoza and González-Espinosa (1992) found that survival of seedlings of Opuntia streptacantha during the first 5 months differed among microsites. Birds and bats consume the fruits of Neobux-baumia tetetzo, but bats more effectively disperse viable

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