Comparing drone and field based methods for assessing Cryptostegia grandiflora density along the Mogalakwena river in South Africa

Abstract Biological invasions pose a global challenge and are one of the most severe threats to biodiversity, rangeland productivity, and human and animal health. In South Africa, recent attention has focused on the invasive woody climber Cryptostegia grandiflora . This study aimed to update data on C. grandiflora abundance in the Mogalakwena River Reserve (MRR). A field survey was conducted from May to July 2023, comparing plant density in 36 quadrats with previous surveys undertaken in 2013 and 2016. Additionally, the study evaluated the effectiveness of drone-based analysis for density assessment. High-resolution mosaics of the study area were created using Drone2Map in ArcGIS and analysed to estimate plant density. The results indicated a significant increase in density over the past decade, with higher densities observed on the eastern bank of the river. Statistical tests confirmed significant increase from both 2013 and 2016 to 2023, with no significant difference between 2013 and 2016. Regression analyses confirmed a strong association between field-based and drone-based estimates, though drone data systematically underestimated density, especially at higher infestation levels. This was supported by Spearman’s correlation ( r = 0.79, P < 0.001), Bland-Altman analysis, and the Wilcoxon Signed-Rank Test ( p = 0.015). Despite this, drone-based monitoring offers valuable potential for large-scale invasion assessment. The study underscores the importance of integrating drone technology with field-based surveys for comprehensive monitoring of invasive species and highlights the need for further calibration to improve drone-based accuracy. Effective management strategies are urgently needed to control the spread of C. grandiflora and mitigate biodiversity impacts.