S .D. Connell, M.J. Anderson J. Exp. Mar. Biol. Ecol. 241 1999 15 –29
19
Anderson and P. Legendre, University of Montreal and RDAs with appropriate permutation tests were done using
CANOCO
E courtesy of C.J.F. ter Braak. 2.3. Univariate tests
Univariate tests of hypotheses for individual taxa were done using multi-factorial ANOVA e.g. Underwood, 1997. The first analysis was a comparison of partial cages
and open panels, to test for artefacts due to cages Factor 5 partial cages, fixed with three levels: small mesh, large mesh, open panels. In the absence of any significant
interpretable caging artefacts, a second analysis compared panels in full cages with open panels, to test for effects of exclusion of fish Factor 5 cage, fixed with three levels:
small mesh, large mesh, open panels. For each of these sets of analyses, the caging factor was crossed with the factor of panel size three levels: 5 cm 3 5 cm, 10 cm 3 10
cm and 20 cm 3 20 cm. The design was balanced: for each combination of treatments there were three sticks Factor 5 sticks, nested in cage 3 panel size and n 5 2 replicate
panels per stick. Univariate ANOVAs and Cochran’s tests were done using
GMAV
5 courtesy of A.J. Underwood and M.G. Chapman.
3. Results
3.1. Composition of assemblages Assemblages on panels were composed primarily of juvenile oysters Saccostrea
commercialis Iredale Roughley and Crassostrea gigas Thunberg, bryozoans Watersipora arcuata Banta, Bugula neritina and Tubulipora pulchra, barnacles
Hexaminius sp., Elminius covertus Foster, and Balanus spp., calcareous tubeworms Spirorbis sp. and Galeolaria caespitosa Savigny, recently settled gastropods Nodilit-
torina acutispira Smith and Bembicium auratum Quoy and Gaimard, and ephemeral
¨ species of algae Oscillatoria sp., Rhizoclonium sp., Enteromorpha prolifera Muller,
Ulvaria oxysperma and Caloglossa leprieurii Montagne. 3.2. Artefacts due to cages
Analyses did not detect any artefacts due to cages. Although assemblages differed
[
among open panels and panels under partial cages F 5 1.580; P , 0.05, pairwise
2, 45
comparisons did not reveal consistent significant differences among the three treatments P . 0.05. Similarly, preliminary univariate analyses did not detect significant effects of
partial cages for any taxon P . 0.05 except oysters. Densities of oysters differed among open panels and cage controls P , 0.05, but SNK tests did not reveal consistent
significant differences among the three treatments. The density of oysters was smaller on open panels than on panels inside partial cages made of small mesh SNK test,
P , 0.05, but no significant differences were detected among the remaining pairwise comparisons SNK test, P . 0.05. The density of oysters did not differ between open
panels and panels inside partial cages made of large mesh, and between panels inside
20 S
.D. Connell, M.J. Anderson J. Exp. Mar. Biol. Ecol. 241 1999 15 –29
Fig. 2. Two-factor nMDS plot Clarke, 1993 comparing centroids of assemblages on panels open to predation symbols not circled to those protected from predation with small mesh symbols inside circles for each patch
size: 5 cm 3 5 cm 5, 10 cm 3 10 cm 10 and 20 cm 3 20 cm 20. The values for each centroid mean of n 5 2 panels per stick were calculated from principle coordinates of Bray–Curtis dissimilarities.
partial cages made of small or large mesh. Importantly, the effect of small cages was greater than the effect of small partial cages Fig. 2. Assuming partial cages provide an
adequate test of artefacts of cages we conclude that cage artefacts were minimal and that the effects of excluding fish could be interpreted.
3.3. Effect of cages Prior to the experiment, we predicted that prey on smaller panels would be less
affected by predation than prey on larger panels. The two-factor nMDS plot of centroids for each stick calculated from the principal coordinates of Bray–Curtis dissimilarities
indicated that multivariate differences in the structure of assemblages between panels open to predation open panels and panels most protected from predation cages with
small mesh were similar in magnitude and direction for each size of panel Fig. 2. That is, centroids representing assemblages on panels inside cages circles are generally
located in the bottom region of the nMDS plot and their separation from centroids representing assemblages on panels open to predation not circled is similar across the
range of panel sizes. Importantly, no multivariate interaction was detected between the factors cage and panel size Table 1a.
The effect of predation was restricted to small predatory fish. Assemblages on panels open to predation by all fish open panels differed from those inside cages with small
mesh no predation by small or large fish, but not from cages with large mesh predation by small fish only, Table 1a and b.
Univariate analyses testing for effects of cages indicated that most taxa were
S .D. Connell, M.J. Anderson J. Exp. Mar. Biol. Ecol. 241 1999 15 –29
21 Table 1
2
a Two-factor distance-based redundancy analysis db-RDA on multivariate data standardised per 100 cm unit area examining effects of cage treatments large mesh vs. small mesh vs. open panels and panel size 5
cm 3 5 cm vs. 10 cm 3 10 cm vs. 20 cm 3 20 cm on assemblages of epibenthic organisms for full cages. See
[ [
text for explanation of MS and F . b Pairwise comparisons of main effects among sizes of mesh and panel.
[ [
The multivariate pair-wise statistic t 5 œF
[ [
df MS
F P
a Source of variation Cage
2 0.059
1.86 0.001
Panel size 2
0.158 4.99
0.001 Cage 3 panel size
4 0.070
1.10 0.157
Residual 45
0.713 Total
53 1.000
[
b Pairwise comparisons t
P Cage
Large mesh vs. small mesh 1.345
0.006 Large mesh vs. open panel
1.073 0.209
Small mesh vs. open panel 1.771
0.001 Panel size
5 cm 3 5 cm vs. 10 cm 3 10 cm 2.081
0.001 5 cm 3 5 cm vs. 20 cm 3 20 cm
2.944 0.001
10 cm 3 10 cm vs. 20 cm 3 20 cm 2.312
0.001
unaffected by cages, except oysters, algae and a herbivorous gastropod, Bembicium auratum Table 2. Oysters were significantly more dense on panels inside cages with
small mesh than on other panels Fig. 3a, Table 2a. No differences in densities of oysters were detected between panels open to predation and panels protected by cages
with large mesh. The density of Bembicium auratum was greater on panels inside cages than on open panels, but did not differ between panels inside cages made from small or
large mesh Fig. 3b, Table 2a. Percentage cover of algae was greatest on open panels, significantly less extensive on panels protected by large mesh and the least extensive on
panels protected by small mesh Fig. 3c, Table 2a. The only other possible effect of predators was on the barnacle Hexaminius sp., which occurred with greater frequency on
panels inside cages with small mesh than on panels in cages with large mesh Table 3:
2
x 5 10.5, df 5 2, P , 0.05 but this species did not recruit in great abundance during this experiment. As found in multivariate analyses, the magnitude of cage effects did not
vary among panels of different size; the cage 3 panel size interaction was not statistically significant for any univariate taxon P . 0.05, Table 2.
We identified the species which primarily contributed to multivariate differences among the caging treatments. This was done by successively omitting taxa shown by
univariate analyses to be most affected by cages Table 2a and repeating the multivariate analysis until no significant multivariate differences were detected among
caging treatments. Oysters were most affected by cages Table 2a; ANOVA: F 5
2, 18
23.45, P , 0.001 but after oysters were omitted from the multivariate analysis,
[
significant multivariate differences remained RDA: F 5 1.244, P , 0.05. Cages
2, 45
had a much smaller effect on the percentage cover of algae Table 2a; ANOVA: F
5 6.22, P , 0.01 and Bembicium auratum Table 2a; ANOVA: F 5 5.04,
2, 18 2, 18
22 S
.D. Connell, M.J. Anderson J. Exp. Mar. Biol. Ecol. 241 1999 15 –29 Table 2
Summary of univariate analyses of variance and a posteriori SNK tests for effects of mesh size large mesh vs. small mesh vs. open panels and panel size 5 cm 3 5 cm vs. 10 cm 3 10 cm vs. 20 cm 3 20 cm on
2 a
assemblages of epibenthic organisms for full cages standardised per 100 cm unit area
b
Cages Panel size
C 3 P Sticks
SNK results a Taxa affected by cages
Bivalves Oysters
23.45 0.78ns
1.31ns 1.10ns
O 5 L , S
c,d,e
Gastropods Bembicium auratum
5.04 6.37
0.30ns 1.52ns
O , L 5 S 20 , 10
f
Algae Total cover
6.22 0.67ns
1.20ns 1.92ns
O . L . S b Taxa unaffected by cages
Bivalve Lasaea australis
0.89ns 2.42ns
0.99ns 6.34
c
Gastropod Nodilittorina acutispira
1.67ns 0.85ns
0.77ns 2.59
c,d
Polychaetes Spirorbis sp.
0.44ns 0.02ns
0.47ns 2.34ns
c,d,e
Galeolaria caespitosa 0.29ns
18.49 1.61ns
2.26ns 20 . 10
Alga Ulvaria oxysperma
2.05ns 9.75
0.50ns 0.99ns
20 . 10 . 5 Other taxa
Copepods 1.58ns
1.73ns 0.92ns
2.46
d
Insect larvae 0.01ns
16.67 0.60ns
1.64ns 20 , 10 5 5
Total no. of taxa 1.17ns
61.58 1.43ns
1.30ns 20 . 10 . 5
a
For each analysis, there were n 5 2 panels on each of three sticks in each combination of 3 3 3 treatments. ns 5 P . 0.05; P , 0.05; P , 0.01; P , 0.001.
b
5 5 5 cm 3 5 cm, 10 5 10 cm 3 10 cm, 20 5 20 cm 3 20 cm; S, small mesh; L, large mesh; O, open panels; inequalities indicate significant differences with P , 0.05, equalities indicate differences are not
significant with P . 0.05.
c
For these variables, patches measuring 5 cm 3 5 cm were not included in the analysis, due to lack of numbers on these sizes of panels.
d
Variable was transformed to y9 5 log y 1 1.
e e
Variances were heterogeneous. Non-significant results are interpretable. Some caution should be used in interpreting significant results.
f 21
Variable was transformed to y9 5 sin œy.
P , 0.05. A significant multivariate effect remained after percentage cover of algae and
[
oysters were removed from the analysis RDA: F 5 1.243, P , 0.05, but not after
2, 45 [
the removal of B . auratum, percentage cover of algae and oysters RDA: F
5 1.203,
2, 45
P . 0.05. These three variables were, thus, responsible for causing multivariate differences in assemblages among caging treatments; cages did not affect the com-
position or relative abundances of other species in the assemblage. A two-factor nMDS plot of the remaining taxa, after removing these three variables, also showed that
multivariate effects of cages were no longer apparent Fig. 4, symbols representing panels inside cages circled are well-mixed with those representing open panels not
circled.
3.4. Effect of sizes of panels In addition to a significant effect of predation, there was an important effect of
S .D. Connell, M.J. Anderson J. Exp. Mar. Biol. Ecol. 241 1999 15 –29
23
2
Fig. 3. Mean abundance per 100 cm 6S.E. versus treatments of fish predation S, full cage, small mesh; L, full cage, large mesh; PS, partial cage, small mesh; PL, partial cage, large mesh; O, open and for three
different sizes of panel.
different sizes of panels on epibiota Fig. 2. Assemblages of epibiota on panels of different sizes were significantly different from each other Table 1. This result was
probably influenced by the very strong and positive effect of increasing panel size on the number of taxa. Many more taxa occurred on 20 cm 3 20 cm panels compared to 10
cm 3 10 cm and 5 cm 3 5 cm panels Tables 2 and 3. Increasing size of panel also had
24 S
.D. Connell, M.J. Anderson J. Exp. Mar. Biol. Ecol. 241 1999 15 –29
Fig. 4. Two-factor nMDS plot comparing centroids of assemblages on panels open to predation symbols not circled and protected from predation symbols inside circles after abundances of oysters, total percentage
cover of algae and abundances of the gastropod Bembicium auratum were removed from analyses. See the legend of Fig. 2 for explanation of symbols.
a positive effect on the densities of the alga Ulvaria oxysperma and the tubeworm Galeolaria caespitosa, but a negative effect on the densities of insect larvae and
Bembicium auratum Table 2.
Table 3 Summary of frequencies of occurrence of taxa across a three panel sizes and b five treatments testing the
effects of predation: S, L, O, PS, PL as described in Fig. 3
a b
Taxa a Patch size
b Cage treatments 5 3 5
10 3 10 20 3 20
S L
O PS
PL Bivalves
Bankia australis 1
12 2
4 3
4 Xenostrobus sp.
6 1
1 1
3 Bryozoa
Watersipora arcuata 8
20 7
5 4
6 6
Bugula neritina 4
7 23
5 6
5 9
9 Tubulipora pulchra
7 6
23 8
5 7
10 6
Barnacles Hexaminius sp.
5 2
10 9
1 3
4 Elminius covertus
2 4
2 1
1 2
Balanus spp. 3
3 8
3 3
2 3
3 Algae
Rhizoclonium sp. 30
30 30
18 18
18 18
18 Oscillatoria sp.
26 29
30 17
16 18
17 17
Enteromorpha prolifera 24
30 30
18 16
17 17
16 Caloglossa leprieurii
20 25
30 15
16 15
12 17
Other taxa foraminifera
1 14
9 4
4 7
4 5
mites 7
7 12
5 7
2 5
7
a
Frequencies out of 30 possible patches 5 treatments 3 2 panels 3 3 sticks.
b
Frequencies out of 18 possible patches 3 sizes 3 2 panels 3 3 sticks.
S .D. Connell, M.J. Anderson J. Exp. Mar. Biol. Ecol. 241 1999 15 –29
25
4. Discussion
