PARENT SESSION
Oral Session #91: Mutualisms -- Facultative, conditional, and complex.
Presiding: W. Wilson
Thursday, August 8. 1:00 PM to 3:45 PM. Cochise Meeting Room, TCC.

Conditional interactions between juniper and mistletoe mediated by shared avian frugivores: parasitism to potential mutualism.

van Ommeren, Ron^*,1,2, Whitham, Thomas¹, ¹ Northern Arizona University, Flagstaff, Arizona² Senna Environmental Services, Phoenix, Arizona

ABSTRACT- Although mistletoe is typically viewed as a parasite of juniper in a 2-way interaction, its role may become neutral or even mutualistic when their common avian seed dispersers are considered in a 3-way interaction. Wintering avian frugivores forage on both juniper (Juniperus monosperma) berries and the fruit of its associated mistletoe (Phoradendron juniperinum). Our findings support a hypothesis of conditional interactions. First, mistletoe provide a stable resource for their shared avian seed dispersers, whereas junipers do not. Juniper berry production varied 10-15 fold over the 3 years of our study, while mistletoe fruit abundance did not vary significantly. Second, the abundance of avian frugivores is tied to the abundance of juniper berries in mast years and mistletoe fruits in all years. The best overall predictor of avian frugivore abundance was the abundance of mistletoe; infected stands attracted up to 3 times more birds than stands with little or no mistletoe. Thus, mistletoe berries can serve as the main attractor for birds that disperse juniper berries. Third, in agreement with the hypothesis that mistletoe can benefit junipers by attracting and supporting greater populations of avian seed dispersers, the number of juniper seedlings was more than 2-fold greater in mistletoe-infected stands compared with uninfected stands. Results suggest that the occurrence of a 3-way interaction, in the presence of environmental variation, may change the ecological roles of associated species.

KEY WORDS: mistletoe, one-seed juniper, seed dispersal, parasitism