The influence of dwarf mistletoe on bird communities in Colorado ponderosa pine forests

Dwarf mistletoes (Arceuthobium spp.) are parasitic plants that are widely distributed in coniferous forests of the northern hemisphere. Because the effects of dwarf mistletoes on their host trees include stunted growth, reduced seed production, and death, these mistletoes may have a substantial influence on forest structure. Studies of the effects of dwarf mistletoe on forest communities have focused primarily on their influence on timber production. We studied the effects of southwestern ponderosa pine dwarf mistletoe (A. vaginatum) on the abundance and diversity of bird communities in central Colorado. Four stands, which ranged in level of mistletoe infestation from none to heavy, were selected at each of two locations. Each stand was surveyed approximately once per week during the avian breeding seasons of 1989 and 1990 by spot mapping. The number of bird registrations and bird species richness were positively correlated with the level of dwarf mistletoe, and this pattern was consistent among 24 of 28 avian species. No species had a significant negative correlation with the level of dwarf mistletoe. The relative abundance of bird species (i.e., evenness) did not differ among stands. The number of cavity-nesting birds detected also was positively correlated with both dwarf-mistletoe levels and number of snags. The number of snags and dwarf-mistletoe levels also were highly correlated. Most snags had been infected as live trees by dwarf mistletoe and the mistletoe probably contributed to their death. While dwarf mistletoe has traditionally been viewed as a forest pest because of reductions in timber volume, we suggest that in areas where management goals are not strictly focused on timber production, control of dwarf mistletoe may not be justified, practical, or even desirable. Our data suggest that dwarf mistletoes may have positive influence on wildlife habitat. Consequently, we suggest that eradication efforts be reconsidered given that dwarf mistletoes have been a part of these forest ecosystems for thousands, and possibly millions, of years.