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RangelandecologyII•  Ecologicalprinciplesofgrazingsystems•  Ecologicalrelationships•  Grazingeffects•  Herbivore-plantdynamics•  Systemdynamics•  State-and-transitionmodels

Ben-GurionUniversityoftheNegev

VegetationEcologyCourse2018/19BertrandBoeken

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Ecologicalprinciples•  Grazingsystemsconsistofherbivores,vegetation,landscape

andresources

•  Interactionscanoccuramongallsystemcomponents

•  Interactionsoccuratmanylevelsofecologicalorganization•  Relationshipsdependonspatialandtemporalscales

•  Changesinsystemcomponentsrepresentstatesandtransitions

• Onethingleadstoanother• Somethingscanbechanged,otherscannot

• Controlabilityisquestionable

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Components•  Livestock

–  Density,selectivity,foragingbehaviorandlandscapeutilization•  Vegetation

–  Productivity,structure,composition,palatabilityandnutritionalvalue

•  Landscape–  Heterogeneityofpatchesandpatternsatvariousscales

•  Resources–  Availabilityandsupplyoflight,water,soil,nutrients,organicmatter

Landscape

Resources

HerbivoresVegetation

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Interactions• Vegetationvarieswithlandscapepatchinessandresources

highandlowproductivitypatches

• Patchinessaffectsgrazingpatterndependingongrazingdensityxdurationandforagingdecisions

• Grazingpatternsalterpatchinessandvegetationduetovaryinglevelsofdisturbanceandconsumption;IDH,GOH

• Patchinessandanimalsdetermineresourcedistributionduetoresourceflowsandsource-sinkrelations,anddistributionoffeces/urine/remains

Landscape

Resources

HerbivoresVegetation

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Scalesofinteractions•  Spatial

–  Sites-Patches-Watersheds-Regions-Biomes

Runoffgeneratedoncrust,Interceptedbyshrubs;Leakagefromareadependsonpatchiness(connectivity).

•  Temporal– Seconds--Years--Centuries

Acowbitesoffafewblades;Afterdecadesofgrazing,grasslandcompositionhaschanged.

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Levelsoforganization

•  IndividualsDigestion,behavior,growth,birth,reproduction,death

•  PopulationsColonization,growth,extinction

•  CommunitiesDiversification,impoverishment

•  LandscapesFormation,erosion,degradation

•  EcosystemsResourceenrichment,depletionOrganicmatterdynamics

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Grazingeffects•  Grazinganddiversity(similartotheIntermediateDisturbance

Hypothesis)–  AtlowGI,dominantsareconsumedandsparsespeciesbenefit

•  Fromreleaseofcompetition

•  Forsites,lightand/orsoilmoisture

–  AthighGI,sparsespeciesarealsoconsumed/trampled

•  Effectsofgrazing–  Includetrophicandstructuraleffects–  Varywithproductivity

Speciesrichne

ss

Intensityofgrazing

Dominantspeciesareconsumedmost

Rarespeciesbenefit

Rarespeciesalsonegatively

affected

BajaCalifornia

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GrazingeffectsGrazingandproductivity(GrazingOptimizationHypothesis)

–  AthighGI,biomassremovaldoesnotpermitregrowth–  AtlowGI,biomassremovalencouragesgrowth:

•  Compensatorygrowth–  Removalofdeadmaterial(self-shading)–  Removalofoldorgans(physiologicalsinks)–  Increasedresourcesupply(recycling/importbyurine,feces)–  Salivaryco-factors(But:whynotalreadymaximized?)(McNaughton1976.Amer.Nat.113:691-703)

•  Changesinallocationpatterns(moreleafBM,lessRAandstorage)•  Changesincomposition

–  Morefast-growingspeciesBiom

assprod

uction

Intensityofgrazing

Removalexceedsregrowth

Removalfavorsgrowth

Serengeti

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EquilibriumbetweenvegetationgrowthGandherbivoreconsumptionC

!  Assuminghomogeneousbiomass

!  Immediate,elasticresponse

Herbivore-plantdynamics

dV/dt=G(V)-cH(V)V=vegetationbiomassH=herbivoredensityc=consumptionrate

BiomassV

Growthand

consum

ption

G(V) cH(V)

Asecondlow-Vequilibriumpoint,duetoungrazable

reserve(seeds)

BiomassV

Growthand

consum

ption

G(V) cH(V)Extinctionbelowturningpoint,ifminimalVneededformaintenance

Noy-Meir,I.1975.JournalofEcology63:459-481.

Vincreasesifgrowthexceedsconsumption,Decreasesifmoreconsumedthanproduced

BiomassV

Growthand

consum

ption

G(V) cH(V)

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Chan

ge

Time

•  Transitions–  Sudden-Gradual;Reversible-Irreversible

•  States–  Stable-Transient

•  Positivefeedbackmechanisms–  Astatestrengthensitselfand/orinhibitsanother–  Enhancingatransitionandresistingtheopposite

direction

•  Hysteresis–  Transitionbyforcegreaterthanfeedbackstrength–  Reversaloftransitionrequiresdifferentforce

•  S-Tmodels–  Localandglobalstability–  Catastrophetheory–  vs.successionalandrangequalitymodels

Systemdynamics

State2State1

+ -

- +Force

State2

State1

States

Prob

ability

State3State1 State2

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RangelandstatesGrassland-Shrubland(SWUS,AUS)

•  Perennialgrassesandshrubsexcludeeachother(posFB+mutualinhibition)

•  Grasseshavehighlyediblebiomass,butarenotresistanttograzing(reservedepletion)

•  Shrubsinvadeifgrassdeclines,butarenotpalatable(forcattle)

•  Trendistermeddesertification,althoughnodeclineinproductivity.

Shrubland-Baresoil(OldWorlddryland)•  Noperennialgrasslands(anymore?)

•  Shrub-patchandcrustexcludeeachother(posFB+inhibition)

•  Shrubdeclineduetobrowse,tramplingandclearing

•  Changesinlandscapestates•  Desertification:lossofresourcesandproductivity.

California

NorthernNegev

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!  Regionsofonestablestateandoftwoalternativestates!  Catastrophicbehavior:collapseandslowrecovery

State-and-transitionVe

getation

biomass

Grazingintensity

collapse

recovery

Fc

Fr Grazingintensity

Wateravailability

collapseLowBM

HighBM

HighorLow recovery

Fc

Fr

Grazingcontrol

+EN

-EN

From:HolmgrenandScheffer.2001.Ecosystems4:151–159LockwoodandLockwood.1993.J.RangeManagement46:282-288.

• AtlowGI,high-BMstate• AthighGI,collapsetolow-BMstate• AtmuchlowerGIandtime+,recovery• BMbetweenFcandFr:dependsfromwhere.

• CollapseandrecoveryoccuratdifferentGIvalues,• Dependingonwateravailability(rainfall)• Effectofgrazingcontroldependsonrainfall(ElNiñoyears)

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!  DoS-Tmodelsexplainrangelanddynamics?!  Dotheypredictchangesbetterthanclimaxorrange-quality

models?

State-and-transition

PredictivesuccessofS-Tmodeldependsondefinitionofstates• Vegetationbiomass(continuousecosystemvariable)-seemstowork• Composition(byordination/clustering)-doesnotfitwell:nostablestatesfoundwithhigherprobability

(Allen-Diaz,B.andJ.W.Bartolome.1998.EcologicalApplications8(3):795-804)• Rangequality(byproductivity/sensitivity)-neither• Landscapestatesaremostappropriate(intheoryandpractice).

Climaxandrange-qualitymodels• applicable,iftheirowncriteriaareused• predictstatesbetter(andthuslocallyuseful)• areequallyphenomenologicalbut• theoreticallydubious(reversibility).

State2?State1?

Explanation• Explainsgeneralbehavior• Manysituationswithalternativestablestates• Nomechanismspecified

Climaxmodelsdonotpredictrangelanddegradationanddesertification;TheyrepresenttheoptimisticBalanceofNatureview.

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Desertification•  Desertificationofrangelandresultsfromcombinationsof

–  Climatechange–  Over-grazing–  Clearcutting–  Inappropriateagriculture

•  Definitionsofdesertificationvary–  Culturalandpoliticalbias

–  Politicaloreconomicgoals–  Includecauses(!)(UNESCO,FAO)

–  Ecological,hydrologicaloragriculturalperspectives

Theextentofdesertification

Andtimeisrunning…