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Adapted from Barbault and Halffter (1981), Delhoume and Maury (1992), Arizmendi and Espinosa de los Monteros (1996), and Valiente-Banuet and Arizmendi (1998).

finishes as a competitive exclusion between the cactus and its perennial nurse plant (Yeaton 1978; McAuliffe 1988). Cyclical dynamics can be important in structuring desert communities, and the process has been defined as Markovian in community dynamics (McAuliffe 1988). Indeed, complex combinations of interactions are widespread in desert communities, and the balance between facilitation and competition, which varies with life stage, physiology, and environmental stresses, deserves further research (Valiente-Banuet and Ezcurra 1991; Bertness and Callaway 1994; Callaway and Walker 1997). For instance, the population dynamics of Ppringlei established in well-developed Pleistocene alluvial soils may drift toward extinction, especially during extended dry periods, whereas in young Holocene soils with more favorable moisture conditions, biotic interactions have a central role along its different life-cycle stages (C. Silva-Pereyra, A. Valiente-Banuet, L. Valiente, P. Dávila, and J. Ortega, unpublished observations).

Pollination and seed production are caused by a wide array of animal vectors, such as bees, hummingbirds, hawkmoths, and bats. In some high diversity areas, cacti maintain different and diverse guilds of cactus pollinators, such as about 90 species of bees that visit Opuntia flowers in the Chihuahuan Desert (Grant and Hurd 1979) and 9 species of nectar-feeding bats in the columnar cactus forests of south-central Mexico (Rojas-Martínez and Valiente-Banuet 1996; Valiente-Banuet et al. 1996).

The nectar-feeding bat Leptonycteris curasoae is a latitudinal migrant throughout its distribution in North America (Cockrum 1991; Fleming et al. 1993) and the most important pollinator of columnar cacti (Valiente-Banuet et al. 1996, i997a,b). It has resident populations in the Tehuacán Valley and the Balsas River Basin (Roj as-Martínez et al. 1999), and its pollination relationships with columnar cacti are tight and coupled. In the northern limit of its distribution (southwestern United States), L. curasoae migrates during fall and winter, probably to the tropical deciduous forests of Sonora, Mexico, following the seasonal availability of floral resources (Rojas-Martínez et al. 1999). The predominance of cactus-floral resources during spring and summer in these ecosystems and during autumn and winter in tropical deciduous forests supports high population densities of nectar- and fruit-feeding bats (Rojas-Martínez and Valiente-Banuet 1996) and is correlated with the arrival of a great number of bird species that consume fruits and seeds (Arizmendi and Espinosa de los Monteros 1996). Indeed, bats play two important ecological roles by pollinating the flowers and dispersing the seeds directly to safe sites. Those biotic interactions that increase the average individual fitness of at least one species with no negative effects on any other species are defined as positive (Hacker and Gaines 1997). Positive biotic interactions apparently have the predominant effects on species diversity in such ecosystems. However, more research needs to be done in these environments to determine indirect effects of species.

Conclusions and Future Prospects

At the population level, biotic interactions that cacti have within and between trophic levels help determine survivorship, mortality, and fecundity patterns observed under natural conditions. Although demographic models indicate that positive interactions (i.e., facilitation) affect the most sensitive life-cycle stages, the contribution of bi-otic and abiotic factors to the population dynamics of a particular cactus species is poorly understood. Much research needs to be done to analyze the importance of a metapopulation approach in which soils occupy a central role (Fig. 6.3). Specifically, soil-geomorphic evolution is a key aspect for evaluating the relative importance of abiotic components versus biotic interactions on demography. Preliminary analyses indicate that soil evolution exerts a strong influence on the predominance of cacti in arid environments.

At the community level, biotic interactions also occupy a central role in the explanation of how species coexistence is maintained. Positive interactions such as nurse-plant associations, pollination mutualisms, and seed dispersal strategies clearly suggest that biotic interactions contribute to species maintenance. However, indirect interactions have received little research attention, and almost all investigations have addressed the role of pairwise interactions on particular species. As multispecies systems, cactus-dominated communities deserve research on top-down processes that consider the role of predators in regulating competitive effects among plants. Moreover, considering the extremely important structural dominance of cacti and the trophic relationships between these plants and animals, bottom-up effects have been suggested but have never been properly analyzed.

Considering the economic, cultural, and religious importance of cacti in Mexico, understanding the factors that control the abundance and distribution of cacti over arid landscapes should be a major objective of ecological investigations in deserts. The information presented indicates that many aspects need to be considered to have a more comprehensive knowledge about how to conserve and to manage this highly diversified group of plants (Chapter 8). Indeed, the protection of positive interactions should be a high priority for maintaining these plants in their natural environments.

Acknowledgments

The authors thank Fondo Mexicano para la Conservación de la Naturaleza (Project Ai-97/36), Dirección General de Asuntos del Personal Académico (Project DGAPA-IN-207798), and Instituto de Ecología UNAM for supporting our studies and María del Coro Arizmendi for comments on the manuscript. A.V.-B. dedicates this chapter to the memory of Dr. Carlos Vázquez-Yanes.

Literature Cited

Alcorn, S. M., S. E. McGregor, and G. Olin. 1961. Pollination of Saguaro cactus by doves, nectar-feeding bats and honey bees. Science 132: 1594-1595.

Alcorn, S. M., S. E. McGregor, and G. Olin. 1962. Pollination requirements of the Organpipe cactus. Cactus and Succulent Journal (U.S.) 34: 134-138.

Altesor, A., E. Ezcurra, and C. Silva. 1992. Changes in the photosynthetic metabolism during the early ontogeny of four cactus species. Acta Oecologica 13: 777-785.

Arias, S. 1993. Cactáceas: Conservación y diversidad en México. Revista Mexicana de Historia Natural 44: 109-ii5.

Arizmendi, M. C., and A. Espinosa de los Monteros. i996. Avifauna de los bosques de cactáceas columnares en el Valle de Tehuacán. Acta Zoológica Mexicana (n.s.) 67:

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