What controls woodland regeneration after elephants have killed the big trees?

Abstract

Summary 1.Top-down regulation of ecosystems by large herbivores is a topic of active debate between scientists and managers, and a prime example is the interaction between elephants Loxodonta africana and trees in African savannas. A common assumption among wildlife managers is that a local reduction in elephant numbers will ultimately allow woodland to self-restore to a desired former state. Such regeneration is, however, dependent on the survival of seedlings of impacted tree species. We conducted a field experiment to investigate seedling predation in the elephanttransformed Chobe riparian woodland of northern Botswana. 2. We planted seedling gardens in (i) complete exclosures that excluded all herbivores except small rodents and invertebrates, (ii) semi-permeable exclosures that excluded ungulates but included primates, lagomorphs, all rodents, gallinaceous birds, etc, and (iii) completely open plots. Seedlings were of two tree species decreasing in the area ( Faidherbia albida and Garcinia livingstonei ) and two that are increasing ( Combretum mossambicense and Croton megalobotrys ). 3. After 9 months, seedling survival ranged from > 75% for all species in the complete exclosure to < 20% for Faidherbia albida in the open plots. Survival of all seedlings except C. megalobotrys declined precipitously in open plots during the dry season when invertebrates are largely dormant but when impalas Aepyceros melampus (locally abundant ungulates) increase the browse components of their diets. 4. Seedling survival in the open plots was negatively related to local impala density but unrelated to that of any other browser. 5. Synthesis and applications . Our findings relate to the current debate about managing elephants to restore southern African savanna landscapes to desired historical states. Various seedling predators, including the ubiquitous impala Aepyceros melampus , regulate the regeneration of trees from seedlings, and our experiments support the hypothesis that tall closed-canopy woodlands originate during episodic windows of opportunity for seedling survival. To artificially recreate such a window would require the decimation of seedling predators as well as elephants, which is impractical at the landscape scale.

Combining paleo-data and modern exclosure experiments to assess the impact of megafauna extinctions on woody vegetation

“…1B) (17,24). The removal of large browsers is thought to generally lead to a net increase in abundance of woody plants (20), but this effect depends in part on the compensatory response of smaller herbivores, which can have strong impacts, particularly on the recruitment of woody species (14,16,(25)(26)(27)(28).…”

Tree cover and biomass increase in a southern African savanna despite growing elephant population

“…During the wet season, when water is abundantly available in natural seasonal pans, this large population of elephants becomes scattered throughout the area. During the dry season the large herds congregate around the few available water sources, and thereby change vegetation structure, through coppicing and thinning, particularly affecting the fringes of the Chobe River (Mosugelo et al 2002, Moe et al 2009). Outside the riparian zone of the Chobe River, however, woody vegetation remains largely unaffected by increasing elephant numbers (Skarpe et al 2004).…”

Woodland Conversion by Elephants in Africa: The Search for Causal Factors, Processes, Mechanisms and Management Strategies

“…The decline in woodland in northern parts of Chobe National Park in Botswana has been attributed to herbivory by increasing elephants and large impala (Aepyceros melampus) populations [34] [35]. Moe et al [36] found that seedlings were primarily predated on by impalas and thereby prevented regeneration of trees in the elephant-transformed Chobe woodland of northern Botswana. The influence of fire was not considered as a dominant factor in northern Chobe National Park [34].…”

“…The combined impact of elephants, other herbivores, fires and droughts converts woodlands to scrublands or grasslands. The same factors may exert their influence in maintaining the equilibrium at grassland or scrubland state (see [36]). Therefore, conversion of woodland to scrubland or grassland and maintenance at the converted state may not be entirely due to elephants alone in most cases in the African savannas.…”