Animal Conservation (2004) 7, 221–228 C© 2004 The Zoological Society of London. Printed in the United Kingdom DOI:10.1017/S1367943004001428

Monitoring the conservation status of an endangered amphibian:the natterjack toad Bufo calamita in Britain

John Buckley1 and Trevor J. C. Beebee1,2,†

1Herpetological Conservation Trust, 655A Christchurch Rd, Boscombe, Bournemouth, Dorset BH1 4AP, UK2 John Maynard-Smith Building, School of Life Sciences, University of Sussex, Falmer, Brighton BN1 9QG, UK

(Received 6 October 2003; accepted 12 January 2004)

AbstractAdequate monitoring of amphibian populations will be increasingly important if global declines are to beunderstood and, where possible, reversed. The natterjack toad, Bufo calamita, is the rarest British amphibianand has been the subject of substantial conservation efforts. Monitoring strategies have aimed to quantify boththe total numbers of populations and the status of individual populations. Between 1970–1999 the number ofnatterjack populations increased from 43 to 48 because successful translocations (11) outnumbered extinctions(6). Efforts to monitor individual populations increased over the period 1970–1999. Between 1990–1999 therewere no detectable trends in B. calamita population size or breeding success in Britain, although power todetect any trends over 10 years was low. Calling activity and short-term trends in spawn counts were unreliablepredictors of long-term population viability in the absence of extra information such as toadlet production. Longperiods (> 10 years) of spawn counts are needed to demonstrate trends that are reflective of real populationchanges at the national level. Efforts to develop reliable methods for monitoring B. calamita in Britain couldprovide useful guidelines for work on some other declining species of amphibians.

INTRODUCTION

Concern about amphibian declines has increased steadilyover the past decade and it is now widely recognisedthat many species decreased or became extinct in thelate twentieth century (e.g. Blaustein, Wake & Sousa,1994; Pounds et al., 1997; Lips, 1998; Houlahan et al.,2000). Of particular concern have been declines in ap-parently pristine environments, such as mountainousregions of North America and tropical rain forests inCentral America and Australia. Research into thesedeclines is ongoing but has already highlighted factorssuch as increased levels of ultraviolet-B (UV-B), chemicalpollutants, emerging diseases, introduction of alienspecies and climate change as potential causes (Alford &Richards, 1999; Blaustein & Kiesecker, 2002; Collins &Storfer, 2003). However, in some parts of the worldamphibian declines began earlier and were particularlysevere in the early–mid twentieth century (e.g. Cooke,1972; Beebee, 1975). This was the case in much of Europe,including Britain, and here the causes were relativelyclear and mostly a result of forestry and agriculturalintensification. The natterjack toad, Bufo calamita, in

†All correspondence to: Professor T. J. C. Beebee, John Maynard-Smith Building, School of Life Sciences, University of Sussex,Falmer, Brighton BN1 9QG, UK. Tel: 01273 606755; Fax: 01273678433; E-mail: t.j.c.Beebee@sussex.ac.uk

Britain is a good example. This species vanished from 70–80% of its British range between the late 1800s and 1970primarily as a consequence of habitat change (Beebee,1976, 1977). By the early 1970s it was restricted toless than 50 localities and in some danger of extinctionin Britain. Populations persisted in coastal regions ofnorth-west England and south-west Scotland and asscattered inland and coastal populations in eastern andsouthern England (Beebee, 1977). Bufo calamita is widelydistributed across western and north central Europe.However, although listed internationally by the IUCNunder the ‘least concern’ category, it is acknowledged thatpopulations are declining and this is particularly true in thenorthern parts of its range. This anuran is currently undersubstantial or serious threat in Ireland, parts of northernFrance, Belgium, Denmark, Sweden and Estonia as wellas in Britain. Further south, particularly in Iberia, largepopulations persist.

The natterjack toad was protected by law in Britainin 1975 and has been the subject of considerableautecological research and conservation ever since,including a species recovery programme in the early1990s (Denton et al., 1997). Efforts have been made tomonitor natterjack populations in Britain since the mid-1970s and there is now a substantial database availablefor use in assessing the results of conservation work onthis amphibian (Beebee & Buckley, 2001). In this paperwe report the results of such an analysis and discuss the

222 J. BUCKLEY AND T. J. C. BEEBEE

merits of approaches taken with B. calamita in Britainin the context of global amphibian declines and otheramphibian monitoring programmes.

METHODS

Natterjack sites

Natterjack sites were defined as areas of suitable habitat(terrestrial and aquatic together) supporting a toad popu-lation. All areas where natterjacks had been recorded inthe past (Taylor, 1948, 1963) were visited during the early1970s, and since then searches have been extended to in-clude all other potential sites based on the well definedhabitat associations of this species. Bufo calamita inBritain occurs almost exclusively on coastal dunes, uppersalt-marshes and inland heaths. Essentially all coastal andinland habitats of these types within the British range ofB. calamita were investigated by herpetologists for evi-dence of spawn, larvae or adults by regular surveys duringthe 1970s and 1980s. Very rarely, natterjacks occur in otherhabitats of similar vegetation structure to heaths and dunesand such sites were usually identified following reportsfrom naturalists not directly involved in searching forthe species. Because of their specialised habitat require-ments, many natterjack populations are clearly discreteand unconnected to others. However, some are less iso-lated. In north-west England, for example, clusters ofbreeding ponds (each cluster being defined as a site) aresometimes separated by 1–2 km of terrestrial habitat thatcould be easily traversed by natterjacks. In these cases sitedefinitions were therefore rather arbitrary, but have beenused consistently since the 1970s. They therefore remainconvenient for monitoring trends. Genetic analyses haveconfirmed that there is significant genetic differentiationeven between such proximal sites, indicating low levels ofmigration between them (Rowe, Beebee & Burke, 1998,2000).

Population monitoring

Monitoring amphibian populations requires a count index(C) of some parameter that is expected to reflect popu-lation size. In fact C = N × P, where N is the true popu-lation size and P is the detection probability of C (Schmidt,2003). For C to be useful, P must be reasonably constant(and preferably high) between years. It is of course alsoimportant that C is an accurate index of populationsize, i.e. that it does not vary independently of N. ForB. calamita we wished to detect changes in adult popu-lation sizes and in the frequency of reproductive successbecause this might subsequently affect population sizes.We also wanted to determine whether long-term trendscould be distinguished from short-term fluctuations inboth of these measures. Two aspects of natterjack popu-lation dynamics have been widely monitored. Spawnstring counts have been used as a surrogate for femalepopulation size, while toadlets emerging from breedingponds have been used as an indicator of breeding success.Female natterjacks usually produce just one spawn string

each year, although very rarely a second, smaller stringis deposited (Denton & Beebee, 1996). The adult male:female ratio averages about unity among populations, butcan vary from 0.7 to > 2.0 (e.g. Tejedo, 1992; Denton &Beebee, 1993a). Spawn string counts can, therefore, neverbe an accurate index of total adult population size, but maybe a reasonable approximator of the female populationsize. Spawn is deposited in shallow water, usually separatefrom that of other females and often on bare sandysubstrates. Spawn strings are therefore relatively easy tocount, although since the species is a protracted breederit is necessary to obtain a cumulative tally each yearover a period typically of 6–8 weeks. Assessment ofcounting accuracy between different recorders indicatedthat detection rates are generally high (> 95%) and errorrates are usually ± 5% (Banks & Beebee, 1988). In a fewsites, numbers of adult males assembling at breeding siteshave been used as surrogates of population size rather thanspawn string counts. Males call loudly and can be detectedindividually by torchlight at night. Numbers are recordedon the night of the breeding season in which they reach apeak. Such estimates of male numbers cannot be relateddirectly to population size, but may be used to detectrelative changes over years assuming constant detectionprobabilities over time within specific populations. Wehave no direct evidence whether or not this probabilityis constant, but it seems reasonable for small populationswith multiple visits per season.

Toadlets emerging from the breeding ponds were moni-tored by careful inspection of damp vegetation around thepool edges. Natterjack toadlets are diurnal and usually arenot difficult to find. In most situations all toadlets emergefrom a particular pond within a week or so and remain inthe same vicinity for at least 2 weeks thereafter. Accuratequantification of toadlet numbers is not possible, however,and estimates have therefore been made on a log scale(0 = none, 1 = tens, 2 = hundreds etc).

Statistical tests

All data were tested for normal distribution of residualsusing the Shapiro–Wilks method and no transformationswere necessary. Analysis of variance (ANOVA) and cor-relation analysis were performed using standard methodsassuming linear models (Fowler & Cohen, 1990), using theSTATISTIX 7 computer programme (Analytical Software,Tallahassee, USA). Power analysis was undertaken usingGPOWER version 2.0f (Erdfelder, Faul & Buchner, 1996).A model-based approach to trend analysis was carriedout for us by Dr B. Schmidt at the University of Zurich.In this approach, models of decline and stability werecompared and Akaike weights (equivalent to Bayesianposterior probabilities) calculated for each model.

RESULTS

Site numbers and population estimates

As shown in Fig. 1, the number of natterjack sites knownin Britain rose continuously between the early 1970s and

Amphibian monitoring 223

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Fig. 1. Numbers of natterjack populations in Britain. �, net totalnumber of sites; �, known native sites; �, successful translocations;�, extinctions.

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Fig. 2. Monitoring effort at natterjack sites. �, number of sitesmonitored; �, percentage of known sites monitored.

1993 as surveyors discovered new populations. Since 1993no new sites have been found and it seems likely that allBritish populations of this species are now known. Bet-ween 1970 and 1999 there were six documented extinc-tions and 11 successful translocations (Beebee & Buckley,2001). Assuming that all of the populations discoveredbetween 1970–1999 were present in 1970, there hastherefore been a net increase of five populations, to atotal of 48, in this period. This is a reasonable assumptionbecause natterjack sites in Britain are mostly isolated inpatches of suitable habitat and not generally accessiblefor colonisation or recolonisation events. However, it isnotable that extinctions were not confined to the early yearsand still occur occasionally despite increased conservationefforts. There have also been several unsuccessful trans-locations (not shown), even in recent years.

Monitoring intensity has increased since 1970 such thatby the late 1990s an average of around 80% of all siteswere visited at least once each year (Fig. 2). This was

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Fig. 3. Changes in spawn string deposition and toadlet production.�, Mean number of spawn strings counted per site at all monitoredsites (27–46) during the 1990s; �, mean number of spawn stringscounted per site at eight thoroughly monitored sites during the1990s; �, percentage of all sites monitored (30–44) with at leastsome toadlet emergence during the 1990s.

achieved by a combination of nature reserve wardens,non-government organisation (NGO) employees and localvolunteers, typically at least 20 people over the entirerange each year. However, less than half of the sites(usually around 20) were monitored sufficiently often eachspring so that spawn string counts could be consideredaccurate indicators of the total numbers laid. Trends inmean numbers of spawn strings laid per site at all moni-tored sites (including those with only occasional visitseach year), and at a selection of eight thoroughly moni-tored sites (with cumulative counts each year), and thepercentages of all monitored sites with toadlet productioneach year, are shown for the 1990s in Fig. 3. Numbersof sites monitored per year varied over this period assummarised in the figure legend. No obvious trends wereapparent, although there were substantial fluctuations inboth spawn counts and toadlet production during thedecade. There was a significant correlation (r = 0.808,d.f. = 8, P = 0.005) between mean spawn counts at allmonitored sites and those at thoroughly monitored sites.Low spawn counts in mid-decade (1995–1996) corres-ponded with unusual climatic conditions, with an excep-tionally high water table in the spring of 1995 followedby an exceptionally low one in the spring of 1996 in north-west England, where most B. calamita populations exist(as detailed in the recorder reports used in Beebee &Buckley, 2001). Spawn counts often correlated betweensites even when, as shown in Fig. 4, they were morethan 100 km apart. In this case two populations in north-west England had similar spawning patterns between1987 and 1999, the period for which reliable datawere available (r = 0.648, d.f. = 11, P =< 0.02). Sincepopulation sizes were unlikely to fluctuate synchronouslyover this geographical distance, it seems probable thatspawn string counts were not always accurate estimatorsof population size. Most probably they reflected a variable

224 J. BUCKLEY AND T. J. C. BEEBEE

600

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01987 1990 1993 1996 1999

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Sandscale spawn count

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Fig. 4. Correlations in spawn string counts. (a) Patterns ofspawn string deposition in Sandscale, Cumbria (�) and Birkdale,Merseyside (�). (b) Correlation in spawn string deposition betweenSandscale and Birkdale 1987–1999.

combination of population size and the proportion of fe-males choosing to breed according to prevailing pond andweather conditions. Such correlations were quite commonamong sites in the same broad geographical area. Never-theless, although of limited value in assessing short termtrends, long term averages probably do reflect relative dif-ferences in population sizes. Natterjacks spawn in similarpools (shallow, with little vegetation) throughout theirBritish range, so detection probabilities for spawn are un-likely to vary significantly, although this has not been for-mally tested. Comparing six populations for up to 15 yearsof reliable records (1985–1999), differences in meanspawn string counts were significant overall (Table 1). TheANOVA result was: F = 14.45, d.f. = 5,80, P = < 0.0001.Comparisons of population pairs by paired t-tests were allhighly significant (results not shown) because, althoughranges of spawn counts were often large, they also oftenfluctuated in synchrony as implied by Fig. 4.

Population trends

As noted by others (e.g. Meyer, Schmidt & Grossenbacher,1998; Alford & Richards, 1999), measurements over short

Table 1. Variations in average spawn string counts for six Bufocalamita populations

Population Mean spawn count Range of spawn counts

Ainsdale 339 17–865Birkdale 108 7–242North Walney 27 2–61Saltfleetby 15 7–29Sandscale 254 65–600Woolmer 39 30–49

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Fig. 5. Pattern of a declining population. Numbers of adults andtoadlets observed at Cockerham. Bars = toadlets (log10).

periods at particular sites may indicate declines that arenot necessarily representative of long term trends. InFig. 4(a), Sandscale natterjacks exhibited a significantdecline in the 8-year period 1989–1996 inclusive(r = − 0.798, d.f. = 6, P = 0.018) but not over the 9-yearperiod 1994–2002 where, if anything, there was an indi-cation of gradual increase (r = 0.510, d.f. = 7, P = 0.197).It remains to be seen whether Sandscale population sizeincreases are sustained and eventually return to numberscomparable with those of the late 1980s, but such completerecoveries have been observed at other sites. Spawn countsat Ainsdale, for example, were between 495–855 in thelate 1980s, fell to less than 200 in the mid-1990s, butreached 865 in 1999. However, in another case it waspossible to detect an unequivocal indication of populationchange. At Cockerham, population size was estimated bycounting adult males assembled at the few small breedingponds. The largest number seen on 1 night was takenas the relative estimate for the year. This was expectedto show less bias than mean number because numbersof monitoring nights varied between seasons, and thisvariation was mostly outside the peak breeding period.As shown in Fig. 5, a downward trend in this smallpopulation was apparent after the construction of a seawall in 1981 that caused deterioration in breeding pondconditions. This decline was significant whether measuredover the 8 years subsequent to the highest count in

Amphibian monitoring 225

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Fig. 6. Patterns of increasing populations. Annual spawn stringcounts at Saltfleetby (�) and Woolmer (�).

1980 (r = − 0.809, d.f. = 6, P = 0.028) or over the entireperiod of 17 years (1976–1992) for which counts wereavailable (r =− 0.935, d.f. = 15, P < 0.0001). No toadletproduction was observed after 1982. The last calling malewas recorded in 1988 and two females were seen in 1990,but then or shortly afterwards the population must havebecome extinct. No signs of any life stages of B. calamitahave been found at Cockerham since 1990. These resultsdemonstrate clearly the kinetics of a terminal decline,which was effectively complete within 6–7 years (whenthe last male disappeared) of the last breeding success.

By contrast, long-term upward trends in spawn stringcounts were commensurate with increasing conservationefforts at two small but well monitored sites, Saltfleetbyand Woolmer (Fig. 6). At Saltfleetby there has beenextensive habitat management (scrub clearance, pondcreation and restoration) together with captive rearing oflarvae through to metamorphosis in most years since 1980.At Woolmer there has been a great effort to improve thequality and number of breeding ponds and to improvethe terrestrial habitat by scrub and tree removal (Banks,Beebee & Denton, 1993). Both of these populations wereapparently close to extinction in the 1970s before this workstarted.

Statistical power

Critical to any monitoring programme is the statisticalpower to detect changes in status (Reed & Blaustein,1995; Storfer, 2003). In terms of site numbers this isnot a substantive issue for B. calamita in Britain becauseevery site is visited every year, rather than a subsamplefrom which estimates must be made. However, statisticalpower is an important issue in assessing overall changesin population size. By examining trends in spawn stringcounts across multiple populations over time (as shown inFig. 3), power analysis indicated a 36% chance of detectinglarge changes (r ≥ 0.5 for correlations with spawn countsand time) over 10 years and a 69% chance of detectingsuch changes over 20 years. Small changes (r ≤ 0.1) would

only have a 7% chance of detection over 20 years, andmoderate changes (r = 0.1–0.3) a 27% chance. Largechanges would have a 40% chance of detection by pair-wise (t-test) comparisons of data from two consecutivedecades.

Arguably more useful than retrospective considerationsof statistical power is trend analysis comparing models ofdecline with models of stability. Such models based on the10-year spawn count data of Fig. 3 (mean numbers acrossall sites) yielded AIC weights of 0.84 (no trend) and 0.16(up or downwards trend). This translates into a 5.25-foldgreater probability of no change relative to change inthe overall numbers of British B. calamita spawn stringsduring the 1990s.

DISCUSSION

Amphibians around the world have declined in recentdecades for reasons that, in many cases, are still underinvestigation. It will be increasingly important to imple-ment conservation measures for amphibians as the re-quisite knowledge becomes available. This in turn willrequire the development of reliable monitoring methodsso that future population trends and thus the successof conservation management, can be determined. Suchmonitoring is, however, fraught with difficulty. In the firstinstance it is desirable to identify all the populations ofa species. Site occupancy estimates can be made usingmethods analogous to mark–recapture (Mackenzie et al.,2002), but this approach still requires follow-up ultimatelyto identify all the specific locations where a species occurs.Discontinuous monitoring across large time intervalsrequires multiple-year resurveys to avoid overestimatesof decline (Skelly et al., 2003). Reduced numbers ofB. calamita in the Penalara Natural Park in central Spainwere noted in just a single year (1999) of monitoring bycomparison with previous estimates between 1982–1986(Martinez-Solano, Bosch & Garcia-Paris et al., 2003), andsurely require more extensive surveys to establish whetherthere has been a real long-term trend. Continuous (annual)survey and monitoring over many years remains the mostreliable way of detecting all populations as well as changesin population size, and this is the approach taken withB. calamita in Britain. Because it declined relatively earlyin the twentieth century, the plight of this species wasrecognised more than 30 years ago, and both monitoringand conservation measures have been developed since thattime. Conservation work has included the creation andrestoration of new ponds, clearance of scrub to maintainopen terrestrial habitats and the translocation of natter-jacks to found new populations (Banks, Beebee & Cooke,1994; Denton et al., 1997).

The underlying principle for a monitoring strategywas to develop methods that would provide a usefulcompromise between precision and practicality. Rigorousestimates of population sizes for most species requiresubstantial resources of both time and money (e.g. mark–recapture protocols). It will usually be impractical to usesuch methods with multiple populations on a regular basis.

226 J. BUCKLEY AND T. J. C. BEEBEE

It may also be undesirable to sustain the high level ofdisturbance that is often involved with these techniquesover long periods with a rare species. Of course thereis no point in collecting unreliable data, but often thevalue of an index cannot be judged in advance. As ithappens B. calamita has some autecological features thatare potentially favourable for monitoring purposes. Malescall loudly during the spring breeding season and can beheard up to at least 1 km away. They cannot be confusedwith any other British species. Females usually deposita single spawn string each year, in shallow water awayfrom the spawn of other females and often in areas devoidof vegetation. Spawn of B. calamita is normally easy todistinguish from that of B. bufo, the only other species inBritain with which it might be confused. Finally, toadletsemerging from ponds are diurnal and remain around thepond edges for a considerable time. Given these usefulfeatures, how successful have monitoring methods basedon them become?

Assessments based on male numbers have provedproblematic for several reasons. They require site visitsat night, the breeding season is protracted and callingsessions are not accurately predictable from climatic data(Banks & Beebee, 1986), satellite males that do not calloccur with varying frequencies (Arak, 1988), and malesmove unpredictably between ponds within a single night(Denton & Beebee, 1993a). In large populations, thesefeatures make calling male quantification impossible.Of course animals also attempt to hide when pondsare disturbed by torchlight searches. Monitoring callingactivity is commonplace with North American anurans,but requires intensive effort to obtain reliable information(Bridges & Dorcas, 2000; Crouch & Paton, 2002).Although linear relationships have been established bet-ween call counts and chorus size in some species, itremains uncertain as to what relationship these have withpopulation size (Shirose et al., 1997). The main attractionof trying to count males is that their attendance at breedingsites seems less variable between years than is spawningby females. In a mark–recapture study of one B. calamitapopulation, all males identified at the site visited thebreeding ponds at some point every year, although notall at the same time (Denton & Beebee, 1993a). In afew cases, particularly where populations are small (asin Fig. 5), systematic male counts have provided reliableindicators of population trends, although this was onlyclear in retrospect.

For amphibians that deposit a single egg clutch eachyear, counting clutches can be a reliable method forestimating the number of reproductive females (Crouch& Paton, 2000). For this reason spawn string counts havebecome the most widely adopted method for monitoringB. calamita populations. This can be carried out in daytimeand is largely weather-independent, so a systematic searchtimetable can be planned well in advance. Nevertheless,as shown in Figs 3 and 4, the variable attendance offemales at breeding sites between years can make spawnstring data difficult to interpret. A mark–recapture studyover 5 years at one site revealed that only between 44–64% of females known to be present actually spawned

in a particular year (Denton & Beebee, 1993a). Ingeneral, the wetter the spring the higher the proportionof females that chooses to spawn. Such climatic effectson reproductive effort are well-known in pond-breedingamphibians (e.g. Jensen et al., 2003). Unfortunately, short-term declines in spawn string counts can, therefore, becaused by factors that include climatic effects on femalebehaviour and these appear to be indistinguishable fromreal trends toward extinction. Fortunately, although theenvironmental conditions that affect spawning patternshave proved too complex for reliable calibration ofthe spawn count index, the circumstances that deternatterjacks from spawning (mostly low water tables inspring, or low spring rainfall) are quite well-known ingeneral terms. However, very high water tables can alsobe problematic because, by flooding large areas, they canmake spawn unusually difficult to find and thus lead toprobable under-recording. In this situation the assumptionof constant detection probability (Pollock et al., 2002)is invalidated, but fortunately such conditions are rareand probably have little impact on trend analysis. Totalspawn string counts averaged over at least 10 years arerequired to generate a reliable indication of relative femalepopulation sizes among sites and, in Britain, sufficientinformation to do this has only accrued within the past 10–15 years. Even this modest achievement is only secure forless than 50% of the populations. Nevertheless, this verysubstantial monitoring effort can now be used as a baselineto compare population trajectories over coming decades.Evidently only large changes will be detected with highpower even then and short-term declines will remaindifficult to distinguish from the natural fluctuations thatare common in pond-breeding amphibians (Marsh, 2001;Storfer, 2003) in the absence of ancillary informationabout other events at each individual site.

Assessing toadlet production cannot be more accuratethan to within an order of magnitude, but is probablya valuable indicator of population viability. It is clearlyvery unusual, even for this species which breeds inephemeral ponds, for more than 2 consecutive years topass without at least some successful metamorphosis.As shown in Fig. 5, at Cockerham the extinction ofmales followed within 6 years of the last breeding success.Bufo calamita is a relatively long-lived anuran, with indi-viduals regularly reaching 7–8 years of age or, occasion-ally, more for females at some sites (Denton & Beebee,1993b). These observations are concordant and useful forconservation monitoring. Three years of breeding failureshould precipitate urgent attention. Although clearly animportant measure, estimates of toadlet production areinherently much less accurate than spawn counts andthus, by themselves, do not constitute an adequate moni-toring index. Furthermore, we still do not know thesignificance of post-metamorphosis survival rates forpopulation viability and there is increasing evidence fromother amphibian species that these could be critical (e.g.Biek et al., 2002). More information about this life-historystage in B. calamita is highly desirable.

A formal species recovery programme for B. calamitain Britain was initiated in 1991 with the objective of

Amphibian monitoring 227

maintaining the species at least at the status it enjoyedin 1970 (Denton et al., 1997). This was a modest aimbecause by that time natterjacks had already undergonemajor declines (Beebee, 1976, 1977). To what extent hasthe recovery programme succeeded? Two main criteriaon which the objective can be assessed are the numberof populations and the overall population size. Since thenumber of populations has increased by about 10% since1970, this criterion has been met. Assessing the secondcriterion requires a robust population monitoring systemand has therefore proved more problematic. Spawn stringestimates suggest that at least within the past decade or sothere has probably been no major decrease in populationsize, but we cannot be certain in the absence of morecomplete data for longer periods and from all the extantsites. Evidently there is some way to go before a crediblejudgement of the second criterion can be made. The workinvolved in developing procedures upon which to judgefavourable conservation status for B. calamita indicatehow difficult it will be to assess amphibian recoveries withany confidence on a broader scale, let alone for species lessamenable than the natterjack to relatively simple methods.

Acknowledgements

We thank the many individuals who have contributedmonitoring information over many years. We also thankT. Halliday and B. Schmidt for constructive commentson an earlier version of the paper and are very grateful toB. Schmidt for carrying out a model analysis on our spawncount data.

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