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ECOLOGY OF Tllii: COJ.'J.;ON 3rAIL lf3LIX ASP:!:RSA I.1JLLER

I N A DISTURB:;!;J DU1'1..8 fil~V I.:tQNJ.:.~t;T

A thesis presented in pa rtial fulfilment of the requir ements fo r the degree of

1.;aster of Science i n Zoology a t 11lassey University

I an Robert lilillar

1978

Helix aspersa J.:iiller on lupi n in

s tudy area 3

iii

A B :..; 'I' _{ .-.. C T

/, s tudy \vas made of the popula ~ ion e colo by of Helix a spersa

l·mller in coa3t <;:. l dunes a t Santof t fores t, near Bulls . The envi ronment

was bein~ a ltered by a f f ores t a tion proces ses.

The di stribu t ion of the c~ni : .. a ls is affec ted by the pres ence

of tree l upin , Lu:linus arboreus Sims , t o which they a r e strongl y

a ttracted. Ar eas pl a nt ed v: i tli lupin a r e capable of supporting a much

grea ter density of s nails than non-lupin areas , a nd thi s is thouGht

to be due to the nutritive va lue of thi s species.

Sna il popu l at ion densitie s ap~ear to increase r apidly after

lupin se ~ding in the dune s and t his r esu lt s in wide spread l upin die- ba ck

aft er as lit t l e a s three yea r s from s eeding. ..fter l upin .die- back the

snail populat ion decr easec aGain. Some sug~es ti ons a r e mad e as to the

ori6in of H. aspersa in the dune country a nd on the eventua l f a te of

the populations under the maturin~ forest .

Using shell characteri ::; tics, it was found tha t most juveniles

in expanlin5 popul a tions r each maturity in little more than one year

whe reas those in high density, declining popul ations generally take t wo

or three years. Ind ividuals in expandin6 populations a lso attain a

gr eater size on ma turity than t hos e in h i gh density popul a tions. The

main f a ctor affecting popu l at ion density a ppears to be adult r e cruitment,

which is considerably higher in expandi ng than in stable or decreasing

popu l a tions.

H. aspersa i s found to be socia lly gr egarious and this is

particularly ma rked over the winter period when adults and large

juveniles aggregate for hibernation. Hiberrn::. tion begins in May and

reaches a pec:k in July. Many· animals are active again in mid-Augus t .

Predation by the song thrush Turdus philomelos was s tudied in

one area. Preda tion occurs throut;hout winter and gene r ally increases

over l a te spring/early SU!TlP.ler.

Sna ils affect nitrogen fixation levels in lupins before lupin

dea th occurs and it is considered tha t this is due primarily to

disrupti on of the phloem tissue of the s tems during sna il feeding.

It is sug ,csted that t h is di :..; ruption of the trans loc.'1. tion ti ssues is

the ul tirr.ate c <-tuse of p l a nt death . The possible economic si t;nifi C·".nce

of H. as oe r sa in du ne fore s try through it s e1·fects on lupin is

discussed , and the need for further investig1t ion indic,'. t ed .

iv

A C K N 0 ~ L 3 D G ~ N ~ H T 5

I i·: ish to tha.nk Dr J . P . "Skip" Ski~y.-:orth for his

supervi3ion and practical a i d d.urin:; the cour3e of this

study. r:r Ian Hunter , at th<lt tirr.~ t!-ie .?or e::::: ter at

E. Z. F . s. Bulls District Office , su.;6ested t:rn to.;>ic , and

he and l·iessr n Jock 3utton and Arnold Terpstra of :3antoft

Forest H. Q. showed a continuing interest in the study and

gave much practica l assist&nce .

Lr H. l:i then-; , Jr ·:1 . Sutton (D • .;; . I . ~ . ), _)r i) Charlton

(1.l • ..i . I..-{. ) and l·'.r C. U. 3 . van Kraayenoord (r.: . H. D. ) d i scussed

aspect.3 of the study. i.:r .\. G. J.obertson made equipment

available for the ace tylene reduction assay. Dr David

Fountain gave much practic-1 assistance in operating t he

gas chromatograph and shovrnd considerable interest in t hi s

and other aspects of the study. l·~r f.~ . GreemiOod (D. S.I. J . )

kindly provideet the ~thizobium culture used in setting up

this experiment .

I would particularly like to thank John Innes who gave

much invaluable aid in drawing diagrams and proof- reading

much of the final text . ~:eville Henderson re.1.d and

criticised part of the initial draft , and Isabelle Verry

transl~ted a p&per from the French.

Ly parents gave much encouragement and pt:.t up with my

erratic working hours durint£ the final yea.r of this study.

r;r s Val Swan typed th~ script 3.nd corrected many of

my errors .

Finally, thanks to a ll of those people who abused,

pleaded and ti1reatened me int o compl eting this work , including

Alistair.

v

V l

T A I3 L E 0 F CONT..:.NTS

Pa(Se

A.B.3T.Ri~CT iii

LI .j'I' OF 1"IGU i1ES x

LIS!' OF T,·.BL;:;s xii

1131 OP PLAT~3 xiii

3ECTION I Introduction

CHl~P-TER

1 • 1

1. 2

1. 3

1. 4 CHAPT~a 2

2. 1

2. 2

2. 3

2. 3. 1

2. 3. 2

2. 3. 3

2. 3. 4

2. 4

2. 4. 1

2. 5 2. 5. 1

2. 5. 2

2. 5.3 2. 6

CHAPTEJ. 3 3. 1

3.2

3. 2. 1

3. 2. 2

SECTION II

CHAPTER 4 4.1

4. 2

4. 3

4. 3. 1

Jand .June !<'orcstry and the aole of Lu~in

Background

Initial dune stabiliza tion

Nutritional role of lupin

Invertebrate pestJ of dune-country lupin.

:-lelix Aspersa

I ntroclu cti on

!tange

Terrestria l life dnd the reJ triction of r a nge

·.-:a ter relutions c:.nd. cstiv;_;.t ion

!-:ibernat ion

il. eprociuction

~estrictions on r a nge

iiabitat preferences

rhe signific -nce of ca lcium

Food und pest- status

Food

Pest-s tatus

Control

~:a tura l enemies

Introduction to Study Areas

Santoft forest

Study area s

Area

Area 2

2

3

4

4

4

5

5 8

9 10

10

11

12

12

12

13

14 16

16

16

19

19

Population Dynamics of H. Aspersa 22

General I ntroduction to Population Dynamics ~tudy 23

Introduction to topic

Aims of the study

Sampling progr am methods

Sampling procedures

23

24

24

24

4.3.1.1

4.3.2

4. 3. 2. 1

4.3.2.2

4.3.2.3

4.4 c:L:\PT~R s

s. 1

s.2

s. 2. 1

s.2.2

s. 2. 3

s.3

s. 3. 1

s.3. 2

s.3.3

s .4

s. 4. 1

s.4.2

s. 4. 2. 1

s.4. 2. 2

s.4.3

s.4.4

s.4.s

s.4.6 CHA?T.::.;:q 6

6.1

6.2

6. 2. 1

6. 2. c 6.2.2.1

6.2.2.2

6.2.3

6.3

6. 3. 1

6.3.2

6.3.2.1

6.3.2.2

6.3.3

,'...re as 1 and 2

vii P<...1.ge_

24 Ghell measurements and ch ar a cteristics -27

:'idul ts and juveniles 27

Shell size 28

Estimati1~ juvenile size at emergence 29

Division of juvenile snails into size groupings 31

Dispersion 3S

Introd~ction 3S

Uethods 36

The distribution of H. as ;) ersa in the environment 36

Aggrega tion for estivation 36

Measurement of dispersion 38

Hesul ts 40

:8nvironmen-:. cJ. l !rnte rogenei ty 40

Ghanges in disper s ion with time 41

iJ isper;:; ion of estiva ting s nails on lupin stems 4S

Discussion SO EnvirorJJlental heterogeneity SO Population dispers ion S1

Disper;:; ion of the 1975-76 cohort 51 Dispersion of adults and l arge juveniles S3 The irr.porta nce of lis,;er3ion to the accuracy of population data S4

Lupin-stem aggregations SS Possible behavioural bases for aG6rega tion S6

1\daptiveness of aggregation S9

Population Processes over the Sampling Period 60

Introduction 60

l·'. ethods 60

3ampling

Ageing methods U3ing shell characteristcs 61

Juvenile ages 61 Adult ages 62

Desiccation &nd heat mortality on bare sand 63

Xesul ts 64

Population sampling 64

Individual ages from shell char·'Lcteristics 64

Juvenile growth-checks 64

Adult grm·;th layers 7 3

Sand-stranding mortality 74

viii

PJ.go

6.4 Discuss ion 7 4

6. 4. 1 Juvenile ;;ro .. th a nd. demii ty 7 4

6. 4.2 ,>dul t longevity 80

6 • .. +. 3 J.iortali ty from environmental haza rds C. 1

6. 4. 4 Hiberna tion 83

6. 4.5 Compari sons of popu l at ions fro m area s 1 and 3 84

6. 4. 6 Ar ea 2 86

CH.:',PTI;H. 7 Long-·rerm Population Processes 88

7. 1 Introduction 88

7.2 r.;ethods 89

7. 2. 1 3LlI':lpling and sampl e s ites 89

7.2.1.1 .hea 4 89

7. 2. 1. 2 Area 3 a nd adj a ce nt a r eas

7. 2.2 Jat a analys is 90

7.3 Resul ts 90

7.4 Di s cu ss ion 96

7. 4. 1 Popul a t ion density increase 96

7. 4. 2 The origin of 3na il infes t a t ions 97

7. 4. 3 rhe ori ~in of sna il s in the s and dunes 98

7. 4. 4 Density effe cts in snai l populations 100

7 . 4. 5 The patterns of snail popula tion groHth and decline in the dunes 104

::l:i:;CTIOK III Predation by the Song Thru sh

CH) .. PTER 8 Predation on H. As)ersa by the Song-Thrush (Turdus Philomelos in Area 2

8. 1

8. 2

8. 2. 1

8.2.1.1

8.2.1.2

8.2.1.3

8.2.2

8.2.2.1

8. 2. 2.2

8.2.2.3

Introduction

Methods

The thrush anvil study

Anvil site loca tions

Shell collection and analysis

Preda tion mortality in relation to snail density

Predation on juv~nile snails

108

108

109

109

109

110

111

112

Collection of f aeces and extra ction of radulae 112

Preparation of standard radulae for comparison 113

Staining and mounting of radulae 113

8. 2.2.4 Description and measurement of the radulae 113

8.3 Results 115 8.3.1 Anvi l study 115

8. 3. 2 Thrush f aeces analysis

8. 4 Discussion

8. 4. 1 Accumula tion of shell s or. <mvils

8. 4. 2 Catch as a function of a rea

8. 4. 3 Preda tion on hibern,~tini; snnils

8. 4. 4 Predation on juvenile s

8. 4. 4. 1 La r ge juveniles

8. 4. 4. 2 Small juvvnile s

ix

Pa~e 123

127

127

129

130

131

131

132

SECTION IV Feeding Studies 134

134

CH.APT.:£~

9. 1

9.2

9. 2. 1

9. 2. 2

9. 2. 3

'I'he Food of Dune Popula t i ons of H. Aspers a and

the ~ffects of Feeding on Lu Jinus Arboreus

Introduction

1.;ethods

Cuticle ani.ilysis

Pala t ability trials

Structural damage to l u)in caused by snail

134

134

135

135

fe eding 137

9. 2. 4 The effects of feedinG by snails on ni t rogen 137

9. 2. 4.1 ~x~erimental pro cedure 138

9. 2. 4. 2 Measurement of nitrogen fix~tion 139

9. 3 9. 3. 1

9. 3. 2

9. 3. 3

9. 3. 4

9. 4 9 . 4.1

9. 4. 2

10

10.1

10 . 1. 1

1 o. 1. 2

10 . 2

Results

Cuticle an2.lysis

Palatability trials

Structural danage to lupi n stems caus -.d by

the fe eding activitie s of sna ils

Bffects on nitrogen fixation

Di scussion

Food of H. a spersa

Effects on lupin of snai l feeding

Concludin5 Discussion and Comments

I ssues for further invest i gation

Population ecology

Economi c signific::tnce : friend or f oe?

Comments on the study

142 142

143

144

145

148

148

151

APfl~~!:UIX

REF6fil.'FCES

152

152

152

153

155

156

157

Figure

2

3

4

5

6

7

8

9

10

11

12

13

14

15 16

17

18

19

20

21

22

23

24

25

L I S T 0 F F I G l; R E S

Loca lity of Santoft fore s t

J'orthern Santoft , showing study si t c loc -tions

3tudy sites in area 1

Lethotl of obtaining shell he i ght measurement wi th verni er calipers

Division of juvenile snail sizes int o s ize classes

Disper s ion of the tota l population of ~rea over the study period

Disper si on of the 1975-76 juvenile cohort (area 1) over the study period

~ispersion of ar ea 1 adults ; adults and l arge juveniles; and hibernat ing adults and large juveniles

Index of d i spersion (I) plotted against s ample mean (x) for the lupin-stem srunples

Population S2mpling results , a rea

Population s .c:ipling results , area 2

Po~ulation sar~plinG r esults , area 3

Population densities in area 1 over t he s ampl ing period

Hibern~tion da ta

ileather data: J anuary , 1976 - October, 1977 Distribution of adul t s and l ar ge juveniles captured in the September sample from area 1

Ar ea 3 :::.nd surrounding a reas , nhm,Jine sites of sam~les A1 - A3 and B1 - B7

Population samplins results, area 4 Populat ion co:·;posi tions of single- site samples, A1 - A3

P~pulation co1,.posi t ions of single-site samples , B1 - B7

Adult height distributions, area s 1, 2 and 3

Method of obta ining snail rad.ula measurements

Pl an of anvil study area, with 10 m radius of activity around each anvi l

Numbers of shells collected from anvils

Plan of anvil study area, with 20 m radius of activity around each anvil

x

Page

17 18

26

30

33

42

44

49

65 66 67

68

69 70

91

93

94

95 116

117

119

120

26

27

Juveni l e r <ldu lae mea .. mrements plott<?d a_;ains t shell height

R2.dula width me a surement plotted aga inst length measurement

xi

124

124

Table

I

II

III

IV

v

VI

VII

VIII

IX

x

XI

XII

XIII

XIV

Y0l

XVI

XVII

XVIII

XIX

xx XXI

XXII

XX III

XXIV

L I S T 0 F T:iBLES

Shell heisht distri bution of all juvenileo exceeding 1. 20 cm c~tured in area 1.

Sample rcsul t::.: froi:i a rea 3,

Jna i l disper~ion in rel~tion to lu~in in Qrca 1

:.; ;JTJ_::>lin0 r esults from a reas 1 anci 2.

I ndices of dispersion for total population results from areaJ 1, 2 and 3.

Indices of dispersion

Indices of dispersion for adults

Age/size compositions of lupin-ste~ sample !)Opul<itions.

Indices of dis?e~sion for lu~i~-stem samples

lfor,1bers of juveni l e '5ro~: th-checks on the shell s of adult ~nd sub- ad.ult anim~ls.

rurnbers of juvenile gro1·1th- checks on the shells of juveniles.

Age/s iz e d i stribution of 11 186 snails str.,ndcd on a 30 m section of bare sand tracks .

Growth- checks nur:ibcrs on the shells of adult snails .

l·iean adult shell heights

Gatch per anvil over the study period .

Anvi l catches as a function of a 10 m radius of thrush activity around each a nvil .

Anvil catches as a function of a 10 m activity radius around each anvil for the period 12/8 -2/12 .

Catch as a function of a 20 m activity r adius around each anvil for the period 12/8 - 2/12.

Percentage of total adult ca tch bearing the remains of a thick epiphragm for each collection.

Proportion of juveniles in anvil catches.

Height distribution3 of juvenile shells from anvils.

lfoasurements of rad.ul ac from thrush f aeces .

Palatabilities of ; reen leaf materi a l from different plant species .

Ethylene/Acetylene peak- he ight ratios .

xii

Page

34

40

4 1

43

45 47

48

48

50

71

73

74

92

96 121

121

122

122

123

125 125

126

143

148

Fronti sp i e ce

Pl a te 1

Pl a te 2

Pl a te 3

Pl a te 4

Pl a t e 5

Plate 6

L I S T 0 F P L A T E ;:3

He l i x aspersa Milller on lupin in study area 3

a . View of a rea 3

b . '.i . as .iersa estiva. ting on l upin

Growth-checks on adul t and l arge juvenile snails from area 3.

Snai l radulae prepar ed from thrush f aeces

I ncubation chamber f or a cetylene r eduction assay

3na il dam~ge to woody lupin s t ems

Sna il-damaged lupin stems

Pa ·e ,;

21

72

11 4

14 1

146

147