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

AfricaWiebke Hanke, Jürgen Böhner, Niels Dreber, Norbert Jürgens, 

Ute Schmiedel, Dirk Wesuls & Jürgen Dengler

Biodiversity, Evolution and Ecology of Plants (BEE)Biocentre Klein Flottbek and Botanical GardenUniversity of Hamburg

Ute.Schmiedel@uni‐hamburg.dew.hanke@biota‐africa.org

Arid

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(Hanke et al. re-submitted to Ecological Applications)

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

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

Disturbance

Div

ersi

ty

Disturbance

Div

ersi

ty

DisturbanceD

iver

sity

Background

Plant diversity vs. grazing: theory

Milchunas, Sala & Lauenroth (1988; Am. Nat. 132: 87-106) “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

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

Olff & Ritchie 1998, Trends Ecol. Evol. 13: 261-265; Mackey & Currie 2001, Ecology 82: 3479-3492; Cingolani et al. 2005: Ecol. Appl. 15: 757-773

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 diversitymeasure (spatial scale; temporal scale)

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 sub-continental transects (funded by German Federal Ministry of Education and Research, BMBF)

Published 2010 in a 3-volume, 1,400-page book series

http://www.biota-africa.org/biotabook/

Study area

Karte: www.google.de/imgres

Succulent KarooNama Karoo

Thornshrub savanna

BIOTA transect

Other biomes

Winter rain

(from Jürgens et al. 2010)

37 standardised BIOTA Observatories (1 km x 1 km)

See: Jürgens et al. 2012: The BIOTA Biodiversity Observatories in Africa – A standardized framework for large-scale environmental monitoring. Environ. Monit. Assess. 184: 655-678

Succulent KarooNama Karoo

Thornbush savannaBIOTA transectOther biomes

Winter rain

Methods: 3 pairs of BIOTA Observatories with fenceline contrasts

Northern Nama Karoo (NNK)290 mm

Central Nama Karoo (CNK)150 mm

Succulent Karoo (SK)250 mm

(from Jürgens et al. 2010)

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) 289 153 252

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) 2 10 12

BIOTA Observatory Narais / Duruchaus Gellap Ost / Nabaos Remhoog. / Paulshoek

Actual stocking rate (ha/SSU) 3.1 1.8 18 8.7 > 20 11.2

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,

goat

Land tenure private private state communal private communal

Number of analyzed plots 19 20 19 20 18 16

► Control for potentially confounding factors (topography, soil):- mostly non-significant; if statistically significant than very small difference

Studied aspects of plant diversity

Metrics Scale

Species vs. functionaltypes

Two spatial scales: 100 m² vs. 1000 m²

Organisational level

Richness vs. evenness

alpha vs. beta diversity5 years of measurement: 

means vs. SD

Vascular plants

Trees Tree

Shrub

Non‐succulent

Woody shrub

Woody dwarf shrub

SucculentSucculent shrub

Succulent dwarf shrub

Herbaceous

Perennial

Geophyte

Perennial grass

Perennial forb

AnnualAnnual grass

Annual forb► All meaningful combinations of metrics, scales & levels

Results

Winter rain

Northern Nama Karoo (NNK)290 mm

Central Nama Karoo (CNK)150 mm

Succulent Karoo (SK)250 mm

Cover

[%]

0

25

50

0

25

50

0

25

50

***

**

**

[%]

[%]

(Permutation test) (NMDS)

Species composition

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

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 forspecies than for functional types and for 1000 m² than for 100 m²)

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 increasedgrazing 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 betadiversity

• Grazing pressure tends to reduce interannual stability of alphadiversity

► Similar standardised multiscale monitoring programmes overmultiple years needed in other biomes to contribute to a globalperspective (SASSCAL to continue and extend BIOTA Obs network)

Thanks to the farmer communities, the permit authorities in Namibia and South Africa and

BIOTA Para-ecologistsThank your for your attention!

Northern Nama Karoo

Annual forb

Annual grass

Perennial forb

Perennial grass

Geophyte

Non-succulent dwarf shrub

Non-succulent shrub

Succulent dwarf shrub

Succulent shrubTree

Cov

er [%

]

0

5

10

15

20

25

30

***

*

***

Light grazingHeavy grazing

Shifts in functional groups

► Decrease of annual grasses

Central Nama Karoo

Annual forb

Annual grass

Perennial forb

Perennial grass

Geophyte

Non-succulent dwarf shrub

Non-succulent shrub

Succulent dwarf shrub

Succulent shrubTree

Cov

er [%

]

0

2

4

6

8 ***

**

***

**

Shifts in functional groups

► Decrease of perennial grasses

Succulent Karoo

Annual forb

Annual grass

Perennial forb

Perennial grass

Geophyte

Non-succulent dwarf shrub

Non-succulent shrub

Succulent dwarf shrub

Succulent shrubTree

Cov

er [%

]

0

2

4

6

8

10

12

14

***

**

Shifts in functional groups

► Decrease of succulent shrubs