Long-term high densities of African elephants clear the understorey and promote a new stable savanna woodland community

摘要

Questions Species defined as ecosystem engineers (e.g. elephant) are able to strongly shape their habitat. In African savannas, elephants have often been shown to reduce woody-plant abundance and diversity. However, recent studies highlight more complex elephant-induced effects on vegetation. Here, we assessed if long-term high elephant densities (>2 km(-2)) in a large open landscape resulted in the depletion of savanna woodland woody communities or if it led to a new alternative equilibrium. Location Woodland savanna of Hwange National Park, Zimbabwe. Elephant densities at the study site have remained high for the past two decades (>2 km(-)(2)). Methods We measured long-term (>15 years) elephant utilisation of woody-plant communities and their effects on vegetation structure, species composition and functional traits (e.g. N leaf concentration, specific leaf area) in 12 vegetation plots. Results We observed opportunistic foraging behaviour by elephants with only a slight temporal shift in species composition, mainly explained by changes in rare species. Further, we did not observe any modification in mean functional trait values, overall height, and stem diameters of the woody-plant communities. However, we found differential changes in woody-plant abundance according to the height layer (decrease in the number of tall plants >200 cm and increase in the number of short plants <50 cm) and a strong reduction in crown diameter for plants in the 50-200 cm height class. Conclusion Our study strongly suggests that long-term high elephant densities have led to a stable state in savanna woodland vegetation in terms of plant community composition and their functional traits. However, high elephant densities did affect vegetation structure, which would have several important indirect effects on this ecosystem (e.g. predator-prey interactions). We hope that this study stimulates more work on the long-term effects of ecosystem engineers in large and open ecosystems.