Reducing agroecosystem vulnerability to weed invasion - Invasive Plants: Ecological and Agricultural Aspects - page 170

Reducing agroecosystem vulnerability to weed
invasion
K. Neil Harker 1 , George W. Clayton 1 and John T. O'Donovan 2
1 Weed Management Research, Sustainable Production Systems, Agriculture and Agri-Food Canada,
Lacombe Research Centre, 6000 C & E Trail, Lacombe, Alberta T4L 1W1, Canada
2 Agriculture and Agri-Food Canada, Box 29, Beaverlodge, Alberta T0H 0C0, Canada
Invasion opportunities
Invasions, whether human, animal or plant, are most successful when weak-
nesses and opportunities in communities are successfully exploited. Invading
species and the habitats they invade can both influence invasion outcomes [1,
2]. In plant communities, the niches that allow avenues of introduction for
invading plants can be reduced by careful agronomic management.
Although strict definitions of plant invasions usually involve species expan-
sion in an area not normally occupied by that species [3], we consider the
annual proliferation of weeds in crop canopies to be an invasion of the space
intended for crop species. Whether it is appropriate to label repeated weedy
incursions in agricultural land as establishment or invasion processes is debat-
able. The subject of this chapter is restricted to the repeated invasions of long-
established common weeds in annual cropping systems, as opposed to inva-
sions by exotic or alien plant species. Nevertheless, management techniques
that reduce agroecosystem vulnerability to long-established weeds will also
restrict invasive opportunities for exotic species.
Herbicide application has traditionally been an effective and relatively eco-
nomical means of managing weeds in crops, and can be important in ensuring
that new weed introductions do not proliferate in cropland and adjacent areas.
However, over-dependence on herbicides can lead to shifts in weed communi-
ty composition. Anecdotal evidence indicates that the widespread use of
growth regulator herbicides in western Canada in the 1950s and 1960s result-
ed in an eventual increase in the incidence of herbicide tolerant dicot weeds
such as hempnettle ( Galeopsis tetrahit L.) and smartweed ( Polygonum spp)
and monocot weeds such as wild oat ( Avena fatua L.) and green foxtail
( Setaria viridis (L.) Beauv.) The incidence of herbicide resistant biotypes has
also increased dramatically over the last twenty years [4]. In addition to vary-
ing in their response to herbicides, resistant and susceptible weed biotypes can
differ in several other respects including competitiveness [5] and seed dor-
mancy characteristics [6]. Thus a more holistic approach to weed management