Department of Natural Resources
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Item Synergistic effects of long-term herbivory and previous fire on fine-scale heterogeneity of prescribed grassland burns(Ecology, 2020-10) Kimuyu, Duncan; Werner, Chhaya M.; Veblen, Kari E.; Sensenig, Ryan L.; LaMalfa, Eric; Young, Truman P.Grassland and savanna ecosystems, important for both livelihoods and biodiversity conservation, are strongly affected by ecosystem drivers such as herbivory, fire, and drought. Interactions among fire, herbivores and vegetation produce complex feedbacks in these ecosystems, but these have rarely been studied in the context of fuel continuity and resultant fire heterogeneity. We carried out 36 controlled burns within replicated experimental plots that had allowed differential access by wild and domestic large herbivores since 1995 in a savanna ecosystem in Kenya. Half of these were reburns of plots burned five years previously. We show here that the fine-scale spatial heterogeneity of fire was greater in plots a) previously burned, b) accessible to large herbivores, and especially c) these two in combination. An additional embedded experiment demonstrated that even small experimental burnfree patches can have strong positive effects on tree saplings, which experienced less damage during controlled burns and quicker post-fire recovery. This work highlights the importance of simultaneously examining the interactions between fire and herbivory on fuel heterogeneity, which can have important impacts on the growth of woody saplings in savanna grasslands.Item Tree resprout dynamics following fire depend on herbivory by wild ungulate herbivores(Journal of Ecology, 2019-04) Kimuyu, Duncan M.; LaMalfa, Eric M.; Sensenig, Ryan L.; Young, Truman P.; Riginos, Corinna; Veblen, Kari E.1. Savanna tree cover is dynamic due to disturbances such as fire and herbivory. Frequent fires can limit a key demographic transition from sapling to adult height classes in savanna trees. Saplings may be caught in a ‘fire trap’, wherein individuals repeatedly resprout following fire top-kill events. Saplings only rarely escape the cycle by attaining a fire-resistant height (e.g. taller than the minimum scorch height) during fire-free intervals. 2. Large mammalian herbivores also may trap trees in shorter size classes. Browsing herbivores directly limit sapling height, while grazing herbivores such as cattle facilitate sapling growth indirectly via grass removal. Experimental studies investigating how meso-wildlife, megaherbivores and domestic livestock affect height of resprouts following fire are rare, but necessary for fully understanding how herbivory may reinforce (or counteract) the fire trap. In our study system, interactive fire–herbivore effects on transitions from sapling (<1 m) to adult tree (>1 m) height classes may be further influenced by plant defences, such as symbiotic ants. 3. We used the Kenya Long-term Exclosure Experiment (KLEE) to investigate how post‐fire resprout size of a widespread monodominant East African tree, Acacia drepanolobium was influenced by (a) herbivory by different combinations of cattle, meso-wildlife (15–1,000 kg) and megaherbivores (>1,000 kg) and (b) the presence of acacia–ant mutualists that confer tree defences. We sampled height, stem length and ant occupancy of resprouts exposed to different herbivore combinations before and after controlled burns. 4. Resprout height of saplings that were short prior to fire (<1 m) was reduced primarily by meso-wildlife. Negative effects of elephants on post-fire resprout height increased with pre-fire tree size, suggesting that resprouts of the tallest trees (with the greatest potential to escape the fire trap cycle) were preferentially browsed and reduced in height by elephants. There were no significant cattle effects. 5. Synthesis. We provide experimental evidence for two potential pathways through which large herbivores exert control over sapling escape from the fire trap: (a) post-fire meso-wildlife browsing of short (<1 m) resprouts and (b) elephant browsing of the largest size class of resprouts, which would otherwise be most likely to escape the fire trapItem Fire disturbance disrupts an acacia ant–plant mutualism in favor of a subordinate ant species(Wiley and sons, 2017-03) Kimuyu, Duncan K.; Sensenig, Ryan L.; Guajardo, Juan C. Ruiz; Veblen, Kari E.; Riginos, Corinna; Young, Truman P.Although disturbance theory has been recognized as a useful framework in examining the stability of ant–plant mutualisms, very few studies have examined the effects of fire disturbance on these mutualisms. In myrmecophyte-dominated savannas, fire and herbivory are key drivers that could influence ant–plant mutualisms by causing complete colony mortality and/or decreasing colony size, which potentially could alter dominance hierarchies if subordinate species are more fire resilient. We used a large-scale, replicated fire experiment to examine long-term effects of fire on acacia–ant community composition. To determine if fire shifted ant occupancy from a competitive dominant to a subordinate ant species, we surveyed the acacia–ant community in 6–7 yr old burn sites and examined how the spatial scale of these burns influenced ant community responses. We then used two short-term fire experiments to explore possible mechanisms for the shifts in community patterns observed. Because survival of ant colonies is largelydependent on their ability to detect and escape an approaching fire, we first tested the evacuation response of all four ant species when exposed to smoke (fire signal). Then to better understand how fire and its interaction with large mammal herbivory affect the density of ants per tree, we quantified ant worker density in small prescribed burns within herbivore exclusion plots. We found clear evidence suggesting that fire disturbance favored the subordinate ant Crematogaster nigriceps more than the dominant and strong mutualist ant C. mimosae, whereby C. nigriceps (1) was the only species to occupy a greater proportion of trees in 6–7 yr old burn sites compared to unburned sites, (2) had higher burn/unburn tree ratios with increasing burn size, and (3) evacuated significantly faster than C. mimosae in the presence of smoke. Fire and herbivory had opposite effects on ant density per meter of branch for both C. nigriceps and C. mimosae, with fire decreasing ant densities per meter of branch and the presence of large herbivores increasing ant density. Taken together, these experiments suggest that major ecosystem disturbances like fire can disrupt mutualistic associations and maintain diversity in partner qualityand identitydespite a clear dominance hierarchy.Item Fire disturbance disrupts an acacia ant–plant mutualism in favor of a subordinate ant species(EcologicalSocietyof America, 2017) Kimuyu, Duncan K.; Sensenig, Ryan L.; Guajardo, Juan C. Ruiz; Veblen, Kari E.; Riginos, Corinna; YOUNG, Truman P.Although disturbance theory has been recognized as a useful framework in examining the stability of ant–plant mutualisms, very few studies have examined the effects of fire disturbance on these mutualisms. In myrmecophyte-dominated savannas, fire and herbivory are key drivers that could influence ant–plant mutualisms by causing complete colony mortality and/or decreasing colony size, which potentially could alter dominance hierarchies if subordinate species are more fire resilient. We used a large-scale, replicated fire experiment to examine long-term effects of fire on acacia–ant community composition. To determine if fire shifted ant occupancy from a competitive dominant to a subordinate ant species, we surveyed the acacia–ant community in 6–7 yr old burn sites and examined how the spatial scale of these burns influenced ant community responses. We then used two short-term fire experiments to explore possible mechanisms for the shifts in community patterns observed. Because survival of ant colonies is largelydependent on their ability to detect and escape an approaching fire, we first tested the evacuation response of all four ant species when exposed to smoke (fire signal). Then to better understand how fire and its interaction with large mammal herbivory affect the density of ants per tree, we quantified ant worker density in small prescribed burns within herbivore exclusion plots. We found clear evidence suggesting that fire disturbance favored the subordinate ant Crematogaster nigriceps more than the dominant and strong mutualist ant C. mimosae, whereby C. nigriceps (1) was the only species to occupy a greater proportion of trees in 6–7 yr old burn sites compared to unburned sites, (2) had higher burn/unburn tree ratios with increasing burn size, and (3) evacuated significantly faster than C. mimosae in the presence of smoke. Fire and herbivory had opposite effects on ant density per meter of branch for both C. nigriceps and C. mimosae, with fire decreasing ant densities per meter of branch and the presence of large herbivores increasing ant density. Taken together, these experiments suggest that major ecosystem disturbances like fire can disrupt mutualistic associations and maintain diversity in partner qualityand identitydespite a clear dominance hierarchyItem Spatial scales influence long-term response of herbivores to prescribed burning in a savanna ecosystem(CSIRO PUBLISHING, 2017-04) Kimuyu, Duncan M.; Sensenig, Ryan L.; Chira, Robert M.; Githaiga, John M.; Young, Truman P.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 andpreferencefortheburnsbyimpalaandGrant’sgazelledecreasedsignificantly.Fortherestofthespecies(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.Item Synergistic effects of fire and elephants on arboreal animals in an African savanna(British Ecological Society, 2015) Sensenig, Ryan L.; Pringle, Robert M.; Kimuyu, Duncan M.; Palmer, Todd M.; Corinna Riginos; Veblen, Kari E.; Young, Truman P.