Meta-analysis identifies native priority as a mechanism that supports the restoration of invasion-resistant plant communities

Biological invasion is considered to be one of the main drivers of biodiversity loss with potential negative socio-economic impacts. Invasive alien plant species are well adapted to rapid establishment and exploitation of the resources of disturbed environments, therefore disturbed and intensively managed habitats may support high levels of invasive species. Ecological restoration – defined by the Society for Ecological Restoration as the process of assisting the recovery of an ecosystem that has been degraded, damaged or destroyed – is increasingly recognized as a relevant tool to combat land degradation and biodiversity loss, and also invasive alien species. As the invasion problem becomes increasingly serious, there is an urgent need to develop more innovative, effective and proactive strategies to help improve the resistance of restored communities to invasion, limiting the establishment and further spread of invasive alien species.

I started my MSc thesis with restoration in 1995 in the Centre for Ecological Research (CER). Since then we have been monitoring these experiments in the framework of Kiskun LTER Restoration Experiments. The long-term results show that seeding with native species has been the best method as opposed to mowing and N immobilization, although our system - open sandy grasslands - still has relatively large remnants in the landscape that act as a source of native propagules. In addition to increasing the richness and cover of target species, seed-based restoration has also been most successful in reducing the cover of invasive alien species, although it has not completely eliminated invasive species. 

Figure 1. Native sand grassland species co-grow with the invasive perennial forb Asclepias syriaca in a field restoration experiment. Seeding with native species reduces invasive alien species to a certain extent based on long-term restoration results, and we have investigated in the literature whether this effect can be enhanced by limiting similarity, seeding density and priority effect.

Based on these results, my three co-authors, Anikó Csecserits (Invasion Division, National Laboratory for Health Security, CER), Katalin Török (Restoration Ecology Research Group, CER) and Orsolya Valkó (‘Lendület’ Seed Ecology Research Group, CER) and I have launched a new study on how to optimize the selection of native species to further reduce invasive alien species. We reviewed the literature and selected functional similarity, seeding density and priority effects to be tested. We launched a competition experiment between native and invasive species in the framework of the National Laboratory for Health Security [grant number RRF-2.3.1-21-2022-00006]. As part of the project, we systematically reviewed the literature and prepared a review for the Communications Biology Collection Ecological Restoration and Rewilding. The editors felt that our study was too opinionated and asked us to present it in Perspectives. We did so, and submitted our manuscript. The reviewers, seeing the systematic review, suggested that the study should be developed into a meta-analysis, and we did so with the help of Péter Batáry (Lendület’ Landscape and Conservation Ecology Research Group, CER). 

Figure 2. Native perennial grass Festuca vaginata (green leaves) and invasive annual grass Tragus racemosus (flowers) grown together in our greenhouse pot experiment studying the potential of limiting similarity, seeding density and priority effects to increase the competitive advantage of native species over invasive alien species.

Based on our quantitative review of 48 papers published in relation to seed-based ecological restoration experiments, we demonstrate the potential of seed-based ecological restoration in controlling the establishment and growth of invasive alien species up to 40 %, that can be increased to 50 % by involving the priority effect. Already providing one week of advantage for native species can visibly suppress invasive alien species, although cannot eliminate them. Seeding functionally similar species generally had a neutral effect on invasive alien species, as also shown by previous reviews in the topic, probably partly due to the fact that resources are often not limiting in restoration and in experimental settings. High-density seeding is effective in controlling invasive alien species, but there can be thresholds above which further increases in seeding density may not result in increased invasion resistance.

Figure 3. Orchard plot of the focal model of the suppression of invasive alien species (IAS) by seeding native species with the studied mechanisms (similarity, density, priority, multiple) as moderators. If the confidence bar falls in the negative side and does not intersect with zero, we interpret that seeding native species affects IAS negatively. 

The study also highlights the need to integrate research across geographical regions, global invasive species and potential resistance mechanisms to improve the predictive capacity of invasion ecology and to identify best restoration practices to prevent and control invasive alien species.