Overgrazing and bush encroachment by Tarchonanthus camphoratus in a semi‐arid savanna

Coetzee, Bernard W. T.; Tincani, Lucrezia; Wodu, Zelalem; Mwasi, Shem M.

doi:10.1111/j.1365-2028.2007.00842.x

Cited by 44 publications

(37 citation statements)

References 5 publications

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“…2008). Encroachment appears to result from any of a number of distinct factors or interactions of multiple factors including overgrazing and recovery from anthropogenic disturbance from earlier (traditional) societies (Scholes & Archer 1997; Angassa & Oba 2007; Coetzee et al. 2008), increases in CO 2 and N deposition (Archer 2010), reduced fire frequency (Scholes & Archer 1997), long‐term climate change (Knapp et al.…”

Section: Introductionmentioning

confidence: 99%

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Impacts of shrub encroachment on ecosystem structure and functioning: towards a global synthesis

et al. 2011

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Encroachment of woody plants into grasslands has generated considerable interest among ecologists. Syntheses of encroachment effects on ecosystem processes have been limited in extent and confined largely to pastoral land uses or particular geographical regions. We used univariate analyses, meta-analysis and structural equation modelling to test the propositions that (1) shrub encroachment does not necessarily lead to declines in ecosystem functions and (2) shrub traits influence the functional outcome of encroachment. Analyses of 43 ecosystem attributes from 244 case studies worldwide showed that some attributes consistently increased with encroachment (e.g. soil C, N), and others declined (e.g. grass cover, pH), but most exhibited variable responses. Traits of shrubs were associated with significant, though weak, structural and functional outcomes of encroachment. Our review revealed that encroachment had mixed effects on ecosystem structure and functioning at global scales, and that shrub traits influence the functional outcome of encroachment. Thus, a simple designation of encroachment as a process leading to functionally, structurally or contextually degraded ecosystems is not supported by a critical analysis of existing literature. Our results highlight that the commonly established link between shrub encroachment and degradation is not universal.

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Section: Introductionmentioning

confidence: 99%

“…The overgrazing hypothesis is based on the premise that sustained heavy grazing reduces above‐ and belowground grass biomass, leading to increased resource availability for the establishment of shrubs, greater shrub recruitment (Coetzee et al. 2008) and therefore reduced fire frequency and intensity (Scholes & Archer 1997; Oba et al.…”

Section: Introductionmentioning

confidence: 99%

Impacts of shrub encroachment on ecosystem structure and functioning: towards a global synthesis

et al. 2011

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“…Specifically, over‐grazing has been shown to cause replacement of palatable grasses with unpalatable grasses and/or woody species (e.g., Coetzee et al. ). However, few studies have documented consequences of the addition of moderate numbers of livestock to systems where these animals likely replace (either completely or partially) a diverse wild herbivore assemblage.…”

Section: Introductionmentioning

confidence: 99%

Are cattle surrogate wildlife? Savanna plant community composition explained by total herbivory more than herbivore type

Veblen

Porensky

Riginos

et al. 2016

Ecological Applications

130

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The widespread replacement of wild ungulate herbivores by domestic livestock in African savannas is composed of two interrelated phenomena: (1) loss or reduction in numbers of individual wildlife species or guilds and (2) addition of livestock to the system. Each can have important implications for plant community dynamics. Yet very few studies have experimentally addressed the individual, combined, and potentially interactive effects of wild vs. domestic herbivore species on herbaceous plant communities within a single system. Additionally, there is little information about whether, and in which contexts, livestock might functionally replace native herbivore wildlife or, alternatively, have fundamentally different effects on plant species composition. The Kenya Long-term Exclosure Experiment, which has been running since 1995, is composed of six treatment combinations of mega-herbivores, meso-herbivore ungulate wildlife, and cattle. We sampled herbaceous vegetation 25 times between 1999 and 2013. We used partial redundancy analysis and linear mixed models to assess effects of herbivore treatments on overall plant community composition and key plant species. Plant communities in the six different herbivore treatments shifted directionally over time and diverged from each other substantially by 2013. Plant community composition was strongly related (R = 0.92) to residual plant biomass, a measure of herbivore utilization. Addition of any single herbivore type (cattle, wildlife, or mega-herbivores) caused a shift in plant community composition that was proportional to its removal of plant biomass. These results suggest that overall herbivory pressure, rather than herbivore type or complex interactions among different herbivore types, was the main driver of changes in plant community composition. Individual plant species, however, did respond most strongly to either wild ungulates or cattle. Although these results suggest considerable functional similarity between a suite of native wild herbivores (which included grazers, browsers, and mixed feeders) and cattle (mostly grazers) with respect to understory plant community composition, responses of individual plant species demonstrate that at the plant-population-level impacts of a single livestock species are not functionally identical to those of a diverse group of native herbivores.

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“…High grazing pressure may also promote shrub invasion because it opens spaces for colonisation allowing net recruitment into the grazing land (Coetzee et al, 2008;Ens & French, 2008;Kraaij & Ward, 2006). High abundance of invasive shrubs at low-elevation landscapes was reported by several researchers worldwide (Averett et al, 2016;Pauchard & Alaback, 2004;Pollnac et al, 2012).…”

Section: Discussionmentioning

confidence: 98%

“…On the other hand, the dominance of poorly palatable species, such as E. plana and S. africanus , and the low occurrence of highly palatable species (e.g., T. triandra ) in the BLs, and in areas with moderate to heavy invasion, are possibly the result of the changes in ecological processes that arise from disturbance such as overgrazing and shrub invasion (Kakembo et al, ). Shrub invasion may accelerate the decline in the abundance of desirable grass species because (a) invasive shrub seedlings may strongly compete with growing grasses for moisture and nutrients in the upper soil layers (Tefera, Dlamini, & Dlamini, ); (b) competition from the invasive species may depress grass seed germination from the seed bank and/or grass seedling establishment; (c) shrub invasion may create space (Coetzee et al, ; Ens & French, ; Kraaij & Ward, ) for other less desirable herbaceous species, adapted to a stressed environment, to increase and apply additional competitive pressure on desirable species; and (d) invasive shrubs may influence the survival of desirable grass species through shading and allelopathic effects (Cipollini, Titus, & Wagner, ).…”

Section: Discussionmentioning

confidence: 99%

Invasion of Euryops floribundus in degraded South African communal grassland: Unpacking the invasion relationship with elevation, soil properties, the quality and quantity of the herbaceous layer

Gxasheka

Tefera

Mota³

2019

Afr J Ecol

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Shrub invasion is a recent occurrence in African grassland ecosystems. This study assessed the distribution of Euryops floribundus (a native shrub species) along an elevation and invasion intensity gradients, and its relationship with soil and herbaceous vegetation in a South African grassland. Shrub density increased (p < 0.05) from the uplands (2,301 plants/ha) to the bottomlands (4,888 plants/ha). Themeda triandra (a highly palatable grass) was dominant at the uplands and sloping sites, whereas Eragrostis plana and Sporobolus africanus (poorly palatable grasses) dominated the bottomlands. Upland soils had the highest organic carbon (OC), phosphorous (P), calcium and nitrogen (N) contents, but the lowest shrub density and herbaceous biomass. Canonical correspondence analysis indicated that elevation, soil OC and N contents determine important vegetation variables along the elevation gradient. In the shrub density gradient, soil P, OC and N responded positively to E. floribundus density. Grass biomass in the moderate and heavily invaded sites was high, but the abundance of palatable grasses declined, suggesting that E. floribundus invasion may be concomitant with a decrease in rangeland condition in terms of quality forage provision.

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Overgrazing and bush encroachment by Tarchonanthus camphoratus in a semi‐arid savanna

Cited by 44 publications

References 5 publications

Impacts of shrub encroachment on ecosystem structure and functioning: towards a global synthesis

Impacts of shrub encroachment on ecosystem structure and functioning: towards a global synthesis

Are cattle surrogate wildlife? Savanna plant community composition explained by total herbivory more than herbivore type

Invasion of Euryops floribundus in degraded South African communal grassland: Unpacking the invasion relationship with elevation, soil properties, the quality and quantity of the herbaceous layer

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