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Table 5 Potential effect modifiers/reasons for heterogeneity

From: Management of UK priority invasive alien plants: a systematic review protocol

VariablePotential effectMethod of testing
Study in greenhouse/fieldStudies of management interventions such as herbicides in greenhouses may be more effective than those in the field because of the lack of exposure to wind/rain that may remove herbicide from target plantsSub-group analysis
Rate of application of herbicide/cutting (management intensity)Higher application rate of herbicide may lead to more effective control of invasive plant speciesSub-group analysis/meta-regression
Type/mechanism of herbicideDifferent types of herbicide may differ in their effectiveness in controlling target invasive plant speciesSub-group analysis
Timing of herbicide applicationTiming of herbicide application may alter their effectiveness in controlling target invasive plant speciesSub-group analysis/meta-regression
Frequency of management intervention (e.g. repeated spraying with herbicide)More frequent application of herbicide or more regular cutting may lead to more effective control of invasive plant speciesSub-group analysis/meta-regression
Experimental vs observationalThe results of experimental studies may be more consistent due to having fewer confounding factorsSub-group analysis
Time since management interventionSome management interventions may decline in success as time since they were last carried out increasesMeta-regression
Time since invasionIn sites where invasive plant species have been present for long periods of time they may be difficult to control because of a build-up of invasive propagulesMeta-regression