The Effect of Active versus Passive Interventions on Riparian Rehabilitation Success

Abstract

Anthropogenically-induced ecosystem transformations, like land-use change and biological invasions, have accelerated throughout the 20th century with significant impact on rivers and associated riparian ecosystems. In response to the impact and imperatives to recover ecosystem function, rehabilitation of riparian habitat is needed either through active (alien [non-native] clearing and revegetation) or passive (alien clearing and spontaneous succession) methods. We asked whether active rehabilitation of riparian ecosystems yields higher success, defined here as increased native species indices, compared to passive rehabilitation. We conducted the study in a Mediterranean-type ecosystem at 11 riparian rehabilitation sites (ranging between 0.4–2 ha in size), two to three years post-rehabilitation. Accounting for differences in soil and geomorphology, we found that the cover, species richness, and diversity of native vegetation was significantly higher following active rehabilitation compared to passive. Conversely, active rehabilitation had no significant benefit in reducing richness, diversity or cover of secondary invaders and re-invaders compared to passive rehabilitation. Both actively and passively rehabilitated sites were characterized by secondary invasions of alien pioneer grasses and herbaceous weed species. Some initial impacts on soil were noted, indicating potential soil legacy effects of alien tree invasions. Importantly, our study has provided evidence to support riparian rehabilitation as an effective response to ecosystem degradation for achieving improved native vegetation recovery in Mediterranean-type ecosystems, with some benefits of active over passive methods in the short-term, where financial resources are available. The documentation and assessment of riparian rehabilitation projects is rare, and this research provides a crucial Global South contribution to the global evidence base.