Is dispersal easier than pollination? Two tests in new Zealand Loranthaceae

We tested the relative frequency of pollen limitation and dispersal limitation for two bird-pollinated and bird-dispersed New Zealand mistletoes, Peraxilla tetrapetala and Alepis flavida, at a South Island site where the bellbird (Anthornis melanura) is the sole pollinator and disperser. There was no evidence of dispersal limitation for P. tetrapetala over four seasons or A. flavida over two seasons. Few ripe fruits were present on plants at any one time (usually <5%), and more than 90% of the fruit crop was removed. A. flavida was not pollen limited, but P. tetrapetala was significantly pollen limited in 6 of 10 years. The presence of pollen limitation but not dispersal limitation, despite both services depending on the same bird, is influenced by the much faster rates of flower ripening per day compared with fruit ripening (15-54 times faster in P. tetrapetala). In New Zealand, pollination failure probably threatens at least as many plant species as dispersal failure. In theory, dispersal should threaten fewer plant species, because dispersal usually involves more animal species, makes smaller quantitative demands on the dispersers (fewer fruits ripening per day, and fruits can wait longer for attention), and may be less obligate for at least some reproduction to occur.