CANCELLED - Diversity-invasibility relationship: Temporal variability in the relationship between exotic plant invasion and species richness over 35 years in mountain beech forest, New Zealand

Background/Question/Methods

This paper is concerned with temporal variability in the diversity-invasibility relationship.  Over the past fifty years, Nothofagus solandri var. cliffortioides (‘mountain beech’) forest understorey has been invaded by an exotic herbaceous species, Hieracium lepidulum, in many sites in the Southern Alps, New Zealand.  The invasion of Hieracium into understorey plant communities has previously been shown to have a strong positive relationship with vascular plant species richness in the early stages of invasion, even after controlling for other invasion-determining factors such as propagule supply and elevation.  We use a data-set compiled from 34 years of ecological monitoring of 9,000 ha of mountain beech forest to test whether this positive diversity-invasibility relationship:

i.                     changes over time as Hieracium becomes more widespread and abundant across the landscape;

ii.                   is dependent on the index used to quantify invasion (comparing Hieracium presence, abundance and the proportion of flowering plants within the community)

iii.                  changes with spatial scale (comparing invasion into plant communities (400 m²) and neighbourhoods (0.75 m²))  

Results/Conclusions

We show that the diversity-invasibility relationship is positive throughout the invasion of Hieracium over 35 years at the spatial scale of both plant community and neighbourhood.  This period spans a change in status for Hieracium from an uncommon exotic plant to the most abundant and widespread species in the forest understorey.  The diversity-invasibility relationship does not change once other invasion-determining factors have been controlled.  For Hieracium invasion measured as both presence and abundance within the community, the ability to predict invasion patterns using species richness became stronger as Hieracium became more widespread and abundant.  However, although the slope of the relationship between the probability of Hieracium presence and species richness remained constant over time, the rate of increase in Hieracium abundance with increasing species richness becomes greater over time.  The proportion of flowering plants within a community was found to be affected by abiotic variables only.  At the spatial scale of plant neighbourhood, the relationship between the rate of increase of Hieracium invasion probability and species richness is also found to increase over time.  Reasons for these observed patterns will be discussed.