Relative Impacts of Elephant and Fire on Large Trees in a Savanna Ecosystem

Shannon, Graeme; Thaker, Maria; Vanak, Abi Tamim; Page, Bruce R.; Grant, Rina; Slotow, Rob

doi:10.1007/s10021-011-9485-z

Cited by 80 publications

(119 citation statements)

References 44 publications

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“…The majority of research has been focused on assessing the recruitment and distribution of height classes of S. birrea due to its commercial usage, but it can be assumed that similar height class distributions will be present for A. nigrescens. Both species have been shown to be experiencing losses of individuals in large height classes within protected areas in which elephant are present (Moncrieff et al 2008, Helm et al 2009, Shannon et al 2011. Focusing management strategies and actions on areas which are conducive to the growth of large trees of each species will be important to maintain populations of each species in reserves (Helm and Witkowski 2012).…”

Section: Discussionmentioning

confidence: 99%

Using Maximum Entropy modeling to predict the potential distributions of large trees for conservation planning

et al. 2012

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Citation: Smith, A., B. Page, K. Duffy, and R. Slotow. 2012. Using Maximum Entropy modeling to predict the potential distributions of large trees for conservation planning. Ecosphere 3(6):56. http://dx.doi.org/10.1890/ES12-00053.1Abstract. Large trees, as keystone structures, are functionally important in savanna ecosystems, and low recruitment and slow growth makes their conservation important. Understanding factors influencing their distribution is essential for mitigation of excessive mortality, for example from management fires or large herbivores. We recorded the locations of large trees in Hluhluwe-Imfolozi Park (HiP) using GPS to record trees along 43 km of 10 m-wide transects. Maximum entropy modeling (MaxEnt) uses niche modeling to predict the distribution of a species from the probability of finding it within raster squares, based on environmental variables and recorded locations. MaxEnt is typically applied at a regional spatial scale, and here we assessed its usefulness when predicting the distribution of species at a small (local) scale. HiP has variable topography, heterogeneous soils, and a strong rainfall gradient, resulting in a wide variety of habitat types. We used locations of 179 Acacia nigrescens and 106 Sclerocarya birrea (large trees ! 5m), and raster environmental layers for: aspect, elevation, geology, annual rainfall, slope, soil and vegetation. A. nigrescens was largely restricted to the Imfolozi section, while S. birrea had a wider distribution across the reserve. Understanding the interaction of environmental variables dictating tree distribution may facilitate habitat restoration, and will assist planning decisions for persistence of large trees within reserves, including options to reduce fire frequency or herbivore impacts. Though the AUC (Area Under the Curve) values used to test model predictions were high for both species, the ground truthing test data showed that distribution for A. nigrescens was more accurate than that for S. birrea, highlighting the need for independent test data to assess model accuracy. We emphasize that MaxEnt can be used at finer spatial scales than those typically used for species occurrence, but models must be tested using spatially independent test data.

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Section: Discussionmentioning

confidence: 99%

Using Maximum Entropy modeling to predict the potential distributions of large trees for conservation planning

et al. 2012

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“…Structural responses of woody canopy to different fire frequencies as a function of vegetation type (Acacia vs. Combretum) and catena hillslope position. As such, the effects of fire on vegetation structure are inextricably linked with the actions of herbivores, both grazers and browsers, and it is the interactive effects of fire and herbivory that provide top-down control on savanna vegetation structure (Staver et al 2009, Midgley et al 2010, Shannon et al 2011. herbaceous biomass to a level that lowers fire intensity (Briggs et al 2002).…”

Section: Fine-scale Effects Of Environment On Vegetation Structural Rmentioning

confidence: 99%

Spatial patterns in the effects of fire on savanna vegetation three‐dimensional structure

Levick

Asner

Smit

2012

Ecological Applications

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Spatial variability in the effects of fire on savanna vegetation structure is seldom considered in ecology, despite the inherent heterogeneity of savanna landscapes. Much has been learned about the effects of fire on vegetation structure from long-term field experiments, but these are often of limited spatial extent and do not encompass different hillslope catena elements. We mapped vegetation three-dimensional (3-D) structure over 21 000 ha in nine savanna landscapes (six on granite, three on basalt), each with contrasting long-term fire histories (higher and lower fire frequency), as defined from a combination of satellite imagery and 67 years of management records. Higher fire frequency areas contained less woody canopy cover than their lower fire frequency counterparts in all landscapes, and woody cover reduction increased linearly with increasing difference in fire frequency (r2 = 0.58, P = 0.004). Vegetation height displayed a more heterogeneous response to difference in fire frequency, with taller canopies present in the higher fire frequency areas of the wetter sites. Vegetation 3-D structural differences between areas of higher and lower fire frequency differed between geological substrates and varied spatially across hillslopes. Fire had the greatest relative impact on vegetation structure on nutrient-rich basalt substrates, and it imparted different structural responses upon vegetation in upland, midslope, and lowland topographic positions. These results highlight the complexity of fire vegetation relationships in savanna systems, and they suggest that underlying landscape heterogeneity needs more explicit incorporation into fire management policies.

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“…; Sensenig, Demment & Laca ) to individual trees (Shannon et al . ). Conversely, grazing and browsing can alter the extent and intensity of fire by affecting fuel load and composition (Van Langevelde et al .…”

Section: Introductionmentioning

confidence: 97%

Synergistic effects of fire and elephants on arboreal animals in an African savanna

Pringle

Kimuyu

Sensenig

et al. 2015

Journal of Animal Ecology

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Disturbance is a crucial determinant of animal abundance, distribution and community structure in many ecosystems, but the ways in which multiple disturbance types interact remain poorly understood. The effects of multiple-disturbance interactions can be additive, subadditive or super-additive (synergistic). Synergistic effects in particular can accelerate ecological change; thus, characterizing such synergies, the conditions under which they arise, and how long they persist has been identified as a major goal of ecology. We factorially manipulated two principal sources of disturbance in African savannas, fire and elephants, and measured their independent and interactive effects on the numerically dominant vertebrate (the arboreal gekkonid lizard Lygodactylus keniensis) and invertebrate (a guild of symbiotic Acacia ants) animal species in a semi-arid Kenyan savanna. Elephant exclusion alone (minus fire) had negligible effects on gecko density. Fire alone (minus elephants) had negligible effects on gecko density after 4 months, but increased gecko density twofold after 16 months, likely because the decay of fire-damaged woody biomass created refuges and nest sites for geckos. In the presence of elephants, fire increased gecko density nearly threefold within 4 months of the experimental burn; this occurred because fire increased the incidence of elephant damage to trees, which in turn improved microhabitat quality for geckos. However, this synergistic positive effect of fire and elephants attenuated over the ensuing year, such that only the main effect of fire was evident after 16 months. Fire also altered the structure of symbiotic plant-ant assemblages occupying the dominant tree species (Acacia drepanolobium); this influenced gecko habitat selection but did not explain the synergistic effect of fire and elephants. However, fire-driven shifts in plant-ant occupancy may have indirectly mediated this effect by increasing trees' susceptibility to elephant damage. Our findings confirm the importance of fire × elephant interactions in structuring arboreal wildlife populations. Where habitat modification by megaherbivores facilitates co-occurring species, fire may amplify these effects in the short term by increasing the frequency or intensity of herbivory, leading to synergy. In the longer term, tree mortality due to both top kill by fire and toppling by large herbivores may reduce overall microhabitat availability, eliminating the synergy.

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Relative Impacts of Elephant and Fire on Large Trees in a Savanna Ecosystem

Cited by 80 publications

References 44 publications

Using Maximum Entropy modeling to predict the potential distributions of large trees for conservation planning

Using Maximum Entropy modeling to predict the potential distributions of large trees for conservation planning

Spatial patterns in the effects of fire on savanna vegetation three‐dimensional structure

Synergistic effects of fire and elephants on arboreal animals in an African savanna

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