Host specificity in parasitic mistletoes (Loranthaceae) in New Zealand

1.We quantify the degree of host specificity for the five extant New Zealand loranthaceous mistletoes (Alepis flavida, Ileostylus micranthus, Peraxilla colensoi, Peraxilla tetrapetalaandTupeia antarctica). 2.Host specificity is highest forA. flavida, P. colensoiandP. tetrapetalawhich primarily parasitize species ofNothofagus, and lowest forT. antarcticaand especiallyI. micranthuswhich parasitize a wide range of host species. 3.These patterns of host specificity support the suggestion that relative host abundance is a key factor determining the degree of host specialization in mistletoes (resource fragmentation hypothesis). While evolutionary history may be important in the specificity of the mistletoe–host relationship in some situations, our data suggest that for New Zealand mistletoes evolutionary history simply reflects the temporal component of relative host abundance. 4.We conclude that it is the stability of host availability through time and space which is the dominant factor determining host specificity patterns.