Abstract
Globally, invasive grasses are a major threat to protected areas (PAs) due to their ability to alter community structure and function, reduce biodiversity, and alter fire regimes. However, there is often a mismatch between the threat posed by invasive grasses and the management response. We document a case study of the spread and management of the ecosystem-transforming invasive grass, Andropogon gayanus Kunth. (gamba grass), in Litchfield National Park; an iconic PA in northern Australia that contains significant natural, cultural and social values. We undertook helicopter-based surveys of A. gayanus across 143,931 ha of Litchfield National Park in 2014 and 2021–2022. We used these data to parametrise a spatially-explicit spread model, interfaced with a management simulation model to predict 10-year patterns of spread, and associated management costs, under three scenarios. Our survey showed that between 2014 and 2021–22 A. gayanus spread by 9463 ha, and 47%. The gross A. gayanus infestation covered 29,713 ha of the total survey area, making it the largest national park infestation in Australia. A. gayanus had not been locally eradicated within the Park's small existing ‘gamba grass eradication zone’, and instead increased by 206 ha over the 7-year timeframe. Our modelled scenarios predict that without active management A. gayanus will continue spreading, covering 42,388 ha of Litchfield within a decade. Alternative scenarios predict that: (i) eradicating A. gayanus in the small existing eradication zone would likely protect 18% of visitor sites, and cost ∼AU$825,000 over 5 years – more than double the original predicted cost in 2014; or (ii) eradicating A. gayanus in a much larger eradication zone would likely protect 86% of visitor sites and several species of conservation significance, and cost ∼AU$6.6 million over 5 years. Totally eradicating A. gayanus from the Park is no longer viable due to substantial spread since 2014. Our study demonstrates the value of systematic landscape-scale surveys and costed management scenarios to enable assessment and prioritisation of weed management. It also demonstrates the increased environmental and financial costs of delaying invasive grass management, and the serious threat A. gayanus poses to PAs across northern Australia.
| Original language | English |
|---|---|
| Article number | 116785 |
| Pages (from-to) | 1-51 |
| Number of pages | 51 |
| Journal | Journal of Environmental Management |
| Volume | 333 |
| Early online date | 7 Feb 2023 |
| DOIs | |
| Publication status | Published - 1 May 2023 |
Bibliographical note
Funding Information:The 2014 distribution data, and the management scenarios presented to park managers/planners directly supported the Park's planning and prioritisation of A. gayanus control. This was carried out via a conservation action planning (CAP) process, and development of an Integrated Conservation Strategy (ICS) (See Ferdinands et al. (2016) for comprehensive planning and management elements of the Litchfield ICS). The ICS process identified fire and invasive grasses as priority threats to the values of Litchfield, with wildfire and A. gayanus ranked as the top two threats, with an ‘extreme’ threat rating (Ferdinands et al., 2016). The Litchfield ICS process identified management targets and threat reduction targets across six management zones in the Park (NTG, 2015). We use these six zones within our study (Fig. 1): Sandstone Plateau North (SPN), Sandstone Plateau South (SPS), and the Lowland North (LN), Lowland Central (LC), Lowland South (LS) and Lowland West (LW). The ICS plan defined a 10,525 ha A. gayanus (gamba grass) ‘eradication zone’ which incorporates the high visitation ‘Tabletop Range’ area in the SPN zone, and two small areas in the SPS management zone (Fig. 1) The Park-wide 2020 target was “All known infestations of gamba grass are eradicated within the eradication zone” (NTG, 2015). Actions to achieve the Litchfield ICS goals are supported by the Park's overarching statutory Plan of Management (NTG, 2017), and annual weed and fire action plans (Ferdinands et al., 2016). However, following the 2014 survey (Adams and Setterfield, 2016) and the 2016 ICS (Ferdinands et al., 2016) there was no substantial additional funding for intensive A. gayanus management, or implementation of a significant control program.Protected areas (PAs) are an essential tool for protecting biodiversity and functioning ecosystems against increasing threats (Pressey et al., 2015); including those posed by invasive alien plant species (Essl et al., 2020; Pyšek et al., 2020). Well-resourced and effective management of invasive alien plants in PAs is critical to mitigate their impacts (Genovesi and Monaco, 2013; Foxcroft et al., 2017; Cheney et al., 2018) and meet CBD global biodiversity framework targets for the protection of the PA ecosystems (Adams et al., 2019; Leadley et al., 2022). However, globally, studies have highlighted the substantial lack of resources for PAs to effectively manage threats (e.g. Adams et al., 2019; Coad et al., 2019; Craigie and Pressey, 2022; Cuthbert et al., 2022) and the poorer biodiversity outcomes that result from this resourcing shortfall (Adams et al., 2019; Craigie and Pressey, 2022). Here we demonstrate the substantial costs of not acting on the threat of invasive grasses, using a case study of the spread and management of ecosystem-transforming invasive grass, in an iconic PA in northern Australia; one that contains significant natural, cultural and social values. Our research shows that failing to fund and implement strategic weed management can lead to rapid expansion of invasive grass infestations, and cost of management can quickly escalate, as can the threat to high-value PA assets.Our study demonstrates the importance of systematic landscape-scale surveys to assess the true scope of the problem and assess the risks to assets from alien plant invasion. The repeated surveys showed an increase in the gross infestation area of ∼9,500 ha within a ∼7-year timeframe across the survey area, and a 148% increase in net infestation area. This dramatic increase in a short timeframe is consistent with the invasion and spread of A. gayanus in the Park over two decades (Figs. 5 and 6), and to our knowledge this is the largest A. gayanus infestation in a national park in Australia. These results support the growing body of research showing the critical role of systematic, landscape-scale surveys, to enable PA mangers to accurately assess the true extent of alien plant invasions, and as a baseline for evidence-based weed management prioritisation, and monitoring programs (Cheney et al., 2018).
Funding Information:
This project was jointly funded by grants from the Northern Territory Government (NRR and SS), and the National Environmental Science Programme (NESP) Northern Australia Environmental Resources Hub (NRR and SS). Thank you to the Litchfield National Park Rangers and NT Parks & Wildlife Planners for access to, and information about the Park. Thanks to staff at the Northern Territory Government Weed Management Branch, Territory Natural Resource Management (TNRM) and Outback Helicopter Airwork NT for supporting the aerial survey. We also thank the NT Parks & Wildlife, NT Weed Management Branch and Northern Territory Gamba Grass Weed Advisory Committee (WAC) for feedback on draft presentations of the survey data, and management scenarios. We also thank Patch Clap and Fiona Freestone for technical support, and Michael Douglas, Keith Ferdinands, and five anonymous reviewers for their helpful comments and suggestions on an earlier draft.
Publisher Copyright:
© 2022 The Authors
