Wednesday, August 5, 2009

PS 57-176: Population dynamics of Ariocarpus fissuratus in populations with different density

M. Rosa Mancilla and María C. Mandujano. Instituto de Ecología. Universidad Nacional Atónoma de México

Background/Question/Methods Density-dependent regulation can be determined by resource availability, environmental variability and competition, which cause positive or negative effects on population growth. In arid environments, there is a negative density-dependent effect when resources are limited, mainly water. The effects consists in an increased mortality rate and decreased birth rate according which has specific effects for each age or stage within of a population. In this study, we analyzed the effect of density-dependence in the population dynamics of Ariocarpus fissuratus in populations with contrasting density (high H vs low L) in Cuatro Cienegas, Mexico, during the period 2005-2008. Results/Conclusions Annual matrices of population projection were constructed, where we found a population decrease, mainly by the low recruitment rate and seedling survival. In addition, there are differences between the two densities: in population L, the intrinsic rate of increase was higher (0.853, 0.98 and 0.99) with respect to population H (0.811, 0.962 and 0.978), were the lowest values correspond to the dry period 2005-06, indicating the sensitivity of A. fissuratus to environmental variability. On average, individuals of population L are large than individuals of population H. However, individuals of population H start to reproduce from small stages classes. The values of elasticity show that in both populations, lambda is sensitive to the survival, while fertility is the demographic process that contributes less to population growth, reflecting the existence of trade-offs between survival and fecundity, where the largest size classes present the highest values of elasticity. Density is a factor that influences of the population dynamics of Ariocarpus fissuratus, and this confirms the premise that in environments with limited resources density-dependence is an important factor in regulating populations.

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