Dispersal of Misteltoes by Honeyeaters and Flowerpeckers: Components of Seed Dispersal Quality

Abstract

The quality of the seed dispersal performed by a dispersal agent has two components. Disperser efficiency is the probability that a seed dispersed by the vector will lodge in a safe site and germinate. Disperser effectiveness is the proportion of seedlings in a population that any one seed vector is responsible for disseminating. Two birds, the Spiny—cheeked Honeyeater (Acanthagenys rufogularis) and the Mistletoebird (Dicaeum hirundinaceum), dispersed the seeds of the host—specific mistletoe Amyema quandang in arid western myall (Acadia papyrocarpa) woodland in South Australia. The Mistletoebird dispersed the mistletoe's seeds by defecating and depositing seeds in a string on the perch, with the majority of seeds being dispersed to branches between 5 and 10 mm in diameter. Honeyeater—dispersed seeds usually fell onto substrates below the defacting bird or were wiped onto a convenient branch, generating a bimodal distribution in the branch sizes to which seeds were dispersed. Mistletoebirds and honeyeaters were similarly efficient in dispersing seeds to the thin (1—6 mm diameter) western myall stems most susceptible to infection (0.9—1.2% of all seeds dispersed, respectively), but Mistletoebirds dispersed four times as many seeds as honeyeaters to the whole range of stem diameters infected (1—16 mm). Germination of mistletoe seeds and seedling growth did not differ significantly in the seeds dispersed by the two birds species across the main fruiting season, but Mistletoebird—dispersed seeds had higher viability and grew more strongly at the start of the season when ripe fruit was scarce. Disperser effectiveness was measured by locating and classifying recently disseminated Amyema quandang seedlings in western myall canopies as Mistletoebird or honeyeater dispersed on the basis of species—specific differences in the stools of the two birds. Honeyeaters had dispersed most of the older (>2 mo) seedlings to live myall branches in three trees, suggesting that they were the more effective disperser of A. quandang. The system is a good model for resolving problems in the ecology of seed dispersal, which are intractable with most terrestrial plant species.