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Population Dynamics of Mammillaria Magnimamma Haworth

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Population Dynamics of Mammillaria Magnimamma Haworth Plant Ecology 170: 167–184,2004. 167 © 2004 Kluwer Academic Publishers. Printed in the Netherlands. Population dynamics of Mammillaria magnimamma Haworth. (Cactaceae) in a lava-field in central Mexico Teresa Valverde*, Sandra Quijas, Manuela López-Villavicencio and Silvia Castillo Departamento de Ecología y Recursos Naturales, Facultad de Ciencias, Universidad Nacional Autónoma de México. Ciudad Universitaria, México, 04510, D.F., México; *Author for correspondence (e-mail: [email protected]) Received 23 October 2001; accepted in revised form 23 August 2002 Key words: Cacti, Demography, Elasticity analysis, Long-lived species, Population projection matrices, Stochas- tic simulations Abstract One of the habitats occupied by Mammillaria magnimamma is a 2000-year old lava-field, in Mexico City. The great ecological interest on this lava-field and the little knowledge there is regarding cacti population ecology have compelled us to analyse the demography of this species to evaluate its present conservation status at this site. We studied two populations of this species within the lava-field: one in a disturbed site (i.e., recently burned) and another one in a well preserved site. For each population we built two size-based population projection ma- trices (1996/97 and 1997/98). Demographic data were gathered directly from observations of plant fates from one year to the next. Additionally, seed germination and seedling establishment experiments were carried out in the field to estimate fecundity values and seedling survival probabilities. The four matrices built were used to perform numerical analyses simulating yearly stochastic demographic variation to project the overall popula- tion’s long-term behaviour under these changing conditions. Three of the four matrices showed ␭ values slightly below unity. In these cases elasticity values were highest for matrix entries corresponding to plants remaining in their same category. The matrix that showed a ␭ value above unity (well preserved site, 1997/98) had higher elasticity values for entries referring to seedling survival and growth. The numerical simulations of demographic stochasticity showed that the population appears to be growing at a slow rate. According to the simulation re- sults, the variation in overall population size over time may be accounted for by yearly variation in seed germi- nation and seedling survival. Population persistence probability might decrease significantly if fire frequency increases. Introduction rather than the exception (Bierzychudek 1982; Cipol- lini et al. 1994; Horvitz and Schemske 1995; Valverde The analysis of demographic patterns in plants dur- and Silvertown 1998). Yet, the way in which this de- ing the last three decades has produced a large mographic variation occurs, the life cycle phases it amount of information about important aspects of the affects, and its long-term population consequences are biology and life history of a number of species (Sil- poorly understood, particularly among long-lived vertown et al. 1993). This information has allowed us plant species. The understanding of the factors that to deepen our understanding of the relevance of dif- determine long-term population dynamics still re- ferent population processes, particularly among an- mains a central issue in ecology (Horvitz and Schem- nual or generally short-lived herbs and shrubs from ske 1995). temperate habitats. One aspect that is now generally Cacti are a plant group that has received little at- accepted is that spatio-temporal variation in the de- tention from a demographic point of view. Recent de- mographic behaviour of plant populations is the rule mographic information on cacti species is adding sig- 168 nificantly to our understanding of plant population dez and Godínez 1994), the demographic analysis of dynamics in nature, since they include long-lived spe- M. magnimamma is expected to offer empirical tools cies that inhabit dry tropical areas, two particularities that could aid in the evaluation of the conservation that are poorly represented within the demographic status of the reserve where the study was performed. literature. The available demographic information on The high demographic vulnerability of cacti is pre- this plant group, as well as on other long-lived plant sumed to be a result of their slow individual growth species, suggest that spatio-temporal fluctuations in rates and frequently high mortality rates during the the success of early life-cycle stages are responsible juvenile phases; apparently these features contribute for long-term population fluctuations (Bowers 1997; to limit their capacity for population growth (Hernán- Pierson and Turner 1998). However, projection ma- dez and Godínez 1994; Contreras and Valverde 2002; trix results and their corresponding elasticity analyses Esparza-Olguín et al. 2002). Additionally, many cacti have shown that, in general, matrix entries corre- species are highly restricted in their distribution and sponding to seed and seedling transitions seem to occupy very specific habitats, which makes them contribute little to population growth rate (Godínez- prone to extinction by habitat destruction. In fact, the Alvarez et al. 1999; Esparza-Olguín et al. 2002). Al- Cactaceae is a plant family with a particularly high though long-term numerical fluctuations appear to be number of threatened species (Hunt 1992; Anderson common within these species, no information is avail- et al. 1994). However, our knowledge of the popula- able as to what kind of ecological processes may ac- tion behaviour of most of these species is very lim- count for them. In this study we analyse the popula- ited. tion dynamics of the cactus Mammillaria In this paper we address the demographic analysis magnimamma Haworth. in a lava-field in Mexico of two populations of Mammillaria magnimamma, City. We incorporated information on the demo- one in a well-preserved and one in a disturbed (i.e., graphic behaviour of this species in different sites and recently burnt) site within the ЉEl XitleЉ lava-field, years into a long-term stochastic numerical simula- through the construction of population projection ma- tion which allowed us to project the potential fate of trices for two growth periods (1996/97 and 1997/98). the population. The aim of this exercise was to con- With the information obtained from the demographic tribute to the understanding of the way in which spa- analysis we carried out numerical simulations, using tio-temporal changes in the demography of plant spe- the stochastic approach developed by Bierzychudek cies give rise to fluctuating population numbers. (1982), incorporating the effect of the spatio-tempo- An additional interest to address the population ral variation in the demography of the species in or- dynamics of Mammillaria magnimamma is related to der to understand the way in which this variation may the ecosystem in which it was studied: a 2000 year result in population fluctuations, as well as to evalu- old lava-field with a particularly high plant diversity ate the long-term persistence probability of this spe- (Carrillo 1995). This lava-field was created after the cies in the area. The stochastic analysis was used to eruption of ЉEl XitleЉ volcano and comprises an im- simulate different disturbance regimes and evaluate portant part of what is now Mexico City, one of the population persistence probability under different most densely populated areas in the world. Unfortu- probability of occurrence of fires, which are the main nately, the lava-field is slowly being turned into ur- cause of disturbance at the site. ban areas and much of its biological value is being lost. In fact, some of its endemic species (e.g., Mam- millaria sanangelensis, Bletia urbana) are virtually Methods extinct. In recent years a small fraction of this lava- field was protected in the form of a reserve, in which The study site this study was carried out. It has been suggested that the analysis of the persistence probability of species This study was carried out at the ЉEl XitleЉ (meaning that are particularly vulnerable to disturbances within ЉnavelЉ in Nahuatl) lava-field, located within Mexico an ecosystem may throw light on the conservation City (19°40Ј N, 99°00Ј W) (Figure 1). This lava-field status of the ecosystem itself by providing informa- is the result of the eruption of the ЉEl XitleЉ volcano, tion on the probability of native species turnover which took place over two thousand years ago (Car- (Menges 1990). Since it has been suggested that cacti rillo 1995). The original area occupied by this lava- are particularly vulnerable to disturbances (Hernán- field was 80 km2, covering an altitudinal gradient 169 Figure 1. Location of the study site (ЉEl XitleЉ lava-field) within Mexico City. from 3,100 to 2,250 m above sea level. The lower part nated by the shrubs Senecio praecox, Verbesina vir- of the lava-field (2,500–2,250 m a.s.l.) was covered gata, Dodonea viscosa and Wigandia urens, the tree by a plant community (described in detail by Rze- Buddleia cordata, and the herbs Muhlenbergia ro- dowski (1954)) dominated by the shrub Senecio prae- busta, Dahlia coccinea and Echeverria gibbiflora, cox. This shrubland, in which 350 plant species could among others (Rzedowski 1954). be found (Rzedowski 1954), comprised 40% of the Within the ЉPedregal de San AngelЉ reserve we original area covered by the lava-field. Today the area chose two sites: a well-preserved (P) and a relatively still occupied by this type of shrubland has been re- more disturbed (D) site. The main difference between duced to only 2.9 km2, most of which is now pro- these two sites is the apparent degree of conservation. tected by the ЉPedregal de San AngelЉ Reserve, be- As noted above, the most prevalent disturbance fac- longing to the National Autonomous University of tor in this area is fire, generally of human origin, oc- Mexico (UNAM). Within this reserve, only 226 of the curring mainly in spring, towards the end of the pro- original plant species were found in 1990, plus 77 longed dry season.
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