A weed risk analytical screen to assist in the prioritisation of an invasive flora for containment

Abstract

Prioritising weeds for control and deciding upon the type of control and its associated investment are fundamental to weed management planning. Risk analysis is central to this process, combining the activities of risk assessment, risk management and risk communication. Risk assessment methodology has a rich history, but management feasibility has typically been a secondary matter, dealt with separately or not at all. Determinants of management feasibility for weeds include the stage of invasion, weed biology, means of control and cost of weed control. Here, we describe a simple weed risk analytical screen that combines risk assessment with species traits that influence management feasibility. We consider stage of invasion, species biological/dispersal characteristics and plant community invasibility in a preliminary analysis of the risk posed by the non-native plant species on Christmas Island in the Indian Ocean. For each of 31 high-risk species considered to be ineradicable under existing funding constraints, we analyse the risk posed to two major plant communities: evergreen closed-canopy rainforest and semi-deciduous scrub forest. Weed risk ratings are combined with ratings for species-intrinsic feasibility of containment (based on a measure that combines time to reproduction with potential for long distance dispersal) to create preliminary rankings for containment specific to each community. These rankings will provide a key input for a more thorough analysis of containment feasibility – one that considers spatial distributions/landscape features, management aspects and the social environment. We propose a general non-symmetric relationship between weed risk and management feasibility, considering risk to be the dominant component of risk analysis. Therefore, in this analysis species are ranked according to their intrinsic containment feasibility within similar levels of risk to produce an initial prioritisation list for containment. Shade-tolerant weeds are of particular concern for the closed-canopy evergreen rainforest on Christmas Island, but a greater diversity of weeds is likely to invade the semi-deciduous scrub forest because of higher light availability. Nevertheless, future invasion of both communities will likely be conditioned by disturbance, both natural and anthropogenic. The plant communities of Christmas Island have undergone significant fragmentation because of clearing for phosphate mining and other purposes. With a substantial number of invasive plant species firmly established and having the potential to spread further, minimising future anthropogenic disturbance is paramount to reducing community invasibility and therefore conserving the island’s unique biodiversity.

A Proposed Method for Evaluating Management Feasibility When Determining Weed Control Priorities after Major Fires and Floods

“…At the other end of the weed risk continuum are those plants, commonly annuals and biennials [5,20] whose occurrence post-disturbance will be transient in nature and therefore not require control. In an examination of the weed flora of Christmas Island, species were ranked as posing "high", "very high" and "extreme" risk [6] . For determining the feasibility of containment of weeds that had restricted distributions on the island, only a few species that posed "extreme" risk (i.e., transformers) were nominated as targets for containment because of the relatively high demand for resources associated with this management goal.…”

“…Prioritising weeds for control and deciding upon the type of control and its associated investment are fundamental to weed management planning [6] . Risk analysis, comprising the activities of risk assessment, risk management and risk communication, is central to this process.…”

“…There has been a gradual move by invasion scientists and practitioners to develop scoring protocols for the assessment of weed management feasibility [6][7][8]11] . A partial analysis of the feasibility of containment for the most impactful weeds in Christmas Island National Park (Christmas Island, Indian Ocean) appeared recently [6] . Only two species-intrinsic factors, namely the time to reproduction and the nature of the dispersal vector suite [12] were utilised.…”

A model that will assist on-ground practitioners in their quest to protect biodiversity values in natural ecosystems

“…Some time ago, Virtue et al (2001) listed the essential criteria for addressing the feasibility of managing weeds as: 1) stage of invasion; 2) weed biology; 3) means of control; 4) cost of weed control; and 5) motivation of land managers. In recent years there has been a move by invasion scientists and practitioners to develop scoring protocols for the assessment of weed management feasibility (Wilson et al 2016, Booy et al 2017,Vanderhoeven et al 2017, Panetta and Grigg 2021.…”

“…In most cases, a practitioner would not be amenable to, or capable of, adapting a scoring system for use on a site-by-site basis. Panetta and Grigg (2021) presented a partial analysis of the weed risk management feasibility of the most impactful weeds in Christmas Island National Park (Christmas Island, Indian Ocean.) Two species-intrinsic factors, namely the time to reproduction and the nature of the dispersal vector suite (sensu Panetta and Cacho 2012) were utilised.…”