Abstract:
Scientists, managers, and policy-makers
need functional and effective metrics to improve our
understanding and management of biological invasions.
Such metrics would help to assess progress
towards management goals, increase compatibility
across administrative borders, and facilitate comparisons
between invasions. Here we outline key characteristics
of tree invasions (status, abundance, spatial
extent, and impact), discuss how each of these
characteristics changes with time, and examine
potential metrics to describe and monitor them. We
recommend quantifying tree invasions using six
metrics: (a) current status in the region; (b) potential
status; (c) the number of foci requiring management;
(d) area of occupancy (AOO) (i.e. compressed canopy
area or net infestation); (e) extent of occurrence (EOO)
(i.e. range size or gross infestation); and (f) observations
of current and potential impact. We discuss how
each metric can be parameterised (e.g. we include a
practical method for classifying the current stage of
invasion for trees following Blackburn’s unified
framework for biological invasions); their potential
management value (e.g. EOO provides an indication of the area over which management is needed); and
how they can be used in concert (e.g. combining AOO
and EOO can provide insights into invasion dynamics;
and we use potential status and threat together to
develop a simple risk analysis tool). Based on these
metrics, we propose a standardized template for
reporting tree invasions that we hope will facilitate
cross-species and inter-regional comparisons. While
we feel this represents a valuable step towards
standardized reporting, there is an urgent need to
develop more consistent metrics for impact and threat,
and for many specific purposes additional metrics are
still needed (e.g. detectability is required to assess the
feasibility of eradication).
