Goss-Custard, John

McGrorty, Selwyn

Reading, Chris

English

NERC Open Research Archive

•• AáInstitute of Terrestrial EcologyNATURAL ENVIRONMENT RESEARCH COUNCILOystercatchers andShellfish:Predator/PreyStudiesJohn Goss-CustardSelwyn McGrorty and Chris ReadingInstitute of Terrestrial EcologyFurzebrook Research StationWarehamPrinted in England byHeffers Printers Ltd, Cambridge, England© Copyright 1977Published in 1977 byInstitute of Terrestrial Ecology68 Hills Road, Cambridge CB2 1LAISBN 0 904282 12 0Dr J. D. Goss-CustardInstitute of Terrestrial EcologyFurzebrook Research StationWarehamDorset BH20 5AS092 93 (Corfe Castle) 361-2The Institute of Terrestrial Ecology (ITE)  was established in 1973, fromthe former Nature Conservancy's research stations and staff, joined laterby the Institute of Tree Biology and the Culture Centre of Algae andProtozoa. ITE contributes to and draws upon the collective knowledgeof the fourteen sister institutes which make up the  Natural EnvironmentResearch Council,  spanning all the environmental sciences.The Institute studies the factors determining the structure, compositionand processes of land and freshwater systems, and of individual plant andanimal species. It is developing a sounder scientific basis for predictingand modelling environmental trends arising from natural or man-madechange. The results of this research are available to those responsible forthe protection, management and wise use of our natural resources.Nearly half of ITE's work is research commissioned by customers, such asthe Nature Conservancy Council who require information for wildlifeconservation, the Forestry Commission and the Department of theEnvironment. The remainder is fundamental research supported byNERC.ITE's expertise is widely used by international organisations in overseasprojects and programmes of research.Cover photographs reproduced by kind permission of Mr. J. B. Bottomley(Oystercatcher — top), and Mr. P. Ainsworth (Mussels — bottom).Predator/Prey Studies onOystercatchers and ShellfishThis leaflet is one of a series describing research which iscurrently being done by the Institute of Terrestrial Ecology(ITE), a component body of the Natural EnvironmentResearch Council. Each leaflet describes a particular aspect ofITE's research and indicates the nature of the problem, themethods used in the research, and possible practicalapplications of the results.In 1976, research was started on the role of the oystercatcherin determining the abundance of the mussel, a commonintertidal shellfish. There were two main reasons for under-taking this research. First, we wished to study a fundamentalecological process, namely the interaction between a predatorand its prey. Second, we wanted to improve our understandingof the impact which these wading birds have on commercialstocks of shellfish, when compared with other factors such asweather or predation by fish. This impact has arousedconsiderable controversy in recent years so the research hasan immediate and practical value as well as adding to ourknowledge of predator/prey relationships generally.The interactions between a predator and its prey are complex,but they can be subdivided into a number of simplerrelationships for the purposes of analysis. For example, thefeeding rate of a predator in relation to the density of its preyis an important part of the overall predator/prey interactionand one form the relationship may take is shown in Figure 1.In the past, ecologists have been content to describe suchrelationships qualitatively rather than quantitatively. Morerecently, attempts have been m6de to describe these relation-ships quantitatively through the use of mathematical models.Once the validity of a model has been tested, the role ofvarious factors, singly and in combination, may be explored bychanging the parameters of the model. It should also bepossible to use the same model to make more precisepredictions of the effects of human impact on naturalcommunities.Collecting data for the construction of models is oftendifficult. Basically, there are two steps. First, the relationships12Number of prey per square metreFigure 1. The number of prey taken per unit time by a hypotheticalpredator at different prey densities.likely to be important in the functioning of the system must beidentified. These relationships can be derived from previousstudies of the species concerned, from experimental tests ofdirect effects and from the biological intuition of the individualresearch worker. Second, these identified relationships needto be described quantitatively over as wide a range ofvariation as possible. Careful design and execution of theways in which data are collected are necessary, so that themathematical expressions of the relationships can be made asprecise as possible. While progress has been made for awhole range of species in achieving the first step, the secondstep may be impossible unless the systems can be studiedwithin the limits of existing field equipment and techniques.Investigations made on estuarine wading birds and a varietyof their intertidal prey animals suggest that they may providerealistic opportunities for the quantitative studiesof thepredator/prey relationship. Furthermore, there is much basicinformation already available on the oystercatcher and theshellfish on which it feeds, with the result that costly time-consuming exploratory studies are unnecessary.The oystercatcher is easy to observe in its natural habitats,while the shellfish are easily sampled (see cover photographs).There is, therefore, an opportunity to examine how theoystercatcher responds to variations in the abundance ofshellfish, and how these responses, in turn, affect themagnitude of the mortality inflicted on the prey by the birds,both in space and through time. The contribution of theoystercatcher to the overall population fluctuations of theshellfish can then be modelled and the nature of the inter-actions explored.Basic Biology of the Species InvolvedOystercatchers are numerous around the coasts of Britain,particularly on estuaries. They breed in many areas of theBritish Isles, especially in the north, but occur in the greatestnumbers in autumn and winter outside the breeding season.Counts made by teams of volunteers as part of the jointBritish Trust for Ornithology/Royal Society for the Protectionof Birds/Wildfowl Trust Estuaries Enquiry have shown that upto 185,000 of these birds may be present in Britain at any onetime. Ringing studies have shown that these wintering flockscontain British birds and many individuals that breed to thenorth in the Faeroes, Iceland and Norway.Oystercatchers feed on several kinds of intertidal invertebratesand in some areas of the country feed in fields as well. On theshore, ragworms  (Nereis diversicolor)  and crabs  (Carcinusmaenas)  are often taken, but the bulk of the diet in most areasseems to be'shellfish. Of these, the edible mussel  (Mytilusedulis),  cockle  (Cerastoderma edule)  and Baltic tellin(Macoma balthica),  are the most important. Mussels lie onthe surface and are easily detected by the birds once thereceding tide has exposed them. Cockles may also bedetected by sight, even though they are buried just below thesurface: presumably, the birds are able to detect the shortsyphon tubes with which the cockles maintain contact with thesurface. Oystercatchers often use their bills to detect cocklesand the deeper-burrowing  Macoma  by touch. Once found,the birds may swallow the smaller shellfish whole, but openthe larger ones with their powerful beaks either by crackingthe shell open or by prising the two halves apart. The birdsthen cut the flesh away from the shell and swallowit.Normally, oystercatchers do not simply take all the shellfishthey encounter, but select particular size classes. For example,they select large  Macoma  and medium-sized cockles(Figure 2) and only take other size classes in large numberswhen the preferred ones are scarce. This selection impliesthat it is important to distinguish between size classes ofshellfish when the role of oystercatchers in the populationdynamics of their prey is being studied.3The main shellfish prey of oystercatchers in this study is themussel. In the summer, each female, depending on its size,produces several thousand eggs which are fertilised in thesea. After a few weeks of growth in the plankton, the ciliatedlarva develops a bivalve shell and sinks to the bottom whereit may settle on one or more substrates before settling on anestablished mussel bed. By delaying final settlement in thisway, many larvae are sufficiently large to avoid beingconsumed by the adult mussels. The growth rate and410 15 20 25 30 35 40Size range (length in mm)rL_r--<2 4 6 8 10 12 14 16 18 20 22Size range (length in mm)Figure 2. The sizes of cockles and Macoma (solid line) taken byoystercatchers in one area on the Wash compared with the sizes ofanimals (dotted line) in the sand where the birds were feeding.survival of the young mussels varies enormously from placeto place and will depend upon the density and size of itsneighbours and on a variety of abiotic factors, such as theperiod of tidal immersion, and hence time available forfeeding. Little is known about the planktonic phase of thelarval mussel, but, on the beach, small mussels are eaten inlarge quantities by crabs and fish, and large mussels are eatenby birds, notably the oystercatcher, and fished by man. Manymussels of all ages may be destroyed by winter storms.Populations of mussels in estuaries are often characterised bylow recruitment but good growth rates, with a relatively highproportion surviving for 5 or 10, or even more, years. Thishigh rate of survival is in contrast with that of populations onrocky shores. At high levels on exposed rocks, there is lowrecruitment and poor growth potential, but, in the absence ofpredators, a high proportion may survive even up to 20 years.At low levels on the shore, recruitment may be very heavy andgrowth rate extremely fast, but few mussels survive beyond1 or 2 years because of intensive predation by a wide rangeof species which penetrate the lower levels of the shore from, the sublittoral when the tide is in. These findings on rockyshores suggest that predators can have a profound influenceon the performance of mussel populations, and that a studyof the interactions between one predator, the oystercatcher,and its mussel prey would be worthwhile.The Study AreaAn important consideration in the choice of study area wasthe efficiency with which the prey population could besampled. Accordingry, a small estuary was selected inpreference to a large embayment such as the Wash orMorecambe Bay. This decision to some extent determinedthe prey species, because, in general, oystercatchers feedmainly on mussels in the smaller estuaries. The Exe estuaryin South Devon was chosen because several features made itparticularly suitable for study. Most oystercatchers in thisestuary feed on mussels, although a variety of other prey isalso taken. The mussel beds on the Exe are extensive (Figure3) and are protected from storms which, in more exposedsites, may destroy entire beds.56Low water mark.1? ordinary tidesC>a Mussel bedsMud & sandflatsrEstuarywaters of riversExe & ClystFigure 3. The Exe estuary, showing mussel beds.High water markordinary tidesInteractionsFor the purposes of study, the interaction between predatorand prey can be subdivided naturally into two rnajorcomponents and a mathematical model constructed for each:—(i) variation in the abundance of each age and size classinto which the total mussel population may be divided;(ii) the responses of the oystercatchers to variations in theabundance of particular age and size classes ofmussels, and their impact on them.As is shown schematically in Figure 4, the output from onemodel comprises the input for the other. In the simplest formof the model the birds respond to variations in the abundanceof the preferred age and size classes of their prey by takingfewer or more of these prey in the various age and sizeclasses and, indeed, of other species. The predation on themussels affects the settlement and growth potential of thesmall animals and the reproductive output of the populationas a whole. These prey responses, in turn, determine thefuture food supplies of the birds. Because factors other thanthe abundance of mussels affect the predatory activities of thebirds and, similarly, factors other than the birds affect thebehaviour of the mussel population, extrinsic variables mustalso be incorporated into both models, if a proper understand-ing is to be obtained.ExtrinsicfactorsModel of mussel Model of bird predationpopulationPopulationdensity ofmusselsNumbersof musselspredatedNumbersof mussel Extrinsictaken factorsper birdNumbers Extrinsicof birds factorsFigure 4. Simplified diagram of the predator-prey interaction betweenoystercatchers-and mussels.Each of these models will itself comprise a series of sub-models which describe the behaviour of particularly criticalparts of the system. Space does not permit more than acursory examination of this point, but an indication of theprocedures and methods employed may be gained bydescribing aspects of the model of bird predation in moredetail.Model of Bird PredationIt has proved useful in studies of predator/prey interactions todistinguish between (a) the behavioural responses ofindividual predators to variations in the abundance of prey,and (b) the numerical response to prey abundance of thepredator population as a whole through survival andreproduction. The combined behavioural responses of all theoystercatchers present on the estuary determine themortalitY that they inflict on the mussels. Some of the basicrelationships that comprise one of these categories ofresponse, the behavioural response, may be illustrated withdata obtained from oystercatchers eating cockles on the Wash.The birds are presented with a food supply which varies indensity from place to place. Direct observation of birds on thefeeding grounds suggest that both the number and weight ofprey taken per unit time is related to the abundance of prey78(a) (b)•• E 500ca 400co„a', 300•rei 200 •100_c.a)200 600 1 000 5 10 15 20 25Numbers of cockles per square metre Weight of cockles per square metreFigure 5. The relationship between (1) the number of cockles, and(2) the weight of cockles eaten per minute by oystercatchers in relationto the abundance of the prey in the substrate. The birds took larger fooditems as the abundance of cockles in the substrate increased.in the substrate (Figure 5). It is likely that the birds prefer tofeed where feeding is easiest and so will go where their preyare most abundant. Indeed, oystercatchers on the Wash weremost numerous where cockles were most abundant (Figure 6).0-1 0-2 0.3 0.4 0.5 0.6 0.7Prey abundance••Figure 6. The relationship between the density of oystercatchers and theabundance of cockles in nine areas on the Wash.-9\ ?80 :I\70dgl,g 6000tO1 60 -0a)  I.'..,..a .0 50  a-E 8.Ir3 4000 ss .0-- 40 c..8 ss0'' 0)._30 ..ga)Iei20 2000 '.c 0 To10 •foI— a2aJuly Aug Sept Oct Nov Dec JanFigure 7. The proportion of oystercatchers feeding in the preferred part oftheir feeding area in relation to the total numbers present.However, this tendency to congregate in the most profitablefeeding areas may, in turn, be counteracted by the birdsbecoming increasingly intolerant of each other as their owndensity rises, so that some birds feed in the less preferredareas (Figure 7). Such a tendency would depress the impactof the oystercatchers on their prey where these are mostabundant. These behavioural responses by the oystercatchersto each other and to their common food supply would resultin their impact on their prey during autumn and wintervarying in magnitude from place to place according to thedensity of prey at the start. One possible form which therelationship might take is shown in Figure 8. The aim of thestudies on the birds' feeding behaviour is to build a sub-modelwhich describes the relationship between the density of theprey and the impact of the oystercatchers on it while, at thesame time, incorporating the effects of any other relevantvariable.Future developmentIt is, of course, only possible to describe a small part of theresearch as it is envisaged at the moment. However, once themodels have been constructed and tested, it will be possibleto explore how the oystercatcher-mussel system wouldrespond to disturbances of various kinds. This explorationshould contribute to the understanding of how predator-prey910Density of prey in AutumnFigure 8. Possible form of the relationship between the density of theprey in autumn and the percentage eaten by oystercatchers by thefollowing spring.systems function, and, at the same time, yield insights to thesolution of practical problems. Currently there are two mainapplications.First, there are a number of proposed schemes for developingparts of major estuaries for water-storage and industry. Birdsdisplaced by such schemes would presumably attempt tosettle in the remaining feeding grounds, with a consequentincrease in bird density. The long-term quantitative effect onboth predator and prey populations of an increase in thedensity of predators that feed preferentially on particular sizeand age class of prey cannot be predicted easily without amathematical model based on a firm understanding of thesystem. Second, it is often claimed that oystercatchers are amajor pest of commercially exploited shellfish populations.The effect of varying numbers of oystercatchers on theperformance of the mussel population will be explored by themodels derived from this research. Although the models willbe constructed for a particular estuary, their properties shouldenable reasonable predictions to be made for other areas.á0 41  0  t". V . tZ •

Oystercatchers and shellfish: predator-prey studies

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Oystercatchers and shellfish: predator-prey studies

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