Measuring the effects of chronic anthropogenic disturbance on threatened cacti.
Martorell, Carlos*,1, Rodríguez-Ortega, César2, Peters, Edward3, 1 Universidad Nacional Autónoma de México, Mexico City, Mexico2 Secretaría del Medio Ambiente y Recursos Naturales, Mexico City, Mexico3 Instituto Nacional de Ecología-Semarnat, Mexico City, Mexico
ABSTRACT- Habitat degradation is usually addressed as a major cause of biodiversity loss, but it and its effects on threatened species are seldom studied. We measured the population density of nine threatened species of Mammillaria (Cactaceae) and the intensity of chronic anthropogenic disturbance in southern Mexico. Disturbance was measured as the result of three disturbance agents: Livestock raising, human activities and soil degradation. Demographic studies were conducted on four species in order to compare population dynamics under different disturbance intensities, and in two species demographic data were obtained both in a fenced area and an area exposed to disturbance agents. We found that half of the species achieved larger densities when exposed to moderate intensities of disturbance, being ruderal in the sense of "plants thriving in disturbed sites". Most species show beneficial effects of one disturbance agent, but the synergic effects of two or more agents seem to be detrimental. Population growth rates decreased with disturbance in two species, but increased in the other two. The elimination of disturbance was found to be beneficial in terms of the population growth rate in a non-ruderal species, but detrimental to a species rarely found in well-preserved areas. Sensitivity analyses show that populations in less disturbed areas are more dynamic, depending strongly on reproduction and individual's growth, while those in disturbed areas rely heavily on the survival and stasis of established cacti. Demography validated the conclusions drawn from the density-disturbance relationships, suggesting that this may be a reliable method for rapid assessments of species under high anthropogenic pressure. The widespread ruderality in the genus suggests that rational land use is not incompatible with conservation. In ruderal species disturbance management and metapopulation-dynamics manipulation seem the most appropriate management strategies.
Key words: Mammillaria, Conservation, Ruderality, Mexico
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