M .G. Chapman J. Exp. Mar. Biol. Ecol. 244 2000 181 –201
185
the total length TL of a tape measure laid flush with the substratum to the 5-m linear length LL of the transect TL:LL.
A few days prior to the start of each experiment, | 30–60 snails of each species in each site were individually marked using numbered plastic labels stuck onto the shells.
Previous studies have shown little disturbance associated with similar handling and marking Chapman, 1986. Nevertheless, the snails were left undisturbed for a few days
prior to the start of each experiment. Movement was measured as the linear distances and directions displaced by individual snails over three different periods of time; 1 day
two tidal cycles, 1 week and 2 weeks, for each of three experiments May, August and September, 1998. These measures were calculated from the subsequent positions of
snails on the shore, measured in situ from two fixed points in each site Underwood, 1977 with no further disturbance or handling. Numbers of marked snails varied among
species and sites according to the proportions expected to be recovered in each site expected sample size was n 5 20; see Chapman, 1986. Because hypotheses were about
variation among species and sites over different periods of time and not specifically about changes in movement over time, the same pool of marked animals of snails was
available for recovery after 1 day, 1 week and 2 weeks. Nevertheless, because not all snails were found on each occasion, samples were different each time.
To test the hypothesis that the different species responded differently to features of habitat, on eight occasions during the three experiments, the use of three different
microhabitats by each species was measured by counting the marked animals of each species in and out of three different microhabitats: in crevices or overhangs, in contact
with or under the canopy of foliose macro-algae or in water 1 cm deep.
3. Results
3.1. Physical differences among sites The different measures of habitat were summarized for each site as i the topographic
index at the largest spatial scale, ii the average topographic index at the intermediate spatial scale, iii the average topographic index at the smallest spatial scale, iv the
average proportion of free-standing water per transect, v the average proportion of macroalgal cover per transect and vi the average measure of TL:LL Table 1. These
Table 1 Mean S.E., where applicable measures of variability of habitat among the three simple S1, S2, S3 and three
complex C1, C2, C3 sites; details in text T.I. 1 m
T.I. 50 cm T.I. 5 cm
TL:LL Prop. water
Prop. algae S1
0.0001 0.0002 0.0001
0.04 0.01 1.05 0.00
0.28 0.08 0.10 0.09
S2 0.0003
0.0018 0.0016 0.02 0.01
1.04 0.01 0.44 0.02
0.09 0.02 S3
0.0010 0.0002 0.0001
0.03 0.01 1.07 0.03
0.34 0.05 0.05 0.01
C1 0.0009
0.0069 0.0014 0.08 0.01
1.21 0.02 0.31 0.05
0.17 0.04 C2
0.0013 0.0090 0.0069
0.08 0.01 1.10 0.02
0.28 0.00 0.11 0.02
C3 0.0028
0.0008 0.0003 0.09 0.01
1.11 0.02 0.25 0.03
0.11 0.02
186 M
.G. Chapman J. Exp. Mar. Biol. Ecol. 244 2000 181 –201
values were compared among sites and between levels of topographic complexity using analyses of variance topographic complexity, fixed factor, two levels; sites, three levels,
nested within topographic complexity. They were also treated as a multivariate set of data and compared using nMDS ordination calculated from a matrix of normalized
Euclidean distances among sites to illustrate variation among sites using all criteria.
The simple sites showed significantly smaller variation in the topographic index and no significant differences among sites at the small spatial scale analyses of variance;
P . 0.05. Similar significant differences at P , 0.05 were not found at the inter- mediate scale because of the relatively smaller topographic index in one complex site
C3, compared to the other complex sites Table 1. Nevertheless, the analysis of variance on data transformed to natural logarithms showed a relatively large difference
between the mean topographic index of each level of topographic complexity F 5 6.90, 2 and 4 df, P , 0.06. At the coarsest spatial scale, the topographic index in S3 was
similar to that in C1 and C2. Therefore, topographic complexity varied from one spatial scale to another among the replicate sites and between the two levels on topographic
complexity.
There were no significant differences in cover of water 25 to 44 or foliose macro-algae 5 to 17 among sites within each level of topographic complexity, nor
among the two levels of topography. Finally, the TL:LL ratio showed significant differences among sites, with no significant difference between the two levels of
topographic complexity over and above differences among sites.
Treating each of these measures as a variable in a multivariate comparison among sites showed that, despite considerable variability among replicate sites within each level
of topographic complexity, the two levels of complexity clearly separated in 2- dimensional space Fig. 1. What had been deemed a priori to be simple sites were
clearly different from complex sites.
3.2. Linear distances displaced during movement Distances displaced during movement were transformed to natural logarithms before
analysis because these measures are generally logarithmically distributed Underwood, 1977. Heterogeneity of variances was tested using Cochran’s test prior to analysis of
variance and appropriate means compared using Student–Newman–Keuls SNK tests Underwood, 1997. ‘Species’ and ‘topography’ were considered fixed factors, ‘experi-
ment’ i.e. May, August or September was a random factor and ‘sites’ were nested in ‘topography’. Because species and topography were fixed factors and experiments and
sites random, there were no tests for the main effects of species or topography or their interaction. Where appropriate i.e. interaction terms had P . 0.25; Underwood, 1997,
interaction terms were eliminated to allow tests for species, topography or their interaction.
There was poor recapture of snails after 1 day and 2 weeks in a few of the sites in May, 1998 because of rough seas. Therefore, distances displaced after 1 day were
examined in two separate analyses. In the first, all sites were used, but there were only 12 snails of each species in each sample in all analyses, snails were omitted at random
to balance the samples. The species moved differently from site to site and among
M .G. Chapman J. Exp. Mar. Biol. Ecol. 244 2000 181 –201
187
Fig. 1. nMDS plot of the three simple S1, S2 and S3 and three complex C1, C2 and C3 sites based on ranked normalized Euclidean distances among sites using the average measures summarized in Table 1.
experiments significant interaction among species, sites and experiments in Table 2a, but of 18 comparisons among species three times, six sites, only three were significant
SNK tests. In each complex site in each experiment, all species moved similar distances i.e. the linear displacement between the start and end of the experiment was
similar. A
. porcata moved further than N. atramentosa and B. nanum in S2 in August and in S3 in May. A
. porcata and N. atramentosa moved further than B. nanum in S1 in September. Because SNK tests showed few significant differences and those found were
not consistent across sites or the different experiments, this interaction was not considered likely to invalidate comparisons of other sources of variation.
Eliminating interaction terms as described, allowed a test for Sp 3 T, which showed that differences among species were consistent across the two levels of topography and
vice versa Table 2a. Averaged across all sites and the three experiments, A . porcata
showed significantly greater average displacement than N . atramentosa and B. nanum,
which moved similar distances. This difference was larger on the simple topography Fig. 2, although there was no significant interaction note that distances were
transformed to natural logarithms prior to analysis. In the second analysis, S1 and one complex site C3; chosen randomly were omitted
because of small recapture in S1 in May and the analysis was repeated with n 5 19 Table 2b. The significant Sp 3 T interaction Table 2b showed that the two species
showed different patterns of movement on the different surfaces. A . porcata moved
further than N . atramentosa and B. nanum in simple sites, whereas in complex sites all
species moved similar distances. Also, A . porcata moved further in simple than in
complex sites, whereas N . atramentosa and B. nanum moved similar distances in all
188
M .G
. Chapman
J .
Exp .
Mar .
Biol .
Ecol .
244 2000
181 –
201 Table 2
Analyses of mean distances displaced transformed to natural logarithms a after 1 day in all sites, n 512, Cochran’s C50.05, P.0.05; b after 1 day omitting S1 and C3, n 519, Cochran’s C50.06, P.0.05; c after 1 week in all sites, n517, Cochran’s C50.05, P.0.05; d after 2 weeks in all sites August and September
only, n 519, Cochran’s C50.06, P.0.05. In this and subsequent tables, ns5P.0.05, P,0.05, P,0.01, P,0.001
a b
c d
df MS
F P
df MS
F P
df MS
F P
df MS
F P
c e
Species5Sp 2
13.90 6.18
2 11.55
2 6.56
No test 2
1.80 0.75
ns
b
Topography5T 1
1.47 0.27
ns 1
1.11 1
28.54 No test
1 14.97
No test Experiments5E
2 24.61
9.98 2
23.42 10.47
2 6.28
25.88 1
1.58 2.48
ns Sitetopography5ST
4 5.30
2.15 ns
2 1.54
0.69 ns
4 4.91
1.19 ns
4 6.57
10.29
a d
d
Sp3T 2
6.55 1.69
ns 2
11.77 5.97
2 0.93
No test 2
0.27 0.39
ns Sp3E
4 1.53
0.68 ns
4 0.10
0.05 ns
4 1.59
1.57 ns
2 2.42
3.47 ns
Sp3ST 8
0.95 0.42
ns 4
1.21 0.61
ns 8
1.36 1.35
ns 8
0.59 0.84
ns T3E
2 0.19
0.08 ns
2 3.71
1.66 ns
2 1.51
0.37 ns
1 3.52
5.51 ns
E3ST 8
2.47 1.91
ns 4
2.24 1.73
ns 8
4.12 4.00
4 0.64
0.66 ns
Sp3T3E 4
3.88 1.72
ns 4
2.54 1.29
ns 4
4.73 4.69
2 0.14
0.20 ns
Sp3E3ST 16
2.25 1.75
8 1.97
1.52 ns
16 1.01
0.98 ns
8 0.70
0.72 ns
Residual 594
1.29 648
1.29 864
1.03 648
0.96
a
Tested against Sp3T3E after elimination of Sp3ST.
b
Tested against ST after elimination of T3E.
c
Tested against Sp3E3ST after elimination of S3E and Sp3ST.
d
Tested against Sp3E3ST after elimination of Sp3T3E and Sp3ST.
e
Tested against Sp3E after elimination of Sp3ST.
M .G. Chapman J. Exp. Mar. Biol. Ecol. 244 2000 181 –201
189
Fig. 2. Mean S.E. linear distance displaced after 1 day by A. porcata A.p., B. nanum B.n. and N. atramentosa N.a. in areas of simple and complex topography n 5108; indicates significant differences
among means from SNK tests P ,0.05.
sites. This is similar to the trends illustrated in Fig. 2a using all sites. Therefore, for the distances displaced over 1 day, species showed significant differences. These varied to a
minor extent according to the topographic complexity, but there were few differences between simple and complex sites.
After 1 week, the species showed different patterns of movement from one experiment to another across the different types of topographic complexity Table 2c. In May, N
. atramentosa moved significantly further than the other two species on the simple
topography and there were no differences among species on the complex topography Fig. 3a. In August, A
. porcata moved significantly further than the other two species on the complex topography, but the three species moved similar distances on the simple
topography Fig. 3b. In September, there were no significant differences among species Fig. 3c. Although in all experiments, all species moved slightly further on simple than
on complex topography, these differences were only significant for A . porcata in
September, N . atramentosa in May and B. nanum in August.
Distances moved after 2 weeks were only available from two experiments August and September because of small sample sizes in a number of sites in May. There were
no significant differences among the three species Table 2d. It was not possible to eliminate or pool terms to obtain a test for topographic complexity relevant P values
, 0.25; see Underwood, 1997. Nevertheless, this level in the analysis had a very large mean square value compared to all other levels Table 2d indicating movement across
the two levels of topographic complexity was very variable. All species dispersed further on simple than on complex topography Fig. 4. The magnitudes of these differences for
190 M
.G. Chapman J. Exp. Mar. Biol. Ecol. 244 2000 181 –201
each species were greater than differences among pairs of species in each type of topography. Therefore, although there was no formal test for topographic complexity,
the distances displaced over 2 weeks appeared to be primarily determined by topog- raphy.
These conclusions were also evaluated by testing movement over 2 weeks in each of the two experiments separately. In August, there were significant differences among sites
F 5 5.46, 4 and 324 df, P , 0.01, leading to a non-significant overall effect of topographic complexity. Nevertheless, movement was considerably greater on average
across the simple sites mean6S.E. 5 261.1617.3 cm than across the complex sites
Fig. 3. Mean S.E. linear distance displaced after 1 week by A. porcata A.p., B. nanum B.n. and N. atramentosa N.a. in areas of simple and complex topography in three different experiments, a May, 1998,
b August, 1998 and c September, 1998 n 551; indicates significant differences among means from SNK tests P ,0.05.
M .G. Chapman J. Exp. Mar. Biol. Ecol. 244 2000 181 –201
191
Fig. 4. Mean S.E. linear distance displaced after 2 weeks by A. porcata A.p., B. nanum B.n. and N. atramentosa N.a. in areas of simple and complex topography n 5114.
162.6610.9 cm. In September, there was a significant difference between the two levels of topographic complexity when the sitestopography interaction was pooled with
the residual P . 0.23. Again, nett movement was significantly greater over simple topography
mean6S.E. 5 243.1617.1 cm
than over
complex topography
mean6S.E. 5 194.9613.7 cm. These results support the above interpretation. The decrease in specific differences in movement between 1 day and 2 weeks is also
illustrated in Fig. 5, which compares the mean distance displaced by each species averaged over all experiments with the topographic complexity of the substratum in
each site for each level of topographic complexity. In five of the six sites, A . porcata
moved further in 1 day than did the other two species Fig. 5a, c and e. After 2 weeks, this difference among species had almost disappeared Fig. 5b, d and f.
Another influence that habitat may have on movement is to alter the variances of distances displaced by members of a population, especially for species that have specific
requirements for habitat, but which might encounter these habitats by chance. To test the hypotheses that variances of displaced distances over 1 day, 1 week or 2 weeks varied
according to species, topographic complexity or both, the variances were calculated for each site using the log-transformed data. These were compared among species,
topographic complexity and experiments, using the value from each site as a replicate.
After 1 day, there was a significant interaction between species and experiments F 5 2.77, 4 and 36 df, P , 0.05, with A. porcata showing greater variability in
distances displaced in May than did the other two species, which showed similar amounts of variability. After 1 week, there was a significant difference among species
F 5 10.93, 2 and 4 df, P , 0.05, but, in this case, N. atramentosa showed greater
192 M
.G. Chapman J. Exp. Mar. Biol. Ecol. 244 2000 181 –201
Fig. 5. The mean distance displaced averaged over all experiments during 1 day a, c and e and 2 weeks b, d and f in relation to a, b topographic complexity at the scale of 1 m, c, d topographic complexity at the
scale of 50 cm and e, f topographic complexity at the scale of 5 cm; d, A . porcata; m, B. nanum; j, N.
atramentosa.
M .G. Chapman J. Exp. Mar. Biol. Ecol. 244 2000 181 –201
193
variability in the distances displaced. After 2 weeks, the variances of the distances displaced did not differ among species, the two levels of topographic complexity, the
two experiments or with any interactions of these variables P . 0.05 for all terms in the analysis.
3.3. Directionality of dispersal The original objective was to test hypotheses that the mean direction displaced would
vary according to species, topographic complexity or their interaction and that directional patterns of movement would be consistent across experiments. This was to be
done using analyses of variance for directional data Underwood and Chapman, 1985. In fact, movement was only directional Rayleigh’s test; Mardia, 1972 for A
. porcata in two sites in August and for N
. atramentosa in two sites in May. Because there is no logic in comparing mean directions when movement is randomly orientated, these
particular hypotheses were not tested. Neither was it possible to test the hypotheses that directionality of movement varied according to species or topography, but was consistent
] across experiments, using the method described in Chapman 1986 because the R
values were too small. Therefore, to test the hypothesis that directionality varied among ]
species, topography or experiment, R was calculated for each species in each site in each experiment. These values were then compared using analysis of variance.
All F-ratios were non-significant P .0.05. Therefore, movement was similarly randomly orientated in all species, on complex and simple topographic complexity and
during the three different experiments. 3.4. Correlations between dispersal and environmental variables
Although the analyses had indicated that topographic complexity was important in determining movement over 2 weeks, comparison across all six sites showed that
movement was not simply related to topographic complexity at any one scale, i.e. centimetres, tens of centimetres or a metre Fig. 5. Similarly, there were few
relationships between the variability in the distances displaced and various measures of topographic complexity or cover of water or macro-algae Fig. 6. The only relatively
consistent trend was that B
. nanum showed greater variability in the distances displaced than either of the other two species as the substratum became more topographically
complex at the smallest spatial scale Fig. 6c. 3.5. Use of different microhabitats during low tide
The use of different microhabitats during low tide was compared among the three species by analysing the proportion of marked animals in each microhabitat in each site,
using the data from the eight independent occasions as replicates. There were no hypotheses about differences among sites because of differences in the relative
abundances of the different habitats from site to site. Nevertheless, these data can test the null hypotheses that a differences among species are consistent over all sites and
194 M
.G. Chapman J. Exp. Mar. Biol. Ecol. 244 2000 181 –201
Fig. 6. The variance among distances transformed to natural logarithms displaced averaged over all experiments during 2 weeks in relation to a topographic complexity at the scale of 1 m, b topographic
complexity at the scale of 50 cm and c topographic complexity at the scale of 5 cm; d, A . porcata; m, B.
nanum; j, N . atramentosa.
M .G. Chapman J. Exp. Mar. Biol. Ecol. 244 2000 181 –201
195
b differences among species vary interactively from site to site, for each microhabitat separately and n 58 measures i.e. times of sampling.
The proportion of marked snails in crevices or overhangs varied significantly among species Table 3a, with a significantly greater proportion of N
. atramentosa found in crevices mean6S.E.50.1560.02 than were B. nanum 0.0460.01 or A. porcata
0.0460.01. The proportion of marked snails in contact with or under the canopy of algae varied
interactively among species and sites Table 3b. In four of the sites, there were no significant differences among species. In S3, a significantly greater proportion of N
. atramentosa were in contact with or under the canopy of algae mean6S.E.5
0.1560.03 than were B. nanum 0.0260.01 or A. porcata 0.0260.02 and in C1, a significantly greater proportion of N
. atramentosa and A. porcata were in contact with or under the canopy of algae mean6S.E.50.1460.03 and 0.1960.07 than were B.
nanum 0.0160.01. The proportion of marked snails under at least 1 cm depth of water also varied
interactively among species and sites Table 3c. Patterns varied inconsistently from site to site. The proportion of snails under water and results of SNK tests are as follows .
indicates a significant difference, P ,0.05; 5 indicates a non-significant difference, P .0.05:
S1 – A. porcata 0.7560.04 . B. nanum 0.4860.05 5 N. atramentosa 0.4060.04 S2 – A. porcata 0.6860.04 5 N. atramentosa 0.6660.04 . B. nanum 0.4960.03
S3 – N. atramentosa 0.6360.05 . B. nanum 0.1760.03 5 A. porcata 0.2460.04 C1 – A. porcata 0.4660.03 5 N. atramentosa 0.4460.07 5 B. nanum 0.1960.05
C2 – A. porcata 0.5160.06 5 B. nanum 0.4260.03 5 N. atramentosa 0.4360.04 C3 – A. porcata 0.6760.05 . B. nanum 0.3060.06 5 N. atramentosa 0.3860.06.
Although patterns of use of water by the three species varied interactively, A . porcata
were more commonly found in water than the other species in all sites except S3 although these differences were not always statistically significant.
Table 3 Analyses of the proportion of marked snails of each species in different microhabitats; a in crevices or
overhangs, data transformed to arc sines, Cochran’s C 50.15, P.0.05; b in contact with foliose macro-algae, data transformed to arc sines, Cochran’s C 50.13, P.0.05; c in water; data untransformed, Cochran’s
C 50.12, P.0.05; n58 independent times of sampling for each analysis
a b
c df
MS F
P MS
F P
MS F
P Species5Sp
2 2529.5
24.16 821.0
3.85 ns
0.55 2.95
ns Topography5T
1 1347.5
2.51 ns
231.7 0.34
ns 0.22
0.85 ns
Sitetopography5ST 4
537.2 6.98
682.9 9.27
0.26 13.95
Sp3T 2
108.2 1.03
ns 514.6
2.42 ns
0.06 0.31
ns Sp3ST
8 104.7
1.36 ns
213.0 2.89
0.19 10.19
Residual 126
77.0 73.7
0.02
196 M
.G. Chapman J. Exp. Mar. Biol. Ecol. 244 2000 181 –201
4. Discussion
