Duncan M. Kimuyu

Native and domestic browsers and grazers reduce fuels, fire temperatures, and acacia ant mortality in an African savanna

Despite the importance of fire and herbivory in structuring savanna systems, few replicated experiments have examined the interactive effects of herbivory and fire on plant dynamics. In addition, the effects of fire on associated ant-tree mutualisms have been largely unexplored. We carried out small controlled burns in each of 18 herbivore treatment plots of the Kenya Long-term Exclosure Experiment (KLEE), where experimentally excluding elephants has resulted in 42% greater tree densities. The KLEE design includes six different herbivore treatments that allowed us to examine how different combinations of megaherbivore wildlife, mesoherbivore wildlife, and cattle affect fire temperatures and subsequent loss of ant symbionts from Acacia trees. Before burning, we quantified herbaceous fuel loads and plant community composition. We tagged all trees, measured their height and basal diameter, and identified the resident ant species on each. We recorded weather conditions during the burns and used ceramic tiles painted with fire-sensitive paints to estimate fire temperatures at different heights and in different microsites (under vs. between trees). Across all treatments, fire temperatures were highest at 0-50 cm off the ground and hotter in the grass under trees than in the grassy areas between trees. Plots with more trees burned hotter than plots with fewer trees, perhaps because of greater fine woody debris. Plots grazed by wildlife and by cattle prior to burning had lower herbaceous fuel loads and experienced lower burn temperatures than ungrazed plots. Many trees lost their ant colonies during the burns. Ant survivorship differed by ant species and at the plot level was positively associated with previous herbivory (and lower fire temperatures). Across all treatments, ant colonies on taller trees were more likely to survive, but even some of the tallest trees lost their ant colonies. Our study marks a significant step in understanding the mechanisms that underlie the interactions between fire and herbivory in savanna ecosystems.

Lessons on the relationship between livestock husbandry and biodiversity from the Kenya Long-term Exclosure Experiment (KLEE)

Although livestock and wildlife share most of their ranges worldwide, little controlled experimental research has been done on their interactions. Since 1995 we have been manipulating the presence of cattle and large wild ungulates in a Kenyan savanna rangeland in order to better understand the nature of competition and coexistence between these two guilds of herbivores and how they affect biodiversity. In a replicated experiment in which different combinations of cattle and wild herbivores are allowed access to large-scale plots, we have been monitoring the impacts of these herbivores on vegetation, on the wild herbivores, and cattle themselves, and on a variety of other taxa. We have also been conducting experimental research to examine other ways in which livestock management in eastern Africa might affect biodiversity. These include studies on the impacts of fire, livestock corrals, and changes in tree density. This research has revealed the following patterns. (1) Cattle suppress many species of wild herbivores, presumably through competition for their shared resources. The nature of this competition, however, is contingent on rainfall and the presence of other herbivores. (2) Wild herbivores both compete with and facilitate cattle, depending on rainfall. (3) The pastoral practice of housing livestock nightly in protective corral enclosures (“bomas”) over time produces long-lived nutrient hotspots preferred by both livestock and wild herbivores. (4) Fire, frequently used by pastoralists in the past, is valuable for improving grass quality, with benefits for many species of wild herbivores. (5) Pastoral practices that reduce woody cover, including burning and boma construction, create local habitat patches that are preferred by wild herbivores, apparently for their greater anti-predator visibility. (6) Despite competition between livestock and wild herbivores, coexistence between these two guilds can be managed, and there are several positive (facilitative) pathways between livestock husbandry and wild herbivores and other biodiversity.

Influence of cattle on browsing and grazing wildlife varies with rainfall and presence of megaherbivores.

In many savanna ecosystems worldwide, livestock share the landscape and its resources with wildlife. The nature of interactions between livestock and wildlife is a subject of considerable interest and speculation, yet little controlled experimental research has been carried out. Since 1995, we have been manipulating the presence and absence of cattle and large mammalian herbivore wildlife in a Kenyan savanna in order to better understand how different herbivore guilds influence habitat use by specific wildlife species. Using dung counts as a relative assay of herbivore use of the different experimental plots, we found that cattle had a range of effects, mostly negative, on common mesoherbivore species, including both grazers and mixed feeders, but did not have significant effects on megaherbivores. The effect of cattle on most of the mesoherbivore species was contingent on both the presence of megaherbivores and rainfall. In the absence of megaherbivores, wild mesoherbivore dung density was 36% lower in plots that they shared with cattle than in plots they used exclusively, whereas in the presence of megaherbivores, wild mesoherbivore dung density was only 9% lower in plots shared with cattle than plots used exclusively. Cattle appeared to have a positive effect on habitat use by zebra (a grazer) and steinbuck (a browser) during wetter periods of the year but a negative effect during drier periods. Plots to which cattle had access had lower grass and forb cover than plots from which they were excluded, while plots to which megaherbivores had access had more grass cover but less forb cover. Grass cover was positively correlated with zebra and oryx dung density while forb cover was positively correlated with eland dung density. Overall these results suggest that interactions between livestock and wildlife are contingent on rainfall and herbivore assemblage and represent a more richly nuanced set of interactions than the longstanding assertion that cattle simply compete with (grazing) wildlife. Specifically, rainfall and megaherbivores seemed to moderate the negative effects of cattle on some mesoherbivore species. Even if cattle tend to reduce wildlife use of the landscape, managing simultaneously for livestock production (at moderate levels) and biodiversity conservation is possible.

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

African savannas support an iconic fauna, but they are undergoing large‐scale population declines and extinctions of large (>5 kg) mammals. Long‐term, controlled, replicated experiments that explore the consequences of this defaunation (and its replacement with livestock) are rare. The Mpala Research Centre in Laikipia County, Kenya, hosts three such experiments, spanning two adjacent ecosystems and environmental gradients within them: the Kenya Long‐Term Exclosure Experiment (KLEE; since 1995), the Glade Legacies and Defaunation Experiment (GLADE; since 1999), and the Ungulate Herbivory Under Rainfall Uncertainty experiment (UHURU; since 2008). Common themes unifying these experiments are (1) evidence of profound effects of large mammalian herbivores on herbaceous and woody plant communities; (2) competition and compensation across herbivore guilds, including rodents; and (3) trophic cascades and other indirect effects. We synthesize findings from the past two decades to highlight generalities and idiosyncrasies among these experiments, and highlight six lessons that we believe are pertinent for conservation. The removal of large mammalian herbivores has dramatic effects on the ecology of these ecosystems; their ability to rebound from these changes (after possible refaunation) remains unexplored.

Spatial scales influence long-term response of herbivores to prescribed burning in a savanna ecosystem

Both wild and prescribed fire in savanna ecosystems influence habitat use by herbivores by creating or maintaining spatial and temporal heterogeneity in forage quality and vegetation cover. Yet little is known about how spatial scales influence long-term persistence of fire effects. We examined changes over a 6-year period in herbivore preference for experimentally burned patches that varied in spatial extent and grain. Avoidance for the burns by elephants and preference for the burns by impala and Grant’s gazelle decreased significantly. For the rest of the species (zebra, eland, oryx, hartebeest, warthog and hare), there were no significant changes in preference for the burns. Changes in preference for the burned areas depended on the spatial extent and grain of the burn, with intermediate-size (9-ha) burns and large (8-ha) patchy burns being more preferred 6–7 years after fire. Grain, but not the spatial extent of the burned area, influenced changes in grass height. Fire resulted in a delayed reduced tree density irrespective of the spatial scale of the burn. Results of this study indicate that, depending on the scale of fire prescription, the impacts of fire on herbivores may last longer than previous studies suggest.

Relationships Between Cattle and Biodiversity in Multiuse Landscape Revealed by Kenya Long-Term Exclosure Experiment

ABSTRACT On rangelands worldwide, cattle interact with many forms of biodiversity, most obviously with vegetation and other large herbivores. Since 1995, we have been manipulating the presence of cattle, mesoherbivores, and megaherbivores (elephants and giraffes) in a series of eighteen 4-ha (10-acre) plots at the Kenya Long-term Exclosure Experiment. We recently (2013) crossed these treatments with small-scale controlled burns. These replicated experimental treatments simulate different land management practices. We seek to disentangle the complex relationships between livestock and biodiversity in a biome where worldwide, uneasy coexistence is the norm. Here, we synthesize more than 20 yr of data to address three central questions about the potentially unique role of cattle in savanna ecology: 1) To what extent do cattle and wild herbivores compete with or facilitate each other? 2) Are the effects of cattle on vegetation similar to those of wildlife, or do cattle have unique effects? 3) What effects do cattle and commercial cattle management have on other savanna organisms? We found that 1) Cattle compete at least as strongly with browsers as grazers, and wildlife compete with cattle, although these negative effects are mitigated by cryptic herbivores (rodents), rainfall, fire, and elephants. 2) Cattle effects on herbaceous vegetation (composition, productivity) are similar to those of the rich mixture of ungulates they replace, differing mainly due to the greater densities of cattle. In contrast, cattle, wild mesoherbivores, and megaherbivores have strongly guild-specific effects on woody vegetation. 3) Both cattle and wild ungulates regulate cascades to other consumers, notably termites, rodents, and disease vectors (ticks and fleas) and pathogens. Overall, cattle management, at moderate stocking densities, can be compatible with the maintenance of considerable native biodiversity, although reducing livestock to these densities in African rangelands is a major challenge.