Spatial Distribution and Temporal Expansion of Prosopis Juliflora, pasture availability and livestock productivity in Ngaremara Ward, Isiolo County

Abstract

Prosopis juliflora is among the most aggressive invasive alien woody species threatening the ecological integrity of arid and semi-arid rangeland ecosystems. Although originally introduced to combat desertification and provide fuelwood and fodder, its rapid expansion has transformed pastoral landscapes by reducing grazing resources and threatening livestock-dependent livelihoods. This study assessed the spatial distribution and temporal expansion of P. juliflora and evaluated their implications for pasture availability and livestock productivity in Ngaremara Ward, Isiolo County, Kenya. A stratified household survey of 413 pastoral livestock keepers, complemented by 7 key-informant interviews, 4 focus-group discussions, and rangeland observations, was conducted to assess household-level impacts and livestock-production outcomes. Google Earth Engine was used to process multi-temporal Landsat imagery to characterize long-term land-use and land-cover dynamics, while higher-resolution Sentinel-2 imagery was used to validate and refine recent invasion patterns. Supervised Random Forest classification was integrated with Normalized Difference Vegetation Index analysis to estimate pasture availability across invasion gradients. Overall accuracy for the land-cover classification was 88.6% and a Kappa coefficient of 0.84. Household-level analysis revealed perceived impacts differed significantly among households, with age, production system, and feed shortage showing threshold-specific associations with impact severity. Older household heads and agro-pastoral households were particularly highly vulnerable, while adaptation effectiveness varied by production system and household-head gender. Long-term analysis revealed continuous expansion of P. juliflora between 2013 and 2025, particularly after 2019. Coverage increased by approximately 249% during dry season and 213% during wet season, while pastoral rangelands declined by approximately 1.2% and 10.2%, respectively. Normalized Difference Vegetation Index (NDVI) analysis indicated declining vegetation productivity within invaded areas, while evergreen P. juliflora sometimes produced high greenness values without indicating recovery of palatable pasture. These findings provide robust evidence to inform invasive species management, rangeland restoration, and sustainable pastoral resource planning across Kenya’s arid and semi-arid lands.