Abstract
Impacts of invasive species are often difficult to quantify, meaning that many invaders are prioritised for management without robust, contextual evidence of impact. Most impact studies for invasive plants compare heavily invaded with non-invaded sites, revealing little about abundance–impact relationships. We examined effects of increasing cover and volume of the non-native herbaceous groundcover Tradescantia fluminensis on a temperate rainforest community of southern Australia. We hypothesised that there would be critical thresholds in T. fluminensis abundance, below which the native plant community would not be significantly impacted, but above which the community’s condition would degrade markedly. We modelled the abundance–impact relationship from 83 plots that varied in T. fluminensis abundance and landscape context and found the responses of almost all native plant indicators to invasion were non-linear. Native species richness, abundance and diversity exhibited negative exponential relationships with increasing T. fluminensis volume, but negative threshold relationships with increasing T. fluminensis cover. In the latter case, all metrics were relatively stable until cover reached between 20 and 30%, after which each decreased linearly, with a 50% decline occurring at 75–80% invader cover. Few growth forms (notably shrubs and climbers) exhibited such thresholds, with most exhibiting negative exponential relationships. Tradescantia fluminensis biomass increased dramatically at > 80% cover, with few native species able to persist at such high levels of invasion. Landscape context had almost no influence on native communities, or the abundance–impact relationships between T. fluminensis and the plant community metrics. Our results suggest that the diversity of native rainforest community can be maintained where T. fluminensis is present at moderate-to-low cover levels.
Highlights
The functional response of native species to the altered conditions of an invaded ecosystem can influence abundance–impact relationships
Our hypotheses were: (1) that there would be a critical threshold in the abundance–impact relationship between T. fluminensis and the native plant community, below which there would be no major impact of invasion; (2) that critical thresholds in the abundance–impact relationship of T. fluminensis would vary among different native plant growth forms; and (3) that abundance–impact relationships of T. fluminensis would be influenced by landscape context
We expected that a low-to-moderate cover of T. fluminensis would need to develop before there were major changes in plant diversity, but that this threshold would be: (1) lower for plant growth forms that most directly compete with T. fluminensis; (2) higher for plant growth forms that may have some resistance to invasion; and (3) lower in areas closer to modified landscape features that could amplify the impacts of T. fluminensis
Summary
The functional response of native species to the altered conditions of an invaded ecosystem can influence abundance–impact relationships. It is likely that landscape contextual factors (such as disturbance) would modulate invader abundance–impact relationships, such that the magnitude of native diversity decline in response to invasion will be greatest in certain landscape contexts. The aim of our study was to evaluate the abundance–impact relationships of the invasive ground-cover herb Tradescantia fluminensis with native temperate rainforest vegetation across variation in landscape context. We expected that a low-to-moderate cover of T. fluminensis would need to develop before there were major changes in plant diversity, but that this threshold would be: (1) lower for plant growth forms that most directly compete with T. fluminensis (such as native spreading herbs); (2) higher for plant growth forms that may have some resistance to invasion (such as climbers or tree ferns that can grow above T. fluminensis); and (3) lower in areas closer to modified landscape features (such as roads, footpaths, and urban gardens) that could amplify the impacts of T. fluminensis
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