Weed control thresholds: a useful concept in natural ecosystems?

F.D. PanettaA and R.F. JamesB

A Alan Fletcher Research Station, Department of Natural Resources, PO Box 36, Sherwood, Queensland 4075, Australia.

B CRC for Weed Management Systems and Centre for Resource and Environmental Studies, Australian National University, ACT 0200, Australia.


The threshold concept has received considerable attention in relation to the management of well-established weeds and other pests in agricultural systems, but its applicability to the management of weeds of natural ecosystems has yet to be examined in any depth. Four aspects that must be considered in relation to the use of thresholds in weed management are identified, namely:
i. benefits provided by the system being managed,
ii. damage relationships resulting from the presence of weeds in the system,
iii. population dynamics of the weeds concerned, and
iv. the treatment of risk.

It is argued that difficulties associated with capturing much of this information would generally preclude such an approach to determining action thresholds for environmental weeds. Significantly, the debate continues as to the usefulness of the threshold concept in the context of weed management in agricultural systems. Recent Australian research has shown that the greatest economic benefits in cereal crops are provided by management actions that aim to minimize weed seed bank populations.

Weed management in natural ecosystems is often a very labour-intensive undertaking and the costs of weed control per unit area can escalate rapidly with increases in weed density. Control efforts are arguably most cost-effective during the earliest stages of weed invasion; management effort should be focused here, given that particular weeds are considered sufficiently serious to warrant intervention (and the natural area considered sufficiently important to warrant protection). An understanding of weed population dynamics may allow the definition of maintenance levels, where a low annual or biennial control commitment would be sufficient to prevent substantial population increase. Better information on the cost (and effectiveness) of control efforts at different stages of weed invasions should assist in defining such levels, which could function as triggers for weed control.


Plant Protection Quarterly (1999) 14 (2) 68-76.