Quantifying the impact and reversibility of exotic plant species in succession

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Quantifying the impact and reversibility of
exotic plant species in succession

• D.A. Peltzer 1, P.J. Bellingham 1, L.R. Walker 2
  and P.A. Williams 1
• 1 Landcare Research, New Zealand
• 2 University of Nevada, Las Vegas NV, USA

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General questions about invasions

• Which species will naturalise?
• 2) Which species will become invasive?
• 3) What impacts are invaders having?

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Nativeplants
Introducedplants

• 25,000 species
• 2,100 naturalised
• 250 weeds

• 2,057
• species

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Concerns about invasive species stem from
understanding and mitigating their negative
economic and biological effects (e.g., Cronk and
Fuller 1995, Vitousek et al. 1996, Pimental et
al. 2000, Mack et al. 2000, Levine et al. 2003).
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Outline for this talk

• Case studies of weed impacts on diversity,
  nutrients and succession in NZ
• Buddleja vs Coriaria
• Pinus vs Nothofagus
• Ulex vs Kunzea
• A framework for reversibility of impacts

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Buddleja vs Coriaria
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Pinus vs. Nothofagus
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Pinus vs. Nothofagus

• Pinus contorta rapidly invades areas previously
  dominated by native Nothofagus forest
• What are the relative impacts of pines vs. beech
  on resources and succession?

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Nothofagus
r20.58
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• Pinus contorta grows more rapidly and matures
  earlier than Nothofagus
• Nothofagus can regenerate under Pinus
• Pinus increases soil N and P relative to
  Nothofagus

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Ulex vs Kunzea shrub successions
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Williams et al. unpublished
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Ulex vs Kunzea

• Native woody plant richness was about one-third
  higher under Kunzea
• Successional trajectories are different under
  Ulex and Kunzea

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Successions under exotic species

• Native woody species can regenerate under
  Buddleja, Pinus and Ulex canopies.
• Successions under exotics may differ from those
  under natives (e.g. Kunzea, Coriaria)
• How can we assess or mitigate negative impacts?

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Removal experiments

• Can quantify ecosystem impacts of individual
  species
• Can be used to determine the outcome of
  interspecific interactions
• Are the primary technique used in restoration and
  by conservation managers

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Removals applied to invaders

• Borrow techniques from field competition studies
• Anything short of extirpation creates a density
  gradient which can be used to quantify the
  impacts of invaders
• Need to have appropriate controls

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Reversibility functions

• Likely non-linear functions
• Quantify using removals with appropriate controls
  (both intact and invaded)
• Determine the impacts of exotics on resources,
  community structure etc.

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Which impacts are reversible?

• Reversible shading, pollinators
• Perhaps reversible alteration of soil nutrient
  (N, P) levels, community composition
• Not reversible extirpations, feedback switches

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Summary

• Exotic plants dominate many successions in New
  Zealand and elsewhere
• Comparisons with native successions can reveal
  long-term impacts of these invaders
• Removal experiments with appropriate controls can
  be used to generate reversibility functions

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