The editor’s choice for our November issue is ‘Timing of invasive species removal influences nonnative biotic resistance and trajectories of community reassembly‘ by Agostina Torres et al. Here, Associate Editor Maud Bernard-Verdier discusses the importance of this research:

---

Invasive species control often targets single species, but as species introductions continue to increase globally (Seebens et al., 2017), multiple invasions have become the rule. Secondary invasions after removal are a major unintended consequence of invasive species control, and yet our understanding of these dynamics remains limited by decades of focus on single invaders.

For a long time, a main hypothesis when dealing with multiple invasions was the threat of invasional meltdown, that is, the facilitation and accumulation of successive introduced species leading to major ecological impact (Simberloff & von Holle, 1999). But, non-natives compete with each other as well, and can offer a type of non-native biotic resistance to further invasions (Kuebbing & Nuñez, 2015; Rauschert & Shea, 2012). Frequently, lesser non-native competitors benefit from a sudden removal of a dominant non-native. Our mechanistic understanding and ability to predict such secondary invasions are still lacking, and experimental manipulations are the most direct way to investigate them.

Torres et al. propose that seasonal timing of removal might be a game changer. The idea is that as the strength of biotic interactions changes over the growing season, the consequences of removing competitors will also change. They tackled this complex problem in a smartly designed removal experiment in which they followed community reassembly after removal in either early summer or late autumn, both in the field and in controlled conditions (mesocosms).

They studied highly invaded shrubland communities in secondary forests of Isla Victoria, in Patagonia (Argentina). A clever choice was to separately target two co-dominant invasive shrubs, Sweetbriar rose and Scotch broom, in a context where one of the dominant invaders might benefit from the targeted removal of the other. They assessed community reassembly a year after removal in 60 plots of shrubland, and analysed not only the change in cover of the other co-dominant invader, but also dynamics in the rest of the community using joint species modelling.

They found that the timing of intervention changed the outcome: late removals in autumn were less susceptible to secondary invasion than early summer removals. Early in the growth season, limiting resources (light, nutrients, water) released by removal benefitted competitors which were in full growth. Interventions in late autumn, when soil water was higher and growth slower, created smaller to non-existent resource pulse effects.

The study offers a clear demonstration of two scenarios of secondary invasions, depending on the competitive hierarchy between non-natives. Broom removal, likely creating a nitrogen pulse, benefitted less abundant non-native plants, but not the co-dominant rose, whose growth appears not to have been limited by broom. Rose removal, on the other hand, directly benefitted the broom, which was likely suppressed by rose via soil water competition. Unsuppressed broom growth appears to have then limited the growth of native species, suggesting that rose was offering a type of non-native biotic resistance limiting broom expansion.

Adjusting the timing of removal to aim for periods when growth and competitive interactions are lowest may be a simple and effective way to mitigate unintentional boosts in other competitive non-native species. The strategy of late fall removal may be applicable in other highly seasonal climates, and deserves to be tested in other systems. After only one year of study, it remains unclear how the long term consequences of late removal compare to those of early removal, yet this study provides new insights on the role of competitive hierarchies and temporal dynamics in resource interference behind some of the unintended consequences of invasive species control.

Read the full article online: Timing of invasive species removal influences nonnative biotic resistance and trajectories of community reassembly

---

References:

Kuebbing, S. E., & Nuñez, M. A. (2015). Negative, neutral, and positive interactions among nonnative plants: Patterns, processes, and management implications. Global Change Biology, 21(2), 926–934. https://doi.org/10.1111/gcb.12711

Rauschert, E. S. J., & Shea, K. (2012). Invasional interference due to similar inter- and intraspecific competition between invaders may affect management. Ecological Applications, 22(5), 1413–1420. https://doi.org/10.1890/11-2107.1

Seebens, H., Blackburn, T. M., Dyer, E. E., Genovesi, P., Hulme, P. E., Jeschke, J. M., Pagad, S., Pyšek, P., Winter, M., Arianoutsou, M., Bacher, S., Blasius, B., Brundu, G., Capinha, C., Celesti-Grapow, L., Dawson, W., Dullinger, S., Fuentes, N., Jäger, H., … Essl, F. (2017). No saturation in the accumulation of alien species worldwide. Nature Communications, 8, 1–9. https://doi.org/10.1038/ncomms14435

Simberloff, D., & von Holle, B. (1999). Positive interactions of nonindigenous species: Invasional meltdown? Biological Invasions, 21–32. https://doi.org/10.1023/a:1010086329619