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Population Dynamics of the Western Prickly Pear, Opuntia Macrorhiza (Cactaceae) Author(S): Kathleen H

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Population Dynamics of the Western Prickly Pear, Opuntia Macrorhiza (Cactaceae) Author(S): Kathleen H Population Dynamics of the Western Prickly Pear, Opuntia macrorhiza (Cactaceae) Author(s): Kathleen H. Keeler and Brigitte Tenhumberg Source: The Southwestern Naturalist, 56(2):147-153. 2011. Published By: Southwestern Association of Naturalists DOI: http://dx.doi.org/10.1894/F02-JB-17.1 URL: http://www.bioone.org/doi/full/10.1894/F02-JB-17.1 BioOne (www.bioone.org) is a nonprofit, online aggregation of core research in the biological, ecological, and environmental sciences. BioOne provides a sustainable online platform for over 170 journals and books published by nonprofit societies, associations, museums, institutions, and presses. Your use of this PDF, the BioOne Web site, and all posted and associated content indicates your acceptance of BioOne’s Terms of Use, available at www.bioone.org/ page/terms_of_use. Usage of BioOne content is strictly limited to personal, educational, and non- commercial use. Commercial inquiries or rights and permissions requests should be directed to the individual publisher as copyright holder. BioOne sees sustainable scholarly publishing as an inherently collaborative enterprise connecting authors, nonprofit publishers, academic institutions, research libraries, and research funders in the common goal of maximizing access to critical research. THE SOUTHWESTERN NATURALIST 56(2):147–153 JUNE 2011 POPULATION DYNAMICS OF THE WESTERN PRICKLY PEAR, OPUNTIA MACRORHIZA (CACTACEAE) KATHLEEN H. KEELER* AND BRIGITTE TENHUMBERG School of Biological Sciences, University of Nebraska-Lincoln, Lincoln, NE 68588-0118 *Correspondent: [email protected] ABSTRACT—Although most cacti that have been studied are long lived, following individually marked plants in Boulder County, Colorado, for .7 years, we determined that average life span of Opuntia macrorhiza, the western prickly pear, is 3 years. A few individuals probably live .10 years. Vegetative reproduction, produced by rooting of cladodes, exceeded reproduction by germination and establishment from seeds. Both types of new recruits, from vegetative reproduction and seeds, had higher death rates than established plants. Size and frequency of flowering increased with age, although size both increased and decreased, sometimes dramatically, between years. Flowering correlated more strongly with size than with age. Estimates of population growth indicated these populations were stable (l 5 1.02). Elasticities suggest that the population was most sensitive to survival of smaller plants. RESUMEN—Si bien la mayorı´a de los cactus estudiados son perennes, luego del seguimiento por ma´s de 7an˜os de plantas marcadas individualmente en el condado de Boulder en Colorado, encontramos que el promedio de vida de plantas de Opuntia macrorhiza, el nopal raizudo, es de 3 an˜os. Pocos individuos probablemente viven ma´s de 10 an˜os. La multiplicacio´n vegetativa a trave´s del enraizamiento de cla´dodes excedio´ a la reproduccio´n por germinacio´n y establecimiento a partir de semillas. Ambos tipos de nuevos reclutas, sean provenientes de multiplicacio´n vegetativa o de semillas, evidenciaron mayores tasas de mortandad que las plantas ya establecidas. El taman˜o y frecuencia de floracio´n aumentaron con la edad, pero el taman˜o de plantas aumento´ y decrecio´, a veces notablemente, entre an˜os. La floracio´n se correlaciono´ma´s fuertemente con taman˜o que con edad. Las estimaciones de crecimiento poblacional indicaron que estas poblaciones fueron estables (l 5 1.02). Las elasticidades sugieren que la poblacio´n fue ma´s sensible a la supervivencia de las plantas ma´s pequen˜as. There are ca. 1,500 species of cacti; many ,20 years, primarily due to failure of recruit- species grow slowly, flower infrequently, and live a ment. Because of environmental stochasticity long time, which matches predictions for spe- long time series are necessary to detect such cialist xeriphytes (Godı´nez-A´ lvarez et al., 2003). trends, but unfortunately, they are not available For instance, cardo´n (Cephalocereus columna- for many herbaceous species and communities. trajani) may live .145 years (Zavala-Hurtado This study reports on an 8-year demographic and Dı´az-Solis, 1995) and Neobuxbaumia macro- study of Opuntia macrorhiza, the western prickly cephala .200 years (Esparza-Olguı´n et al., 2002). pear. Even tiny button-like Corypantha robbinsorum Opuntia is in the Cactaceae and contains ca. were estimated to average 17 years of age 150 species (Pinkava, 2003) that are generally (Schmalzel et al., 1995). Despite species richness dry-habitat specialists. The genus is native to the of cacti, there are relatively few demographic New World, but some species introduced else- studies of cacti (Godı´nez-A´ lvarez et al., 2003; where have naturalized and are invasive (Benson, Valverde and Zavala-Hurtado, 2006; Mandujano 1992; Pinkava, 2003; Reyes-Agu¨ero et al., 2006). et al., 2007). Demographic studies are important Many species of Opuntia live for a long time; e.g., to evaluate viability of populations and species Mandujano and coworkers (Mandujano et al., and guide management decisions. For instance, 1996, 1998, 2001) reported that Opuntia rastrera Rae and Ebert (2002) documented a decreasing (nopal) lived for decades. This species domi- population trend of the rare Florida species nates the nopal ecosystem through its long life Harrisia fragrans, and extinction was expected in span and ability to reproduce vegetatively by 148 The Southwestern Naturalist vol. 56, no. 2 rooting of cladodes (Mandujano et al., 1998). plant was recorded the previous year. Plants that were Another Opuntia known to be long-lived is O. entirely yellow were conservatively recorded as present and alive. Absence of O. macrorhiza where one was engelmanii, which lived an average of 20 years in recorded the previous year was noted as death of the the Sonoran Desert (Bowers, 1996). For most plant. In a few instances, the plant may have been species, ages are not known but are inferred kicked by cattle and moved to a new location, and so be from size of plants (e.g., Godı´nez-A´ lvarez et al., falsely reported as dead. However, most vegetative reproduction occurred close to the parent plant 2003; Reyes-Agu¨ero et al., 2006). because big cladodes do not easily move far and the In this study, we describe reproductive pat- long spines of O. macrorhiza are not barbed enough to terns, estimate life expectancy, and rates of cling. growth of populations of O. macrorhiza. This We estimated flowering effort of each plant by species is a common cactus of the plains, Rocky counting buds, flowers, and fruits. We usually made only 1 survey/plot/year, late enough to count devel- Mountain foothills, and Great Basin of the oping fruits. Consequently, we have undercounted United States ranging southward into northern flowering effort by missing flowers that did not result in Mexico (Chihuahua; Benson, 1992; Anderson, seeds. 2001; Pinkava, 2003). It is an endangered species We analyzed survival with Cox proportional-hazards models (Cox and Oakes, 1984; Kalbfleish and Prentice, in Iowa and salvage-restricted in Arizona (United 1990). Plants for which we did not know date of States Department of Agriculture Natural Re- germination, date of death, or both, were included as sources Conservation Service, http:plants.usda. censored data; e.g., if a plant for which we do not know gov). date of germination died in the fourth year of our study, we knew that this plant had lived $4 years. Censoring allowed us to include this information in the MATERIALS AND METHODS—We mapped and measured analysis. Plants with unknown germination were left- plants of Opuntia macrorhiza var. macrorhiza annually censored data, plants that were still alive at the end of in five plots in the Open Space and Mountain Parks, the study were right-censored data, and plants with Boulder, Boulder County, Colorado. The study was unknown date of germination and that were still alive conducted 1998–2005, although some plants were at the end of the study were interval censored. In Cox observed in1995. Numbering of plots conformed to the proportional-hazards models, the observed rate of permanent plot system of J. and C. Bock (Bennett, hazard is the product of a baseline hazard and a factor 1997; Bock and Bock, 1998). Three plots (28, 57, and that gives the joint effect of a set of covariates z1,….., zp. 61) were flat with gravelly soil. One plot (52) had General form of the model is: similar soil but was on a hilltop (Davidson Mesa). The fifth plot (102), in Chataqua Park Meadow, was on a Xp ~ grass-covered east-facing slope. Two plots (28 and 61) htðÞ, z l0ðÞt exp ðÞbizi were grazed by cattle in summer, one plot (52) was i~1 grazed in winter, and two plots (57 and 102) were not where h(t, z) denotes the observed rate of hazard, l0(t) grazed. All plots were 100 m2 except one (57), which is the baseline hazard, t is time since the last sample, was 92 m2 and had the highest density and absolute and b1,… bp are relative contributions of covariates. number of plants. Our sample was too small to include a year effect, so we New plants were added to the study when they only included plot as a covariate. Survival analysis was appeared within the plots. Cladodes of O. macrorhiza performed in R (Version 1.8.0; R Development Core can root when separated from the plant (Benson, Team, 2006). 1992). Following Mandujano et al. (1998), we counted We calculated mean and median sizes of plants using adjacent cladodes as two plants if they were not Microsoft Excel. We tested for correlations among physically connected. This was the most-parsimonious plots, between number of plants and annual precipi- definition and ensured straightforward comparison of tation (National Oceanographic and Atmospheric both studies. Thus, we defined a plant as a physiolog- Administration, 1995–2005), and between number of ically independent unit.
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