Large herbivores promote habitat specialization and beta diversity of African savanna trees

Abstract

Edaphic variation in plant community composition is widespread, yet its underlying mechanisms are rarely understood and often assumed to be physiological. In East African savannas, Acacia tree species segregate sharply across soils of differing parent material: the ant-defended whistling thorn, A. drepanolobium (ACDR), is monodominant on cracking clay vertisols that are nutrient rich but physically stressful, whereas poorly defended species such as A. brevispica (ACBR) dominate on nutrient-poor but otherwise less-stressful sandy loams. Using a series of field experiments, we show that large-mammal herbivory interacts with soil properties to maintain this pattern. In the absence of large herbivores, transplanted saplings of both species established on both soil types. Browsers strongly suppressed survival and growth of ACDR saplings on sandy soil, where resource limitation constrained defensive investment. On clay soil, ACBR saplings established regardless of herbivory regime, but elephants prevented recruitment to maturity, apparently because trees could not tolerate the combination of biotic and abiotic stressors. Hence, each tree species was filtered out of one habitat by browsing in conjunction with different edaphic factors and at different ontogenetic stages. Browser abundance was greater on sandy soil, where trees were less defended, consistent with predicted feedbacks between plant community assembly and herbivore distributions. By exploring two inversely related axes of soil "quality" (abiotic stress and nutrient content), our study extends the range of mechanisms by which herbivores are known to promote edaphic specialization, illustrates how the high cost of a protection mutualism can constrain the realized niche of host trees, and shows that large-scale properties of savanna ecosystems are shaped by species interactions in cryptic ways that mimic simple abiotic determinism. These results suggest that ongoing declines in large-herbivore populations may relax spatial heterogeneity in plant assemblages and reduce the beta diversity of communities.

Woody plant biomass and carbon exchange depend on elephant‐fire interactions across a productivity gradient in African savanna

“…Other differences across the sites such as soil fertility and community composition may also influence the response of savanna woody plants to fire and elephants (Pringle et al . ). In KNP, the more fertile soils of Mopani and Satara may also contribute to greater woody growth at those sites.…”

A thorny issue: Woody plant defence and growth in an East African savanna

“…The exclosures consisted of an 11‐strand, 3‐m tall electrified fence with additional mesh and electrified wires from 0 to 0.5 m height and excluded herbivores larger than 2 kg for 17 years (Augustine & McNaughton, ; Sankaran, Augustine, & Ratnam, ). The savannas at these sites occur on red, sandy loam soils developed from basement, metamorphic parent materials (Augustine, ; Pringle, Prior, Palmer, Young, & Goheen, ). Topography consists of gently, rolling hills, interspersed with occasional granitic inselbergs (Augustine & McNaughton, ).…”

Conservation lessons from large‐mammal manipulations in East African savannas: the KLEE, UHURU, and GLADE experiments

“…unpublished data). Second, wild LMH can slow or prevent the recruitment of saplings into adult size classes, and reduce densities of at least some species of woody plants . Third, wild LMH increase mortality rates of all species in both sapling and adult size classes .…”

“…In contrast to understory plants (where responses are related to overall grazing pressure; see above), both the release from suppression and the increase in recruitment of woody individuals were often specific to both the woody plant species and the herbivore guild excluded. Over a decade, the combination of sapling release, reduced woody plant mortality, and increased growth rates of existing woody individuals generated a much larger response of woody cover and biomass to LMH exclusion on red versus black cotton soils, likely due to a combination of greater palatability of woody plants and higher abundances of browsing, wild LMH on the red soil …”