Weed species have a variety of adaptations for dispersal. Many members of the Compositae and other groups (e.g., Asclepias and Epilobium spp.) have hairs attached to the seeds that provide buoyancy to aid dispersal by wind. In other species, the plant breaks off at ground level (e.g., Amaranthus albus, Sisymbrium altissimum), or the inflorescence detaches as a unit (e.g., Panicum capillare) and rolls with the wind over the ground surface. Seeds of some species have hairs, spines, or hooks that adapt them to disperse in the fur of animals, and these are often equally effective at attaching to clothing (e.g.,
Cenchrus incertus, Arctium lappa). A few agricultural weeds, many of them woody, have fleshy fruits that entice birds to swallow the seeds (e.g., Solanum spp., Toxicodendron radicans). Since fruit-feeding birds usually lack the sort of alimentary tract required to digest seeds, these are mostly regurgitated or passed out with the feces, often at a considerable distance from the parent plant. Another small group of weed species have explosive dehiscence mechanisms that catapult seeds as much as a few meters from the parent (e.g., Oxalis stricta - Lovett Doust, MacKinnon & Lovett Doust, 1985). A very few weeds have oily bodies attached to the seeds (eliasomes) that entice ants to carry the seeds to their nests (e.g., Fumaria officinalis, Euphorbia esula - Pemberton & Irving, 1990). After the ants have bitten off the eliasome, the seeds are then discarded and may subsequently germinate. Seed-feeding ants also regularly disperse seeds accidentally during foraging. The effectiveness of these dispersal mechanisms are evaluated with further examples in Salisbury (1961, pp. 97-143) and Cousens & Mortimer (1995, pp. 55-85).
Although some agricultural weed species show obvious adaptations for dispersal, most do not. Of the 50 weeds of arable land discussed by Salisbury (1961), 76% lack any apparent adaptation for dispersal. Consequently, most weed seeds fall close to the parent plant. For example, Howard et al. (1991) found that Bromus sterilis and B. interruptus seeds shed in a winter wheat field fell in a normally distributed pattern about the parent plant with standard deviations of 31 cm and 19 cm, respectively.
Prior to dispersal by humans (see next section), species without obvious dispersal adaptations probably dispersed in soil washed along streams, in mud clinging to large animals, and in the guts of birds and mammals. The seeds of many weed species pass through the digestive tracts of grazing animals without damage (Kirk & Courtney, 1972; Takabayashi, Kubota & Abe, 1979; Blackshaw & Rode, 1991), and may be retained in the gut for several days (Burton, 1948; Ozer, 1979), thereby allowing deposition at sites distant from their point of origin. Although most of the seeds ingested by seed-eating birds are probably destroyed, a few apparently pass through the digestive tract unharmed (Proctor, 1968; Aison, Johnson & Harger, 1984).
The frequency distribution of distance traveled by wind-dispersed seeds is typically very skewed (Smith & Kok, 1984; Feldman & Lewis, 1990). Consequently, most wind-dispersed seeds land within a few meters of the parent plant (Plummer & Keever, 1963; Michaux, 1989), but a few seeds may be caught in updrafts and occasionally travel far enough to reach nearby fields. Whereas only 10% of the arable weeds discussed by Salisbury (1961) are wind-dispersed, 28% of the species he lists as common in British upland grasslands have appendages that facilitate wind dispersal. Many of these wind-dispersed grassland weeds thrive on road margins, ditch banks, fence lines, and hedgerows where they are relatively free from trampling and grazing by animals and cutting by mowing machines. From there, they disperse into fields, especially during the establishment year of leys and during periods when pastures are rested from grazing. Consequently, preventing fruiting of these weeds in ruderal habitats adjacent to farm fields is an important part of their management.
Was this article helpful?