Documenting the chronology of ecosystem health erosion along East African rivers

Pristine tropical ecosystems are currently suffering under extreme demographic pressure. This has led to habitat destruction and disturbances, an important precondition for the invasion of exotic species into pristine ecosystems, with detrimental effects on biodiversity. In this study, we analysed the chronology of land cover changes along rivers in south‐eastern Kenya using historical aerial photographs and current aerial pictures taken from a drone. We assessed quantity and quality of riparian thickets along these rivers in 1961, 1980 and 2015/2016 and analysed effects of fundamental land cover changes on arthropod abundance and diversity. Our data show effects from demographic pressure by the increasing number of human settlements rising by 425% from 1961 to 2015/2016 (i.e. from 36 to 189 settlements). 58.5% of the land surface originally covered by pristine riparian forests was cleared and transformed into agricultural land. After clearance and agricultural usage it frequently became fallow land and was invaded by the exotic shrub species Lantana camara . While the total coverage of riparian vegetation did not change over time, its quality declined dramatically. Currently 36.6% of the cleared pristine riparian forest is covered by L. camara thickets, and only 22.5% of the pristine riparian forest has remained unchanged over time. This modification of the vegetation structure impacts biodiversity. The abundances of three guilds of arthropods significantly differed between pristine and L. camara ‐dominated riparian vegetation, with herbivorous arthropods and ants being more abundant in pristine than in L. camara ‐dominated vegetation. Our data show that habitat destruction and disturbance is the prerequisite for subsequent invasion by exotic plant species, having a strong effect on species composition and diversity at higher trophic levels.