**Mónica E. Queijeiro-Bolaños** and Zenón Cano-Santana. Facultad de Ciencias, Universidad Nacional Autónoma de México

**Background/Question/Methods **Populations are strongly influenced by the competition between the species. This can be seen through many processes such as the involved species distribution. It has been suggested that this interaction can be studied from changes in abundance and non-overlapping distribution (Connell, 1990). In the Zoquiapan National Park (ZNP) in Central Mexico two dwarf mistletoe species are distributed, Arceuthobium vaginatum subsp. vaginatum and A. globosum subsp. grandicaule which parasitize Pinus hartwegii. It has been reported large infestation frequencies of these mistletoes and that it exist a negative and significant correlation between the frequency of A. globosum and A. vaginatum. This lead us to the hypothesis that these two dwarf mistletoe species have a competitive relation, so the goal of this work is to determine if the distribution of the two dwarf mistletoe species is due to competition. In July 2009 we took the infested tree percentage (ITP) by A. vaginatum and A. globosum from 75 plots in the ZNP. Also we obtained the altitude and slope of each plot. With these data we tested a model with a path analysis and structural equation modeling in which we related the ITP of each species with the ITP of the other specie, the altitude and the slope.
**Results/Conclusions **We detected the presence of A. globosum in 18 plots and of A. vaginatum in 65 plots. Only seven plots were free of infestation. The ITP by A. globosum was up to 62% of the trees and by A. vaginatum 88%. The model for A. globosum had a good adjustment (chi-square = 12.46, d.f. = 5, p = 0.029, r2 = 0.21), while the model for A. vaginatum was not significant (chi-square= 2.428, d.f. =2 , p= 0.297, r2 =0.2). But in both cases the only significant path was the ITP by the other specie in a negative way (=-0.45, p<0.0001). Hence, we conclude that the distribution of these two dwarf mistletoe species is influenced mostly by the competition among them and that A vaginatum is the dominant specie in this relation.