Seed dispersal of fleshy-fruited environmental weeds in New Zealand

Fruit-eating animals play a key role in spreading non-native environmental weeds, via seed ingestion and subsequent dispersal. We reviewed available information on dispersal of fleshy-fruited environmental weeds in New Zealand. We found almost a third (32.9%) of 295 environmental weed species in New Zealand have fleshy fruits adapted for internal dispersal by animals. Fruiting phenology differs between weeds and native plants, with many weed species fruiting from late autumn until early spring (May to September) when native fruits are scarce. Weed fruiting duration does not differ from natives. Eight birds and two mammals are the main dispersers of weed seeds in New Zealand: blackbirds (Turdus merula), silvereyes (Zosterops lateralis), starlings (Sturnus vulgaris), kereru (New Zealand pigeons, Hemiphaga novaeseelandiae), song thrushes (Turdus philomelos), tui (Prosthemadera novaeseelandiae), bellbirds (Anthornis melanura), mynas (Acridotheres tristis), brushtail possums (Trichosurus vulpecula) and feral pigs (Sus scrofa). All 10 species include significant quantities of fruit in their diet annually or seasonally and are widespread in New Zealand. The bird dispersers rarely damage ingested seeds. Possums and pigs destroy some seeds, but possums in particular are likely to disperse several smaller-seeded weeds. Blackbirds, silvereyes and starlings are probably the most important dispersers; they are the most widespread birds, and all disperse more than 20 weed species. Starlings pose additional risks because they disperse seeds long distances to roost sites, including offshore islands. The kererū is a significant disperser of weed seeds also, and the key disperser for three species with large, single-seeded fruits. Most seeds are likely to be dispersed less than 100 m by birds, with some dispersed hundreds of metres or even several kilometres (by birds and mammals). Reducing the spread of fleshy-fruited weeds via animal ingestion is challenging. Potential management tools include reducing invasive mammal abundance and seed availability using traditional or biological control (particularly in populations that are major seed sources for dispersal to high value sites), providing alternative native food sources by planting natives that fruit at peak weed fruiting times, and targeting favoured roost sites for surveillance and control.