RESEARCH NOTES
Consistency in hand-searching for terrestrial snails
J.F. Ward-Booth and G.B.J. Dussart
Canterbury Christ Church University College, Canterbury, CT1 1QU
email: [email protected]
A range of techniques are available for sampling terrestrial
molluscs. Soil and litter can be sieved and examined under
the microscope to find micro-molluscs which mainly comprise adults 3 mm1 Hand searching is often used in conjunction with litter and soil sampling.2 For example, hand
searching has been used to sample quantitatively snail species
richness above ground level in Madagascan rainforests.3
However, when undertaking ecological monitoring in habitats of restricted size, the removal of litter and soil samples
can be destructive and consequently interfere with the objectives of the investigation, in which case, hand-searching may
be the only option.
Field surveys may be separated by time and geography.
For example, distributions of Cepaea nemoralis (L.) in
Dorset were compared with those of Cyril Diver, collected
seventy years earlier.4 By contrast, in a different sampling
exercise,5 three observers searched one hectare at different
geographic locations. In his 1938 paper, Boycott6 summarised a lifetime of malacological field studies in which he
had hand searched sites all over Britain over a long period of
time. In these and other studies, there is a tacit assumption
that hand searching is done consistently over time and space.
The aim of this study was therefore to investigate the
consistency of hand-searching for macromolluscs in an agricultural area of South East England. Two aspects were of
interest. Firstly, do different observers find similar amounts
and types of molluscs in replicated sampling; secondly does a
single observer find similar results across replicated samples?
It was also intended to investigate ancillary factors which
could affect sampling performance e.g. time of day.
Sampling comprised intense fifteen-minute hand searches
for macromolluscs (adults 3mm maximum shell length)
across 1 3 metre areas of field margin on 27/9/1996. Eighteen field margins of varying soil type, topography and land
use were included. For most investigations, there were three
replicates giving a total of 45 minutes at each site. Molluscs
were collected in the field by hand and with forceps, sorted
in the laboratory and counted. The few slugs found were
not included in analyses. Data were recorded for (i) live
snails (‘snails’) (ii) dead empty shells (‘shells’) and (iii) snails
plus shells (‘snails+shells’). When appropriate, data were
log10(x 1) transformed to normality.
In order to investigate the consistency of inter-operator
performance, four operators were used. Operator A was
an experienced malacologist with knowledge of the area.
Operator B was an experienced malacologist without such
knowledge. Operator C had a little experience of field malacology and D was inexperienced. The operators made contemporaneous, 15-minute hand searches for macromolluscs
on adjacent, and apparently similar areas at each of ten sites
J. Moll. Stud. (2001), 67, 502–506
(soil temperature 13–17C; relative humidity 52–66%) The
ten sites covered a range of habitat types in order to include a
range of snails densities. After sorting and counting, the collected material was grouped into four size classes by operator
A in the laboratory.
In order to investigate the consistency of single operator
performance, operator A made three replicated 15-minute
searches at each of the ten sites previously mentioned
between 11–14th of August 1997. In order to investigate the
effect of vegetation type on operator performance, operator
A carried out three replicated 15 minute searches at each of
four sites along a ditch on clay soil with varying amounts of
vegetation. Sampling took place on 2–3/07/1997 (temperature 14–15C; humidity 90–100 % with light drizzle). Molluscs were collected, sorted, counted and marked with nail
varnish before being randomly distributed back in the same
area. The procedure was repeated so that each animal could
have been captured three times. Operator performance at
each site was expressed as:Number of macromolluscs caught three times
Total number of individual macromolluscs found
100
This was not treated as a formal mark-recapture experiment
as there was no intention to estimate population density.
An experiment to compare sampling performance for
night-time sampling compared with day-time sampling was
conducted. Operator A carried out ten 15-minute daytime
searches along seventy metres of ditch on the margin of a field
of hay on clay soil between 09:30 and 13:00 on 01/09/1997.
During similar weather conditions, adjacent, similar areas
were sampled at night with a torch between 20:30 and 24:00
on 03/09/1997. There was a time lapse to allow the snails to
resettle.
During the investigation of inter-operator performance,
40.1% of the 1981 snails and shells found in this experiment
were collected by operator A and 75% of operator A’s data lie
above the median of operators B, C and D (Fig.1a). 53.6% of
the snails shells were found at site 10. Here, molluscs were
so abundant that handling became a limiting factor for all
operators. Operator A and site 10 were obviously exceptional. When site 10 was omitted from the data set (Fig.1b)
there was no significant variation in the performance of the
operators (F2,16 1.9 P 0.17) but significant differences
between the sites (F8,16 15.3 P 0.001) (log10 transformed
data). These results were confirmed by tests for concordance
which showed highly significant agreements between the site
ranking by all the operators for each of ‘snails’, ‘shells’ and
‘snails shells’ (P 0.005 in each case).
Between 3 and 7 species were found at each of the 10 sites.
© The Malacological Society of London 2001
RESEARCH NOTES
Operator D with no experience of field malacology found
fewer species than A, B or C. Indeed, 75% of the other
searchers’ data lay above this operator’s median. The null
hypothesis was rejected for all four operators across sites 1 to
10 inclusive (Operators F3,27 7.3 P 0.001; Sites F9,27 3.1
P 0.01). However, there was no significant difference
between the two most experienced operators in the number of
species found (F1,9 4.4 P 0.05).
All operators collected similar numbers of snails in the four
size categories across ten sites (F3,9 3.0 P 0.05) but there
was significant variability in the numbers of snails caught
(F3,9 22.9 P 0.001). The peak abundance of 5–10 mm
sized molluscs (Fig. 2) perhaps reflects the fact that this category includes adults of a small species (Oxychilus helveticus
Blum, adult shell breadth 8–10 mm) and juveniles of a large
species (C. nemoralis L. adult shell breadth 18–25 mm). Few
molluscs in the smallest size category were found, which
could be due to lack of acuity in the method or because the
smaller molluscs were actually absent. In a separate investigation on a cold day in January 1998 however, a 15m2 site
was hand searched for four hours and 489 juvenile ( 5mm)
Monarcha cantiana (Montagu) were found.
During the investigation of single operator performance,
analysis of log10 transformed data for numbers of individual
molluscs showed no significant variation between operator
A’s replicates but as expected, there was variation between
sites (Table 1).
When the effect of vegetation on operator performance
was investigated, there was no significant variation in the
mean number of species recorded between sampling sessions
for snails (F2, 3 2.1 P 0.21) or shells (F2, 3 4.2 P 0.07).
The percentage of shells and snails found on all three occasions varied depending on the circumstances (Table 2). As
might be expected, sites which were easiest to search gave the
highest yield of thrice-caught molluscs .
Thirteen species of snail were found during night sampling
and eleven species by day. The difference comprised one shell
of Cernuella virgata (da Costa) and one live Helix aspersa
(Müller). The animals were contagiously distributed and
Wilcoxon paired-rank test showed that although more
animals were found in daylight, the differences between night
and day sampling were not significant for snails (W 14
n10 P 0.20) and shells (W 9.5 n10 P 0.10). However,
there was a significant difference for snails shells (W 7
n10 P 0.05). Mating molluscs were only seen at night. In
later, comparable hand searches by day across this field
margin from June 1996 to July 1997, sixteen species of macromolluscs were caught, with three species being recorded only
once. Aegopinella nitidula (Draparnaud) is a small, brown,
discoid, snail 8–10 mm. broad.7 When dead and in the soil it is
Figure 1. a. Box and whisper plot to compare collecting capability of four operators at ten different sites. b. The same data, recalculated without the extreme values from Site 10.
503
RESEARCH NOTES
difficult to find at night. Data for this species were somewhat
variable between samples. However, when A. nitidula was
removed from the data set, analysis of variance of log10 transformed data showed no significant differences between the
mean number of snails found by day and by night (F1,9
4.8 P 0.05).
Methods for quantitatively sampling terrestrial molluscan
populations have been reviewed.8 Where a complete species
list is the prime objective, for example in a taxonomically
focussed study, extraction of molluscs from soil and litter is
essential. However, there are sampling problems. For example, there can be large differences in results from bulkand hand-sorted samples of molluscan communities from
Pleistocene deposits.1 Despite these seminal works, there
seems to be little research which compares the performance
of different operators in hand searching for terrestrial snails,
or which has quantitatively assessed factors such as day and
night sampling.
Although soil and litter sieving are a preferred approach, in
some situations, removal of soil and litter may not be appropriate. Constraints of time, money and ecological strategy
may limit the investigator to hand-searching, for example
where the experimental design dictates repeated visits to the
same, small habitat. In the study described here, a number of
parameters of hand-searching were investigated.
Table 1. Consistency of performance of operator A;
results of two factor analysis of variance for sites of varied
topography.
Operator A’s
replicates
snails shells
snails
shells
F2, 9 0.21 P 0.81
F2, 9 0.44 P 0.65
F2, 9 0.54 P 0.59
Sites
F2, 9 21.0 P 0.001
F2, 9 10.8 P 0.001
F2, 9 25.4 P 0.001
For total abundance of molluscs, the experienced operator
who was familiar with the area found more snails than the
other operators. Vegetation form and density was important;
sites with thick, tufted vegetation might need bigger search
areas or longer times in order to improve consistency in numbers of animals collected. Depending on the location,
between 47 and 73% of the molluscs were found in three successive searches on different days; in total however, fewer
animals were found in each successive search. The number of
species recorded was not similarly affected, ‘more of the
same’ being found by different operators. Although snails
tended to quit areas of recently disturbed vegetation, no
species was outstanding in this respect.
The two experienced operators found statistically similar
numbers of species across the ten sites. Despite the different
soils, typography and land use, these sites would appear to
support a similar number of niches throughout the year9 As
might have been expected, the operator with no experience of
field malacology found fewer molluscs in terms of both
species and abundance. Although training of operators in
species recognition is beneficial, our research indicates that
such training need not be too prolonged, since both the experienced and the relatively inexperienced operators, unfamiliar with the area, had similar results. In one sense,
operator A was too efficient. Mary Seddon (pers.comm.) has
provided a week’s training course before using local labour in
fixed time, hand-searches for snails in the eastern African
rainforest. Where the objective of a survey is a comparative
investigation of, say, anthropogenic environmental impact in
different habitats, it might be necessary to address the issue of
hyper- or hypo-efficient searchers. If the objective was to
obtain a complete species list, then other sampling methods
would be used and attention would focus on these highly
efficient operators.
With respect to mollusc size, a large number (489) of small
juvenile M. cantiana were found in mid-winter. It thus
appears that it is possible to successfully hand search for juve-
Table 2. Yield of snails shells during three searches of sites with differing vegetation. Samples were marked and
returned to the site.
Site character
In all cases, the dominant vegetation in the immediate
sampling area comprised grass. All samples were taken
in the same locality.
Searching
% found
three times
% of first
batch found
three times
Number of
molluscs
Chalk soil on the edge of a wheat crop;
sampling in a ditch with small grass patches 2–3 cm apart.
Easy
72.6
91.8
124
Sandy soil on the edge of a wood; when the grass in this
flat area was parted by hand, patches of soil were revealed
Medium
54.2
82.1
24
Clay soil on the edge of a hay field; grass tufts were growing
densely in this ditch but could be parted to reveal small
areas of soil
Medium/
difficult
51.1
73.1
45
Sandy soil adjacent to a hay field; growing on an extensive
flat area, the grass grew in tufts which were too dense to
reveal soil when parted.
Difficult
46.9
72.2
81
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RESEARCH NOTES
Figure 2. Size comparison of number of shells and snails collected by four different operators searching the same ten sites.
nile macro-molluscs above ground level, even in inclement
weather.
The distribution of live animals was contagious but dead
shells were more evenly distributed. When close to the earth,
A. nitidula was found more easily in daylight. Night-time
sampling recorded two more species, and some mating molluscs were seen, including two pairs of M. cantiana and two
pairs of Arion ater (Linné). The results reported here indicate
that in general, unless mating observations are required,
most sampling could be undertaken during daylight.
At sites with abundant molluscs, the less experienced
operators focussed on the most abundant species, with the
result that they slightly underestimated species richness.
There was also a considerable difference in the rate of collection of snails by the operators (ranging from 20.6 to 52.9
snails per minute). Here, dexterity influenced the results and
created statistically significant variation between operators.
A shorter search time might ease the problem but also result
in a reduction in the number of species found. A compromise
might be to increase the number of samples taken but to
reduce the time spent on each one.
In conclusion, it can be observed that almost every malacologist with an interest in terrestrial molluscs at some time
undertakes hand searches,10 and this paper attempts to
answer the question ‘How reliable is such a searching technique?’. The answer seems to be ‘It depends.’ Even minute
hand searching will be unlikely to reveal micromolluscs such
as Vertigo moulinsiana and is therefore inadequate for some
investigations eg. demographic studies. However, timed hand
searching produced consistent results for snails 3mm in
that, depending on experience, operators found similar
numbers of snails and all operators ranked sites similarly on
snail abundance. Except for the untrained operator, similar
numbers of species were found across the sites, and all operators found similar size ranges. Small, juvenile snails were
found by intensive hand-searching, even in poor weather.
Unless there is a specific need to investigate aspects such as
nocturnal behaviour, night searching conveyed little benefit
over intensive daytime searching.
On the basis of these results, we suggest that hand searching can be useful in situations where more destructive
sampling cannot be used or for comparative investigations
which involve a range of personnel; for example, assessment
of changes in land use on macromolluscan communities in
remote locations. However, care should be taken to arrange
training such that there are neither hyper-efficient, or hypoefficient searchers on the team.
505
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RESEARCH NOTES
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