Info

16 n.s.

Unpublished observations of E. Mellink and M. E. Riojas

-López.

The white-throated packrat is one of the vertebrates that relies most heavily on platyopuntias for its food (Vorhies and Taylor 1940; Spencer and Spencer 1941; Rangel and Mellink 1993). Although Opuntia cladodes are its preferred food, seasonal variations in preference occur. In Arizona, this species exhibits clear peaks in consumption of cladodes in May and November (Vorhies and Taylor 1940), which coincides with the driest period of the year.

The vertebrate most popularized as a cladode eater is the collared peccary. Such fame is not undue, as platyopun-tias are one of its most important food resources in arid lands. Indeed, Opuntia cladodes are the most common food for the peccary throughout the southwestern United States and northern Mexico (Neal 1959; Leopold 1959; Sowls 1984; Hoffmeister 1986). Peccaries are not ruminants, but their ruminant-like digestion allows them to use profitably such a high-fiber forage (Langer 1979). Despite their year-round high consumption of platyopuntias, collared peccaries exhibit seasonal variation, consuming them in greater quantities during the summer (Corn and Warren 1985) or fall (Eddy 1961). In northern San Luis Potosi, Mexico, collared peccaries steadily increase the amount of cladodes in their diet from 25% in June to 66% in September (Luevano et al. 1991). Cladode consumption during the summer may be associated with high temperatures, and peccaries as well as white-tailed deer presumably require the water for thermoregulation (Zervanos and Hadley 1973). During drought, a collared peccary must consume at least 2,300 g of succulent plants per day to thermoregulate effectively (Langer 1979).

Deer are not always regarded as consumers of cladodes, but in some arid regions they can rely heavily upon them, at least during certain times of the year. In Big Bend National Park, Texas, Opuntia engelmannii makes up 14% of the yearly diet of mule deer and 10% of that of white-tailed deer (Krausman 1978). At other localities in Texas, Opuntia cladodes form 30 to over 50% of the white-tailed deer's diet (Everitt and Gonzalez 1979; Quinton and Horejsi 1977). Deer consume the cladodes both for energy and as a source of water for thermoregulation (Arnold and Drawe 1979). Maximum cladode consumption by deer has been reported to occur in the spring (Krausman 1978), summer (Arnold and Drawe 1979), and summer/fall (Quinton and Horejsi 1977). Even within a given season, great month-to-month variation occurs in the amount of Opuntia cladodes consumed by deer (Luevano et al. 1991). Differences in the timing of peak consumption reflect water needs by the deer, along with the availability of free water and other succulents that might provide water. Less-studied animals also change their dependence on cladodes during the year, depending on other available resources. For example, the northern pocket gopher consumes cladodes the entire year, but in dramatically higher percentages during the winter, when the land is covered by snow and little other food is available (Vaughan 1967).

Nutritional Qualities of Cladodes

Platyopuntia cladodes make a rather poor forage. Their protein content varies between 3 and 13% (on a dry weight basis), depending on the species, time of the year, whether the plant bears fruit, the particular cladode, and the age of the plant (Sowls 1984; Retamal et al. 1987; Flores Valdez and Aguirre Rivera 1989; Pimienta-Barrios 1990; Gregory and Felker 1992; Theimer and Bateman 1992). Moreover, while some platyopuntias exhibit variations in protein and phosphorus content associated with cladode age, others do not (Gregory and Felker 1992). Still, at even the highest level, the protein content is generally not sufficient for a substantive diet. For example, when collared peccaries are fed exclusively a cladode diet, they lose weight, but when protein is supplemented, not only do they maintain their weight, but some even gain weight (Sowls 1984). Collared peccaries also demonstrated vitamin B deficiencies when fed only cladodes (Sowls 1984). Regardless of its low nutritional value, the water provided by the cladodes is often critical for the survival of consumers and appears to be more important than any nutritional shortcomings at various times of the year. In any case, vertebrates seldom feed only on platyopuntias, and the inclusion in their diet of other plants with more protein, or invertebrates, prevent them from severe undernutrition.

Not only are Opuntia cladodes a less than optimal food for vertebrates, but there are also other risks associated with their consumption. When collared peccaries are forced to consume large quantities of nopales, large amounts of water flow through the digestive system, causing an almost continuous diarrhea (Sowls 1984), which can have serious consequences in arid lands. Probably the best known negative consequence of feeding on platyopuntias comes from the high levels of the oxalates in them (Hodgkinson 1977; Sowls 1984; Gibson and Nobel 1986). Dietary oxalates bind calcium (Ca), magnesium (Mg), sodium (Na), and potassium (K) as highly insoluble compounds and, in sufficiently high doses, commonly leads to hypocalcemia (James 1972; Hodgkinson 1977; Sowls 1984; Gibson and Nobel 1986). The binding with Ca and Mg is likely the strongest (E. Ezcurra, personal communication). Ill effects of high oxalic acid intake by mammals include nephritis and respiratory failure (Hodgkinson 1977; Sowls 1984). Although calcium oxalate crystals may cause mechanical damage to the digestive system (James 1972), those of Opuntia cladodes are rather rounded, as opposed, for example, to crystals in agaves, and may not be sufficiently abrasive to be a major problem (E. Ezcurra, personal communication).

However, herbivores that have intestinal or ruminal microflora capable of digesting cellulose can also degrade calcium oxalate and absorb the calcium, if given enough time for their digestive microbiota to adapt (Allison and Cook 1981; Justice 1985). This appears to be the case with packrats and other rodents (Shirley and Schmidt-Nielsen

1967; Justice 1985). This ability is provided by the adaptability of the digestive microflora itself, and does not represent an evolutionary adaptation of the herbivores (Justice 1985). Herbivores cope with dietary oxalates in at least one other way. Collared peccaries select inner cladodes that have lower levels of oxalates (although lower spininess seems to be also involved in this selection), but because these cladodes also have less protein and more lignin, the peccaries sacrifice diet quality (Theimer and Bateman 1992). Nopales also contain high quantities of alkaloids (Meyer et al. 1980; Gibson and Nobel 1986). These substances could harm consumers of platyopuntias, but not enough is known about them and their potential effects on herbivores.

Spine Effects on Herbivory

Spines can potentially inflict wounds that can become infected (Anthony 1954). Several vertebrates, e.g., the collared peccary (Theimer and Bateman 1992) and white-throated packrats (Rangel and Mellink 1993), feed less on the more spiny cladodes. In the Galápagos Islands, spines effectively prevent young arborescent platyopuntias from being consumed by tortoises (Biggs 1990). However, Berlandier's tortoises feeding on cladodes are not hindered much by spines, as several individuals with large spines in the masseter muscles on both sides of the neck have been observed (Auffenberg and Weaver 1969).

In addition to selecting cladodes that are less spiny, vertebrates often scrape the spines off. For example, collared peccaries sometimes bite through the entire cladode, but most commonly they step on it, peel the skin (epidermis plus hypodermis) off one side, and then eat the pulp (Sowls 1984). Captive Galápagos land iguanas fed cladodes often scrape the surface with a front foot to remove the spines before biting into the pad (Carpenter 1969). In platyopuntia orchards in Jalisco, desert cottontails discard the areoles and spines along with little pieces of cuticle when feeding on cladodes. This explains the abundant pieces of cuticle found at the bases of platyopuntias, together with cottontail rabbit fecal pellets. Platyopuntias that have only jackrabbit pellets and no cuticle pieces suggest that the jackrabbits handle the spines in a different manner. White-throated packrats, in addition to selecting less spiny plants, most often gnaw across the cladode, beginning somewhere along its edge, and then work their way inward, possibly discarding the spines and glochids along the way (Fig. 7.1A). At other times they feed by scraping the pulp from the side of the cladode (Fig. 7.1B). In spite of the hazards that might be involved, the animals that rely on Opuntia cladodes for an important part of

Figure 7.1. Cladodes exhibiting signs of feeding by packrats near Las Papas, Jalisco, Mexico: (A) feeding on the edge and (B) feeding on the side.

their diet are able to cope with this resource, and death induced by such consumption in free-ranging conditions is rather uncommon.

Consumption of Reproductive Structures

The reproductive structures of platyopuntias, especially fruits and seeds, are the major source of attraction to consumers. All the reptilian and bird consumers eat them, as do most of the mammalian consumers (Table 7.1). Actually, given a chance, all mammals that consume vegetative platyopuntia structures may readily eat flowers and fruits as well. Flowers and fruit are available seasonally, but the seeds can be available all year, because fruits ripen asynchronously, and because seeds can remain on the ground for long periods.

Consumers of reproductive structures are of three foraging types: (1) primary foragers—those that directly reach the flowers or fruits to feed on them; (2) secondary consumers—those that that feed on flowers or fruits once they have been made available by a previous consumer; and (3) tertiary consumers that feed on seeds, including those of platyopuntias that are in the feces of other consumers, e.g., kangaroo rats, pocket mice, and canyon towhees, among others (Gonzalez-Espinoza and Quintana-Ascencio 1986). A primary consumer may open a fruit, allowing a secondary consumer to reach its interior, or it may cause petals, fruits, or seeds to fall to the ground. Once a fruit has been pried open by a primary consumer, such as a packrat, a host of other users might eventually feed on it. These secondary consumers will include not only insects—ants, bugs, wasps (Fig. 7.2)—but also birds and rodents (Gonzalez-Espinoza and Quintana-Ascencio 1986).

Flower Consumption

Flowers provide more energy for herbivores than do the cladodes or the fruits but only modest amounts of protein (Christian et al. 1984). So the nutritive quality does not seem to explain the "enthusiasm with which the iguanas scramble for a newly fallen flower, nor does it demonstrate the willingness of the iguanas to travel from tree to tree to consume fallen flowers of Opuntia" (Christian et al. 1984). Indeed, two-thirds of the reptiles that consume platyopuntias specifically eat the flowers (Table 7.2). Pollen and nectar contained in the flowers might be part of the explanation for such a preference.

Some bird species on the Galápagos Islands consume pollen and nectar during the dry season, switching to fruits, seeds, and arthropods during the rainy season (Grant and Grant 1979, 1981; Millington and Grant 1983). During the dry season when other foods are scarce, the cactus finch on Daphne Major relies almost exclusively on pollen and nectar, which seem to provide sufficient nutrients for pre-rain breeding. This is an advantage because pairs of this finch that start breeding before the rains produce more offspring (Grant 1996). This finch is a true cactus specialist. To some extent, it excludes the other finches from feeding on pollen and nectar of platyopuntias, defending larger territories year-round, which is in contrast

Figure 7.2. Nopal fruit used as food source by a wasp after a primary consumer has pried it open, near Las Papas, Jalisco, Mexico.

with other finches on Daphne Major, who defend smaller territories and only for part of the year (Grant and Grant 1981; Grant 1996). Indeed, the ground finch on Daphne Major consumes much lower amounts of nopal pollen and nectar, and the onset of its breeding season is after the rains (Grant 1996).

Doves on the Galápagos Islands feed on Opuntia flowers, possibly first removing the stamens and nectar, and then tearing and eating the petals. This seems to be a rather unusual feeding habit—not often observed and not exhibited by all dove populations (Grant and Grant 1979). Lava lizards on Pinta Island climb up platyopuntias and tear the petals to obtain the pollen. In contrast, the lava lizard on Daphne Major does not climb platyopuntias but obtains pollen opportunistically by eating the pollen-impregnated petals that fall when the Daphne Major cactus finch feeds (East 1995). The lizards probably obtain an important portion of protein from the pollen, especially as the onset of Opuntia flowering occurs during the dry season when arthropods are in short supply (East 1995). Even the Lagarto Tizón, an omnivorous lizard of the Canary Islands, Spain, feeds on platyopuntia pollen (Valido and Nogales 1994).

Pollen has variable levels of protein. Notably, pollen from bat-pollinated cactus flowers has a high protein content, e.g., up to 44% in the saguaro, but pollen of the non-bat-pollinated Opuntia versicolor has only 9% protein (Howell 1974). In addition to their protein content, pollen grains are valuable because they contain essential amino acids and vitamins (Howell 1974; Grant and Grant 1981; Millington and Grant 1983; Richardson et al. 1986; Grant 1996). Galápagos finches digest over 90% of the pollen they consume (Grant 1996). The physiology of such high digestibility has not been clarified, but either the Galápagos finches and lava lizards (which also exhibit a high digestion of pollen) are especially efficient, or platyopuntia pollen is quite easy to digest (Grant 1996). Several paths are theoretically possible in the digestion of pollen; for a small, pollen-eating marsupial (Tarsipes rostratus), pollen seems to be digested directly through the pores in its exine coat (Richardson et al. 1986). Consuming nectar along with pollen, in addition to providing energy, may also help the ingested pollen germinate, facilitating its digestion (Grant 1996).

Fruit Consumption

Opuntia fruits are a valuable food resource for animals and are readily eaten when available. Over 60% of platyopun-tia consumers in any taxa eat the fruits. Platyopuntia fruits (tunas) have 9 to 18% sugar and large quantities of vitamin C (Pimienta-Barrios 1990). Fruits of Opuntia lindheimeri from southern Texas have 7% protein, 0.15% phosphorus (P), 2.5% Ca, 0.93% Mg, 3.4% K, and 0.02% Na (Everitt and Alaniz 1981). Vertebrates may consume fruits as a "gourmet" food, when they encounter them, as the Shasta ground sloth did (Hansen 1978). On the Canary Islands, the endemic Lagarto Tizón consumes fruits of Opuntia dil-lenii during May (Valido and Nogales 1994). As different platyopuntia species bear fruits of different sizes, color, and spininess, foraging preferences of vertebrates differ (Janzen 1986).

The seeds are used by at least 13 vertebrate herbivores, mostly birds (Table 7.2). They are also an important resource for many rodents (González-Espinoza and Quintana-Ascencio 1986). Such seeds are rich in oils and proteins (Pimienta-Barrios 1990). Although seeds may be available year-round, certain vertebrates eat them only when other resources are in short supply. In the Galápagos Islands during the dry season, seed consumption by the Daphne Major cactus finch declines as flower feeding increases (Millington and Grant 1983).

Evolutionary and Ecological Context

Past and Present Herbivory Pressures

In arid and semiarid lands, platyopunitas often constitute one of the most conspicuous elements of the landscape, and it is easy to find relationships among them and some vertebrates. Platyopuntias provide protein, carbohydrates, and water to vertebrates, and these vertebrates in turn act as pollinators and dispersers (Grant and Grant 1979; Gonzalez-Espinoza and Quintana-Ascencio 1986; Biggs 1990; East 1995). However, these relationships do not explain the evolution of the traits currently exhibited by platyopuntias and their fruits. Rather, platyopuntias on continental America are the ghosts of past interactions that involve currently extinct megaherbivores (Janzen and Martin 1982; Gonzalez-Espinoza and Quintana-Ascencio 1986; Janzen 1986). On the Galápagos Islands, the evolutionary pressures, which are still operational, are different.

According to Janzen (1986), not only did Pleistocene megaherbivores shape the form and anatomy of platy-opuntias, but they also could have dispersed them from South to North America, or vice versa, after the closure of the Central American bridge. In addition to being longdistance dispersers in Pleistocene communities, some megaherbivores probably munched their way through dense patches of platyopuntias, creating clearings that would be colonized by other plants and, perhaps, small mammals, reptiles, and invertebrates. After most of the megaherbivores of the Americas vanished at the end of the Pleistocene, platyopuntias have maintained most of the traits developed under the pressure of their former consumers. Certainly, important changes in distribution and abundance resulted from the absence of their principal dispersers, but the species survived. In a few cases, erosion of anachronistic traits (sensu Janzen and Martin 1982) seems to be occurring; e.g., some platyopuntia species have "spiny" fruits that are not eaten by herbivores and which are mostly sterile (Anthony 1954).

Although they might have contributed little to the past shaping of platyopuntias, the extant opuntiofagous vertebrates do currently serve as dispersers of seeds. Birds remove only modest amounts of seeds—less than 5% of the total crop (Janzen i986)—but rodents can remove more seeds from the fruits once they fall to the ground (Gonzalez-Espinoza and Quintana-Ascencio 1986). Rodent caches often become the source of new platyopuntias away from the mother plants. The seed shadows produced by extant vertebrate dispersers are surely much different—and at smaller geographical scales than from those that can be presumed for Pleistocene megaherbivores (Janzen 1986). Nonetheless, these extant vertebrates may substantially increase the cover of platyopuntia communties, especially when patterns of competition among different plants is altered by the introduction of alien grazers, such as cattle or sheep (Riegel 1941; Timmons 1942). The effects of direct removal of platyopuntia parts by extant species on the system are difficult to assess. For example, collared peccaries can remove 2 to 5% of the cladodes (Bissonette 1982). As platy-opuntias are well armored against grazing, direct removal of large parts of their vegetative structure is unlikely (and probably was unlikely even during the Pleistocene), except during severe drought.

Galápagos Islands

Nowhere is the association between platyopuntias and vertebrates as intense as on the Galápagos Islands. The morphology of nopales on different islands is a clear adjustment to avoid herbivory on vegetative structures (Thornton 1971; Biggs 1990; Hicks and Mauchamp 1995). Arborescent platyopuntias predominate on specific islands of the Galápagos that support, or have supported, tortoise and land iguana populations. These plants have large scaly trunks (> 60 cm in height), bearing rounded compact crowns with lower cladodes strongly armored with spines and the upper cladodes almost spineless. When young, spines protect these nopales from grazing by tortoises. On islands that have never supported tortoises or iguanas, the plants are decumbent and have weak or no spines, some cladodes bearing only tufts of glochids (Biggs 1990). During the rainy season, some cladodes of arborescent platyopuntias become heavy, turgid with water, and break off from the mother plants, falling to the ground. There they maintain a high water content, even through the following dry season, when they are the main food source for land iguanas and tortoises (Biggs i990).

A particularly strong relationship exists between birds and platyopuntias in the Galápagos Islands. In its simplest form, differences in size and hardness of platyop-untia seeds may be a partial response to predation by finches. Conversely, the size and shape of the beaks of finches may reflect an adjustment to forage efficiently for pollen and nectar, as well as the ability to break seeds (Grant and Grant i989). Some finches contribute significantly to the cross-pollination of platyopuntias, transporting pollen from plant to plant. Such transport is important, as flowers that receive pollen from flowers of the same plant produce significantly fewer seeds than those that receive pollen from more distant plants (Grant and Grant 1981). Consequently, these finches promote larger seed crops.

However, this pollinating service is not without negative repercussions. When feeding on flowers, the finches often snip off the styles, presumably to facilitate access to the pollen, which prevents fertilization of the ovules. As a result, by obtaining energy and protein from nectar and pollen, the finches benefit by having an early onset of their

Figure 7.3. Feral burro eating the fruit of Opuntia aff. megacantha near Las Papas, Jalisco, Mexico.

breeding season, but, in doing so, threaten the seed supply for their dry-season feeding (Grant and Grant 1981). The negative effect of snipping styles might be only partial: the later the style is snipped, the greater the chance that the pollen has already reached the stigma and some ovules are fertilized (Grant and Grant 1981). Despite their overall value as pollinators, finches sometimes cause damage to platyopuntia trunks and young cladodes when pecking into them to drink fluid, eat storage tissues, and take insect larvae (Grant and Grant 1981). As a result, the damaged pads are vulnerable to infection by bacteria and fungi, eventually leading to necrosis.

The preceeding is a highly simplified picture of the relationships that have shaped the Galapagos Islands platy-opuntias and their communities. The real picture is more complex and involves variations at different time scales. Regrettably, the conditions that promoted the evolution of the local relationships have not remained intact. The land reptiles have been hunted, sometimes to extinction, and alien species have been introduced (Thornton 1971; Hicks and Mauchamp 1995). These events have already affected the permanence of such relationships and probably will cause others that might be detrimental to the conservation of the entire platyopuntia-associated system.

Alienized Relations

Whenever platyopuntias have been introduced to other parts of the world, they have been readily accepted by local vertebrates. Ten animals have been reported to consume introduced platyopuntias (Leopard tortoise, Lagarto Tizón, ostrich, emu, black magpie, little raven, baboon, "monkeys," camel, and steenbok; Table 7.1), but there are probably many more unreported consumers. Frequently, native vertebrates may become dispersers of alien platyopuntias, which is the case for the Lagarto Tizón (Valido and Nogales 1994) on the Canary Islands, the black magpie in Australia (Darnell-Smith 1919), and primates and birds in South Africa (Weed Section 1940). Other native vertebrates that consume alien platyopuntia fruits may also disperse seeds. Alien vertebrates may also disperse alien platyopun-tias, as demostrated by feral European rabbits in Australia (Darnell-Smith 1919). The roster of opuntiofagous vertebrates in areas were platyopuntias are alien is surely much larger than what has been reported so far (Table 7.1), and it is unlikely that it will ever be fully reported.

The other side of the coin in alienized relations is that of the impact of alien vertebrates on native platyopuntias. Four alien species currently consume cladodes in the wild in the Americas (black rat; feral burro, Fig. 7.3; wild boar;

and feral goat). When cattle and horses roamed wild two centuries ago, they likely also engaged in cladode consumption, as domestic individuals of these species do today. In continental America, the introduction of large domestic vertebrates (namely, cattle and horses) restored functions interrupted by the extinction of megafauna at the end of the Pleistocene (Martin 1975; Janzen 1986), although this view is not always accepted. In insular contexts, however, the introduction of alien herbivores or omnivores often causes conservation hazards, if not mass extinction, even for well-protected species, such as platyopuntias.

Three alien vertebrates threaten platyopuntias on the Galápagos Islands. Goats feed on the pads of nopal (Hicks and Mauchamp 1995). They munch through the trunks of arboreal platyopuntias, up to 50 cm in diameter, causing them to fall (Eliasson 1968). Medium and large platyopuntias are subject to a higher grazing pressure, because they are less spiny, and this selection can seriously impair platyopuntia populations, as it leads to the killing of the plants before they reach reproductive age (Hicks and Mauchamp 1995). Fallen cladodes can produce new trees, but the goats eagerly eat the pads before any rooting takes place (Hicks and Mauchamp 1995). Burros have also affected the distribution of platyopuntias on the Galápagos Islands (Van der Werff 1982; Hicks and Mauchamp 1995). In the case of seedlings, their heavy spiny armour can prevent grazing by native reptiles but does not prevent goats from considering them a "favorite" (Schofield 1989). While not evident at first consideration, mice (probably Mus mus-culus) are also a threat to platyopuntias on the Galápagos Islands (Snell et al. 1994). They burrow among and into the roots, weakening their hold on the soil. The effect of such activities and the success of prevention programs in the long run is difficult to predict.

Concluding Remarks

Platyopuntia cladodes offer not only food and water, but also protective cover, den anchorage, and den building materials to wild vertebrates. Several species are closely associated with platyopuntias: Berlandier's tortoise, white-throated packrat, collared peccary, and deer, as well as finches, land iguanas, and tortoises on the Galápagos Islands. Other vertebrates rely on cladodes for survival during critical periods such as drought. Still other species, although not using the cladodes themselves, use the native platyopuntia nopaleras as habitat.

Despite the fact that several relationships between platyopuntias and vertebrates exist, there has been a paucity in the efforts to understand them, except for the highly creative studies on the Galápagos Islands. For the continental Americas, the advances in understanding platyopuntia-vertebrate interactions notably include the proposal of Janzen (1986) on the evolution of platyopuntias, their communities, and their dispersal in North America as well as the work of González-Espinoza and Quintana-Ascencio (1986) on Opuntia seed dispersal for nopaleras in the Mexican plateau. Most other work has focused on the role of cladodes in the diet of selected vertebrates, especially the collared peccary. Clearly, much research remains to be done to understand the function of nopaleras in the continental Americas. If such an understanding is to be generated, action should be taken soon, as nopaleras are being modified at accelerated rates to raise agricultural products or livestock, or as an inevitable side effect of human population growth.

Acknowledgments

We thank Rosy Licón, Roberto García-Benitez, Dolores Sarracino, and Park Nobel, who helped obtain some of the bibliographic sources, and Amadeo Rea and Exequiel Ezcurra for insightful reviews.

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