Table Of ContentOdonatologica27(3): 307-316 September7, 1998
Dragonflyspecies-richnessandtemperature:
nationalpatternsandlatitudetrendsinBritain
B.C.Eversham andJ.M.Cooper
BiologicalRecords Centre, NERC Institute ofTerrestrial Ecology, Monks Wood,Abbots Ripton,
Huntingdon,Cambs PE17 2LS,United Kingdom
ReceivedAugust21, 1997/Reviewed andAcceptedFebruary 16, 1998
The patternofOdonata species-richness in Britain is mappedat 10km resolution.
This isstronglycorrelated withmeanairtemperatures. Therelationshipwithseasonal
and monthlymeantemperatures isexplored: summer temperaturesarebetterpredic-
tors ofoverall dragonflyrichness than are wintertemperatures. However, there ap-
pears tobe somelatitude variation in the relationship. Thus,in northern Britain,in-
creasingOdonata richnessiscorrelated with increasingsummer temperaturesfornon-
-boreal species, and with decreasingwinter temperatures for predominantly-boreal
species;physiologicaladaptationsofindividual species areproposed asa possible
explanation.
INTRODUCTION
Thegeographic distributionofindividualdragonfly species in Britainhasbeen
documented sincetheearly nineteenthcentury (LONGFIELD, 1960).Of the 40
currently resident species, 36 hadbeenrecordedin Britainby 1900,andnational
distribution maps showing theoccurrence ofeach species within the 112 vice-
-counties werepublished by CORBET etal.(1960).Anationalrecording scheme
was launched in 1968,andpreliminary nationalmaps,using as the mapping unit
the2860 10km squaresofthe BritishOrdnanceSurvey grid, havebeenpublished
atintervals from 1971 onwards. Acomprehensive set ofspecies maps, with de-
scriptive text,has recently been published (MERRITT etal., 1996),andit is this
dataset, summarisedat 10km square resolution,whichprovides thebasis for the
analyses whichfollow.Atthesametimeas recordingofdragonflies hasadvanced,
theprecision and reliabilityofinterpolations ofclimate data forBritain, derived
from356meteorological stations, hasimproved, so thataccurateclimatedataat10
km square resolutionare nowavailable(VINER& HULME, 1994).
308 B.C.Eversham &J.M.Cooper
NATIONALPATTERNS OF SPECIES-RICHNESSAND CLIMATE
Ithaslong been known thatmore species ofdragonfly occur in southern Eng-
land than inthenorth ofBritain. Thisis anexample ofa commontrend in many
parts oftheworld,whereby species richnessdeclinesathigherlatitudes(SCHALL
&PIANKA, 1978).Thethoroughness ofrecording hasonly recently becomesuf-
ficient formore detailedpatterns ofspecies richness tobe explored. Thegeneral
pattern,ofadeclineinspecies-richness fromS orSEtoNorNW,is foundinmany
taxa,andiscorrelatedwithseveralenvironmentalfactors(LAWTON etal., 1994).
InBritain, manyphysical variables— temperature,rainfall, sunshine,topography,
geology andsoils— showastrongSEtoNW gradient,theSEbeingwarmer, drier,
sunnier, predominantly lowland, with more basic rocks and alluvial soils (VIN-
CENT, 1990;EVERSHAM &ROY, inpress). Probably as aresultofthesetrends
inenvironmental factors, Odonata diversity ‘hotspots’ are concentrated insouth-
ern England, although several ofthe scarcer species are confinedto the north
(PRENDERGAST etal., 1993).Large-scale patterns in species-richness havethus
been explained interms ofmajor gradients, whereas smaller-scale variationsare
consideredto be due to current and historical land use (EVERSHAM, 1993;
LAWTON etal., 1994).Diversity ‘hotspot’ distributionis probably causedby an
interactionbetweenthetwo.
Phytogeographers havepreviously suggested thattheeffectsofclimateare much
greaterthan otherfactors(such assoil) in determining large-scalepatternsofspe-
cies distribution(CAIN, 1944;HILL & DOMINGUEZ-LOZANO, 1994). Some
zoologists haveproposed thatsolarenergyinputis thesingle factorresponsible for
latitudegradients inspecies-richness (TURNERetal., 1987).Recentconcern about
possible futureclimatechange hasfocused attentionon therelationship between
species distributionand climate(United Kingdom ClimateChange impacts Re-
view Group [UK CCIRG], 1991;ASPINALL & MATTHEWS, 1994).Previous
studieshaveillustratedacorrelationbetweendragonfly species-richness andtem-
perature(WATTetal., 1990),noting thatthe smoothedspecies-richness followed
closely thepattern ofApril mean daily temperatures insouthern England. Forin-
stance, all areas containing over 20species were withinthe 9°C April meaniso-
therm(UKCCIRG, 1991).Acorrelationwith spring temperatures madesense in
termsofspecies ecology, becausetheseason couldbe seenas atime-windowbe-
tweenlarvaldiapause andadultemergence.Many Britishspecies undergo awinter
diapause, brokenby temperaturerise inspring (thethresholdforrecommencement
of development varying between species) (CORBET, 1962).The main flight pe-
riod ofall but oneBritish species, including ‘boreal’species (see definitionsbe-
low), begins between early May and mid-June; the exception is arecent colonist
withasoutherndistribution,Aeshnamixta,whichusuallyemergesfromearly July
onward (MERRITTetal., 1996).
Asapreliminary toastudy modellingthepatternsofspecies-richness intheflora
Distribution patternsand latitude trends inBritishOdonata 309
and faunaofBritain withre-
spect toawiderangeofenvi-
ronmental variables
(EVERSHAMetal., inprep.),
the relationship between
Odonataspecies-richness and
temperature was examined.
This was partly foritsintrin-
sic interest, and partly as a
sensitivity testofthe choice
of temperature variablesfor
modelling.
RESULTS
MONTHLYTEMPERATURES
ANDDRAGONFLY RICHNESS
Figure 1 showstherecorded
pattern of dragonfly species-
richness inBritain, basedon
data summarised in
MERRITT et al. (19%). It
alsoindicates4latitudebands
usedinlateranalyses. Figures
2 and 3 show the pattern of
temperatureinsummer(July-
-September mean)andwinter
(January-March mean), the Fig. I. Species-richness ofdragonfliesin Britain, shown as
number ofspecies ineach 10x10 km square ofthe British
choice of temperature vari- NationalGrid.Thelines oflatitude shownareinNationalGrid
ables following HILL & units. Areasdiscussed in thetextare:area 1=SalisburyPlain;
DOMINGUEZ-LOZANO area2=Dorsetheaths;area3=SurreyheathsandNewForest;
(1994). The main difference area4=Norfolk Broad;area5=intensive arable area ofeast-
between summer and winter ernEngland.Based ondata in MERRITTetal.(1996).
spatial patternsoftemperature
in Britain is that summer shows a clearSE-NW gradient, which is confused in
winterby awarm-coast (especially inthewest), cold-interioreffect.Thus,theSE
haswarmer summers but colderwintersthantheSW.
Examinationofmapsofmonthlymeantemperaturessuggested thatcertainmonths
weremuchbettercorrelatedwith dragonflyrichness,butthatthis variedregionally.
TableIshows the correlationbetweendragonfly richness and monthlymean tem-
peratures, nationally andforthefourlatitudebandsshownon Figure 1;thesecor-
310 B.C.Eversham & J.M.Cooper
relationsareplotted inFigure
4. Dragonfly richness and
temperature are positively
correlated at the national
scale,and dragonflyrichness
varies much more closely
with summer temperatures
than with winter tempera-
tures.TableIIshows thecor-
relationswithin thetempera-
ture data.Thespatial patterns
of most months are closely
similar(coefficients mostly>
0.9),except whenwinterand
summerare compared (coef-
ficientsof0.6-0.7).
Summer temperatures are
betterpredictorsofdragonfly
species-richness than winter,
nationally and within each
latitude band. In southern
England and northern Scot-
land,thecorrelationwithtem-
perature in wintermonths is
negative, albeit not signifi-
cantly so. Although varying
slightly withlatitude,there is
no clear tendency for the
monthofpeak correlationto
Fig. 2.Mapofsummer(July-September)meantemperatureof belaterintheNthantheS, as
10x10km national gridsquares.
mighthvebeenexpected from
trendsinflightperiod, somespecies flying laterinthe NthantheS(MERRITT et
al., 1996).FromFigure 4itcanbe seenthatdragonfly richness inthemostnorth-
ern latitudeband(10km squaresNof56°07’N)generally hastheweakestcorrela-
tions with mean monthly temperatures. However, thesecorrelationsare all non-
significant, except May, JuneandJuly wherep< 0.01 foreach.
The weaknessof correlationin northern Scotland was investigated further, by
examining thefaunalcomposition oftheregion. The dragonfly faunaof northern
Scotland is morebiogeographically divergent thanelsewhere.Atleastfourofthe
20residentspecies areboreo-montane((Aeshnacaerulea)or moregenerally north-
ern(Coenagrion hastulatum, Somatochloraarctica, Leucorrhinia dubia),andare
rare orabsentfromsouthernBritain.A furthertwo (Aeshnajuncea andSympetrum
Distribution patterns and latitude trends in British Odonata 311
danae) are more frequent in
theN than intheS inBritain
andEurope (ASKEW, 1988;
MERRITTetal., 1996)scare
also likely to beto some de-
gree cold-adapted, and their
distributions might be ex-
pectedtocorrelatenegatively
with temperature. (For con-
venience, this assemblage is
referred to as ‘boreal’, al-
though we recognise that
most ofthe species also oc-
cur in mountains further
south,andsomehavelowland
southern populations). Con-
versely, most other species,
such as Coenagrion puella
andAeshnacyanea, aremore
wide-ranging but generally
southerninBritain, andprob-
ably require higher tempera-
turesfordevelopmentandac-
tivity.Thetwosetsofspecies
seldomco-occur inScotland,
so thattotalrichness ofdrag-
onflieshere is anamalgama-
tionofseparatesubfaunas.To
investigate this, the two
groups were analysed sepa- Fig. 3. Mapofwinter (January-March)meantemperature of
rately. The correlations of 10x10kmnational gridsquares.
thesetwo groupswith mean
monthly temperatures are plotted in Figure 5. The southern group has a highly
significantpositive correlationwithsummermonthly temperatures(foreachmonth
May-August, r >0.12,n= 775,p<0.01),but no correlationwithwintermonthly
temperatures(foreachmonthNovember-March, r <0.03, n=775,p>0.5).Con-
versely, theborealgrouphasa highly significant negative correlationwithwinter
monthly temperatures(for each month November-March, r < -0.17, n = 775, p
<0.01), and no correlation with summer monthly temperatures, or a very weak
negative correlation(foreachmonthMay-July, r<0.06,n=775,p>0.05;August,
r = -0.09, n = 775,0.05 >p> 0.01). Therefore, the species-richness ofsouthern
Odonata increaseswith increasing summer temperatures, andthat ofboreal spe-
312 B.C.Eversham &J.M.Cooper
cies increases with de-
creasing wintertempera-
tures. Overall, tempera-
ture is astrongpredictor
of dragonfly species-
richness in Britain, ex-
plaining 34.1% of the
variance (derived from
theresidualsofalog-lin-
ear regression fitted to
the data using the least
squares method in
GENSTAT) inrichness.
DISCUSSION
Fig, 4.Corelationbetween dragonflyrichness and meanmonthly
dailytemperaturesforBritainasawholeanddividedintofourlati-
tudebands based onthe NationalGrid (cf. Fig. 1). The lower absolute
valuesofcorrelationco-
efficients in the latitudebands compared with the nationaldatasetmay be due
simply tosamplesizeandtothetotalclimaterangewithineachband.Theseasonal
effect is greatest in southernEngland, whereaweak negative relationship with
wintertemperaturesis probably duetotherelatively lowspecies-richness ofdrag-
onfliesin theextremeSW, aregion ofmildwintersbutrelativelycool summers.
The very weak corre-
lations in all months of
thenorthernScottishlati-
tudeband is unlikely to
be an artefact of lower
recording effort
(MERRITTetal., 1996).
Rather,itarises fromthe
combination of boreal
and non-boreal species,
which have increasing
Odonata richness with
decreasingwinterandin-
creasing summer tem-
peratures respectively. Fig. 5. Correlation between dragonflyrichness and monthly mean
TURNER et al„ (1987), dailytemperatures N ofthe BritishNational Grid latitude line 70
finding a negative rela- (cf. Fig. 1).— The boreal species are Coenagrionhastulatum,
Aeshnacaerulea,Aeshnajuncea,Somatochlora arctica,Sympetrum
tionship between mean danae and Leucorrhinia dubia. The southern species areall those
winter temperature and notincludingthe sixborealspecies.
Distribution patterns andlatitudetrendsinBritish Odonata 313
moth species-richness, Table I
suggested thatmildwinters Correlation between dragonflyrichness and meanmonthlytem-
were detrimental to peraturein 10km squares forthe whole ofBritainandfor four
latitude bands based onthe British National Grid. Correlations
diapausing mothsbecause
withsummer(July-September)andwinter(January-March)mean
theirenergy reserves were temperaturesarealso shown
depleted by araisedmeta-
bolic rate. The six boreal Month Latitude Latitude Latitude Latitude All
Odonata species, possibly 0-20 20-40 40-70 >70 latitudes
being better adapted to
January -0.074 0.056 0.172 -0.070 0.324
lower temperatures, may February -0.028 0.104 0,194 -0.071 0.349
becomemoreactiveduring March 0.022 0,157 0.231 -0.041 0.444
mild winter weather than April 0.124 0.203 0.279 0.024 0.513
their non-boreal counter- May 0.196 0.239 0.317 0.116 0.564
June 0.192 0.237 0.295 0.121 0.568
parts in northernlatitudes.
July 0.169 0.252 0.282 0.094 0.576
Iffoodsupplies are low in August 0.112 0.223 0.272 0.063 0.565
winter, activity would be September 0.014 0.186 0.245 0.016 0.535
detrimentalto larval sur- October -0.039 0.145 0.223 -0.008 0.489
vival, and warmer winter November -0.087 0.082 0.198 -0.053 0.421
December -0.093 0.052 0.161 -0.053 0.343
temperaturesmaylimitthe
Summer 0.105 0.225 0.268 0.060 0.563
distributionofboreal spe- (Jul-Sep)
cies.Conversely, theboreal Winter -0.034 0.110 0.202 -0.063 0.377
species may be better (Jan-Mar)
adapted to low summer
temperatures than the non-borealspecies, resulting in a positive correlationbe-
tween species-richness andsummer temperatureinthelatterbutnot intheformer.
Itisunfortunatethatthereappeartobenopublisheddataonthetemperaturethresh-
olds forlarvaloradultactivity in borealdragonflies.
Inotherareas, thepatchinessofdragonflyspecies-richness isnotfullyexplained
by temperature,andtheeffectsoftopography, geology, landuseandhabitatavail-
abilitycanbedetected.Forexample, Salisbury Plain(area 1 inFig. 1)is anexten-
sive areaof dry calcareous grassland, with very littlestanding or flowing water,
and withvery few dragonfly species as aresult. TheDorset heathsandthe New
Forest(area2 inFig. 1)containextensiveacidic wetlands,as well asclean,base-
richrivers, supporting scarcespecies such asCoenagrion mercurialeandCeriagrion
tenellum.Area3 comprises theheathsofSurrey, withC. tenellumandanoutlying
population ofLeucorrhiniadubia,andtheWeald, anareaofclay andsandcharac-
terisedby fast-flowing lowlandrivers,abundantwoodland,andrich lowlandlakes
and ponds, which form the British headquarters for Corduliaaenea and Soma-
tochlorametallica.TheWealdisboundedbythe chalkhillsoftheNorthandSouth
Downs, whichsupportrelativelyfewspecies ofdragonfly.TheNorfolkBroads and
adjacent grazing marshes (area4) comprise extensivefens derivedfrommedieval
peat digging, andrich lowlandrivers and dykes, whichsupport the only British
314 B.C.Eversham &J.M.Cooper
populationsof.AeshnaisoscelesandconcentrationsofLibellulafulvaandBrachytron
pratense.Much ofthe rest ofeastern England (area5) is very intensively farmed
arableland,withfew surviving wetlands.
It is not surprising thattemperature is a strong predictor ofdragonfly species-
-richnessinBritain, inviewofthegradient inspecies-richness ataEuropean scale
whichmaybe deducedinbroad terms fromAGUILAR etal.(1986)andASKEW
(1988), bothofwhichindicatefarmoreMediterraneanthanborealspecies. Itcould
also suggest that thepaucity oftheBritish fauna(40 spp., compared with 45 in
Denmark,64 inThe Netherlands,65 in Belgium and81 inFrance [MERRITT et
al., 1996]) is due as muchto climaticfactors as to Britainbeing anisland(VIN-
CENT, 1990; EVERSHAM & ARNOLD, 1992). Ifthiswere the case, then the
predictedincreaseinBritishsummertemperaturesassociatedwithglobal warming
wouldbeexpected topermittheestablishmentinBritainofadditionalspecies from
mainlandEurope, ifthey are dispersiveenough toarrive.Some,suchasSympetrum
fonscolombei and S.flaveolum, havebredin thepast (LONGFIELD, 1949), and
arrivedin large numbersin 1995,ayearin whichtherewerealso large influxesof
migrantLepidoptera. Both species emerged from breeding sites in southernEng-
landin 1996,anditseemslikely thatthismaybeapermanentcolonisation,atleast
until summertemperatures fall again.
Dragonfly species-richness is an important criterionintheselectionofsites for
conservation protection in Britain. Thresholds have been setfor the numberof
species necessary ifa site is to merit declarationas a Site ofSpecial Scientific
Interestonthegrounds ofitsdragonfly fauna(NATURECONSERVANCY COUN-
CIL, 1989).Theseare currentlyregional,basedonpolitical boundaries.This study
suggests it shouldbe possible to definesuch thresholdsofimportance basedon
localclimate.For somepurposes,itmaybepreferable touse abroaderanalysis of
Table II
Correlations betweenmonthlymeandailytemperatures ofBritish 10x10 km NationalGrid squares
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
Jan 1
Feb 0.990 1
Mar 0.931 0.963 i
Apr 0.848 0.892 0.976 1
May 0.735 0.790 0.917 0.978 i
Jun 0.641 0.703 0.861 0.944 0.986 i
Jul 0.610 0.672 0.837 0.923 0.973 0.995 1
Aug 0.664 0.722 0.870 0.940 0.978 0.991 0.994 1
Sep 0.759 0.806 0.920 0.962 0.975 0.969 0.968 0.987 1
Oct 0.891 0.917 0.969 0.967 0.936 0.895 0.885 0.921 0.968 1
Nov 0.969 0.971 0.963 0.919 0.845 0.776 0,759 0.807 0.882 0.97 1
Dec 0.996 0.982 0.924 0.846 0.739 0.648 0.621 0.675 0.770 0,90 0.977 1
Distribution patternsand latitudetrendsinBritish Odonata 315
environmentalfactors(as inthedefinitionofbiogeographic zones, whicharethem-
selvesbased mainly on climaticfactors [CAREY etal„ 1995]),butthiswouldbe
influencedby factors such as rainfall, which are ofless significance toOdonata
thanto certainothertaxa (EVERSHAM etal., inprep.).
ACKNOWLEDGEMENTS
We aregratefultoDAVID ROYand MARKHILLforbiometricadvice,toHENRYARNOLDfor
databasemanagement, toPAUL HARDINGforcommentsonanearlierdraftofthepaper; andespe-
cially tothe manyhundreds ofdragonflyrecorders whose datawehave used.
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