Woody plant encroachment drives population declines in 20% of common open ecosystem bird species

• Published in: Global change biology Vol. 30; no. 6; pp. e17340 - n/a
• Main Authors: White, Joseph D. M., Stevens, Nicola, Fisher, Jolene T., Reynolds, Chevonne
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01.06.2024
• Subjects: Agricultural ecosystems; Animal populations; Animals; Biodiversity; Bird populations; Birds; Birds - physiology; Conservation of Natural Resources; Ecosystem; Ecosystem management; Ecosystem services; Ecosystems; Encroachment; functional traits; grassland; Habitats; Nesting; occupancy models; Plant communities; Plants; Population decline; Population Dynamics; Population growth; population trends; Protected areas; savanna; South Africa; Structure-function relationships; Threatened species; Trends; woody plant encroachment; Woody plants; South Africa; functional traits; population trends; savanna; woody plant encroachment; occupancy models; birds; grassland
• ISSN: 1354-1013; 1365-2486
• DOI: 10.1111/gcb.17340

Grassy ecosystems cover more than 40% of the world's terrestrial surface, supporting crucial ecosystem services and unique biodiversity. These ecosystems have experienced major losses from conversion to agriculture with the remaining fragments threatened by global change. Woody plant encroachment, the increase in woody cover threatening grassy ecosystems, is a major global change symptom, shifting the composition, structure, and function of plant communities with concomitant effects on all biodiversity. To identify generalisable impacts of encroachment on biodiversity, we urgently need broad‐scale studies on how species respond to woody cover change. Here, we make use of bird atlas, woody cover change data (between 2007 and 2016) and species traits, to assess: (1) population trends and woody cover responses using dynamic occupancy models; (2) how outcomes relate to habitat, diet and nesting traits; and (3) predictions of future occupancy trends, for 191 abundant, southern African bird species. We found that: (1) 63% (121) of species showed a decline in occupancy, with 18% (34) of species' declines correlated with increasing woody cover (i.e. losers). Only 2% (4) of species showed increasing population trends linked with increased woody cover (i.e. winners); (2) Open habitat specialist, invertivorous, ground nesting birds were the most frequent losers, however, we found no definitive evidence that the selected traits could predict outcomes; and (3) We predict open habitat loser species will take on average 52 years to experience 50% population declines with current rates of encroachment. Our results bring attention to concerning region‐wide declining bird population trends and highlight woody plant encroachment as an important driver of bird population dynamics. Importantly, these findings should encourage improved management and restoration of our remaining grassy ecosystems. Furthermore, our findings show the importance of lands beyond protected areas for biodiversity, and the urgent need to mitigate the impacts of woody plant encroachment on bird biodiversity. Our study utilised a comprehensive citizen science dataset to analyse the relationship between woody plant encroachment and bird occupancy in southern Africa's grassy ecosystems from 2007 to 2016. Of 191 bird species examined, 34 experienced declines in their occupancy due to increased woody vegetation, while only 4 species showed gains. Our predictions suggest that if current rates of encroachment persist, some vulnerable bird populations could see a reduction of up to 50% in their 2016 occupancy levels within the next 52 years. These findings place renewed focus on the impacts of encroachment on biodiversity and the importance of mitigating these landscape‐level changes.