A standardized set of metrics to assess and monitor tree invasions

A standardized set of metrics to assess and monitor tree invasions
John R. Wilson1,2
1Centre of Excellence for Invasion Biology, Private Bag X1, University of Stellenbosch, Matieland 7602
2Invasive Species Programme, South African National Biodiversity Institute, Private Bag X7, Claremont, 7735. Email:

Metrics that are useful for scientists, managers, and policy makers are needed to improve our ability to understand and manage biological invasions at a global level. Here we discuss some of the fundamental features of tree invasions, and what methods can be used to record and monitor them. The aim is develop a standardized set of metrics to describe tree invasions appropriate to meeting specific management goals and increasing compatibility across administrative borders, and between invasions. We recommend six basic metrics: a) status per region (including an interpretation for trees of Blackburn's unified framework for classifying invasive species according to their invasion status); b) the number of populations as an estimate of the number of foci requiring management (defined by a separation of >10km); c) compressed canopy area (i.e. area of occupancy; AOO; net infestation) as a measure of abundance; d) range size (i.e. extent of occupancy; EOO; gross infestation) as an estimate of the total affected area that needs to be considered for management; e) qualitative observations of current and potential impact; and f) a species distribution model to estimate the potential range that could be occupied. These metrics can be used in concert (e.g. we describe a proposed method of categorising invaders based on AOO and EOO), but they represent a basic level of information.  For many purposes, additional information will be required (e.g. on potential status, and predicted future population growth rates and spread rates). We hope this represent a step towards a standardised method of reporting invasions along the lines of those already seen in conservation science, but even at a basic level more work is required to develop standard metrics for impact and threat.