Density and environmental effects
on shell size in some sand dune
snail populations
P. TATTERSFIELD
1~ej)arltrienlof Zoology and Comparative PhysioloQ,
C'niuersity of
Birtninghnm, P.O. Box 363, Birmingham, BIS 2TT
I'o~~ulaiions
01' tlircc species 01' land snail, Helicelln ilola. C'nndiduln infersecln and Cochlicelln nculn are
sand duiics on Coll in the Inner Hebrides.
I'~i~iiilati~~ii
dciisity and mean shell size were estimated on 18 sample sites. The sites were ranked
lei. six eii\~ii-oiiiiiriitaltictors. Environmental tactors which are related to the dune vegetational
\uccwioii i i c c~u iitlor iiiuch of the variation in the densities of C'. inlcrsecla and C.nculn. The density
(11 / I . i / h , Iiowcvcl-, sliows no strong association with these factors. Both snail density and shell size
AN* i.cl.itivcly iiidrpcndrnt ol'total soil calcium levels. For each of the three species, mean shell size is
iicgitivvly ;issociatrtl with population density; the environmental factors account for little of the
v m i i l i o i i i i i slicll sizr.
E'(iui.~ x i ~ s i lincclianisiiis
~lc
are suggested to account tor the variation of shell size with density. I t is
argued that a direct inHuence of density on shell size, possibly mediated by mucus conditioning of
ilic c-iivil-oiiincnt is tlic most probable mechanism. There is some evidence to suggest an inter\lie( ilic cllcct wliclrby (;. inlerseclo density atlects H . ilalo shell size.
ciidictl o i i
KI:Y WOKI)S:--l)rnsity ell'ects -landsnails
- shell sue - sand dunes.
CONTENTS
Iiiiixductioii
~l'llc\ l U t l }
rvlvlllodh
RC~hlllI\
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . .
iIlC?l
. .
. . .
. . .
l)iwi\\ioii
. . . . . . . . . . . . . . .
/\c.tllo\\~lrtlgcIllcllth
. . . . . . . . . . . .
K(4iwiicc.s
. . . . . . . . . . . . . . .
R ~ ~ I ~ i t i ~ i i i I,ctween
slii~n
environmental variables
1'11(~ vitl-iati~iiii n snail density
. . . .
I'lic v m i i t i o i i i i i shell size
. . . . .
. . . .
. . . .
. . . .
. . . .
. . . .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
. .
. .
. .
.
.
. . . . . . . .
7I
7'
72
7.4
7I
7.i
70
77
xo
XO
INTRODUCTION
The distribution and abundance of many species of land snail have frequently
been related to both physical and biotic factors in the environment. Soil calcium,
shade, human disturbance, humidity and rainfall levels have been demonstrated
to be important influences on both geographical distribution (Boycott, 1934) and
in iiiore local habitat differences between species (Cameron 8c Palles-Clark, 197 1 ;
0024-4066/X 110.5007 1 + 1 1$02.00/0
71
0 198 1 The Linnean Society ol'London
72
P. TATTERSFIELD
Caiiieron & Morgan-Huws, 1975; Cameron, 1973). Considering the poor powers
ot' dispersal of many species, it is also likely that historical accidents may be
iiiiportant in distribution patterns (Cameron, Down & Pannett, 1980; Cameron,
Carter & Palles-Clark, 1980).
The nature of the forces limiting density, however, have remained more
obscure. The role of competition has remained controversial in spite of much
circumstantial evidence (Cain, Cameron & Parkin, 19691, and the occurrence of
absolute food shortages has been doubted (Williamson, Cameron 8c Carter,
1976). Recently, however, it has been demonstrated that population density can
inlluence growth rate, adult size, activity levels and fecundity in certain species of
pulllionate mollusc. Both field studies (Williamson, Cameron 8c Carter, 1976;
Yoin-Tov, 1972; Butler, 1976) and work on laboratory populations (Oosterhoff,
1977; Caineron & Carter, 1979; Herzberg, 1965) have tended to show that high
dciisity leads to a reduction in activity levels and growth and reproductive
processes. Food shortages, either of a quantitative or qualitative nature may be
iiiiplicated in some cases (Pomeroy, 1969; Butler, 1976). However, the other
authors considered food supply to be ample and suggest either behavioural
interaction and/or mucus conditioning of the environment as possible
iiiechanisins for such effects.
I n this study, the variation in shell size and population density of three species
of helicid mollusc Candidula intersecta, Cochlicella acuta and Helicella itala are
cxaiiiiiied and related to various environmental variables.
T H E STUDY AREA
All sampling was done during the first two weeks of September 1979 on two
said dune system on Coll, an island of the Scottish Inner Hebrides, lying about
70 kin north-west of Oban (National Grid ref NM 20 57). Both dune systems are
situated at the west end of the island (Fig. 1); the largest stretches in a northsouth direction across the width of the island, and it leads into the sea at two
sandy bays, Feall Bay to the north and Crossapol Bay to the south. The other
dune system is separated from the Feall-Crossapol system by about 3.5 km of
grazing farinland and heathland. It leads into the sea at Hogh Bay and extends
lor about 1 kin inland. Both systems contain a variety of habitat types ranging
along the dune vegetation succession from mobile dune areas, characterized by
large tussocks of inarrain grass, Ammophila arenaria (L.),interspaced by patches of
bare sand, to machair. The machair typically lies on the Hatter, lower parts of the
duiies; its vegetation is distinctive with complete cover by a short sward
containing a large variety of herbs and grasses. Marram grass is absent and the
ar-cas are heavily grazed in comparison with the mobile dune areas.
METHODS
Eighteen sites, C1-C18, each approximately 200 m2 were sampled. Each site
was chosen so as to avoid obvious discontinuity in either vegetation o r
topography. Random, square 0.25 m2 quadrats were taken at each site and all
living snails resulting from a careful search were collected. The numbers ofeach
species in each quadrat were recorded and the whole site sample retained for
slielt size ineasureinent. O n sites where snail density was low, extra samples were
DENSITY EFFECTS I N SNAILS
.~igiirc
..
.I .
^.
I
..
I
I.
5kctCli iiiap 01 LOII
13
.
..
. ,. .
..
r .
inaicating tne approximate positions or tne eignteen sample sites.
Table 1. The number of snails found on
the soil surface and buried from five
random 25 x 25 cm turves taken on site
c1
c. /ICUl/l
'I'url'
011
surlicr
Buried
H . ilala
On surface
Buried
J
5
3
5
2
0
3
0
I
0
0
0
1
15
Total
13
1
50
1
2
Y
3
16
0
9
0
1
1
1
3
taken by collecting every snail found during a search after quadrats had been
taken. Snails collected in this manner were placed in a separate container. Five
25 x 25 cin turves, approximately 7 cm in depth were also sieved from site C 1.
Table 1 gives the numbers of snails found buried and on the surface. Considering
the few snails found buried, no further sieving was done.
Each site was scored for six environmental variables:
( 1) The density of marram grass was estimated by counting the number of leaf
blades in the quadrats used for snail sampling.
P. TATTERSFIELD
14
(2) Tlie inclination of’each site was estimated by eye. Slopes were ranked from
sliallow to steep slope.
(3) Tlie proportion of bare ground or sand was also estimated by eye.
(4) Grazing pressure was estimated by the presence and relative frequency of
lierbivore tracks and faeces.
( 5 ) Surface soil colour was ranked from light to dark from soil samples taken
troiii 16 of the sites. Ranking was accomplished by direct comparison in the
hbordtor)’.
( 6 ) Total calcium content was also measured from the soil samples. Dried,
weighed samples were dissolved in excess hydrochloric acid. Flame
photoinetry was then used to measure Ca*+concentration. The analysis was
repeated and the mean values ranked.
All sites were visited on one morning and ranked for variables 2 , 3 and 4. Sites
wliicli were particularly similar were revisited and a decision made as to their
d a t i v e position. The ranking procedure used allows the use of non-parametric
statistics, thus avoiding assumptions about the sampling distributions of the
variables under study.
Vernier callipers were used to measure shell sizes in the three snail species.
Sliell diameters of C. intenecta and H . italn and the shell height of C. aculn were
iiieasured. It was felt that these dimensions would show greatest variation as a
result ot’ growth. Shell sizes were measured to the nearest 0.1 mm.
Tlic relationships between all variables have ben investigated with the use of
Spcariiian rank correlation coeflicients (rs).A correction factor has been applied
where tied ranks occurred (Siegel, 1956). Regression methods have been used to
determine the nature of the relationships between snail density and shell size.
Nomenclature for snails follows Kerney 8c Cameron ( 1979).
RESULTS
Relationships between environmental variables
Rank correlation coetlicients between the environmental variables are given in
Table 2. With the exception of total soil calcium, all the environmental variables
are highly inter-correlated, only the correlation between bare ground and
1’aI)lc2. R a n k
correlation coetlicients (rs between the environmental variables
with i i u i i i h ol’saiiiples (n)and probability levels ( P )
Spc.;iriiiaii
Bare
ground
C; razi iig
prcxsurc
liidiiic
oliiic
”
I,
I’
IN
0.XlO
<0.001
Soil
colour
P
n
I,
P
I8 -0.1162
(0.001
18
0.639
(0.01
IX -0.82.5
<0.001
I8
0.449
I/
Is
Soil
calcium
P
n
rL
P
16 -0.816
(0.001
16
0.197
>0.1
<0.1
16 -0.791
iO.001
16
0.359
>0.1
(0.01
16
<0.001
16 -0.Oti5
>0.1
(0.01
16
>0.1
n
I s
M ;I1.1 iiiii
tlciisity
I iidiiiv
01 s i w
Giiiiiiig
1”
esslII c
18 -0.671
0.855
H;ii.c.
#I.(
J1111(1
16 -0.703
0.246
Soil
(
(JIOIII.
16 -0.462
(0.1
75
DENSITY EFFECTS I N SNAILS
inclination having a probability level greater than 5%. This suggests that
variation in each of these factors is related to the overall successional stage of the
sites. Total soil calcium is unrelated to this successional sequence, and
exaiiiination of calcium distribution with respect to geographical location
suggests that the distribution is patchy. The three sites on the Hogh Bay dune
systein, C1, C11 and C10 are ranked very close at the top of the calcium scale;
sites situated close together on the Feall-Crossapol system also tend to have
siiiiilar soil calcium levels. Overall, soil calcium ranges from c. 4 to 15%.
The variation in snail density
Table 3 suininarizes the density estimates of each species on the 18 sites. One
way analyses of variance are highly significant (all P < 0.001) between sites for all
species (even when sites with zero densities are excluded). Table 4 gives rank
correlation coethcients between the environmental variables and snail densities.
None ofthe species’ densities correlate well with total soil calcium. The density of
(,’. ncula shows a inarked association with typical semi-tixed or mobile dune
Iiabitats. Cnndidula inlerseclu shows a reverse pattern, being found at highest
tleiisities on inachair sites; it is absent from six sites, three of which are on the
tIogli Bay systein where no C. intersecla were found in spite of an intensive search.
This systein is isolated from the Feall-Crossapol system and since there
appeared to be suitable habitats for C. inlersecla (an introduced species in Britain
(Caiiieron & Redfern, 197611, its absence may be the result o f a failure to disperse.
Two 01’ the reinaining sites are typical mobile dune, but the third is anomalous, a
rnachair site lying adjacent to others carrying high densities of C. intersecla.
Ifelicella italu occurs o n all sites sampled and shows a great range of densities
wliicli, however, do not correlate well with any of the environmental variables.
Table 3. Mean snail densities (no. per 0.25 m2)and standard errors for
the 18 sample sites
$iic.
11. 1lrrlfl
MCAII
dcti\ity/quddrat
9.55
0.80
I .90
1 .oo
2.40
2.62
3.17
I.5.0X
0.83
2.33
9.73
2.36
0.43
2.36
7.31
0.25
5.56
I.50
c. flculfl
C. inlersecln
S.E.
1.21
0.36
0.50
0.45
0.62
0.57
0.68
2.04
0.28
0.86
1.25
0.69
0.23
0.52
0.71
0.14
1.02
0.43
Mean
density/quadrat
S.E.
Mean
density/quadrat
S.E.
-
-
2.10
0.73
0.20
0.20
8.20
2.90
2.69
3.75
0.13
0. I3
2. I5
0.60
0.64
0.76
-
-
9.36
7.43
4.00
0.75
1.20
1.58
0.55
0.25
7.81
5.70
1.08
0.75
-
-
-
-
-
-
0.10
1.60
0.10
0.86
0.08
9.83
0.11
1.00
13.64
0.29
0.07
4.86
0.75
4.69
5.19
0.60
0.08
1.87
0.10
0.33
2.32
0.22
0.07
0.84
0.35
0.76
0.87
0.34
-
-
P. TATTERSFIELD
76
Table 4. Rank Spearman correlation coeficient (rS)between the
environmental variables and snail densities with number of samples
(n) and probability levels ( P )
It. ilnla
density
P
/I
~
n
C.inlersecla
C . acula
density
density
TI
P
~~~~~~~~~~
n
rs
P
~~~~~~~
M .,111.1111
...
tlciisil)
18
0.213
>0.1
12
-0.709
(0.01
15
0.767
(0.001
18
0.428
(0.1
12
-0.578
(0.05
15
0.789
<0.001
I8
-0.083
>O.l
12
0.518
(0.1
15
-0.704
(0.01
18
-0.151
>0.1
12
-0.595
(0.05
15
0.633
(0.01
16
-0.075
>0.1
10
0.675
(0.05
13
-0.742
(0.01
>0.1
10
-0.571
>0.05
I3
0.440
I iicliiic
01 siic
Gr.iLiiig
~)I'CS$LII'C
IhlI~
gl.OUIIII
Soil
('OIOUI.
Soil
talriuiii
16
0.078
>0.1
Associations between species are weak. Candidula inlersectn density is
independent of both C. acuta (rs =-0.217, n = Q , P>0.1) and H. itala (r, =-0.119,
n= 12, P > 0.1) densities. Cochlicella acuta and H . itala, however, show a weak
association (rs =+0.574, n= 15, P < 0.05).This correlation remains when zero
densities are included.
The variation in shell sire
The snails collected on each site represent a sample of all size classes in the
population. Frequency histograms have been drawn for each species on each site
where numbers permitted ( n > 3 5 ) . Size classes of 1.0 mm were used for C. nculn
and C. intersecta. For both these species, size class distributions were in general
syiiiiiietrical about the mean and unimodal. Size classes of 1.5 mm were used for
If. itnln. The distributions of this species were more complex, being bimodal on
some sites and skewed on others. No trend in form of size class distribution could
be found with respect to either snail density or the environmental variables; it
was therefore decided to use mean shell size as an estimator of shell size. The
lollowing analyses for H. ilala have been repeated using ranked modal classes of
sliell size and the results are essentially similar to those using the ranked means.
Table 5 shows that mean shell size and the environmental factors are in general
uiirclated, the only significant correlations being between H . itala size and bare
ground and soil colour (positively with light coloured soil).
Table 6 and Fig. 2 show the relationships between shell sizes and density. All
three species show significant negative regressions of'shell size on log density. For
each of' the species, regressions of shell size on the logarithm of density give a
better lit than a linear model. A curvilinear response of shell size with density is
thus suggested. Inter-specific relationships are in general insignificant, except
that C. intersecta density appears to affect H. itala shell size (mean shell size on the
natural logarithm of density: r=-0.663, n= 12, P(0.02). I t is of interest in this
context that C. intersecla density is negatively correlated with bare ground and soil
colour, i.e. significantly, but in the opposite direction to the correlations between
tliose environmental variables and H. itala shell size, leaving open the possibility
DENSITY EFFECTS I N SNAILS
77
Table 5. Rank Spearman correlation coefficients (rs) between the
environinental variables and ranked mean shell sizes with number
of samples ( n ) and probability levels ( P )
Ii. ilnln
shell diameter
I1
TI
C. inlersecla
c. nculn
shell diameter
shell height
P
n
rr
P
n
rs
P
M .II'I'.IIII
tlriiriiy
I I N liiw
ol'\iir
(; i..txi
18
0.310
>0.1
12
0.354
>0.1
8
-0.119
>0.1
18
0.082
>0.1
12
0.357
>0.1
8
-0.143
>0.1
ng
I)IX'~\III'C
18
-0.323
>0.1
12
-0.182
>0.1
8
0.167
>0.1
I8
0.507
(0.05
12
0.302
>0.1
8
0.132
>0.1
16
-0.503
(0.05
10
-0.455
>0.1
7
0.179
>0.1
16
0.196
>0.1
10
0.576
(0.1
7
-0.321
ILIlr
~I-OUIIC~
Soil
COIOII~
Soil
1
C.. I ( I:
IIIII
>O.l
Table 6. Regression equations for plots of mean shell size on
the natural logarithm of density. Number of samples (n),
intercept (a),slope (b)and correlation coeficient ( r )
I/
llll~ll
I
1 1 1 l 1 ~ 1! f ? l l l
(. Ill
lrlrr
n
a
b
18
12
8
10.92
7.67
11.45
-0.36
-0.49
-1.10
s.E.(~) P
0.30
0.09
0.20
(0.005
<0.001
(0.05
r
P
-0.674
-0.793
-0.709
(0.005
(0.005
(0.05
that the latter are an indirect consequence of this apparent inter-specific
interaction.
DISCUSSION
The distribution of both C.acuta and C. intersecta show marked association with
distinctive habitat types. Cochlicella acuta predominates in semi-tixed or mobile
dune habitats, whereas C. intersecta is found at highest densities on machair.
Similar habitat associations have been reported for these species on dune systems
elsewhere: Baker (1968) considered the distribution of C.intersecta at Braunton
Burrows, Devon, to be restricted by high soil water content in slack areas and by
lack of shelter on the exposed mobile dune. Annual water-table profiles for the
sites on Coll are not known. However, the vegetation on the machair sites was
not typical of areas which are frequently submerged. The island of Coll is
regularly exposed to high winds and it seems likely that the distribution of
C. intersecta may be limited by its intolerance to wind-blown sand on the mobile
dune sites. The association of C. acuta with habitats in which there is much
iiioveinent of' sand is in agreement with Boycott (19341, and Lewis (1977) who
surveyed populations of this species along the west European seaboard.
Although Boycott (1934) considered H . itala to be the most extreme xerophile
found in Britain, its distribution on Coll is not closely related to any of the
P. TATTERSFIELD
18
15.0
-
14.C
-
A
4
c
5
r
w
9.0 8.0 -
10.0
9.0
8.0
7.0
6.0
13.0
-
12.0
-
10.0
11.0
-
C
lo
~
1
I
1
I
I
0
I
Figwc 2. Plots 01' iiieaii shell size on log. density for (A) H. ifah, (B)C. Intersecfa and (C) C. nnrla.
Vcrt i c d 1);u.s iiitliratc oiic standard error about the mean shell size. Regression equations are given
i i i 'I';ilik
6.
environmental variables recorded. I t may be that all sites studied are equally
open to colonization by H. itulu. Although the range of soil calcium content is
relatively large, it accounts for little of the variation in either shell size or snail
density. Similar habitat separation has been demonstrated in other helicids:
Cameron ( 1969) shows that dampness of habitat has a striking intluence on the
DENSITY EFFECTS IN SNAILS
79
distribution of Arianta arbustorum. The distributions of the closely related species
horlensis which often occur in the same general area, may
usually be separated by habitat or topography (Cain & Currey, 1963; Carter, 1968;
Cameron 1969, 1970).
The negative relationships between shell size and population density suggest
that some form of density dependent process is affecting mean shell size in each
species. There is a suggestion of a curvilinear response, density having most
infiuence on shell size when it is low.
Before considering possible mechanisms, it should be noted that changes in
mean shell size may arise either as a result of a simple shift in the population size
structure or a more complex change in the distribution of size classes. A complex
change in size class structure seems unlikely at least for C. acuta and C. intersectn
whose size class distributions were similar for all sites studies. Oosterhoff (1977)
and Williamson el al. (1976) avoided such complications by considering final
adult size in C. nemoralis, but adults cannot easily be distinguished in the species
studied here.
At least four different, but not mutually exclusive, mechanisms can be
suggested to account for the shell size/density relationships.
( 1) Competitive interaction for some limiting resource.
(2) Genetical differences between populations.
(3) Variation in annual growth or reproductive patterns between sites.
(4) A direct influence of density on growth rate or adult shell size.
Ultimately, these hypotheses can only be tested experimentally, but the
balance of circumstantial evidence favours the fourth possibility. Slower growth
(or reduced final size) resulting directly from a resource shortage ( 1) would only
give a simple relationship between shell size and density if the latter fortuitously
correlated with nearness to carrying capacity. The latter clearly varies between
habitats, as indicated by the associations between density and environmental
tictors in C. acuta and C. intersectu.
Siinilarly, while genetic differences between populations or environmental
difterence between sites could (and probably do) influence growth-rates and adult
size, they could only account for the observed correlation between shell size and
density if‘ they themselves are correlated with density. The extent of genetic
ditterences are unknown (see below). In C.nemoralis populations, at least, the
shell colour and banding polymorphism appears unconnected either to variation
in density between populations or to changes in density within a population
(Williamson el al., 1976, 1977). Environmental differences between sites in this
study do correlate with densities in two species, but not with shell size as such.
As the populations were only sampled on one occasion, the influence of
temporal changes in resource levels or variation in the seasonal growth or
reproductive patterns cannot be determined. Differences in the frequency or
duration of‘ resource shortages between sites might be expected to have a
cumulative eft’ect on a continuous variable such as shell growth. In this case
however, snail distribution would seem to be unrelated to such a process. Carter,
JefYery & Williamson ( 1979) suggest that seasonal variation in the abundance of
preterred food types may put mixed populations of C. nemoralis and C . horlensis
under competitive stress. Baker (1968) notes that the breeding season of
C.intersecta is variable and suggests that its onset and duration may be influenced
by environmental conditions.
Cej,nen nemoralis and C.
80
P. TATTERSFIELD
The observations reported here provide circumstantial evidence for a causal
relationship between population density and processes influencing shell size in
populations of three species of helicid land snail. Similar negative correlations
between shell size and population density have been reported in C.nemoralis
(Williainson et al., 1976; Cook 8c Cain, 1980). Other ecologically important
characteristics such as activity levels, survival rates and clutch sizes have also been
related to density in this and several other species of pulmonate mollusc
(Oosterhoft’, 1977 gives a review; Cameron 8c Carter, 1979). Several mechanisms
have been suggested to account for such relationships; where food supply was
considered to be adequate these have often rested on the presence of inhibitory
compounds in the mucus. Mucus pre- treatment experiments tend to support
such conclusions (Oosterhoff, 1977; Cameron 8c Carter, 1979; Dan, 1978).
Laboratory experiments to determine the influence of population density on the
activity of the three species involved here (Tatterstield, in prep.) suggest that the
activity levels of H . itala and C.intersecta are suppressed at high densities.
Cochlicella acuta activity however showed no change under the density treatments
used.
Shell size has been shown to have a hereditary component in some species
(Cook,1965, 1967). Large individuals of C. nemoralis in general lay more eggs and
iiiore frequent clutches (Oosterhoff, 1977; Wolda, 1970), and thus have a
selective advantage. However, larger snails may also be more prone to predation
(Cook8c O’Donald, 1971; Bantock, Bayley 8c Harvey, 1976). Cook 8c Cain (1980)
have suggested that a negative relationship between size and density and a
positive one between output and size may regulate size in C. nemoralis. The data
presented here suggest that the influence of density is very large for each of the
three species. There is evidence to suggest that C. intersecta density influences
ff. ilnln shell size. Cameron 8c Carter (1979) and Dan (1978) demonstrate
interspecific inHuences of density on activity although, as in this study, they
generally seem to have less magnitude than intra-specific effects.
The significance of such adaptations is not known, but it seems possible that
the ability to respond to the presence of other snails might, along with other
environmental cues, allow snails to optimize their behavioural strategy. Snail
species which live in relatively dry habitats such as sand dunes can survive long
periods of dry weather in aestivation. Internal food reserves may therefore be
crucial to survival in dry habitats (Pomeroy, 1969). The ability to make a rapid
assessiiient of the availability of resources and the degree of potential
competition for those resources when activity becomes possible may thus be
highly advantageous so as to avoid wastage of energy reserves.
ACKNOWLEDGEMENTS
I would like to thank Dr R. A. D. Cameron for his help, encouragement and
supervision throughout this study. The research was carried out while in receipt
of‘a Science Research Council studentship.
REFERENCES
BAKER, R. E., 1968. The ecology of the wrinkled snail. Helicellu cuperulu (Mont.)on the Braunton Burrows sand
t l u i i c . systciii. Proceedings o/the hfdatolO@d Society of London, 17: 41-54.
UANTOCK, C. R., BAYLEY, J. A. & HARVEY, P. H., 1976. Simultaneous selective predation of two features of
ii iiiixctl sildiiig population. Evolulion, 29: 636-649.
DENSITY EFFECTS IN SNAILS
81
BOYCOTT, A. E., 1934. The habitats of'land mollusca in Britain. Journal of Ecology, 22: 1-38.
BIi'I'LkX, A. J.. 1976. A shortage off'ood for the terrestrial snail Hclicella virgata, in South Australia. Oecolohia,
Ijrrliii, 25: 349-37 I.
CAIN, A. J., CAMERON, R. A. D. & PARKIN, D. T., 1969. Ecology and variation in some helicid snails in
iiortlierti Scotland. Proceedirqp of the MdQCOlOhiCd Society of London, 38: 269-301.
al
oflhe Royal
CAIN, A. J. & CURREY, J. D., 1963. Area effects in Cepaea ncmoralis (L.). P h d ~ ~ o p h i ~Transactions
Socidy OJ /.ondon 8 . 246: 1-81.
CAMERON, R. A. D., 1969. The distribution and variation of three species of land snail near Rickmansworth,
1icwIi)rdsliire. Zoo~o~icdJOurna~
of the L i m a n Society, 48: 83-1 11.
CAMERON, R. A. D., 1970. Dittkences in the distributions of three species of helicid snail in the limestone
district 01' Derbyshire. Proceedings oflhe Royal Society of London B, 176: 131-159.
CAMERON, R. A. D., 1973. Some woodland mollusc faunas from southern England. MalacoloGa, 14: 355-370.
CAMERON, R. A. D. & CARTER, M. A., 1979. Intra- and inter-specific effects of population density on
growth and activity in some helicid land snails (Castropoda: Pulmonata). Journal of Animal Ecology, 4 8 :
237-246.
CAMERON, R. A. D., CARTER, M. A. & PALLES-CLARKE, M. A., 1980. Cepaea on Salisbury Plain: patterns
ol'variation, landscape history and habitat stability. ~ i o l o ~ i c a ~ J o uofr nthe~ L i m a n Society, 14: 335-358.
CAMERON, R. A. D., DOWN, K. & PANNElT, D. J., 1980. Historical and environmental influences on
lictlgcrow snail faunas. Biolohical Journal of the Linncan Society, 13: 15-87.
CAMERON. R. A. D. & MORGAN-HUWS, D. I., 1975. Snail faunas in the early stages of chalk grassland
succession. Biolohica1Journal of the Linncan sociely, 7 : 215-229.
CAMERON, R. A. D. & PALLES-CLARKE, M. A., 1971. Arianta arbustorum (L.) on chalk downs in southern
Eiiglaiid. Proceedings of lhe Malacolo&d Sociely of London, 3 9 : 31 1-318.
CAMERON, R. A. D. & REDFERN, M., 1976. British Land Snails.Synopses ofthe British Fauna (New Series), No. 6.
Liliiiran Society ot'London. London: Academic Press.
CAKI'ER, M. A., 1968. Studies on Cepaea. 11. Area effects and visual selection in Cepaea m r a l i s (L.)and Cepaea
hrLmi.\ (Mull.). Philosophical Transactions ofthe Royal Society of London B, 253: 397-446.
CAKTER, M. A., JEFFERY, R. C. V. &WILLIAMSON, P., 1979. Food overlap in coexistingpopulations ofthe
latitl snails Cepaen nemoralis (L.) and Cepaea hortensis (Mall.). B i o l o & l J o u d o j the Linncan Society. 11:
169-1 76.
COOK, L. M., 1965. Inheritance of'shell sue in the snail Arianta arbustorum. Evolution, 19: 86-94.
COOK, L. M., 1967. The genetics of'Cepaea nnmalis (L.). Heredity, 22: 397-410.
COOK, L. M. & CAIN, A. J.. 1980. Population dynamics, shell sire and morph frequency in experimental
populations 01' the snail Cepaea rumoralis (LJ. B i o l o A i c d J o u d ofthe Linnean Society, 14: 259-292.
COOK, L. M. & O'DONALD, P., 1971. Shell size and natural selection in Cepaeawnoralis. In R. Creed (Ed.),
/*kilu&d Genelics and Evolution. Essays in Honour of E. B . Ford: 93-108. Oxford: Blackwell.
DAN, N. A., 1978. Studies on the Growth and Ecology of Helix aspersa (MUU.). Ph.D. Thesis. University of
Maiicliestcr.
Ii ERZEBERC, F., 1965. Crowding as a factor in growth and reproduction o f H e l i x upersa. American Zoologist, 5 :
354.
KERNEY, M. P. & CAMERON, R. A. D., 1979. A Field Guide to the Land Snails ofBritain and North-Wesl Europe.
Lotitloti: Collins.
LEWIS, G., 1977. Polytnorphisin and selection in Cochlicella acuta. Philosophical Transactions of the Royal Society of
/ . d O i i , B, 276: 399-45 1.
OOSrERIiOFF, L. M., 1977. Variation in growth rate as an ecological factor in the land-snail Cepaea nemoralis
(L.1. ,VetherlandsJournal of Zoology, 2 7 : 1-192.
I'OM EKOY, D. E., 1969. Some aspects of the ecology of the land snail, Helicella virgata, in south Australia.
Aurlrrdiiin Journal o/ Zoology. 17: 495-514.
SI ECEL, S., 1956. Non-Parametric Statisticsfor the Bchavioural Sciences. New York : McCraw-Hill.
WILLIAMSON, P., CAMERON, R. A. D. & CARTER, M.A., 1976. Population density affecting adult size of
siiail Cepiea neinoraks L. Nature, 263: 496-497.
WILLIAMSON. P., CAMERON, R. A. D. & CARTER, M. A., 1977. Population dynamics of the land snail
(.'qil,nm iuiiiornlis L.: a six-year study. Journal of Animal Ecology, 46: 181-194.
WOLDA, Ii., 1970. Variation in growth-rate in the land snail Cepaea nemoralis. Researches on Population Ecology,
E;YO/U. 12: 195-204.
YOM-TOV, T., 1972. Field experiments on the effect of population density and slope direction on the
reproduction 01' the desert snail Trochoidea (Xcromassa) seetzni. Journal of Animal Ecology, 41: 17-2 1.
6