Seed dispersal mechanisms and dispersal potential of environmental weeds in New Zealand

Seed dispersal distances drive the rate of spread for invasive plants and are strongly influenced by dispersal mechanisms. We investigated weed seed dispersal mechanisms and dispersal potential in New Zealand. We collated data on dispersal mechanisms for 318 species that produce seed in New Zealand. We modelled maximum seed dispersal distances of 286 species for which data were available using dispersal mechanism, growth form, seed release height, and seed mass. Internal dispersal by vertebrates was the most frequent mechanism, both among weed species (56%, 60% of which had fleshy fruits) and all weed species × dispersal mechanism interactions (34%). Wind was the next most frequent mechanism (39% of species, 24% of mechanisms), followed by water (29%, 18%), attachment (16%, 10%), ant (8%, 5%), and ballistic dispersal (5%, 3%). Most weed species (56%) have only one known dispersal mechanism, 37% are dispersed by multiple mechanisms, and 7% have no known mechanism. Non-standard mechanisms (those not predicted by plant morphology) were used by 44% of weed species and comprised 51% of all mechanisms. Modelled maximum seed dispersal distances show that most weed species have the potential for long-distance dispersal: 70% of species were predicted to be dispersed > 100 m, 17% > 1 km, and 0.3% > 10 km. Dispersal distances were greatest for vertebrate-dispersed seeds (mean = 973 m), then seeds dispersed by wind (146 m), ants (75 m), and ballistic mechanisms (14 m). Many weed species are dispersed by non-standard mechanisms (especially vertebrates) that have the potential to move seeds much further than standard mechanisms and thus accelerate range expansion. We predict that non-standard mechanisms are even more frequent than reported to date, as they have been rarely studied in New Zealand. Examining the combined effect of multiple mechanisms is critical to understanding the historic and future spread of weeds.