Dry matter yield of herbaceous rangeland plants and livemass gain of Tswana steers in eastern Botswana in response to stocking rate and phosphorus supplementation

Tacheba, G.; Mphinyane, W. N.

doi:10.1080/10220119.1993.9638332

Cited by 6 publications

(2 citation statements)

References 10 publications

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“…Be that as it may, the main patterns regarding impacts of livestock on species composition noted by Illius and O'Connor are that grazing is associated with a change from a perennial to an annual sward, often with a high incidence of forbs (e.g. Kelly & Walker, 1976; Le Houérou, 1989; Tacheba & Mphinyane, 1993; O'Connor, 1995), and a decline in ‘palatable’ species (e.g. Abel, 1993).…”

Section: Species Compositionmentioning

confidence: 99%

On non‐equilibrium in arid and semi‐arid grazing systems

Sullivan¹,

Rohde²

2002

Journal of Biogeography

200

148

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A critical comment on A. Illius and T. G. O'Connor (1999) On the relevance of non-equilibrium concepts to arid and semi-arid grazing systems. Ecological Applications, 9, 798-813.A debate in ecology rages over the sources and types of dynamic behaviour driving ecological systems. Drylands have become a particular focus of this debate. In these environments extreme and unpredictable variability in rainfall are considered to confer non-equilibrium dynamics by continually disrupting the tight consumer-resource relations otherwise considered to pull a system towards equilibrium. This implies that livestock grazing in drylands, widely thought to cause degradation and ÔdesertificationÕ through bad management practices leading to overstocking, might not be causing irreversible ecological change through over-use of vegetation. An article recently published in Ecological Applications (Illius & O'Connor, 1999), however, argues that variability in arid and semi-arid grazing systems is not the outcome of qualitatively different dynamical behaviour, and that livestock do cause negative change through ÔnormalÕ density-dependent relations. The authors maintain that these operate primarily in key resource areas and during drought periods.We contest these arguments on several grounds: that key terms are poorly applied in ways which suggest inconsistencies in the internal logic of the arguments; that the paper is unjustifiably selective in the choice and interpretation of ÔevidenceÕ on which it builds; and that the authors do not engage critically with the crucial policy implications of the debate, particularly as they relate to pastoral landuse by African herders in areas under communal tenure and management. We do not suggest that degradation never occurs in arid and semi-arid rangelands, but that Illius and O'Connor's analysis of the mechanisms via which this might take place is misleading and at times, we feel, theoretically bizarre. In particular, we suggest that drought periods may be the times when density-dependent mechanisms are least likely to occur and that key resource areas exist because of ungrazeable reserves which effectively cannot be degraded, although subject to heavy grazing. We also attempt to draw out the theoretical implications of attributing change caused by biotic-abiotic effects to biotic-biotic interactions. We contend that non-equilibrium concepts remain crucial for both natural and social science approaches to understanding dryland environments and their multiple, dynamic uses by pastoralists.Illius and O'Connor's critique of non-equilibrium concepts as a means for understanding ecosystem behaviour in African drylands seeks to address a deepening conceptual fault line separating factions of the rangeland science and livestock development community. Epitomizing one side of this divide are understandings of semi-arid and arid environments in terms of relationships between their biotic components, emphasizing the potential for grazing by domestic livestock to perturb the ÔsystemÕ from a knowable and desi...

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Section: Species Compositionmentioning

confidence: 99%

On non‐equilibrium in arid and semi‐arid grazing systems

Sullivan¹,

Rohde²

2002

Journal of Biogeography

200

148

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“…The loss of perennial grasses was, therefore, a response to the combined influence of drought and the grazing conditions of communal tenure, and not a response to drought alone. Tacheba and Mphinyane (1993) showed that, in the years following a two‐year drought in Botswana, annual grasses and forbs rose from ∼20 to >50% of the herbaceous biomass under intensive stocking, but only increased to 28% under low stocking rates.…”

Section: Do Grazing Animals Have Impacts On Semiarid Systems?mentioning

confidence: 99%

On the Relevance of Nonequilibrium Concepts to Arid and Semiarid Grazing Systems

Illius

O’Connor

1999

Ecological Applications

473

262

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The dynamics of arid and semiarid grazing systems are prone to the effects of highly variable rainfall, with droughts causing frequent episodic mortality in herbivore populations. This has led to the suggestion that they are nonequilibrium systems, in which animal impacts on plants are strongly attenuated or absent. We examine the utility and appropriateness of nonequilibrium concepts for understanding ecosystem processes in African rangeland, attempt to distinguish disequilibrium from nonequilibrium, and argue that such concepts do not justify the view that herbivory has little impact in climatically variable systems. We present evidence for an alternative view of African rangeland function. We argue that, despite the apparent lack of equilibrium, animal numbers are regulated in a density‐dependent manner by the limited forage available in key resource areas, which are utilized in the dry season. This model asserts that strong equilibrial forces exist over a limited part of the system, with the animal population being virtually uncoupled from resources elsewhere in the system. Spatially and temporally, the whole system is heterogeneous in the strength of the forces tending to equilibrium, these diminishing with distance from watering and key resource areas and during the wet season. We argue that wet‐season range is more heavily utilized by animal populations sustained by key resource areas than would apply in the absence of key resources, and that uncoupling of the animal population from vegetation carries an increased risk of degradation. Droughts may impose intense and localized defoliation on vegetation, and this may result in altered species composition, reduced rain‐use efficiency, soil erosion, and loss of productive potential. Rather than ignoring degradation, policy‐makers and ecologists should seek to identify the characteristics of grazing systems that predispose some systems toward degradation, while others appear to be resistant. Development policies should focus on the spatial heterogeneity in susceptibility to grazing impacts and on preserving the productive capacity of key resource areas.

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Use of Landsat Thematic Mapper Data to Assess Seasonal Rangeland Changes in the Southeast Kalahari, Botswana

Ringrose

Musisi-Nkambwe

Coleman

et al. 1999

Environmental Management

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/ Management problems arise in semiarid rangeland that are characterized by marked wet and dry seasons because of forage deficiencies in the dry season. These natural vegetation rangelands can sustain livestock all year long when forage and senesced grass are available into the dry season. Seasonal range condition data are required to provide a basis for pasture management to help locate dry season cover and thereby minimize overstocking and degradation. The generation of seasonal data using Thematic Mapper (TM) imagery was undertaken to assess changes in natural vegetation cover in the southern Botswana Kalahari. Visual analysis of spectral reflectance curves, the development of spectral separability indexes, and conventional classification analysis techniques were used to identify and differentiate rangeland features. Results from reflectance curves indicated that most rangeland cover types could be preferentially distinguished using mainly wet season data, especially on the longer TM wavebands, and that range feature differentiation was more problematic on darker soils than on lighter soils. Spectral separability indexes (SSIs) confirmed that range feature separation varied considerably as a function of waveband and was more effective in the wet than the dry season. The SSIs also showed that range feature differentiation in both seasons was most effective using a combination of the chlorophyll absorpance band (TM3) and two mid-infrared bands (TM5 and TM7). Wet season data were more effectively classified in terms of range features than dry season data although some class similarity was inferred across the two classified data sets. The work shows that overall trends may be generated by comparing seasonal data sets, thereby providing an overall basis for dry season decision making. However, particular problems arise within the dry season data sets probably because of spectral similarities between shadow and darkened vegetation cover, thereby implying that further work is needed. KEY WORDS: Semiarid rangelands; Botswana; Kalahari; Spectral differentiation; Seasonal change; Darkened vegetation cover

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Dry matter yield of herbaceous rangeland plants and livemass gain of Tswana steers in eastern Botswana in response to stocking rate and phosphorus supplementation

Cited by 6 publications

References 10 publications

On non‐equilibrium in arid and semi‐arid grazing systems

On non‐equilibrium in arid and semi‐arid grazing systems

On the Relevance of Nonequilibrium Concepts to Arid and Semiarid Grazing Systems

Use of Landsat Thematic Mapper Data to Assess Seasonal Rangeland Changes in the Southeast Kalahari, Botswana

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