The spatial scale of dispersal events

Various seed dispersal mechanisms work on greatly differing spatial scales (Figure 2.7). Dispersal of seeds by rain splash or explosive dehiscence of the fruit is generally not effective for more than a few meters, unless assisted by wind. At the other extreme, contaminated crop seed and other agricultural commodities have regularly transported weeds many hundreds of kilometers and have been major sources of intercontinental weed introductions.

Three categories of scale can be usefully distinguished for purposes of managing the spread of weeds: dispersal within fields, between fields, and between regions. Some natural dispersal processes operate primarily at the within-field scale (Figure 2.7). For a weed to spread long distances, other agencies must come into play. Wind, birds, mammals, and water may transport seeds between fields. Natural processes, however, rarely transport species between regions or continents, which is a large part of why regional floras were highly distinct prior to human commerce.

If a newly arrived weed persists within a field, its spread is likely because both natural processes and machinery will tend to disperse the species out

Within fields Between fields Between regions

Wool trade -

Livestock (transported) -

Contaminated seed -

Human

Irrigation water --transport

Manure -

Combines -

Livestock (walking) -

Soil wash -

Birds -

Tillage implements -

Natural

processes

Explosive dehiscence -

Rain splash -

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