Even within relatively pristine systems such as New Zealand’s forests and shrublands, non-native species make up a large proportion of all plants. From the National Vegetation Survey (NVS) databank, administered by scientists at Manaaki Whenua, we find that around 37% of over 60,000 vegetation survey plots across New Zealand contain at least one non-native plant species, with 56% of these invaded plots containing multiple non-native species.
At present we lack experimental data to properly guide the management of ecosystems that have experienced multi-species invasions. To address this problem, ecologists at Manaaki Whenua, the University of Auckland, the University of Canterbury, and international colleagues in Australia, the USA and Singapore, are investigating how co-occurring plant invaders interact and the consequences of those interactions for the invaded ecosystems.
Because ecosystem impacts tend to increase with the abundance of a weed, how co-occurring weeds affect each other’s abundance, as well as total weed abundance at any site, will affect their combined ecosystem impacts. An additional weed at a site can influence total weed abundance at the site or the abundance of the first weed to invade. For example, nitrogen-fixing legumes like Scotch broom or clover can help non-native sward grasses grow more prolifically. Complicating matters, we know that some native plants can also facilitate weed invasion.
However, the impacts of co-occurring weeds can also directly interact – such as where pines and non-native grasses can create a ladder of fire into the canopies of forests.
Co-occurring weeds can therefore have profound, but sometimes unanticipated, effects on plant communities and ecosystems. One weed might suppress the impact of another through competition, keeping a perhaps more problematic weed in check. Different management strategies are needed to effectively mitigate these invasion impacts in systems with multiple weeds.
The researchers suggest that adopting a community ecology framework, which considers the complexity of interactions among all the non-native and native species at a site, might help better identify target weeds for management. Choosing to first manage weeds that have high potential to interact with other invaders and natives could reduce the likelihood of unexpected outcomes for ecosystem functions and processes.
However, more empirical data are needed. Aligning more invasion impact research with management activities such as weed removal would also help to determine how co-occurring weeds interact across a wide variety of systems and feed in to helping practitioners implement the most effective strategies to reduce their impacts.