COS 141-6 - The vulnerability of urban green spaces to plant invasions

Background/Question/Methods

The likelihood of being invaded (‘invasibility’) differs among urban green spaces (UGSs). This is because of varying environmental and societal influences that drive abiotic and biotic characteristics, as well as propagule pressure of prospective invasive species. Reactive management to thwart invasion is common, but proactive approaches are rare. Practitioners need tools to prevent invasion before incurring management costs; however, in UGSs it is not clear what promotes high invasibility.

Here I present a framework to evaluate UGS invasibility and the results of an observational study. This framework divides invasibility drivers into three pillars, each influenced by human activity: abiotic context, biotic interactions, and potential propagule pressure. The study took place in ‘The Meadoway’, a 16 km stretch of meadows undergoing restoration in Toronto Canada by repeated mowing then sowing using native seed mixes. One invasive species present in the region is Vincetoxicum rossicum (dog strangling vine; DSV) and difficult to eradicate. The goal was to quantify the relative importance among different drivers on invasibility across two management regimes (tilling and restored sites): tilling disturbs soil to release resources for the growth of DSV, seed bank density resists the colonization of DSV, and seed rain from nearby DSV disperses propagules.

Results/Conclusions

First, tilling during the early stages of restoration practitioners significantly lowers invasibility of meadow habitats by 15% compared to restored sites. Structural equation modelling shows that tilling facilitates spontaneous non-native plant species (which are not invasive) by increasing available resources for growth. This saturates the available niches through higher non-native species richness, leaving fewer opportunities for future invasion events. Tilling also reduces potential propagule pressure by depleting the seed rain of DSV. Second, we found that a higher seed bank density increases the resistance to invasion by 15% across management regimes. Seed rain from nearby DSV did not affect invasibility. DSV also never emerged from the seed bank due to either dispersal limitation or a “sit-and-wait” strategy until favorable conditions occur for germination.

Overall, we show invasibility is lowered with active management strategies and native seed mixes. A proactive strategy is to modify existing mixes before restoration, by using cost-effective native plants that grow quickly and produce many seeds to reduce germination of invasive species. These findings also show the flexibility of this invasibility framework to different scenarios with clear actionable outcomes and can be applied to other UGS with different properties that affect its vulnerability to plant invasions.