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A standardized set of metrics to assess and monitor tree invasions

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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).

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Acknowledgments

This paper resulted from the workshop “Tree invasions—patterns & processes, challenges & opportunities” held in Bariloche, Argentina in 2012. We thank all participants at the meeting for valuable discussion. Daniel Simberloff and three reviewers provided valuable comments that improved the manuscript. JRUW acknowledges funding from the South African Working for Water Programme of the Department of Environmental Affairs. IAD was supported by Core funding for Crown Research Institutes from the New Zealand Ministry of Business, Innovation and Employment’s Science and Innovation Group. AP is funded by Ministry of Economy, ICM P05-002 and Conicyt, PFB-23. DMR acknowledges support from the National Research Foundation (Grant 85417), the DST-NRF Centre of Excellence (partly though the collaborative project with the Working for Water programme on “Research for Integrated Management of Invasive Alien Species”) and the Oppenheimer Memorial Trust. CH was supported by the CPRR 81825 of the NRF. BDM was supported by NSF- WildFIRE PIRE, OISE 09667472. BLW was supported by the CSIRO Climate Adaptation Flagship. RDZ was supported by CNPq-Brazil and The University of Tennessee.

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Authors and Affiliations

  1. Invasive Species Programme, South African National Biodiversity Institute, Kirstenbosch National Research Centre, Claremont, 7735, South Africa

    John R. U. Wilson & Dian Spear

  2. Department of Botany and Zoology, Centre for Invasion Biology, Stellenbosch University, Private Bag X1, Matieland, 7602, South Africa

    John R. U. Wilson, Cang Hui, David M. Richardson, Dian Spear & Brian W. van Wilgen

  3. Department of Physical Geography and Ecosystem Sciences, University of Lund, Sölvegatan 12, 22362, Lund, Sweden

    Paul Caplat

  4. Landcare Research, Box 69040, Lincoln, 7640, New Zealand

    Ian A. Dickie

  5. Bio-Protection Research Centre, Lincoln University, Box 85084, Lincoln, 7647, New Zealand

    Ian A. Dickie

  6. Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, MT, 59717, USA

    Bruce D. Maxwell

  7. Laboratorio Ecotono, INIBIOMA, CONICET, Univ. Nacional del Comahue, Quintral 1250, CP 8400, San Carlos de Bariloche, Argentina

    Martin A. Nuñez

  8. Facultad de Ciencias Forestales, Institute of Ecology and Biodiversity (IEB), Universidad de Concepción, Concepción, Chile

    Aníbal Pauchard

  9. Department of Evolution and Ecology, University of California, Davis, Davis, CA, 95616, USA

    Marcel Rejmánek

  10. Department of Zoology and Entomology, Centre for Invasion Biology, University of Pretoria, Private Bag X20, Pretoria, 0028, South Africa

    Mark P. Robertson

  11. CSIRO Ecosystem Sciences, Private Bag 5, P.O. Wembley, WA, 6913, Australia

    Bruce L. Webber

  12. School of Plant Biology, The University of Western Australia, 35 Stirling Highway, Crawley, WA, 6009, Australia

    Bruce L. Webber

  13. CSIR, Natural Resources and the Environment, PO Box 320, Stellenbosch, 7599, South Africa

    Brian W. van Wilgen

  14. Department of Ecology and Evolutionary Biology, The University of Tennessee, Knoxville, TN, 37996, USA

    Rafael D. Zenni

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  1. John R. U. Wilson
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  2. Paul Caplat
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  3. Ian A. Dickie
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  4. Cang Hui
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  5. Bruce D. Maxwell
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  6. Martin A. Nuñez
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  7. Aníbal Pauchard
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  8. Marcel Rejmánek
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  9. David M. Richardson
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  10. Mark P. Robertson
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  11. Dian Spear
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  12. Bruce L. Webber
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  13. Brian W. van Wilgen
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  14. Rafael D. Zenni
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Corresponding author

Correspondence to John R. U. Wilson.

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Appendices

Appendix 1: Example of species report (Acacia paradoxa DC. in South Africa)

Species: Acacia paradoxa DC. example herbarium record: (Slater 7035, BOL). No subspecific information available.

Location: South Africa.

Status: Invasive; D2 under Blackburn; (in cultivation?): not known to be cultivated recently (possibly introduced for ornamentation 100 years ago).

Potential: 6–13 % of South African land area; ~70–160 M ha (Zenni et al. 2009; Moore et al. 2011).

Abundance: ~12,000 plants (2010); 0.7 ha (condensed area); 70,000–700,000 seeds (2010).

Population Growth Rate: Few large individuals, 60–80 % of population <1 m and not reproductive in 2009; only 50 individuals >3 m.

Extent: 1 population; 350 ha (condensed polygon) in terms of uncertainty, a range of values of 155–1,550 ha was used in one modelling exercise (Moore et al. 2011).

Spread: natural radial increase of 100 m year−1 (assumed value), mostly gravity. Potential for seeds to be transported by road vehicles (not realized as yet).

Impact: Monoculture created; nuisance thorns. Impact ZAR 1,701 year−1 ha−1 (uncondensed area, monetary values from 2000) extrapolated from (de Wit et al. 2001). For a completed Australian Weed Risk Assessment see Zenni et al. (2009).

Threat: If potential area is multiplied by impact get to ZAR 100 billion year−1.

Survey method(s) used: Systematic walked transects over ~700 ha to generate point distributions. At a national scale this distinctive species has been included in general field-guides for invasive plants for many years, and dedicated leaflets asking for sightings have been distributed nationally since 2009. Any records should also have been picked up by the substantial on-going research, surveillance, and management into Australian acacias in South Africa.

Notes: eradication plan in place.

Contact: invasivespecies@sanbi.org.za.

Information compiled by: John Wilson, jrwilson@sun.ac.za.

Refs:

de Wit MP, Crookes DJ and van Wilgen BW (2001) Conflicts of interest in environmental management: estimating the costs and benefits of a tree invasion. Biological Invasions 3: 167-178.

Moore JL, Runge MC, Webber BL and Wilson JRU (2011) Contain or eradicate? Optimizing the management goal for Australian acacia invasions in the face of uncertainty. Diversity and Distributions 17: 1047–1059.

Zenni et al. (2009) Evaluating the invasiveness of Acacia paradoxa in South Africa. South African Journal of Botany 75: 485–496.

Appendix 2: Example of species report (Pinus contorta Loundon. in New Zealand)

Species: Pinus contorta Loudon.

Pinus contorta Loudon subsp. contorta = Pinus contorta Loudon var. contorta.

Pinus contorta Loudon var. contorta.

Pinus contorta subsp. latifolia = Pinus contorta var. latifolia Engelm. ex S.Watson.

Pinus contorta var. latifolia Engelm. ex S.Watson.

Location: New Zealand (numerous locations).

Status: Invasive; E under Blackburn; All four subspecies of lodgepole pine (contorta, bolanderi, latifolia and murrayana) have been planted (Miller and Ecroyd, 1987) and all regenerate naturally. (Ledgard 2001) (in cultivation?): Not known to be cultivated recently. Introduced in 1880 and established widely for erosion control during 1960s and 70s on a few thousand hectares and self-sustaining since then (Miller and Ecroyd 1987, Ledgard 2001). Suggested as possible covering ~100,000 ha by late 1990s (Ledgard 2001).

Potential: all already invasive. 10–15 % of New Zealand land area (i.e. >2.5 M ha) suitable although could be greater.

Abundance: Various density stands. Seeds freely to high elevation and cones relatively young.

Population growth rate: Published information on estimated extent of cover (Miller and Ecroyd 1987, Ledgard 2001) suggests extent may be increasing at between 5 and 8 % per annum despite control efforts.

Extent: Numerous populations (many large and >1,000 hectares) totalling >100,000 ha extent at all densities. Many populations are found in remote locations as a legacy of where their establishment attempted to protect erosion-prone land from mass-movement. Due to their remoteness and potential cost there is little incentive address control or removal.

Spread: Natural radial increase of ~5,000 ha year−1 (assumed value), mostly wind and gravity.

Impact: Major visual transformation of iconic grazed grasslands into forest, with consequent recreational value loss and aesthetic impact. Invasions most problematic in low-stature native vegetation (Froude 2011), with up to 100 % loss of native plant biodiversity from high elevation grasslands (Ledgard & Paul 2008), strong shifts in fungal communities (Dickie et al. 2010) and, based on results from Pinus nigra strong effects on soil invertebrate diversity even at low tree-densities (Dickie et al. 2011). Economic loss through reduction in land for low-intensity grazing (sheep, beef-cattle). Loss of water a serious concern in some areas (Fahey & Jackson 1997).

Threat: Highest threat is in conservation grasslands and alpine zone where removal will have high non-target impacts.

Survey method(s) used: No national objective survey or monitoring. One province (Canterbury Regional Council) has systematic estimates of extent of cover and density in 11 representative catchments ~70,000 ha to generate point and polygon distributions. Department of Conservation records the presence of weed species in a 10 × 10 km grid.

Notes: Limited control in a few locations.

Contact: Ian Dickie, ian.dickie@lincoln.ac.za.

Information compiled by: Larry Burrows, burrowsl@landcareresearch.co.nz.

Refs:

Benecke, U. 1967: The weed potential of lodgepole pine. Tussock Grasslands and Mountain Lands Institute Review 13: 36–43.

Dickie IA, Bolstridge N, Cooper JA, Peltzer DA 2010. Co-invasion by Pinus and its mycorrhizal fungi. New Phytologist 187: 475–484.

Dickie IA, Yeates GW, St John MG, Stevenson BA, Scott JT, Rillig MC, Peltzer DA, Orwin KH, Kirschbaum MUF, Hunt JE, Burrows LE, Barbour MM, Aislabie J 2011. Ecosystem service and biodiversity trade-offs in two woody successions. Journal of Applied Ecology 48: 926–934.

Fahey B, Jackson R 1997. Hydrological impacts of converting native forests and grasslands to pine plantations, South Island, New Zealand. Agricultural and Forest Meteorology 84:69–82.

Ledgard, N. 2001: The spread of lodgepole pine (Pinus contorta, Dougl.) in New Zealand. Forest Ecology and Management 141:43–57.

Ledgard NJ, Paul TSH 2008. Vegetation successions over 30 years of high country grassland invasion by Pinus contorta. New Zealand Plant Protection 61: 98–104.

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Wilson, J.R.U., Caplat, P., Dickie, I.A. et al. A standardized set of metrics to assess and monitor tree invasions. Biol Invasions 16, 535–551 (2014). https://doi.org/10.1007/s10530-013-0605-x

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