The impact of livestock grazing on plant diversity in drylands: an analysis across biomes and scales in southern Africa

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1 Biodiversity, Evolution and Ecology of Plants (BEE) Biocentre Klein Flottbek and Botanical Garden University of Hamburg Arid Zone Ecology Forum 2013 Kimberley 2 5 September 2013 The impact of livestock grazing on plant diversity in drylands: an analysis across biomes and scales in southern Africa Wiebke Hanke, Jürgen Böhner, Niels Dreber, Norbert Jürgens, Ute Schmiedel, Dirk Wesuls & Jürgen Dengler Ute.Schmiedel@uni hamburg.de w.hanke@biota africa.org (Hanke et al. re-submitted to Ecological Applications)

2 Background Plant diversity vs. grazing: theory Grazing as a disturbance Removal of biomass, Trampling, Excrements Intermediate disturbance hypothesis IDH (Grime 1973; Connell 1978) Maximum diversity at intermediate disturbance Diversity Disturbance Diversity Disturbance Diversity Disturbance Dynamic equilibrium model DEM (Huston 1979; Kondoh 2001) Location of diversity peak depends on productivity of the system Grazing should increase diversity in highly productive habitats Grazing should decrease diversity in low productive habitats

3 Background Plant diversity vs. grazing: theory Milchunas, Sala & Lauenroth (1988; Am. Nat. 132: ) MSL Also grazing history of an ecosystem should play a role The longer the evolutionary history of grazing, the weaker the response of the plant diversity

4 Background Empirical proofs? Thousands of studies of grazing intensity effects on plant diversity (reviews Olff & Ritchie 1998; Mackey & Currie 2001; Cingolani et al. 2005) Any possible response type occurs (positive, negative, unimodal, u-shaped, none) Overall responses appear to be idiosyncratic and inconclusive with regard to the theoretical models Why this idiosyncrasy? Axes (disturbance, productivity, evolutionary history) of the conceptual models (IDH, DEM, MSL) do not contain a clear scaling Models generally refer just to biodiversity and ignore the multiple facets of biodiversity (alpha/beta/gamma; species/phylogenetic/ functional; different measures) Comparisons mostly ignore scale-dependence of any diversity measure (spatial scale; temporal scale) Olff & Ritchie 1998, Trends Ecol. Evol. 13: ; Mackey & Currie 2001, Ecology 82: ; Cingolani et al. 2005: Ecol. Appl. 15:

5 Background For a general understanding of grazing effects, we need: Standardised sampling across ecosystems/biomes instead of compilation of local studies conducted with different methods Combined analysis of various aspects of biodiversity Such studies are cost- and time-intensive and therefore largely inexistent We used the data of the BIOTA Southern Africa project One decade of interdisciplinary biodiversity research on subcontinental transects (funded by German Federal Ministry of Education and Research, BMBF) Published 2010 in a 3-volume, 1,400-page book series

6 Study area Winter rain Thornshrub savanna Succulent Karoo Nama Karoo Other biomes BIOTA transect Karte: (from Jürgens et al. 2010)

7 37 standardised BIOTA Observatories (1 km x 1 km) See: Jürgens et al. 2012: The BIOTA Biodiversity Observatories in Africa A standardized framework for largescale environmental monitoring. Environ. Monit. Assess. 184:

8 Methods: 3 pairs of BIOTA Observatories with fenceline contrasts Northern Nama Karoo (NNK) 290 mm Central Nama Karoo (CNK) 150 mm Winter rain Succulent Karoo (SK) 250 mm Thornbush savanna Succulent Karoo Nama Karoo (from Jürgens et al. 2010) Other biomes BIOTA transect

9 Nama Karoo (northern) Nama Karoo (central) Succulent Karoo Vegetation unit Highland savanna Dwarf shrub savanna Namaqualand blomveld Topography Slightly undulating Slightly undulating Rocky hills, sandy valleys Dominant soil group Calcisols Regosols Leptosols Annual rainfall (mm) Aridity index (UNEP) 0.20 (semi-arid) 0.10 (arid) 0.12 (arid) Rainfall season Summer Summer Winter Contrasting management since 1980s 1980s 1950s Recomm. stocking rate (ha/ssu) BIOTA Observatory Narais / Duruchaus Gellap Ost / Nabaos Remhoog. / Paulshoek Actual stocking rate (ha/ssu) > Grazing intensity lighter heavier lighter heavier lighter heavier Grazing regime rotational continuous rotational continuous rotational continuous Dominant kind of livestock cattle, goat sheep sheep, cattle goat, donkey sheep, goat, cattle sheep, Land tenure private private state communal private communal Number of analyzed plots Control for potentially confounding factors (topography, soil): - mostly non-significant; if statistically significant than very small difference goat

10 Studied aspects of plant diversity Metrics Richness vs. evenness Scale Two spatial scales: 100 m² vs m² Organisational level Species vs. functional types alpha vs. beta diversity 5 years of measurement: means vs. SD Vascular plants Trees Shrub Nonsucculent Succulent Tree Woody shrub Woody dwarf shrub Succulent shrub Succulent dwarf shrub Geophyte Perennial Perennial grass Herbaceous Perennial forb All meaningful combinations of metrics, scales & levels Annual Annual grass Annual forb

11 [%%]%]25Results Species composition Northern Nama Karoo (NNK) 290 mm *** 5Winter rain Central Nama Karoo (CNK) 150 mm Succulent Karoo (SK) 250 mm Cover[ ** ](Permutation test)(nmds) **[

12 Results: mean values over 5 years Functional diversity more sensitive than species diversity Abundance-based measures more sensitive than richness-based Alpha diversity decreases, beta diversity increases Little difference between 100 m² and 1000 m²

13 Results: inter annual variability (SD) over 5 years Stability of cover depends on biome Alpha diversity less stable in heavily grazed systems (but weaker effect for species than for functional types and for 1000 m² than for 100 m²)

14 Conclusions Different aspects of biodiversity react differently to grazing pressure Compare only results for the same parameter at the same scale Study a set of different parameters Different rangeland ecosystems react differently in many respects More arid system seems to be more negatively affected by increased grazing pressure (Dynamic equilibrium model) Some diversity parameters react more sensitively than others Functional diversity > species diversity Cover-based metrics > richness Grazing pressure tends to decrease alpha and increase beta diversity Grazing pressure tends to reduce interannual stability of alpha diversity Similar standardised multiscale monitoring programmes over multiple years needed in other biomes to contribute to a global perspective (SASSCAL to continue and extend BIOTA Obs network)

15 Thanks to the farmer communities, the permit authorities in Namibia and South Africa and BIOTA Para-ecologists Thank your for your attention!

16 Non-succulent shrub Succulent dwarf shrub Succulent shrub Tree Shifts in functional groups *** Northern Nama Karoo Cover [%] 5 Annual forb Annual grass Perennial forb Perennial grass Geophyte Non-succulent dwarf shrub 0 Decrease of annual grasses *** * Light grazing Heavy grazing

17 Shifts in functional groups 8 Central Nama Karoo *** 6 4 ** Cover [%] Annual forb Annual grass Perennial forb Perennial grass Geophyte Non-succulent dwarf shrub Non-succulent shrub Succulent dwarf shrub 2 Succulent shrub Tree Decrease of perennial grasses 0 *** **

18 Shifts in functional groups Succulent Karoo Cover [%] 4 2 *** Annual forb Annual grass Perennial forb Perennial grass Geophyte Non-succulent dwarf shrub Non-succulent shrub Succulent dwarf shrub Succulent shrub Tree 0 Decrease of succulent shrubs **