A little goes a long way when controlling invasive plants for biodiversity conservation

Invasive species, particularly animals, are being eradicated from islands at ever more ambitious scales. In order to protect island biodiversity and the essential ecosystem functions that it provides, however, plant invasions should be given more management attention. While many advances have been made, plant eradication is inherently more difficult than animal eradication due to persistent seed banks, and eradication may not be possible for more extensive populations. While maintenance control has been successful, critics question the sustainability and priority of these efforts, and targets vary widely. Developing consistent and informed targets requires an understanding of how biodiversity varies with invader cover, yet little is known about this topic. Our research suggests that limited control efforts may be highly beneficial. We conducted a meta-analysis of 54 studies to investigate the effects of plant invasions on invertebrate diversity, incorporating invader cover and residence time as potential causal mechanisms. We also contrasted restored plots with otherwise native plots. We found that invertebrate species richness was 31% lower in exotic plots than in native plots, and that there is a threshold at around 70% invader cover after which the negative effects are significant across all studies. Furthermore, these negative effects tended to decrease with time, and invertebrate richness was even greater in restored plots. The implication is that by removing 30% or less of invasive plant cover and restoring natives, we can achieve many of our conservation goals. We argue that by maintaining invasive patches at or below 70% exotic cover at a site in the near term, we can buy time for both the islands’ insect herbivores to adapt to use the invader, and for managers to continue improving plant eradication technologies. By retaining native diversity in this way, we can help to increase the resistance and resilience of these systems to global change and other stressors.