A standardized set of metrics to assess and monitor tree invasions

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) observa-tions 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).

Spread of the Introduced Sitka Spruce (Picea sitchensis) in Coastal Norway

“…Shorter edge lengths in the north might also explain the different pattern of establishment compared to the west, but we did not find any connection. A combination of AOO and EOO reflects pattern and dynamics of spread in a better way, as pointed out by Wilson et al [20]. However, the probability for establishment within 100 m is much higher in the west than compared to the north (Table 5).…”

“…The most conflict-generating species are those which are both useful and invasive [4]. Still, there is little scientific consensus about what is meant by an invasive species [17][18][19][20][21]. "Introduced species that spread rapidly in a new region" and "Introduced species that have harmful environmental impacts on native species" are often used by ecologists or in legislation papers, respectively.…”

“…Based on the definition of Pyšek [19], an observed spread distance of 100 m within 50 years has been proposed as a benchmark criterion for being an invasive tree species. A more quantitative approach including abundance and spread is needed for definition and monitoring of tree invasions, as proposed by Wilson et al [20].…”

Citizen Science as a Tool in Biological Recording—A Case Study of Ailanthus altissima (Mill.) Swingle

“…Among invasive plants, trees are generally easiest to map because of their size [34], although there is currently no methodical, reasonably priced protocol for the estimation of tree invasions that can be used in invasive species management or scientific research [35]. Conducting invasive plant inventories is a critical component of an integrated approach to invasive plant management [26].…”

Stratégies et potentiel d’invasion des massifs forestiers par <i>Hopea odorata</i> Roxb. (Dipterocarpaceae) : cas du Parc National du Banco en Côte d’Ivoire

“…Elles représentent une des plus grandes causes d'érosion de la biodiversité qui aboutit dans certains cas à des extinctions d'espèces (Lefeuvre, 2013). Actuellement parmi les espèces introduites, existe un grand nombre d'espèces d'arbre (Wilson et al, 2014). Ces plantations d'arbres exotiques se sont développées comme une partie intégrante et indispensable de nombreuses économies nationales et de programmes de reboisement (Haysom et Murphy, 2003).…”