Full text of DOI:10.1093/mollus/67.4.463

LAND SNAIL COMMUNITIES IN MEDITERRANEAN UPLAND
GRASSLANDS: THE RELATIVE IMPORTANCE OF FOUR SETS OF
ENVIRONMENTAL AND SPATIAL VARIABLES
CORINNE LABAUNE AND FRÉDÉRIC MAGNIN
Institut Méditerranéen d’Ecologie et de Paléoécologie – UMR 6116 du C.N.R.S., Case 461, Faculté des Sciences de Saint-Jérôme,
F-13397 Marseille Cedex 20, France. (e-mail:labaune.corinne@libertysurf.fr)
(Received 16 October 2000; accepted 20 April 2001)
ABSTRACT
It is difficult to define precisely determinant environmental factors that explain land snail distribution
and abundance, and most variables are often interrelated. In addition, studies have generally examined
only a limited number of environmental variables, yielding a partial and local habitat description.
Recognised determinant factors are: climate, vegetation and soil. Competitive interactions, historical
factors and spatial structure are rarely included, although they could generate biogeographic patterns
on a local or regional scale. The goal of the present work is to evaluate land snail habitat complexity by
examining a wide range of environmental variables. Land snails from open environments were studied
in a Provence range, the Grand Luberon, characterised by low variations in pH and calcium content.
Grasslands on the ridges are maintained by sheep grazing. A stratified sampling was chosen according
to altitude and vegetation structure. Sites were sampled throughout the range. Different vegetation
structure types were analysed, from low grasslands to shrublands. Thirty-eight environmental variables
were noted for each site. They described: (1) topography and climate, (2) habitat structure, (3) flora, and
(4) spatial structure of snail communities. The main analytical method used was the partial Canonical
Correspondence Analysis which yielded a partition of species variation into several independent
components. The four environmental variable groups were used separately. We obtained a partition of
ecological variation which emphasised the predominance of habitat structure over floristic composition
and the important role played by environmentally independent spatial variables. Nevertheless, it
remains difficult to discriminate between the influence of climatic factors s.s., historic events and/or
biotic factors within the altitudinal gradient.
these complex gradients into different explanatory
variables (Outeiro, Aguera, & Parejo, 1994; Barker &
INTRODUCTION
Mayhill, 1999). However, even these explanatory vari-
ables are generally not diversified enough to reflect the
true complexity of the habitat. In general, factors
identified as determinant are: climate (temperature,
rainfall, humidity), vegetation (floristic structure and
composition) and soil (pH, calcium content and tex-
ture). Competitive interactions, historical factors and
spatial structure are rarely included, although they
could generate biogeographic patterns on a local or
regional scale (Cameron, Down & Pannett, 1980;
Magnin, 1993; Magnin, Tatoni, Roche & Baudry, 1995).
The goal of the present work is to evaluate land snail
habitat complexity by examining a wide range of en-
vironmental variables. In particular, we: (1) define the
respective roles of flora and habitat structure; (2) quan-
tify climatic and topographic parameters which could
be limiting factors for land snail species and (3) demon-
strate the value of non-environmental factors used to
explain community spatial structure.
Many studies on the distribution and abundance of
land snails in relation to environment have been pub-
lished since Boycott’s classic review (Boycott, 1934).
However, it is difficult to precisely define determinant
environmental factors (Wallace, 1912), and the vari-
ables are often interrelated (Goodfriend, 1992). In
addition, most studies have only examined a limited
number of environmental variables (e.g. the soil),
yielding a partial and local habitat description. Some
authors (Bishop, 1977; Coney, Tarpley, Warden &
Nagel, 1982) have analysed a wider number of vari-
ables and concluded that snail distribution and abund-
ance are related to regional climate factors and local
physical and chemical characteristics of soil. Multi-
variate analysis has revealed the case by case relation-
ships between snail community organisation and
environmental gradients (Dillon, 1980; André, 1982).
The development of Canonical Correspondence Analy-
sis (ter Braak, 1986) makes it possible to break down
J. Moll. Stud. (2001), 67, 463–474
© The Malacological Society of London 2001

CORINNE LABAUNE & FRÉDÉRIC MAGNIN
on the works of Evans (1972), Puisségur (1976) and André
Open-environment communities were studied in a
Provence range characterised by low variations in pH
and calcium contents. The main analytical method used
was the partial Canonical Correspondence Analysis
(Borcard, Legendre & Drapeau, 1992) which yielded a
partition of species variation into several independent
components (Økland & Eilertsen, 1994).
(1981, 1982) was used to collect less than 5 mm diameter
shells: vegetation, litter and surface soil down to 5 cm were
sampled over a 25 ꢁ 25 cm plot. Five samples were taken
within the 5 ꢁ 5 m square already sampled for large species.
Samples were dried in an oven, then immersed in water.
Floating particles were collected in a 0.5 mm mesh sieve and
shells were then separated from plant material under a
magnifying glass for the smallest portion. The list of species
with their occurrence and abundance is given in Appendix 2.
Floristic and environmental records
MATERIAL AND METHODS
A standard procedure of description was used, based on
Godron (1968). Four variable groups were used. Variable
descriptions and coding are shown in Appendix 1.
The first group (CLIMATE) consisted of 12 climatic or
topographic variables: altitude, local humidity, grassland
patch size, flat site, halfway up location, slope top, summit,
no defined exposure, north facing, south facing, east facing,
west facing.
The second set of variables (STRUCTURE) consisted
of 14 habitat structure variables. Six variables describing
overall soil surface cover were used: bare ground, stones,
boulders, rock, litter, vegetation. Five variables described
different plant layer covers: 0–5 cm, 6–15 cm, 16–25 cm,
26–50 cm, 51–100 cm. Three variables were used to describe
different plant types: cryptogams, herbaceous plants, low
woody plants.
The third group (FLORA) included seven of the most
common dominant plant species, i.e. having the highest cover
(Buxus sempervirens L., Carex humilis Leyss., Bromus erectus
Huds., Festuca ovina s.l., Helianthemum oelandicum (L.),
Thymus vulgaris L., Anthilis vulneraria s.l.).
Both floristic cover and habitat structure variables were
estimated using a visual surface ratio chart (Folk, 1951).
The fourth variable group (SPACE) consisted of five vari-
ables characteristic of site position in space and compared to
other sites. The first two variables were the UTM coordinates
(latitude and longitude) of the sites. The other three variables
described distance between sites. A Hierarchical Cluster
Analysis was performed to obtain these three variables using
the Euclidian distance between their coordinates. Qualitative
variables were then created corresponding to the groups
defined by the hierarchical cluster at different truncation
levels (Dist5, Dist10, Dist20), i.e. divisions into respectively
5, 10 and 20 categories.
Study area
The Luberon is a limestone range approximately 40 kilo-
metres long running from east to west and located in Pro-
vence (France), 50 kilometres from the Mediterranean sea.
Our study focused on the eastern side, known as the ‘Grand
Luberon’, which reaches an altitude of 1125 m.
Due to its length, altitude, climate and orientation, the
Luberon is a climatic barrier on the southern edge of the Oro-
Mediterranean climate zone. There is a larger amount of
precipitation than in other Provence mountains and tempera-
tures are cooler (Livet, 1965). Mean annual temperatures for
the Grand Luberon range between 11ꢀC and 12.5ꢀ at the foot
and between 7ꢀC and 9ꢀC at the summit. There is only one dry
month (according to Gaussen’s definition) and 4–5 cold
months when temperatures fall below 7ꢀC (C.N.R.S., 1975).
Rainfall was estimated at 802 mm per year at the summit
(Silvestre, 1977). The harsh climate is accentuated on the
ridges and northern slopes by a fierce north-westerly wind
(‘Mistral’).
The Grand Luberon’s ridges are covered by a thin band
of low grasslands between 900 and 1125 m. The Luberon
Regional Natural Park maintains these grasslands by en-
couraging grazing both to create a firebreak between the two
slopes and to preserve a remarkably rich biological area. The
slopes themselves are highly wooded (primarily Quercus
pubescens and Quercus ilex formations) and open spaces are
limited to variable-sized clearings.
Soils are superficial and gravely on the ridges and slope
heights with a pH between 6.8 and 7.6. CaCO3 content varies
between 0 and 2%.
Land snail sampling
A stratified sampling was chosen according to altitude and
vegetation structure. Sites were sampled throughout the range.
Only open environments were studied. Different vegetation
structure types were analysed, from low grasslands to Buxus
sempervirens shrublands, including high ungrazed grasslands
with or without low woody vegetation. Sites were distributed
equally between 8 altitude categories, from 700 m to the sum-
mit. When possible, each different vegetation structure type
was equally represented for each altitude category. A total of
80 sites was studied from March 13 to May 6, 1997.
Multivariate analysis
Statistical analyses were performed using CANOCO 4.0 (ter
Braak & Smilauer, 1998).
The species-sites matrix was first studied by Correspond-
ence Analysis (CA). Abundance data were transformed
logarithmically to make their distribution correspond to a
normal law (Legendre & Legendre, 1984). 35 snail species
were studied, but rare species abundance was down-weighted
by an algorithm available in CANOCO.
A 5 ꢁ 5 m plot was examined by two researchers at each
site for 15 min. All living or fresh dead snails were sampled,
then determined and counted in laboratory. A method based
The effect of environmental variables on the species matrix
was then studied using Canonical Correspondence Analysis
464

LAND SNAIL COMMUNITIES IN MEDITERRANEAN UPLAND GRASSLANDS
(CCA) (ter Braak, 1986). The four environmental variable
groups were used separately. This made it possible both to
limit the number of variables for each analysis, as suggested
by Borcard et al. (1992) and Prodon and Lebreton (1994),
and analyse all available variables.
The best descriptive variables were chosen by step-by-step
selection. The significant characteristic of each variable
selected and ordination axes were verified by a permutation
test available in CANOCO (Monte Carlo test). The signifi-
cance level was set at p ꢂ 0.05.
Partial canonical correspondence analyses were used to
define the amount of variation explained by the four variable
groups analysed in pairs (Økland & Eilertsen, 1994). Borcard
et al.’s (1992) method was used. When two variable groups
are analysed using this method, one is treated as a set of co-
variables, while the other is considered to contain explana-
tory variables. This made it possible to calculate variation
percentages due to either one or both variable groups
studied.
terised by sites at an altitude of more than 1050 m,
i.e. near the summit. There is a significant correlation
between site altitudes and their rank on axis 1 of the
CA (r ꢃ 0.684, p ꢃ 0.01, n ꢃ 79).
Axis 2 is a habitat structure gradient. The sites with
the strongest contributions on the negative side show
the highest diversity in vegetation stratification and
soil cover. These are Buxus shrublands where the dif-
ferent layers are relatively equally represented with an
extremely heterogeneous soil cover. On the other hand,
the positive end has the sites containing low grasslands
where the lowest herbaceous layer has a cover of more
than 90%.
Gastropod species distribution is as follows for the
two gradients (Fig. 1A):
The highest contributions to axis 1 are Trochoidea
geyeri (Soós) on the negative side and Candidula uni-
fasciata (Poiret) on the positive end which are mainly
responsible for the swarm’s horizontal shape of the
plot. These two Hygromiinae are the most common in
the Grand Luberon and have altitude related distribu-
tions: T. geyeri occupies the highest sites and is re-
placed by C.unifasciata at sites lower than 900 or
1000 m. Typical mountain species of southern France
can be found on the negative end, next to T. geyeri,
i.e. Pupilla triplicata (Studer), Clausilia rugosa parvula
Férussac and Abida secale (Draparnaud). On the posi-
tive side, C.unifasciata is associated with southern
species less frequent at high elevation, i.e. Candidula
gigaxii (Pfeiffer), Jaminia quadridens (Müller) and Abida
polyodon (Draparnaud).
RESULTS
Ordination of land snail communities along
environmental gradients
Axes 1 and 2 of the CA (Fig. 1A & B) respectively
explain 21.4% and 13.5% of between-species variation.
The total of eigenvalues is 1.872.
Axis 1 represents an altitude gradient. It opposes low
altitude and high altitude sites. Sites at the foot of the
sampled gradient at less than 800 m are at the far posi-
tive end of axis 1. The negative end of axis 1 is charac-
Figure 1. Correspondence Analysis (CA) of 79 land snail records on the Grand Luberon grasslands. A. Gastropod species ordination on the
first factor plane (species labels as in appendix 2). B. Site ordination on the same plane.
465

CORINNE LABAUNE & FRÉDÉRIC MAGNIN
The negative side of axis 2 contains species such as
Variation explained by different sets of variables
Zebrina detrita (Müller), which prefers mountain shrub-
lands, Pomatias elegans (Müller), which lives on loose
soils and eats dead leaves, Vallonia costata (Müller)
and Urticicola glabellus (Draparnaud) which are also
common species in preforest habitats. Species from
very open environments can be found on the positive
side, such as T. cylindrica, Vitrea narbonensis (Clessin)
and T. geyeri.
CCA made it possible to precisely define the role of dif-
ferent groups of environmental and spatial variables
determining the preceding gradients.
Seven of the initial 12 variables in the CLIMATE
group were kept after a step-by-step selection (Table
1). The sum of the canonical eigenvalues of this CCA is
0.487, i.e. 26% (0.487/1.870) of species variation is
explained by this variable group. The first axis (22%
of total inertia) represents an altitude gradient (Fig. 2).
In fact, the altitude variable is the one best correlated
with this axis which separates alpine affinity species
(T. geyeri, P. triplicata, T. cylindrica) collected in alti-
tude sites from more Mediterranean species (C. gigaxii,
Solatopupa similis (Bruguière), A. polyodon) associated
with lower altitude sites. Axis 2 (11.5% of total inertia)
represents a gradient of patch size. The positive side is
associated with sites sampled at halfway up in small
clearings and young shrublands where forest affinity
species were found (e.g. P. elegans and Pagodulina pago-
dula (Des Moulins)). The negative side is associated
with sites at ridge tops and upper slopes corresponding
to the largest grasslands with very open environment
species (V. narbonensis, T. cylindrica, T. geyeri).
Thus the 1–2 factorial plane reveals three land snail
community types:
–mountain assemblages on low grazed grasslands
(with T. geyeri, P. triplicata, T. cylindrica),
–open Mediterranean low altitude assemblages (with
C. unifasciata, Monacha cantiana (Montagu), J. quadri-
dens),
–preforest assemblages with a more complex habitat
structure from the point of view of strata and soil cover
(with Xerosecta cespitum (Draparnaud), A. secale,
Cochlostoma septemspirale (Razoumowsky), Acanthin-
ula aculeata (Müller), Punctum pygmaeum (Drapar-
naud)).
Thus, two of the most important factors to explain
species distribution seems to be altitudinal gradient
and habitat structure complexity gradient.
Seven of the 14 initial variables were selected from
the STRUCTURE group (Table 1). The CCA yields
Table 1. Significance of environmental variables in different groups. P ꢃ significance
probability in a Monte Carlo permutation test based on 99 permutations.
Variable groups
‘CLIMATE’
Significant variables:
altitude
grassland patch size
halfway up location
local humidity
east facing
Code:
P
ALTITUDE
SIZE
M.SLOPE.
HUMIDITY
E.EXPO.
S.EXPO.
SLOPE.T
0.01
0.01
0.01
0.01
0.01
0.03
0.05
south facing
slope top
‘STRUCTURE’
vegetation layer 4 (26–50 cm)
cryptogams
bare ground
vegetation layer 1 (0–5 cm)
woody plants
boulders
LAYER.4
CRYPTOG
EARTH
LAYER.1
LIGNEOUS
BOULDERS
LITTER
0.01
0.01
0.05
0.01
0.02
0.02
0.05
litter
‘FLORA’
‘SPACE’
Carex humilis
Buxus sempervirens
Bromus erectus
Festuca ovina
Heliantemum oelandicum
CHU
BSE
BER
FOV
HOE
0.01
0.01
0.01
0.01
0.01
distance between sites (5 groups)
latitude
longitude
DIST.5
0.01
0.01
0.01
0.01
LATITUDE
LONGITUDE
DIST.20
distance between sites (20 groups)
466

LAND SNAIL COMMUNITIES IN MEDITERRANEAN UPLAND GRASSLANDS
the sum of the canonical eigenvalues: 0.428, i.e. 22.8% cover due to vegetation (cover ꢄ 70%) including
(0.428/1.870) of explained species variation. Axis 1 cryptogams (cover ꢄ 15%). They are associated both
(13.6% of total inertia) is a gradient of environmental with Mediterranean species (C. gigaxii, Zonites algirus
openness (Fig. 3). It compares forest-affinity species (Linnaeus), J. quadridens) and forest affinity species
associated with high woody cover and higher plant (Merdigera obscura (Müller), M. cantiana, Vitrea con-
layer with more open-environment species associated tracta (Westerlund)). On the positive side, the sites for
with a 0–5 cm layer cover. Axis 2 (11.2% of total in- low, stony, grazed grasslands can be found, associated
ertia) represents a gradient of habitat structure accord- with high altitude open environment species (T. geyeri,
ing to altitude. The sites on the far end of the negative P. triplicata, T. cylindrica).
side are located at low altitudes and have high soil
Five plant species in the FLORA group were kept
after step-by-step selection (Table 1). The CCA of the
species-sites matrix constrained by these variables
yields the sum of canonical eigenvalues: 0.333, i.e.
17.8% (0.333/1.870) of explained species variation. It
reveals the plant species which play an important role
in habitat structure (Fig. 4). The first axis represents
12.9% total inertia. The negative side is associated with
B. sempervirens, the highest woody species in our sites,
which provides shelter and leaf litter for forest-affinity
snails (e.g. P. elegans and P. pymaeum). Plant species
making up the lowest layers and snails from open en-
vironments, such as T. geyeri, T. cylindrica and V. nar-
bonensis, are on the negative side. Axis 2 (9.2% of total
inertia) compares dense herbaceous formations with
discontinuous grass and shrublands. Grasslands with a
high herbaceous cover can be found on the negative
side (B. erectus and F. ovina). The densest vegetation
houses mesophilous or forest-affinity snails (Sphy-
radium doliolum (Bruguière), M. obscura, M. cantiana),
while the least dense harbours Mediterranean open
Figure 2. Canonical Correspondence Analysis (CCA) using the cli-
mate and topographic variable group as explanatory variables
(species labels as in Appendix 2 and variable labels as in Table 1).
Figure 4. Canonical Correspondence Analysis (CCA) using the
florisitic variable group as explanatory variables (species labels as in
Appendix 2).
Figure 3. Canonical Correspondence Analysis (CCA) using the the
habitat structure variable group as explanatory variables (species
labels as in Appendix 2 and variable labels as in Table 1).
467

CORINNE LABAUNE & FRÉDÉRIC MAGNIN
environment species (C. gagaxii, J. quadridens, V. nar- Axis 2 (8.3 % of total inertia) is a latitude gradient. On
bonensis). On the positive side are the extremely dis- the negative side are both sites from the ridges asso-
continuous high-altitude grasslands with mountain ciated with mountain species (T. geyeri, P. triplicata)
species (T. geyeri, P. triplicata, C. rugosa) and Buxus and same latitude sites from the eastern and western
sempervirens shrublands with forest-loving species ends of the range, but at lower altitudes and associated
(Cepaea nemoralis (Linnaeus), P. elegans, P. pyg- with southern species (A. polyodon, V. narbonensis). On
maeum).
the positive side are sites from the northern slopes and
The SPACE group has four selected variables (Table mesophilous or forest affinity snails (Z. algirus, U. gla-
1). The CCA of the species-sites matrix constrained by bellus, C. nemoralis).
these variables yielded a sum of canonical eigenvalues
A Monte Carlo test was performed simultaneously
of 0.356, i.e. 19% (0.356/1.870) of explained species for each of the four canonical analyses (99 permuta-
variation. The first axis (17.9 % of total inertia) is tions) on the first axis and on the first four axes together.
strongly correlated with spatial variables based on These axes were significant in each case (p ꢂ 0.05).
Euclidean distance between sites, particularly Dist5
Variation explained by flora and habitat structure
which corresponds to a classification of sites into
5 groups (Fig. 5). These site groups are well-ordinated
along axis 1: on the negative side are sites sampled in
the central part of the range (two groups) including
species located only in this sector (i.e. Z. detrita, P. ele-
gans, V. costata and U. glabellus). Around the origin of
the axis are sites from the eastern and western ridges
(one group) consisting of the most common species.
Finally, the positive side contains sites from the eastern
and western ends of the range (two groups), associated
with southern species with a limited low-altitude distri-
bution. The spatial pattern using the five groups best
explains the species distribution pattern. If the finest
grid (Dist 20) was associated with this axis, this would
mean that site location in the study area has little in-
fluence on the composition of land snail community.
The floristic (FLORA) and habitat structure (STRUC-
TURE) variables were compared and each group was
successively analysed as a covariable set. The relative
importance of habitat structure and floristic composi-
tion were calculated to explain variation in the species-
sites matrix. The results are shown in Fig. 6A. These
two variable groups explain 31.8% of matrix variation.
Nearly half (14%) is due solely to habitat structure
variables which explain more than flora (8.8%). 9% of
explained variation is common to both floristic and
habitat structure variables.
Variation explained by climatic and spatial variables
The two CLIMATE and SPACE variable groups
explain a total of 36.3% of variation, i.e. more than the
habitat variables (Fig. 6C). 17.3% of variation is due
solely to topographic and climatic variables, 10.2 % to
site spatial situation and 8.8% is common to both
groups.
Partition of the variation between the four
explanatory variable sets
To obtain an overall view of the relative importance of
each of the four explanatory variable groups, we
performed a final series of analyses where habitat
description variables (FLORA + STRUCTURE) and
geographic variables (CLIMATE + SPACE) were
combined to form two new groups. As before, two
CCAs were performed: one was constrained by geo-
graphic variables (36.3% explained variation) and the
other by habitat description variables (31.8% explained
variation). The amount of variation explained by each
of these groups is equal to the total percentage of
explained variation obtained in the first two series of
partial CCAs (Fig. 6B). Two partial CCAs made it pos-
sible to calculate the amount of explained variation in
the species-sites matrix due to each of these two large
groups.
Figure 5. Canonical Correspondence Analysis (CCA) using the
spatial variable group as explanatory variables (species labels as in
Appendix 2 and variable labels as in Table 1).
468

LAND SNAIL COMMUNITIES IN MEDITERRANEAN UPLAND GRASSLANDS
Figure 6. Variation partitioning of the species data matrix according to different variable groups. [a]: Amount of variation explained only by
first variable group. [b]: Amount of variation explained by both variable groups. [c]: Amount of variation explained only by second variable
group. [d]: Amount of unexplained variation.
The fraction of species variation explained by all supplemented by occasional mushrooms, lichens or
variables was 54.5%. Overall, the geographic variables fresh vegetation (Mason, 1970; Wolda, Zweep &
(CLIMATE + SPACE) explain most of the variation Schuitema, 1971; Williamson & Cameron, 1976). The
(22.7%). This is mainly due to species distribution low amount of fresh vegetation eaten can be explained
according to altitudinal gradient which was demon- by the presence, in numerous living plants, of unpalat-
strated in the CA and CCA constrained by the able or chemically repulsive parts (Grime, Blythe &
CLIMATE variable group. Habitat description vari- Thornton, 1970). Heller (1988) demonstrated that
ables (STRUCTURE + FLORA) explain one-third of snails prefer annual plant debris in semi-arid environ-
variation (18.2%). 13.6% of explained variation was ments over perennial plants which develop xero-
common to both environmental and spatial variables.
morphic traits making them difficult to eat or digest.
Generally, taste tests have not been very conclusive:
out of 52 plant species offered to C. nemoralis, only 10
appear to be relatively palatable (Grime, MacPherson
Stewart & Dearman, 1968). South (1992) also noted
that variation in snail diet was dependent on food
available at a particular site. Mason (1968) studied
seven forest snail species and concluded that they prim-
arily feed on dead vascular plants with differences in
secondary food source choices. These differences are
significant and occasional use of lichens is frequently
mentioned (Mason, 1974; South, 1992) which could
explain the importance of the ‘cryptogams’ variable in
the CCA constrained by the STRUCTURE variable
group. Currently, we can only note a certain selectivity
for some plant species or plant families by some
gastropods (e.g. Chondrinidae are lichen-feeding). It is
impossible to make any conclusions about genuine
associations or specialisation which could affect malaco-
fauna distribution. The possible relationships between
gastropods and plant species in a habitat are more
DISCUSSION AND CONCLUSION
Relative importance of flora and habitat structure
Axis 1 of the two CCAs constrained by floristic com-
position and habitat structure ordinates the sites and
snail species according to a structure and vegetation
complexity gradient. The results of the CCAs and par-
tial CCAs demonstrate that floristic composition plays
a much less important role in gastropod communities
than habitat structure, as the latter is primarily con-
ditioned by vertical and horizontal vegetation struc-
ture. This concurs with results obtained by most
authors who indicated that habitat floristic composi-
tion is not a determinant factor in explaining land
gastropod distribution. Generally, neither quantity nor
quality of a plant food source influences snail distribu-
tion (Boycott, 1934). Most species have a diet essen-
tially consisting of different kinds of dead plants,
469

CORINNE LABAUNE & FRÉDÉRIC MAGNIN
dependent on microclimate due to vegetation structure and horizontal vegetation structure and thus rein-
than on species composition (Boycott, 1934; Paul, 1978; forced an harsh microclimate.
Cameron et al., 1980). In the present study, Buxus
It is difficult to clearly demonstrate the direct role of
sempervirens plays an important role: its growth in the current climatic gradient. The altitudinal gradient
grasslands severely modifies habitat structure, through illustrated by our analyses could be due to a complex
the addition of a low woody layer rich in leaf litter.
The problem arises of the overlap between floristic
composition and vegetation structure variables. These
two groups of variable are strongly interrelated as
vegetation structure naturally depends on the species in
it. This relationship is emphasised by the amount of
explained variation shared in common by these two
groups (9%), which was lower that the STRUCTURE
group contribution (14%), but higher than FLORA
(8.8%).
combination of climatic, historical and biotic factors.
Relative importance of spatial explanatory variables
Partial CCAs have revealed the importance of com-
munity spatial structure which is independent of the
environmental variables analysed, i.e. climatic vari-
ables s.l., habitat-structure variables and grassland
floristic composition. This environmentally independ-
ent spatial variation in community composition is
obvious in some of the study zones where several
species are strictly localised, although this localisation
is not related to the particular environmental factors.
Actual complexity of the altitudinal gradient
The altitudinal gradient is the main determinant factor This phenomenon could be due to diverse factors, such
for land snail species distribution in the Luberon. as between-species competition, low dispersal ability,
Alpine communities can be found at higher altitudes, undefined historic factors or a combination of several
in particular T. geyeri, P. triplicata, C. rugosa and of the above factors. Particular spatial patterns, e.g.
A. secale, which were also the basis for low-altitude spot distribution of T. geyeri and C. unifasciata at mid
land snail assemblages during the last glacial period altitudes, could be explained by passive dispersion
(Magnin, 1989, 1992). The other end of the altitudinal caused by sheep, followed by settlement of populations
gradient is marked by the presence of thermophilous maintained by local between-species competition (Pfen-
and Mediterranean species (C. gigaxii, A. polyodon, ninger, 1997). Historical causes and low colonisation
J. quadridens).
capacity could also explain localisation of species such
Temperature decrease with altitude is definitely a as V. costata. However, the more or less recent history
limiting factor for some species. Several studies have of the communities and species dispersal ability are
indicated the impact of altitude (from the climatic per- rarely considered to explain the composition of land
spective) on snail community richness and composition snail communities. When community spatial structure
(Cameron & Greenwood, 1991; Dyduch-Falsniowska, is analysed on any scale, it is generally linked to en-
1991; Magnin, 1991). The presence of high-altitude vironmental heterogeneity (Harvey, 1974; Paul, 1978;
species or combinations of species characterising land Murray, Johnson & Clarke, 1982; Dyduch-Falniowska
snail communities during the Pleistocene cold periods & Tobis, 1989; Goodfriend, 1992). When ‘historical
at low altitudes is generally explained by an altitudinal factors’ are mentioned, it is usually to explain a possible
shift of the climatic gradient (Metcalf, 1984; Gitten- temporal difference between environmental changes
berger & Goodfriend, 1992; Magnin, 1992) and these and the consecutive response of snail communities
changes in distribution patterns can be used to recreate (Bishop, 1977; Thomas, 1985). However, Cameron et
palaeoclimates (Harris, 1978; Magnin, 1992).
al. (1980) and Magnin et al. (1995) have demonstrated
However, factors other than climate can explain the that the landscape structure and its evolution over a
observed altitudinal distribution patterns. The sharing hundred year time period could explain part of spatial
of the altitudinal gradient by T. geyeri and C. uni- changes in snail communities. The present study con-
fasciata raises the following problems: although there firms that historic factors have a great influence over
appears to be a strong climatic determinism, recent the spatial structure of communities.
studies (Magnin, 1993) have shown that historical
factors (climate changes) and competition between the quantify the amount of variation explained by different
two species could explain this distribution pattern. groups of variables which control land gastropod
The CCAs and partial CCAs made it possible to
The impact of human activities could also play an abundance and distribution in open mountain environ-
important role. The altitudinal gradient in the Luberon ments. Our results concur with accepted studies and
(as in many Mediterranean ranges) coincides with demonstrate that habitat structure is a more important
changes in landscape management styles: above 900 m, explanatory factor than floristic composition. Our
grazing by sheep has considerably changed the vertical analyses emphasise the important role played by non-
470

LAND SNAIL COMMUNITIES IN MEDITERRANEAN UPLAND GRASSLANDS
DILLON, R.T. 1980. Multivariate analysis of desert snail
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expressed by independent spatial variables. Neverthe-
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of climatic factors s.s., historic events and/or biotic
factors within the altitudinal gradient.
DYDUCH-FALNIOWSKA, A. 1991. The gastropods of
the Polish Tatras. Studia Naturae seria A, 38: 1–111.
DYDUCH-FALNIOWSKA, A. & TOBIS, H. 1989. Spatial
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ACKNOWLEDGEMENTS
EVANS, J. G. 1972. Land snails in archaeology. Seminar
This work was performed through financial support from the
C.N.R.S. Ecological Systems and Human Activities Pro-
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Frédéric Heurgué for his help in sample treatment and Erol
Vela for his contribution to the floristic records, as well as
Thierry Tatoni and Roger Prodon for their critical notes. We
are indebted to Robert Cameron for helpful and stimulating
comments on the manuscript.
Press, London.
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APPENDIX 1
Environmental and spatial variables
CLIMATIC VARIABLES (12) :
quantitative variables (3):
Nominal Variables (9):
ꢅ Altitude:
(eight 50m categories between 700 and 1100m)
ꢅFlat
ꢅNorthern exposure
ꢅHalfway up location
ꢅSouthern exposure
ꢅUpper slope
ꢅEastern exposure
ꢅSummit
ꢅ Humidity: (Apparent site humidity):
1- Very dry
2- Dry
3- Relatively dry
4- Average site
ꢅWestern exposure
ꢅNo defined exposure
ꢅ Grassland patch Size:
1– ꢂ 50 m2
4– ꢄ 250m2– ꢂ 500m2
5– ꢄ 500m2– ꢂ 1ha
6– ꢄ 1ha
2– ꢄ 50m2– ꢂ 100m2
3– ꢄ 100m2– ꢂ 250m2
STRUCTURE VARIABLES (14):
Surface cover:
Layer cover:
Vegetation type cover:
ꢅ Hard rock
ꢅ Layer 1: 0–5 cm
ꢅ Low woody
ꢅ Grasses
ꢅ Cryptogams
ꢅ Boulders(ꢄ 20 cm)
ꢅ Broken stone (ꢂ 20 cm)
ꢅ Vegetation
ꢅ Layer 2: 6–15 cm
ꢅ Layer 3: 16–25 cm
ꢅ Layer 4: 26–50 cm
ꢅ Layer 5: 51 cm–1 m
ꢅ Litter
ꢅ Bare ground
Coding used for these 14 cover variables is:
1–0–3%
2–4–10%
3–11–25%
4–26–50%
5–51–75%
6–76–100%
FLORISTIC VARIABLES :
Cover of three dominant species for each site is coded as follows:
1–10–25%
2–26–50%
3–51–75%
4–76–100%
SPATIAL VARIABLES :
ꢅ Latitude: UTM coordinates
ꢅ Longitude: UTM coordinates
ꢅ Dist5: Division of the Hierarchical Cluster Analysis into 5 groups
ꢅ Dist10: Division of the Hierarchical Cluster Analysis into 10 groups
ꢅ Dist20: Division of the Hierarchical Cluster Analysis into 20 groups
473

CORINNE LABAUNE & FRÉDÉRIC MAGNIN
APPENDIX 2
List of the gastropod species with their abundance and occurrence
(nomenclature after Kerney et al., 1999)
SPECIES AND ABBREVIATIONS
NUMBER OF SHELLS
NUMBER OF SITES
CUN Candidula unifasciata (Poiret 1801)
TGE Trochoidea geyeri (Soós 1926)
PTR Pupilla triplicata (Studer 1820)
7487
2482
4513
1856
281
2683
649
1034
267
880
723
404
296
76
63
65
101
38
63
33
86
50
48
44
39
38
38
36
35
34
25
21
18
17
14
12
12
10
8
TCA Truncatellina callicratis (Scacchi 1833)
CAC Cecilioides acicula (Müller 1774)
GVA Granaria variabilis (Draparnaud 1801)
MCA Monacha cantiana (Montagu 1803)
CRP Clausilia rugosa parvula (Férussac 1807)
JQU Jaminia quadridens (Müller 1774)
TCY Truncatellina cylindrica (Férussac 1807)
XCE Xerosecta cespitum (Draparnaud, 1801)
ASE Abida secale (Draparnaud 1801)
CSE Cochlostoma septemspirale (Razoumowsky 1789)
AAC Acanthinula aculeata (Müller 1774)
PPY Punctum pygmaeum (Draparnaud 1801)
VCN Vitrea contracta (Westerlund 1871)
VNA Vitrea narbonensis (Clessin 1877)
PPA Pagodulina pagodula (Des Moulins 1830)
UGL Urticicola glabellus (Draparnaud 1801)
CNE Cepaea nemoralis (Linnaeus 1758)
PEL Pomatias elegans (Müller 1774)
7
6
6
MOB Merdigera obscura (Müller 1774)
LCY Lauria cylindracea (Da Costa 1778)
VCO Vallonia costata (Müller 1774)
CGI Candidula gigaxii (Pfeiffer 1850)
PMA Phenacolimax major (Férussac 1807)
ZDE Zebrina detrita (Müller 1774)
13
5
4
4
4
3
3
2
1
109
996
1135
3
257
6
ODO Sphyradium doliolum (Bruguière 1792)
ZAL Zonites algirus (Linnaeus 1758)
1
APO Abida polyodon (Draparnaud 1801)
CAV Chondrina avenacea (Bruguière 1792)
SSI Solatopupa similis (Bruguière 1792)
1
2
39
1
1
1
474