Management of invasive species to limit population growth or spread may differ

Background/Question/Methods Management is usually carried out with the stated objective of ‘control’ for an invasive species.  However, demographic processes contribute to both local dynamics and to spatial spread, while dispersal processes contribute only to spread.  This suggests that optimal control of local abundance and population growth may require different management strategies than optimal control of spatial spread.  We synthesize field data on the invasive plant, Carduus nutans, from different parts of its native and invaded range, and use coupled demographic-dispersal models to test a suite of biological control agents against these two main control objectives. 

Results/Conclusions Population growth and spread are both most strongly affected by the same life cycle transitions within an invaded range (Australia or New Zealand).  However, certain vital rates disproportionately affect either spread or growth.  In both invaded ranges, key reproductive life cycle transitions contribute more to spread than to population growth.  In New Zealand, life cycle transitions involving the seed bank contribute more to population growth, because the contribution to spread of seeds retained in the soil is delayed.  As they have different modes of action, this means that the relative ranking of some biocontrol agents shifts depending on whether control of population growth or population spread is the desired outcome.