Weed life history: identifying vulnerabilities - Ecological Management of Agricultural Weeds - page 77

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 mecha-
nisms 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 disper-
sal 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; Özer, 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