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first, and then progress to the inside of the plants. This process is more as a result of the architectural arrangement of the plant, rather than due to differences in palatability of the cladodes (Hoffman et al. 1993).

When different platyopuntia species are available in the habitat, herbivores apparently prefer some over others. In a test with three platyopuntia species and one cholla species typical of nopaleras (nopal communities) in the San Luis Potosi-Zacatecas Plateau, Mexico, white-throated packrats prefer Opuntia robusta and O. streptacantha over O. leucotricha and O. imbricata (Rangel and Mellink 1993). Opuntia robusta and O. streptacantha have the lowest amounts of protein and highest amounts of fiber (Flores Valdez and Aguirre Rivera 1989), so the higher consumption by these packrats is not a response to nutritional quality. Rather, it apparently results from these cacti having fewer spines and glochids, which facilitates their consumption. Collared peccaries prefer one of two different morphs of the same subspecies of O. phaeacantha, again favoring the one with fewer spines (Theimer and Bateman 1992). However, in New Mexico, spinescence apparently does not affect grazing intensity by lagomorphs; rather, plant size and grazing history appear to be the factors that most determine grazing intensity; the proportion of grazed pads increases for plants that have more than seven cladodes (Hoffman et al. 1993).

Nopal growers and researchers indicate that some varieties of cultivated platyopuntias are more prone to consumption by wildlife than are others. So, 'Copena Fi,' a spineless forage nopal, is preferred by rodents and lago-morphs over other spineless varieties in central Mexico (C. A. Flores-Valdes and F. Torres, personal communication). In some cases, lagomorphs can entirely wipe out a commercial orchard of this variety. However, spines are not a requirement for effective mechanical defense. Near Jacumba, California, black-tailed jackrabbits will eat almost anything except Opuntia basilaris (A. M. Rea, personal communication). This species does not have large spines; rather, it is densely covered with fine glochids (small, easily detached spines), a nasty encounter for herbivores.

To determine if white-throated packrats had foraging preferences among different cultivated varieties of Opuntia ficus-indica, rows of cultivars 'Roja,' Amarilla,' and 'Blanca' were examined at Las Papas de Arriba, Jalisco, Mexico. Packrats clearly prefer to consume cladodes of 'Roja' over 'Amarilla' and prefer 'Amarilla' over 'Blanca' (Table 7.3). Packrats build their dens on the ground against and around the trunks of platyopuntias using various materials (e.g., cladodes, twigs, dung, garbage). As there are no differences in the percentage of plants associated with packrat dens among different varieties (Table 7.3), differences in consumption must be attributed to the forage quality of the plants. The higher sugar content of 'Roja' presents one logical explanation for packrats' preference.

Seasonal Effects for Herbivory

In xeric habitats where free water is scarce, cladodes become an important source of water, when they are available. Not unexpectedly, vertebrates increase their use of Opuntia cladodes during the dry season, or use them only then. For example, rabbits and black-tailed jackrabbits consume Opuntia cladodes during the dry season, or when annuals are scarce and other perennials have not developed new growth (Hoffman et al. 1993). Berlandier's tortoises consume more cladodes during the summer, as a source of water (Auffenberg and Weaver 1969). Galápagos land iguanas reduce their preference for cladodes from about 32% of bites in the dry season to 5% after the rains (Christian et al. 1984). Even finches, doves, and mockingbirds drink fluids and eat moist pulp from cladodes in the Galápagos Islands (Grant and Grant 1981). Other animals consume platyopuntias under special conditions; e.g., pronghorn antelope feed readily on cladodes after a wildfire has burned off the spines (Stelfox and Vriend 1977).

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