Transformation and Degradation of a Savanna Riverine Environment Driven by Multiple Agents Including Elephants

Abstract

Riverine environments in protected semi-arid African savannas support a rich biodiversity and serve as a critical dry-season habitat for many mammalian herbivores. A study of the lower Runde River in Gonarezhou National Park, Zimbabwe, used a set of indicators to assess the extent to which chronic elephant utilization, human legacy effects, droughts, floods, and chronic grazing pressure had changed this environment since the park was formed in 1968. Vegetation and environmental changes were stark. (i) Total woody cover increased until 2005 but declined thereafter; an initial legacy effect of previous cultivation disappeared with time; elephant impact on riverine woodland was mitigated by distance from permanent water along seasonal rivers and the availability of alternative foodstuffs. (ii) Based on repeat photography, 37% of riverine trees were lost, shrub abundance was maintained, and annual forbs and grasses replaced a perennial grass sward. (iii) Five selected indicator species all declined sharply: the number of Ficus sycomorus trees decreased by 61% between 1994 and 2022; Vachellia tortilis woodland was eliminated; 81% of Faidherbia albida trees were lost between 2005 and 2024; Pluchea dioscoridis shrubland and Phragmites mauritianus reedbeds were lost. (iv) Gully erosion proliferated on alluvial terraces. Although elephants are attributed with having caused most changes in woody vegetation, cyclone-derived floods are considered responsible for the loss of reedbeds, most F. sycomorus and some F. albida trees, whereas chronic grazing pressure of elephants and other herbivores was the purported cause of extensive gully erosion. Multiple agents have therefore resulted in a loss of elephant foodstuffs that will likely exacerbate elephant use of remaining riverine vegetation. Areas heavily impacted by gully erosion have likely been irreversibly transformed within a century-long timeframe, whereas recovery of severely impacted woodlands and grasslands will depend on reduced elephant impact, reduced flood impact, and a decrease in browsing and grazing pressure.