Plant dispersal: the role of man.
Studies were conducted to determine which plant species are commonly dispersed by the following vectors: soil carried on motor vehicles; topsoil; sugar factory topsoil; horticultural stock; and garden throw-outs. The ecological traits of the species associated with each vector were compared with those of a representative sample of the regional flora. Traits examined were life history, canopy height, lateral spread, flowering start, flowering period, seed persistence in the soil, vegetative reproduction, wind dispersal, log seed weight, and specific leaf area. Two major anthropogenic dispersal pathways were identified, each associated with a clearly defined group of species. Species associated with topsoil, cars and horticulture depended essentially on soil movement, and are often small and fast-growing, but their most consistent unifying characteristic is the production of numerous, small, persistent seeds. In contrast, garden throw-outs, which are themselves functionally similar to increasing garden escapes, tend to be tall, spreading perennials with transient seed banks; attributes which are almost the exact opposite of the soil-borne group. Some recent studies of the British flora have failed to find any dispersal-related differences between those species with increasing or decreasing ranges, or between natives and invasive aliens. Others have found contradictory attributes of alien plants; they were more likely to have bigger seeds than natives, but also more likely to have a persistent seed bank. These findings are consistent with the suggestion that there exist two contrasting groups of successful alien invaders: tall, spreading competitors and small, short-lived, fast-growing species with high reproductive outputs. The parallel with the two groups of species identified is remarkable, and is further evidence of the probable importance of anthropogenic dispersal in the modern landscape.