Cayford: Wader disturbance:a theoretical overview

Wader disturbance: a theoretical overview

John Cayford

Cayford, J. T. 1993. Wader disturbance:a theoreticaloverview. Wader Study Group Bull. 68' 3-5.

This paper briefly reviews some work on foraging efficiency,competition and dispersion,which may help predictthe effects of disturbanceto winteringwaders.

John Cayford, Royal Society for the Protection of Birds, The Lodge, Sandy, Bedfordshire SG19 2DL, UK. (Present address: British Trust for Ornithology,The Nunnery, Thetford, Norfolk IP24 2PU, UK.)

INTRODUCTION on disturbance,with the aim of encouragingresearchers Each year hundredsof thousandsof waterfowl to consider disturbance in the context of some current ovenNinteron, or migrate through, the estuaries of theory. north-westEurope (Prater 1981). These estuaries are dynamichabitats subject to intenseanthropogenic This paper deals with disturbanceto waders on their pressureswhich pose increasingthreats to winter feeding grounds. It should be borne in mind that ovenNinteringpopulations of waders. Barrages (to disturbancecan also affect waders duringtheir breeding generate electricity,control tidal surges or provide season and duringtheir migrations,and that the overall leisure facilities) and marina developmentsare impacton populationdynamics of breedingseason proposedfor a number of estuariesand representone disturbanceand/or disturbanceon migrationstaging type of potentialthreat to many wader species through areas could greatly compoundthat duringthe non- habitat loss and reduced feeding opportunities.Other breedingseason. threats include pollution,climatic change and risingsea levels which erode or inundate estuarine habitat (e.g. Burd 1992). Disturbance,particularly that caused by DEFINING DISTURBANCE recreationalactivities, is increasinglyperceived as a threat, especiallysince many recreationalactivities In any field of inquirythere is need for unambiguous appearto be increasingin intensity,coverage or period definitions. Disturbance is, however, a rather nebulous of the year in which they take place. Direct evidence concept which loosely describescausal relationships linkingdisturbance with populationchanges in waders between a wide range of (usually)anthropogenic stimul• is, however, generally lacking. and the responsesthey elicit in animals. Disturbance can be defined operationallyas any relatively discrete Recent research has predicted the effects of event in time that disrupts ecosystems, communities or developmentsand associatedloss of feeding habitat populations,where disruptionrefers to a change in on wader populations(Goss-Custard & Moser 1988; behaviour,physiology, numbers or survival.Disturbance Goss-Custardet al. 1991). Thiswork has provideda varies in its magnitude,frequency, predictability, spatial theoreticalframework for interpretingthe distributionand duration. Moreover, species (and consequencesof loss of feeding habitat on individuals individualswithin species) vary greatly in their and populations.Brief disturbance can resultin birds susceptibilityto disturbanceand this susceptibilityis beingtemporarily deprived of feedinghabitat (see Smit likelyto vary with age, season, weather and the degree & Visser 1993). Sustaineddisturbance can also result of previousexposure (habituation). in long-termloss of feedinghabitat and a reductionin feeding opportunity.In theory,the only difference As a general rule, disturbanceto waders is highly between disturbance and development is that its Iocalised in time and space. For example, recreational effects are usually more Iocalised, temporary and disturbanceis concentratedusually in the upper shore reversible. zones of estuaries and is restrictedmainly to daylight hours, especiallyweekends in summer. It has been In this paper I discussand brieflyreview some of the suggestedthat birds, being highlymobile, are less work on wader dispersion,competition, feeding susceptible to the effects of disturbance than many efficiencyand prey depletionthat is relevant to research other groups of organisms. Cayford: Wader disturbance:a theoretical overview

EFFECTS OF DISTURBANCE ON THE INDIVIDUAL contributesto the disproportionatelyhigh mortalityof young birds, especiallyduring cold weather. One of the consequencesof sustained,Iocalised disturbanceis that waders shift to alternative feeding Sustained levels of disturbance can force birds to sites. A wading bird must feed to meet its immediate change feeding sites and may reduce intake rates. energy requirementsand if disturbancereduces intake However,waders have a phenomenalcapacity to vary rate below a criticalthreshold, it must emigrate or their intake rate in responseto changingenvironmental starve. Dispersionof the populationis simplythe sum of and physiologicaldemands. For example, Swennen the foragingdecisions of all individualswhere natural et aL (1989) have shownexperimentally that captive selection has shaped individualsto maximise the rate at OystercatchersHaematopus ostralegus increase their which they gather energy from food. Understandingthe feeding rates as the time availablefor feeding is factorsthat determinedispersion and limitfeeding reduced. Many wader species are knownto increase densities is key to an understandingof the possible their daily food intake by feeding in fieldsadjacent to consequencesof disturbanceat the level of the estuariesduring high water and feedingat night(Dugan individual. 1981). Such behaviourmight buffer the worst effectsof disturbance. Optimalforaging theory (OFT) is the theoretical framework,which has proved extremelyvaluable in generatingtestable hypothesesabout prey selectionand EFFECTS OF DISTURBANCE ON THE POPULATION dispersionin relationto food supply in waders. Waders must choose what prey species to feed on, whichsize- The maximumnumber of birdsan area can support classes to select and which food patches to exploit. (carryingcapa. city) will be determinedby the densityand availabilityof preferredprey, ratesof preydepletion and Numerousstudies have shown that waders forage the levelof competitionbetween individuals (interference). efficientlyby feeding in the best areas and selectingthe Only where populationsare limited,or are close to most profitablesize-classes of prey (e.g. Cayford & limitation,by the qualityand availabilityof winteringhabitat Goss-Custard1990). The result is that waders generally can disturbanceimpact negatively in such placeson concentratewhere prey density, prey availabilityand waderpopulations by increasingmortality or decreasing intake rates are relativelyhigh and where energy recruitment.A key questionis whetherwader populations expenditure is relatively low (Goss-Custard & Charman are limitedby the availabilityof prey, butthis is a difficult 1976). As overall numbers increase, dens•ies tend to questionto answer. It is relativelyeasy to measurethe reach a maximum on the most preferred feeding areas behaviouralresponses of birdsto disturbance,but it is (Goss-Custard et aL 1982). This pattern of dispersion much more difficultto quantifythe effectthat these approximatesto the 'ideal-free distribution'(Fretwell & changesin behaviourhave on populationdynamics. Lucas 1970) where differences in prey density are Goss-Custard& Moser(1988) have shownfor Dunlin cancelled out by differentialinterference (the immediate Calidrisalpina at least,that the decline in the overall and reversible reductionin intake rate as bird density winteringpopulation in Britainbetween 1983-1986 was a increases)and feeding rates are similarfor all sites. functionof habitatloss resulting from spread of cord-grass Spartina,supporting the view that feedinghabitat is a A more useful constructfor interpretingwader majorlimiting factor for thisspecies and that carrying dispersionis the 'ideal despotic model' (Fretwell & capacityhad been reached.Clearly, more researchis Lucas 1970) which recognisesindividual differences in needed on the carryingcapacity of estuariesand the competitiveability and assumes that individuals extent to whichwaders are limitedby food supplybefore compete for the best sites in an attempt to maximise we can predictthe effectsof disturbanceon populations. intake rates. If disturbance forces birds to move (temporarilyor permanently)the question arises as to whetheralternative feeding areas can accommodate MEASURING DISTURBANCE displacedindividuals and what effect increasedbird densitywill have on intake rates, body conditionand, Most studies have attempted to establishcausality ultimately,the fitnessof those individualswhich move. between disturbanceand dispersionby measuringthe Evidencesuggests that as bird densityincreases, behaviouralresponses of waders to disturbingstimuli. average intake rates decline in some species as a result This approach providesdata on relative levels of of increasedcompetition, increased prey depletionand disturbanceat differentsites and comparative data on a greater proportionof the populationfeeding in the susceptibilityof species to different stimuli.The rate, sub-optimalareas (Goss-Custard1980). However, not predictabilityand severityof disturbingstimuli are, all individualsare affected to the same degree. however, highlyvariable and the effects of disturbance Juveniles and sub-dominant adults are most susceptible are likelyto be additive,so frequencydata are limitedin to the effects of interferenceand this probably their usefulness. Cayford: Wader disturbance:a theoretical overview

As we have seen, quantifyingthe effects of disturbance REFERENCES on populationsis problematicbecause of the difficultyin isolatingkey variables. One way roundthis is to take a Bell, D.V. & Fox, P.J.A. 1991. Shootingdisturbance: An assessment behavioural measure such as feeding rate (which is of its impactand effecton overwinteringwaterfowl populations highlycorrelated with the dependent measure being and theirdistribution in the UnitedKingdom. WWT/BASC Report. sought), and make the assumptionthat a reductionin feeding opportunitymight reduce feeding rates. This Burd, F. 1992. Saltmarshchange in Essex and north Kent. Erosion would affect body conditionand, consequently,survival and vegetationchange on the saltmarshesof Essex and north or productivity(Owen 1993). Kent between1973 and 1988. Research& surveyin nature conservationNo. 42. Nature ConservancyCouncil, Peterborough. 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Towardspredicting wading bird densitiesfrom pre- One suggestionis that experimentalfield manipulations dictedprey densitiesin a post-barrageSevern estuary. J. Applied Ecol. 28:1004-1026. may offer an alternativemethod for establishinglinks between disturbance,dispersion and population Owen, M. 1993. The UK Shootingand WildfowlDisturbance Project. dynamicsin waders. This approachhas the advantage Wader Study Group Bull:68: 35-46. that species can be targeted and confoundingvariables Prater, A.J. 1981. Estuary Birds of Britainand Ireland. T. & A.D. such as habituation,season, time, temperatureand Poyser, Carlton. even individualvariation can be experimentallyor statisticallycontrolled. More importantly,data collection Smit, C.J. & Visser, G.J.M. 1993. Effects of disturbance on shoreb•rds. is not constrainedby the infrequencyor unpredictability a summaryof existingknowledge from the DutchWadden Sea and Delta area. 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