Ben Gooden1,2, Luke O’Loughlin1, Dane Panetta3 and Louise Morin1
1Commonwealth Scientific and Industrial Research Organisation, GPO Box 1700, Canberra, ACT 2601, Australia
2University of Wollongong, Wollongong, Australia
3The University of Melbourne, Melbourne, Australia
It is well known that alien plant invasions significantly reduce the diversity and alters the composition of native forest communities. However, most extant studies examine impacts of invasion by comparing differences in native vegetation diversity between heavily invaded and native (non-invaded) reference sites, and often in a restricted number of landscape contexts. We have very little understanding of how native vegetation responds to invasion across gradients of invasion intensity or different landscape or habitat conditions. Furthermore, there is a need to determine how invader impacts vary amongst different functional groups within the native community, as this may inform which species to target for restoration post invader management.
We examined the effects of invasion by an alien, semi-succulent, stoloniferous, shade-tolerant herb Tradescantia fluminensis Vell. (commonly named wandering trad) on the diversity and composition of a cool temperate rainforest community in south-eastern Australia. Wandering trad was introduced to Australia from its native range of Brazil as an ornamental groundcover species and subsequently escaped suburban gardens into adjacent native forests. During 2018, we recorded the number and identity of native rainforest plant species from 90 plots (4m2) across a gradient of increasing wandering trad biomass. We also randomly allocated plots across a gradient of anthropogenic development surrounding the invaded patches of rainforest (forest to cleared agricultural land to suburban areas) to test whether invader impacts are modulated by landscape context.
We found that native plant species diversity and richness declined significantly with increasing wandering trad cover, but only after 50% foliage cover, which indicates an impact threshold relationship. In plots with greater than 80% wandering trad foliage cover the number of native species was 90% lower than non-invaded areas. We also found that the landscape context significantly modulates native vegetation responses to invasion; i.e. the magnitude of native species decline in response to invasion was greater in suburban landscapes with low cover of native forest surrounding the plots. We speculate that a reduction in forest cover in the surrounding landscape reduces the resistance of remnant patches of rainforest to invader impacts, perhaps by preventing the replenishment of seed stocks to replace plants displaced by the invader.
In terms of native plant functional group responses, we found that the richness and cover of all plant growth forms declined significantly with increasing wandering trad biomass. However, the magnitude of decline in response to invasion was higher for species with a similar growth form (i.e. ground-cover, stoloniferous, herbaceous) to wandering trad. Species with divergent growth habits (i.e. tufted herbs, ferns, deep-rooted shrubs) were more resistant to invasion and only began to decline when wandering trad cover exceeded 80%.
Our results show that impacts of alien plant invader are non-linear and can be modulated by landscape context and the functional identity of resident native species. Management should focus upon limiting invader abundance to below the threshold abundance level in order to curtail significant native species losses. In terms of ecosystem restoration, revegetation may be required to replace native species in suburban areas that suffer the greatest decline in response to invasion after the target invader has been controlled.