Native species can reduce the establishment of invasive alien species if sown in high density and using competitive species

Abstract

Invasion of alien species is one of the main drivers of land degradation threatening both natural and managed ecosystems. Ecological restoration is crucial in controlling invasion to improve biotic resistance and avoid further land degradation. We investigated the possibility of controlling the establishment of invasive alien species (IAS) by native seed addition. We tested if trait similarity or increased propagule pressure of native species results in the suppression of IAS at the early stage of development. We set up a sowing experiment with three widespread IAS in Hungary of different life forms and functional groups (Asclepias syriaca, Conyza canadensis, Tragus racemosus) and four Pannonic sand grassland species (Festuca vaginata, Galium verum, Gypsophila paniculata, Saponaria officinalis). We found no significant differences in germination ability and seedling emergence between native species and IAS, despite the differences in thousand-seed weight. Using univariate general linear models, we found that the seedling establishment of IAS can be reduced by adding native species at high densities but also depending on the species identity. Instead of species of similar traits, the seeding of a competitor perennial grass of sand grasslands (F. vaginata) reduced the seedling emergence of all studied IAS the most. Our results confirm that IAS can be effectively controlled by native seed addition in the early establishment stage, especially applying higher densities and competitive species. We conclude that invasion-resistant restoration can be achieved by the combination of several factors, including high-density sowing of native species that match IAS in the early stage of development.

Delaying a prescribed burn to scale up the restoration of alien‐invaded Lowland Sand Fynbos in South Africa

“…The extent of native recovery and the ability of the native cover to reduce acacia cover indicates how building ecosystem resilience through sowing native species and follow‐up clearing of acacia can minimize acacia resurgence. This emphasizes the importance of restoring a good cover, abundance, and diversity of native species in combination with regular follow‐up clearing of acacia to suppress the resurgence of invasive species cover (Csákvári et al., 2023; Gaertner et al., 2012).…”

“…Nevertheless, if available, increasing the sowing rates and species diversity in the mix could improve overall germination percentages and species richness (Hall, Bastos, et al., 2021; Hall, Holmes, et al., 2021; Ngwenya et al., 2023). While it is not required to reintroduce all species to recover ecosystem function, it is advisable to recover species‐diverse ecosystems since the long‐term resilience of ecosystems increases with an increase in species richness and diversity (Csákvári et al., 2023; Holmes & Richardson, 1999).…”

Antecedent climate explains divergent, long-term restoration outcomes in the Western Great Plains, USA.

“…When these opportunities arise, high diversity seed mixes, especially when there are multiple species in each functional group, may be a good bet-hedging strategy to promote the enhanced function and structure of the ecosystem. In the context of climate change, mixes planned for the warm/dry end of the spectrum could be more heavily weighted towards high abundances of competitively dominant native graminoids and forbs to ward off competitively dominant introduced plants (Leger, Goergen and Forbis De Queiroz, 2014;Csákvári et al, 2023), with the hope of establishing diverse mixtures in subsequent years, or even later in the same year, with a repeated seeding approach (Shriver 2018). Another approach may be to strategically consider seed source location when developing the seed mix.…”

“…For example, the perennial grass species Elymus elymoides and native forbs compete favorably against one of the most notorious introduced annuals, cheatgrass (Bromus tectorum), by outcompeting B. tectorum for soil water in the fall (Booth, Caldwell and Stark, 2003;Leger, Goergen and Forbis De Queiroz, 2014;Porensky et al, 2014). Including species in seed mixes that are able to compete against the local introduced annuals may speed the development of stable perennial cover (Csákvári et al, 2023).…”

“…Seed mixes tailored towards wet/cold conditions might take advantage of the opportunity to have success with high-diversity mixtures, with multiple species in each functional group as a bet-hedging strategy to promote the enhanced function and structure of the ecosystem. Mixes planned for warm/dry conditions could be weighted towards high abundances of competitively dominant natives to ward off introduced plants (Leger, Goergen and Forbis De Queiroz, 2014;Csákvári et al, 2023), with the hope of establishing diverse mixtures in subsequent years, or even later in the same year, with subsequent treatments. Another approach may be to strategically consider seed source location when developing the seed mix.…”