SAGUARO Ecology of the Saguaro: II NPS Scientific Monograph No. 8

Limiting Factors

The saguaro giant cactus at Saguaro National Monument is not some rare exception to the fact that environmental limiting factors modify all terrestrial plant species from population to population wherever located within the ultimate limits of the species distribution, at which boundary such factors finally become 100% limiting. Moreover, Saguaro National Monument is not only just near the saguaro's distribution limits, the saguaro species boundary line runs through the monument (Figs. 1, 10A). The monument (east) population is a marginal one in every sense of the term.

Temperature, precipitation, topography, soil, and predators are examples of powerful ecological factor groups that operate as density-dependent or density-independent limiting factors. Wiggins (1937) noted the population disaster of the subtropical organpipe cactus at the northern extremity of its range in southwestern Arizona during the serious winter freeze of 1937. In a concise rephrasing of Taylor's (1934) observations on the importance of extremes on the geographical limits of the distribution of species, Wiggins (1937) emphasized that ". . .it is the greatest extreme of a year of unfavorable conditions or the greatest extreme of a series of unfavorable years that finally limits the distribution of species. . ." This is especially true for cold-limited subtropical species as they occur, for example, at their northern latitude and elevational extremities in the southern parts of California, Arizona, New Mexico, and Texas.

Thus the extremes, not the means, are in final control. Short of becoming limiting, the extremes alter the population age-structure and growth according to the intensity and periodicity of the factor. For pertinent discussions on the ecological problem in general and southwestern communities in particular, see Thornber (1911, 1916), Shreve (1911, 1915), Good (1931), MacGinitie (1933), Taylor (1934), Mason (1936, 1947), Wiggins (1937), Turnage and Hinckley (1938), Cain (1944), Went (1957), Lowe (1959, 1964), Daubenmire (1964), Harper (1967), and Odum (1971).

Among the many physical and biotic factors that are controlling, climate is the most clearly overriding. In the light of such effective modifiers of population systems through natural selection such as climate, topography, soils, predators, et al., there is no mystery to the geographical and ecological distribution of the saguaro, nor is there mystery to the wide geographic variation in structure, function, density, and fluctuation of saguaro populations within the huge southwestern range of the species (Fig. 1).

We devoted the first of these volumes on the ecology of the saguaro to the role of freezing weather—the most periodically damaging as well as finally limiting density-independent climatic factor affecting the saguaro at Saguaro National Monument and elsewhere at the edge of its range in Arizona. Throughout the present volume, limiting factors appear continuously as we discuss drought, freezing, soil-type, slope exposure, animals, et al., in relation to experiments and observations on germination, establishment, growth, and survivorship in natural populations of saguaros.

Ecological Perspective

We have previously reported on various aspects of the ecology of the saguaro since these continuing investigations were initiated in 1951. The subjects of previous reports include critical factors during the first years of life (Steenbergh and Lowe 1969), bacterial decomposition (Steenbergh 1970), osmotic characteristics of tissue fluids (Soule and Lowe 1970), lightning-caused destruction (Steenbergh 1972), and the role of freezing weather (Lowe 1959, 1964, 1966; Steenbergh and Lowe 1976).

In this report we include results and conclusions on additional experiments and observations on saguaro reproduction and germination, establishment, survival, and growth to the age of first reproduction together with related results and conclusions from our earlier reports. In the concluding section of this report we offer a brief review and a discussion of natural and human historical factors that bear on the question of the past, present, and future of the saguaro at Saguaro National Monument and elsewhere in the northern portions of its distribution in Mexico and the United States.

The results and the conclusions presented here offer a basis for management policy and decisions affecting the saguaro populations in question. In addition, this report provides information that can serve as a basis for an interpretive program that offers an understanding of basic ecological concepts that underlie the interactions of all living things with each other and with the environment in which they live—and die.

Management recommendations based on our investigations, offered in Appendix I, are of particular interest to the National Park Service not only as they relate specifically to saguaro populations at Saguaro National Monument but also to populations in the three other national monuments in Arizona where the saguaro also occurs: Casa Grande National Monument; Tonto National Monument; and Organ Pipe Cactus National Monument. To a greater or lesser degree, saguaro populations in the latter national monuments are and have been responding to natural and man-caused environmental factors. To a large extent, these same recommendations are applicable to the management of Organ Pipe Cactus National Monument populations of the closely related and similarly responsive organpipe and senita cacti.

The saguaro is the long-lived integrator of the natural environment and the historic uses and abuses of its habitat within Saguaro National Monument and elsewhere throughout most of the Sonoran Desert. Thus the saguaro provides a long and useful on-site record of the historic environment that spans nearly the entire range of the climatic extremes of temperature and moisture of the Sonoran Desert. Knowledge of the ecology of this responsive species, therefore, offers valuable insights to the understanding of the adaptive evolution and the interrelationships of other associated desert species and to their relationships to each other and to the environment, past and present. It is within that perspective that we have examined and report here our findings and conclusions, and it is within that framework that we offer management recommendations on the saguaro giant cactus.