Intertidal habitat loss and wildfowl numbers ... - Wiley Online Library

Journal of Applied Ecology 0887\ 24\ 46Ð52

Intertidal habitat loss and wildfowl numbers] applications of a spatial depletion model S[M[ PERCIVAL\ W[J[ SUTHERLAND$ and P[R[ EVANS% The Ecology Centre\ University of Sunderland\ Science Complex\ Sunderland\ SR0 2SD^ $School of Biological Sciences\ University of East Anglia\ Norwich\ NR3 6TJ^ and %Department of Biological Sciences\ University of Durham\ South Road\ Durham\ DH0 2LE\ UK

Summary 0[ A spatial depletion model of the responses of grazing wildfowl to the availability of intertidal vegetation at Lindisfarne National Nature reserve\ north!east England\ was used to investigate the capacity of the site|s beds of Zostera and other intertidal vegetation to support brent geese Branta bernicla hrota and wigeon Anas penelope[ 1[ Recent total winter counts of brent geese and wigeon were both only 39) of the maximum that the food supply at the site could theoretically support[ Other factors must have been restricting their numbers[ Earlier arrival of brent geese at the site could increase the number of brent goose!days which could be supported\ but would have only a slight negative e}ect on the wigeon!days[ 2[ The model was used to examine three conservation issues] encroachment of Spartina anglica\ sea level rise and loss of food plants from the whole site "which could result from increased autumn storms or plant disease#[ Loss from the top of the shore through encroachment by Spartina anglica had the greatest e}ect on the site|s capacity to sustain geese and wigeon[ Loss from the bottom of the shore\ as would occur through sea level rise\ had less impact[ Increased loss of vegetation over the whole site would have an intermediate e}ect[ 3[ This work has important implications for the management of the site[ Factors such as hunting\ that may be restricting current numbers below those that could be supported by the food supply\ require urgent investigation[ Model predictions indicate that encroachment of Spartina is likely to depress local populations of brent geese and wigeon under current conditions only if it results in the complete loss of Zostera from the top 499 m of the shore[ Key!words] Anas penelope\ Branta bernicla\ conservation management\ Spartina control\ Zostera[ Journal of Applied Ecology "0887# 24\ 46Ð52

Introduction

Þ 0887 British Ecological Society

Many animal populations are limited by their food supply and their distribution is often heavily in~u! enced by food abundance "Krebs 0867^ Newton 0879#[ Understanding the relationship between animal dis! tribution and abundance\ and that of their food sup! ply is therefore fundamental for conservation man! agement[ Once the in~uence of the food supply has been taken into account\ other important factors can be identi_ed more readily[ Predicting the conse! quences of habitat change and habitat loss on animal and plant populations is important in the provision of e}ective recommendations for management[ A number of recent studies have adopted a modelling approach based on the extent of interference between

and depletion by foraging animals to address this problem "Sutherland + Allport 0883^ Goss!Custard et al[ 0884a\b#[ In this paper we use a spatial depletion model of the responses of grazing wildfowl to the availability of intertidal vegetation "Percival\ Suth! erland + Evans 0885# to consider some important conservation issues[ Lindisfarne National Nature Reserve\ north!east England\ is an internationally important site for win! tering waterfowl\ particularly for light!bellied brent geese Branta bernicla hrota "Muller# and wigeon Anas penelope "L[# "Batten et al[ 0889#[ The numbers of brent geese have increased whilst the numbers of wig! eon have declined over the last 14 years "Percival + Percival 0884#[ At the same time\ the birds| main food supply\ Zostera noltii "Hornem# and Z[ angustifolia 46

47 Habitat loss and intertidal wildfowl

Þ 0887 British Ecological Society\ Journal of Applied Ecology\ 24\ 46Ð52

"Hornem#\ has declined through a combination of change in sedimentation pattern and encroachment by cord grass Spartina anglica C[E[ Hubbard[ Herbicide spraying has been used at the site to control the further spread of Spartina "Corkhill 0871#[ The site may also be a}ected by a number of other environmental changes[ Houghton et al[ "0884# have predicted that climate change will cause a rise in sea level of 9[4 m by the year 1099\ which will result in the permanent submergence of the lower part of the shore[ It has been suggested that global warming may also result in an increase in storm frequency and severity\ though there is little consensus as to the prediction of the models or of the evidence "Kattenberg et al[ 0884#[ For example\ Hall et al[ "0883# predicted more intense storms especially on the west coast of Europe while Lambert "0884# predicted reduced storm activity over Europe[ Zostera populations in Britain have been sev! erely a}ected by disease in the past "Philippart + Dijk! ema 0884#[ The dynamics of this disease are not under! stood and another epidemic could occur which might reduce the Zostera populations[ Our model incorporates the numbers of brent geese and wigeon\ their feeding rates\ the abundance of intertidal vegetation "their main food resource#\ the non!grazing loss of this vegetation and the birds| feed! ing preferences "see Percival\ Sutherland + Evans 0885 for details#[ In this paper\ we apply the model to explore the impact of current population levels of wildfowl on the food supply at Lindisfarne[ We exam! ine the consequences of various forms of habitat change on the numbers of bird!days for which brent geese and wigeon could be supported at the site] 0[ We investigate the numbers of bird!days for which brent geese and wigeon could be supported at the site\ given the present levels of available food supply[ 1[ We examine the numbers of brent goose and wigeon days that have been supported each autumn and win! ter over the past 5 years to see how closely the bird numbers approached their theoretical maxima pre! dicted by the model[ 2[ We focus on future changes that might a}ect the birds| local food supply and predict the consequences of any such habitat change on waterfowl usage[ These include further encroachment by Spartina anglica\ the e}ects of climate change and the e}ect of disease out! break in the Zostera populations[ All of these threats would reduce the intertidal vegetation\ but their impact would be on di}erent parts of the shore[ 3[ We investigate how changes in the phenology of the use of the site by brent geese might a}ect the system[ The geese have arrived progressively earlier at the site over the last 14 years "Percival + Percival 0884#\ so we use the model to see what might happen to the maximum numbers of brent goose and wigeon bird!days that could be supported if these trends con! tinue[ The consequences of changes in numbers "but not phenology# of wigeon and brent geese have been

explored in Percival\ Sutherland + Evans "0885# and Percival + Evans "0886#[

Materials and methods OVERVIEW OF THE MODEL

The model was produced to predict the consequences of annual variation in the maximum late summer abundance of intertidal vegetation and bird numbers\ using a depletion modelling approach "Sutherland + Anderson 0882^ Sutherland + Allport 0883^ Suth! erland 0885#[ The model is described in detail by Per! cival\ Sutherland + Evans "0885#[ The site was divided into 499 × 149!m blocks "sectors of the mud~ats# which varied in food density and time exposed by the tide[ By incorporating the patterns of food depletion\ the model predicted the number of individuals of the two main grazers "brent goose and wigeon# that could be sustained through the autumn and winter[ The data incorporated in the model were determined from extensive _eldwork and comprised] the total numbers of brent geese and wigeon present each week^ the initial abundance and distribution of intertidal veg! etation\ the rate of the non!grazing loss of intertidal vegetation through senescence and storm damage^ the grazers| daily food intake rates and their feeding pref! erences[ The birds fed on the tide edge where possible "which not only maximized their distance from the shore to avoid disturbance\ but at the same time pro! vided the best access to food as ~oating Zostera is easier to graze#\ but fed elsewhere when the plants on the tide edge were depleted[ The model was run for the entire period that the grazers are normally present at Lindisfarne "Sep! tember 0st to the end of February#[ The model was iterated for each of these 15 weeks and for each iter! ation the grazers were allocated to blocks of the mud! ~at according to the block|s plant biomass and tidal ~ooding pattern[ The model made a number of assumptions "all of which were based on _eld data\ Percival\ Sutherland + Evans 0885#] "0# the birds| feeding behaviour was determined by a tidal rather than a diurnal cycle^ "1# the birds had no preference for particular plant species^ "2# below!ground rhi! zomes were utilized only by the brent geese and not by the wigeon^ and "3# the non!grazing mortality of the intertidal vegetation was at a constant rate through the season "Percival + Evans 0886#[ The model was tested by comparing the observed and pre! dicted distributions over the whole season in 0889:80 and 0880:81[ It was successful in predicting the observed changes in distribution and was not dis! proportionately sensitive to changes in any one par! ameter "Percival\ Sutherland + Evans 0885#[ F IELD DATA

Wintering wildfowl populations were counted at Lin! disfarne 1Ð3 times per month from 0878:89 to 0883:84

48 S[M[ Percival\ W[J[ Sutherland + P[R[ Evans

as part of the reserve monitoring programme and for the BTO:RSPB:WWT Wetlands Birds Survey[ Where counts were occasionally missed for a particular week the data were interpolated using cubic regression "the lowest order polynomial to give a reliable _t to the data#[ When more than one count was made in any one week\ the maximum count was used[ The intertidal vegetation was surveyed in the win! ters of 0889:80 and 0880:81 "Percival\ Sutherland + Evans 0885#[ In each of these two years\ surveys were carried out immediately before the birds| arrival at the beginning of September "just after the maximum summer biomass had developed#[ An all!terrain vehicle was used to drive a series of transects across the main Zostera bed parallel to the shore[ The percentage cover of Zostera and Enteromorpha was recorded by eye for a total of 0369 0!m1 quadrats for each survey[ These data were converted to dry biomass using cali! bration curves "see Percival\ Sutherland + Evans 0885# and were also used to estimate below!ground biomass[

values for the initial Zostera and Enteromorpha biomass in this zone ranging from 19 to 049) of the observed 0889:81 mean[ This modelled both the increasing loss of habitat from Spartina encroachment and the e}ect of Spartina removal[ Reduction in plant biomass at the bottom of the shore "simulating sea level rise# This was simulated by altering the model to remove vegetation biomass from the lower tidal zones "up to 649 m from the mean low water mark#[ A range of values for the initial plant biomass of 9Ð099) of the observed 0889:81 mean were used in the model[ Pre! dictions of sea level rise of 9[4 m by the year 1099 "Houghton et al[ 0884# would be equivalent to the permanent ~ooding of about 049 m of shore up from the mean low water mark[ Values greater and smaller than these were included here to give total vegetation losses comparable with the other simulations[ Reduction in plant biomass from the whole shore

MODEL APPLICATION

We used the model to explore a number of questions about brent goose and wigeon numbers and their food supply[ First\ we sought to determine the maximum numbers of bird!days for brent geese and wigeon that the site could support\ given the available food supply[ We ran the model with a range of brent goose numbers "with the 0883:84 phenology and wigeon numbers# to determine the maximum number of bird!days that could be supported without exhausting the food sup! ply before the end of February when the geese migrate to Denmark[ This simulation was run again for wigeon\ holding brent goose numbers at the 0883:84 level[ We repeated both these simulations for both species for each year 0878:89Ð0882:83 and compared the predicted maximum number of bird!days of each species with the observed numbers[ The mean veg! etation biomass of the two years for which detailed data were available "0889:80 and 0880:81# was used in all these simulations[ The second application was to use the model to explore a number of scenarios involving habitat loss to investigate the e}ect on the numbers of brent goose and wigeon bird!days that could be supported[ We ran the model with a range of values for the initial intertidal plant biomass\ to determine the relationship between this and the maximum numbers of bird!days for which brent geese and wigeon could be supported[ The following simulations were carried out]


Change in plant biomass at the top of the shore "simu! lating Spartina encroachment and control# Habitat change was constrained to the upper part of the shore\ within 499 m of the mean high water mark "the main Spartina zone#[ The model was run with

There are two possible scenarios which could result in a reduction in initial plant biomass from the whole shore] "0# increased frequency and severity of autumn storms and "1# disease a}ecting the Zostera popu! lations[ The area is already subject to autumn storms\ which result in losses of Zostera and Enteromorpha\ but the frequency of these may change with the pre! dicted climate change "Kattenberg et al[ 0884#[ Zos! tera populations have been a}ected by a wasting dis! ease in the past "Philippart + Dijkema 0884#^ if this were to recur it would have a similar e}ect to that of storms by reducing the food availability to the birds across the whole site[ This reduction in plant biomass was modelled as a proportionate "9Ð59)# loss of veg! etation from the whole site at the start of the season[ The e}ect of up to 19) additional biomass was also included\ again to give a range of values comparable to the other simulations[ Lastly\ brent geese have shown a pattern of arriving earlier in recent years "Percival + Percival 0884# and we used the model to explore the e}ect of further changes in their migration phenology\ to predict what might happen if this trend continues[ We used the same numbers of birds in the model as recorded in 0883:4\ but ran the model with the main arrival of the brent geese moved between 0 and 7 weeks ahead[ A number of goose populations have shown changes in phenology of this magnitude in recent years "Van Eerden 0873#[ As before\ we ran the model to deter! mine the maximum numbers of brent goose and wig! eon bird!days that could be supported without exhau! sting the food supply[

Results The numbers of brent geese and wigeon recorded dur! ing 0878:89Ð0883:84 were consistently well below the


maximum numbers that the site could support "Fig[ 0#[ Over this period the two species averaged 32 and 34) of their estimated maxima\ respectively[ There was a linear relationship between the initial food biomass and the predicted numbers of both brent geese "Fig[ 1# and wigeon "Fig[ 2# that the site could support[ The slope of these relationships was di}erent in the three habitat loss scenarios[ It was greatest when the loss was from the top of the shore "Spartina encroachment# and least when it was from the bottom of the shore "sea level rise#[ Thus\ for a given loss of plant biomass\ the consequences are more severe if lost

from the upper shore[ The simulated loss of vegetation from all zones at the start of the season gave an inter! mediate result[ This pattern was the same for both brent geese and wigeon[ The simulated removal of Spartina\ increasing food availability at the top of the shore\ gave a corresponding linear increase in the maximum number of birds that could be supported[ Figure 3 shows that earlier arrival of the brent geese allowed greater numbers of goose!days to be sup! ported through the whole autumn:winter period[ This relationship was not linear] the rate of increase slowed as the main arrival date was moved earlier[ The e}ect

Fig[ 0[ Comparison of the observed number of brent goose and wigeon bird!days\ and the predicted maximum numbers that could be supported by the food supply 0878:89Ð0883:4[


Fig[ 1[ Model predictions of the e}ect of change in initial intertidal vegetation biomass on the maximum numbers of brent geese that the site could support\ simulating "a# Spartina encroachment and removal\ "b# sea level rise\ and "c# increased autumn storm frequency[


Fig[ 2[ Model predictions of the e}ect of change in initial intertidal vegetation biomass on the maximum numbers of wigeon that the site could support\ simulating "a# Spartina encroachment and removal\ "b# sea level rise and "c# increased autumn storm frequency[

Fig[ 3[ Model simulation of the e}ect of progressively earlier arrival of brent geese on the maximum number of brent that the site could support[ Each symbol represents a model simulation[

of earlier brent geese arrival on the maximum number of wigeon!days that could be supported is shown in Fig[ 4[ Earlier brent geese arrival had a slight negative impact on the wigeon!days\ but this impact was much less marked than the increase in brent goose!days[

Discussion


In all recent years the numbers of brent goose and wigeon bird!days at Lindisfarne were well below the predicted number that could be supported\ suggesting that factors other than the food supply were holding their numbers down[ It is possible that disturbance may be such a factor\ as indicated by the birds| pref! erence for feeding as far down the shore as possible[

Lindisfarne is used intensively for autumn and winter hunting "around 399 permits are issued annually# and wigeon are the main quarry[ No shooting of brent geese is allowed[ Recent work in Denmark has shown that shooting disturbance can reduce the number of wildfowl at a site very signi_cantly "Madsen 0884^ Madsen + Fox 0884^ Fox + Madsen 0886#[ At the same time the introduction of refuges at other sites on the wigeon|s ~yway may also be making those sites more attractive[ Numbers of wigeon on the Ribble Estuary\ north!west England\ have increased greatly following the introduction of a refuge at that site "Cranswick et al[ 0884#[ Modelling of habitat loss showed the relationship between maximum bird numbers that could be sup!


Fig[ 4[ Model simulation of the e}ect of progressively earlier arrival of brent geese on the maximum number of wigeon that the site could support[ Each symbol represents a model simulation[


ported and food biomass to be linear\ but that the spatial location of any habitat loss was important in determining the slope[ The steepest slope\ where changes in the initial biomass had the greatest e}ect\ occurred when the loss was from the upper shore[ This zone is the main area for Zostera noltii\ the dominant Zostera species "Percival\ Sutherland + Evans 0885# and where the food is also most available as it is least covered by the tide[ Since most of the shooting is done from the shore "Denny + Percival 0885#\ upper shore zones are also the most likely to be a}ected by dis! turbance by hunters[ Given the marked e}ect of simu! lated reductions in food availability at the top of the shore\ the potential indirect e}ects of increased hunt! ing on brent goose and wigeon numbers are obvious[ Consideration of the spatial location of the birds| food resources\ as well as the abundance of those resources\ is important in predicting the consequences of habitat loss[ Similar results have been found for shorebirds "Charadrii# exploiting intertidal areas with downshore gradients of increasing invertebrate biomass density "Davidson + Evans 0875^ Evans et al[ 0880#[ The fac! tors a}ecting habitat quality are subtle and do not depend just on food density but certainly include dis! turbance\ predation risk and sediment type[ Changing the phenology of brent geese to extrapo! late the current trend of earlier arrival showed that more brent goose!days could be supported[ This is because they are utilizing the food supply more e.ciently by arriving earlier[ When more birds arrived earlier\ a higher proportion of the vegetation was con! sumed by the brent geese and less was washed away or senesced[ The relationship is non!linear because the non!grazing loss of Zostera is disproportionately greater earlier in the season when its biomass is grea! test[ The slight negative impact on the maximum num!

ber of wigeon that could be supported resulted from a slight decline in food available to this species as more brent geese were consuming food before the main wigeon arrival in October[ The results suggest that the one important con! servation management priority should be to address why brent goose and wigeon numbers are currently so much below what the food supply could support[ It is possible that disturbance\ particularly hunting at the top of the shore\ could be important[ Although the food supply is not currently limiting bird numbers\ it may become more important in the future if\ for example\ hunting disturbance were reduced[ In this case\ the model predictions suggest that the most e}ective management of the food supply would be to at least maintain or\ if possible\ improve the veg! etation biomass on the upper shore[ Spartina control would be important to prevent further encroachment and would be even more bene_cial if it were done in such a way as to promote Zostera recolonization at the same time "Evans 0875#[ Climate change could a}ect future numbers of wild! fowl that Lindisfarne can support\ particularly through increased frequency of autumn storms "which would reduce food availability across the site#[ Sea level rise is less important in its immediate impact\ but it may in the longer term lead to a redistribution of intertidal vegetation which could reduce the numbers of wildfowl that the site can support[ Outbreak of disease in the Zostera populations would have the same e}ect as increased storms\ reducing food avail! ability across the site[ This study\ along with others "Sutherland + Allport 0883^ Goss!Custard et al[ 0884a\b#\ shows how simple models based on depletion\ interference or both can be used to make testable\ quantitative predictions[ We


believe that this approach could be applied to a wide range of systems to facilitate e}ective habitat man! agement[

Acknowledgements We would like to thank the many people who helped with this project including Digger Jackson\ Rob Evans\ Guy Anderson\ Matthew Denny and the wild! fowl wardens for their help with the data collection[ Thanks to the sta} of English Nature\ particularly Phil Davey\ Wesley Smyth\ Martyn Howat\ David O|Connor\ Simon Cooter and Douglas Gilbert for their assistance\ and to Tracey and Barney Percival\ David Jarvis\ Nicola Crockford\ Michael Bone and two anonymous referees[ The project was funded by the Agriculture and Food Research Council as part of their Joint Agriculture and the Environment Pro! gramme[

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