The absence of keystone indigenous trees inhibits bird recovery up to a decade after invasive tree removal from riparian habitats

Abstract

When invasive alien trees are removed, ecosystems are usually left to “self-repair”. Little is known about the extent of recovery or whether plant and animal taxa respond in a similar way. In most cases, the absence of a historical condition makes it difficult to measure restoration success and a flexible approach is usually followed using practical target communities. We explored these issues by sampling bird and plant assemblages after the removal of invasive trees, using a chronosequence (space-for-time substitution) approach. We used the Berg River, one of the most invaded riparian systems in the Cape Floristic Region of South Africa, as a case study. Study sites – cleared of Eucalyptus camaldulensis in 2005, 2007, 2008 and 2014 – were sampled in 2014 and compared to invaded and near-pristine areas. In total, 27 native plant species (four trees, six shrubs, seven forbs, four graminoids, four geophytes and two vines) and 26 alien plant species (four trees, three shrubs, twelve forbs and seven graminoids) from 50 genera and 31 families were recorded across all sites and years. Cleared sites had significantly more native plant species than invaded sites, but this was similar to near-pristine sites. Cleared sites had the highest plant species richness, driven by significantly more alien species, but canopy cover was significantly lower than in invaded or near-pristine sites. In total, 2049 birds from 52 species were recorded across all sites and years. A decade after clearing, bird species richness, abundance and community composition are different to near-pristine sites. This is due to the lower abundance and diversity of trees in cleared sites, which could be important as habitat or a food source for birds in an agricultural landscape. From a bird perspective, we support the approach of selectively clearing invasive trees over time to allow native trees to recover through succession. We highlight the importance of monitoring fauna to evaluate recovery after invasive alien plant clearing and to guide further management interventions.