Glade cascades: indirect legacy effects of pastoralism enhance the abundance and spatial structuring of arboreal fauna

Studies in community ecology are typically conducted over the span of a few years, and results are often interpreted as the product of contemporary processes and interactions. All landscapes have histories, however, and observed patterns of distribution and abundance frequently reflect enduring legacies of past ecological events, the existence and influence of which may not be obvious to investigators. In East Africa, most wildlife occurs outside national reserves and often coexists there with livestock, which are traditionally corralled at night in temporary thorn fence enclosures, or bomas. After being abandoned, bomas develop into nutrient-rich, treeless glades that can persist for more than a century. These hotspots of primary productivity attract both native and domestic large herbivores, but the extent to which their effects cascade to other consumers is unknown. Here, we document positive edge effects of glades on the mean size and growth rates of Acacia trees and show that the density and biomass of arboreal geckos (Lygodactylus keniensis) are elevated near glades and decrease with distance from glades. The edge response of geckos is an indirect effect arising from the positive influence of glades on arboreal arthropod biomass (a trophic effect) and average tree size (a non-trophic effect). By clearing plots of trees to simulate glades, we experimentally demonstrate that these legacy effects arise from the elevated nutrient content of glades as opposed to their distinctive structural features. Finally, we investigated interactions among glade edges, showing that legacy effects are dampened (rather than enhanced) by the presence of other glades nearby. Collectively, our results show that legacy effects of traditional pastoral practices cascade into the treetops, imparting spatial structure across multiple trophic levels in an otherwise homogeneous Kenyan savanna ecosystem.