The distribution of epifauna on Ecklonia radiata (C. Agardh) J. Agardh and the effect of disturbance

J. Exp. Mar. Biol. Ecol., 1983, Vol. 71,

pp. 205-220

205

Elsevier

THE DISTRIBUTION

OF EPIFAUNA

ON ECKLONZA

RADZATA (C. Agardh)

J. Agardh AND THE EFFECT OF DISTURBANCE

WARRICK

J. FLETCHER’

and ROBERT W. DAY

Deparrment of Zoology. Universiry of Melbourne. ParkviNe. Victoria. 3053. Auwaliu Abstract: The epifaunal community of the kelp, Ecklonia rudiara (C. Agardh) J. Agardh, was investigated at Portsea, Australia. The per cent cover of epifauna on the primary blade of Ecklonia was significantly

greater on plants from the inner and outer pilings than on those from low on the pilings, under the pier and from an adjacent reef. Species diversity, but not the per cent cover of epifauna, was significantly greater on crinkled than on smooth plants. Various factors of disturbance were measured, including lashing by other plants, which increased with the density of plants. Other abrasive forces were found to increase closer to the bottom, as did the density of gastropods on the fronds. For each site, the number of disturbance factors present was summed giving a disturbance index (D), which was negatively correlated with per cent cover. Transplants and laboratory experiments indicated that three of these disturbance factors reduced the abundance of hydroids, but that increasing the amount of plant lashing had no effect on Tricellaria poneri MacGill. Seven species were chosen for further analysis. The abundance of most species correlated with either plant length or bushiness, although the variation accounted for had a range of l-43%. For all species, the percentage of zones which were colonized on the Ecklonia blade increased distally, following the age gradient. While the abundance of some species also increased distally, that of hydroids decreased. Membranipora membranacea L. was found to smother the hydroids on 60% of their encounters but neither the distribution nor the abundance of hydroids was altered in the absence of Membranipora. It appears that disturbance rather than competition is the major modifying influence of this community.

INTRODUGTION

Studies on the epifaunal communities on marine algae are quite limited (Seed & O’Connor, 198 l), with most concentrating on the distribution and abundance of species along the algal frond. Such studies concluded that there was interspecific competition among the component species leading to segregation both within and among plants (Boaden et al., 1975, 1976; O’Connor el al., 1979,198O). Few manipulative studies have been done to test these hypotheses, yet these communities “can be readily manipulated and transplanted” (Seed & O’Connor, 1981). This study investigates the epifaunal community of the kelp, Eckfonia radiata (C. Agardh) J. Agardh, at Portsea, Australia, with respect to the distribution of epifaunal species and the effects of disturbance. Preliminary observations indicated that the abundance of epifauna varied considerably among areas. These differences may have arisen because of diRering rates or intensity of settlement or because the probability of survival on the frond varied among sites. Benson (1979) found that disturbance due ’ Present address: School of Biological Sciences (A08), University of Sydney, Sydney, 2006, Australia. 0022-0981/83/503.00 Q 1983 Elsevier Science Publishers B.V

206

WARRICK

J. FLETCHER

AND ROBERT W. DAY

to the lashing atTect of algae varied along the stipe of Cjxtoseira neglectu and that this affected the species diversity on different sections of the stipe. Furthermore, the distributions of the individual epifaunal species were also analysed to test the importance of plant characteristics and interspecific competition. Boaden et al. (1976) and O’Connor et al. (1979, 1980), suggested that these variables are important in structuring the community of Fucus serrutus. MATERIAL

AND

METHODS

This study was done at Portsea Pier and on an adjacent reef, situated in the southeastern corner of Port Phillip Bay, Victoria, Australia (Fig. 1).

*

Fig. 1. Port Phillip Bay, showing the location of the site studied at Portsea.

MORPHOLOGY

OF ECKLONIA

Ecklonia radiata is a laminarian alga with a single erect stipe, one primary and numerous secondary blades (Fig. 2). An intercalary meristem is located near the tip of the stipe, so that elongation of the primary blade occurs at its base, while erosion occurs at its tip (see also Mann & Kirkman, 1981). Plants collected for analysis had either a crinkled or a smooth primary blade, the former being the juvenile or stage 2 of development (Mann & Kirkman, 1981).

EPIFAUNA OF ECK~~~~A

207

SECONDARY BLADES

PRIMARY BLADE

Fig. 2. Diagram of a mature plant of the kelp Eckloniu radiata.

SAMPLING

Five plants were collected from each of the five sites (Table I) at both sampling periods (April and August, 1980). Plants from the pier were selected by using random TABLEI Sampling sites. Location

Symbol

Depth

Reef Both sides of the inner pilings The outside of the outer pilings The base of the pilings Under the pier

R PI PO LP UP

sea bed (3 m) 1.5 m 1.5 m 0.3 m from sea bed sea bed

co-ordinates to specify both the piling from which the plants would be selected and the position on the piling. Similarly, plants from under the pier were selected using co-ordinates to specify the distance and direction from a piling. Plants were selected from the reef by swimming along a randomly selected line and taking every third plant. These plants were immediately preserved in a 10% formalin-sea water solution for later analysis. For each plant a number of morphological characteristics was measured (Fletcher, 1980). The fauna on the primary blade of each plant was recorded, but no examination of the secondary blades was made. No attempt was made to collect the mobile species

208

WARRICK

J. FLETCHER AND

ROBERT

W. DAY

present. The abundances of the sessile species on the primary blade were quantified in a fashion similar to that of Boaden et al. (1975). (1) Solitary species - these were counted and, for each species, the average diameter of ten individuals chosen at random was determined. (2) Arborescent species - the area covered by the colony was measured. (3) Encrusting species - the per cent cover was estimated. (4) Interactions - i.e. growth of one colony over another was recorded. The area covered by each species was used to calculate the percentage of the entire primary blade covered, and the percentage of each plant segment or zone covered (where one segment is one-tenth of the length of the primary blade). The Shannon-wiener diversity index (H’ = - P, 1 logPi, where Pi is the proportion of the ith species; Shannon & Weaver, 1949) was calculated for each plant using the area covered by each species. Results from the plants were analysed with respect to site, plant morphology (crinkled or smooth) and sampling time, using a three-factor analysis of variance (site and morphology were futed factors, sampling time was a random factor). The percentage of the primary blade covered by all the species was transformed using the arcsin transformation to normalize the data. MEASUREMENT

OF ABRASION

AND

LASHING

Two measurements were made to quantify the level of disturbance at the frond surface. (1) Abrasion - a relative estimate of both the amount of water movement and abrasion at each site was made by the use of both shielded and unshielded plaster balls attached to the same plant (see Fletcher, 1980). (2) Lashing by algal fronds - this was measured by attaching foam strips, into which pins had been pushed, to the primary blade of plants and counting how many remained after 24 h. These strips were placed on six plants, plus control areas away from plants at each of the sites.

TRANSPLANTS

Individual plants with extensive amounts of epifauna were transplanted to a number of positions to test the effects of various environmental factors (such as light, sand, gastropods, and lashing by other plants). Two series of transplants were done. First, plants with established colonies of hydroids were photographed and their holdfasts were strapped to blocks of wood with electrical wire. These blocks were then moved to the various test positions (Table II). They were left in these positions for 4 wk, then re-photographed. The initial and final abundances of the hydroids were estimated from these photos using a ranking scale of O-3, where 0 = absent, 1 = present, 2 = common, 3 = abundant. A second series of transplanted plants had colonies of the arborescent bryozoan Tricefhria porteri MacGill. The initial number of colonies on each side of the plant was counted, the plants were then moved to positions on the outside of the pier in regions

EPIFAUNA OF ECKLONIA

209

TABLE II

Details of transplant positions. Position moved to Inside pilings Pilings outside (low density) Pilings outside (high density) Bottom of pilings Sand area Reef area

Factor(s) tested

Factor(s) controlled

Nil Light

All Sand, lashing and gastropods

Lashing and light

Sand and gastropods

Sand and gastropods Sand and light Sand, light and gastropods

Light and lashing Gastropods and lashing Lashing

of high and low plant density, and also to control positions back on the inner pilings. After four weeks the number of colonies was counted again. DISTRIBUTION

AND ABUNDANCE

OF GASTROPODS

The number of gastropods (mostly the turbinid Phasienella) on the blades of ten plants at four heights up the pilings (0,0.5, 1,2 m from the bottom) was counted. The lengths of the plants were also recorded, allowing estimates of the density of the gastropods (per unit length of plant) to be made. EFFECT OF GASTROPODS

ON HYDROIDS

No effective manipulation of gastropods could be done in the field, due to the ditliculties of maintaining mobile animals in treatments. An experiment was, therefore, designed whereby eight plants with established colonies of hydroids, plus a large number of gastropods were brought back to the laboratory. The number of hydroids on each plant was counted and the plants were placed into lighted sea-water aquaria. There were two tanks each divided in half by nylon fly wire mesh. Two plants were placed into half of each tank; after four days, the plants were removed and the number of hydroids remaining was counted.

RESULTS ABUNDANCE

AND DIVERSITY OF THE EPIFAUNA

The per cent cover of epifauna on the primary blade of the Eckloniu plants from the five sites was analysed with respect to site, plant morphology (smooth or crinkled), and time of sampling. A significant difference in the cover of epifauna was found among sites (F = 19.2 with 4, 33 d.f., P < 0.01) but there was no difference between crinkled and smooth plants or between sampling times (F = 0.18 with 1, 33 d.f., n.s.). The cover of

210

WARRICK

J. FLETCHER

AND

ROBERT

W. DAY

epifauna was significantly greater on plants from both the inner and outer pilings than from any other site (Student-Newman-Keuls tests, Table III). TABLE III Comparisons of the mean values of the per cent cover of epifauna from the five sites using the Student-Newman-Keuls method: for this and all further analyses, *P < 0.05; **P< 0.01; ns., P > 0.05; for symbols see Table I. Difference between means

Comparison

Significance

PI versus:

** ** *

R UP LP PO

24.3 23.4 18.4 6.0

n.s.

R UP LP

18.3 17.4 12.4

** ** **

UP

5.9

n.s.

PO versus:

LP versus:

The species diversity of epifauna on each plant was also analysed with respect to site, morphology, and time of sampling. In this case, there were no significant differences among sites but diversity was greater on crinkled than on smooth plants. PHYSICAL

DISTURBANCE

The mean density of Ecklonia differed significantly among sites; the outer pilings and the reef area had a greater density than the other sites (Fletcher, 1980). When lashing

; Fig. 3. The relationship

1;

lb DENS,,”

OF

ECKLONIA

lNo./d

between the density of Eckloniu and the number of pins removed r = 0.96, P < 0.05): vertical bars indicate k I SE.

after 24 h (n = 5,

EPIFAUNA

211

OF ECKLONIA

by the plants was measured using the pads with pins (see p. 208), significantly more pins were removed from the plants on the reef and from those at high density on the outer pilings than from the other sites (F = 5.7 with 4, 46 d.f., P -C 0.01). No pins were lost from the control pads away from plants. There was a significant linear relationship between the number of pins removed and the density of the plants at each site (Fig. 3). Thus, the amount of lashing a plant surface receives (measured as the number of pins removed), increased with the density of the plants. Other abrasive forces were measured using shielded and unshielded plaster spheres. In this case, plants from the inner pilings were subject to less abrasion than those from the other three sites close to the substratum (F = 7.2 with 3, 8 d.f., P < 0.05). Abrasive force, therefore, differed from lashing in that it was independent of the density of the plants. It will, however, be affected by abrasion caused by suspended particles of sand and also by the plants hitting the sea bed. DISTRIBUTION

AND

ABUNDANCE

OF GASTROPODS

The density of gastropods decreased with increasing height from the bottom (r = 0.582, n = 20, P < 0.05). Thus, the level of biological disturbance may differ between sites, with smaller values of disturbance for plants higher on the pilings. TRANSPLANTS

A significantly greater loss of hydroids occurred on plants moved to sandy areas, the reef, the base of pilings, and into areas of high density on the outer pilings than on the control plants (Kruskal-Wallis analysis of variance, Table IV). No difference was

Results of transplant experiments: mean rank loss of hydroids after four weeks at each transplant site; Kruskal-Wallis analysis of variance; H’ = 18.83 is the test statistic for the Kruskal-Wallis ANOVA; Ri is the sum of the ranks of the ith sample.

__ .-

Transplant .-

site -

Inside pilings Outside pilings (low density) Outside pilings (high density) Base of pilings Sand area Reef area

.~.

Mean rank loss --

n

0.22 0.50 1.16 1.50 1.75 2.25

9 4 6 6 4 4

Ri .-

-

.-

.-

74.0 44.0 104.5 120.5 89.5 108.5

found between the control plants and plants moved to areas of the outer pilings at low density. The second series of transplants using colonies of Tricelluriu gave different results; increases in the amount of lashing by other plants did not reduce the number of colonies (F = 0.07 with 1, 12 d.f., n.s.).

212

WARRICK J. FLETCHER AND ROBERT W. DAY

The effect of gastropods on hydroids was examined in a laboratory experiment. Significantly more hydroids were lost from plants with gastropods present in the tank than from plants without snails (F = 19.27 with 1, 7 d.f., P < 0.05). Thus, it appears that each of these types of disturbance removes at least one group of epifauna, the hydroids, from the kelp surface. Other groups may not be affected by all of these factors, as Tricellaria was not affected by the lashing of plants. RELATIONSHIP

BETWEEN DISTURBANCE

AND THE ABUNDANCE

AND DIVERSITY OF

EPIFAUNA

It was shown previously that the abundance of the epifauna varied among sites. To relate this to the amount of disturbance present, an overall measure of disturbance at each site was required. The sites were scored on a three point scale for the presence of each factor of disturbance using the results of the statistical analyses presented earlier, while the disturbance factors were ranked according to the results of the transplants. This indicated that abrasion had twice the effect of both lashing and gastropods. The sum of these scores was termed the disturbance index (D) of a site (Table V). The relationship between the per cent cover of epifauna on Ecklonia and this index is shown in Fig. 4. The abundance of epifauna decreased with increasing levels of disturbance (I = - 0.832, n = 50, P < 0.01). Species diversity also decreased with increasing amounts of disturbance (r = - 0.31, n = 50, P < 0.05). TABLEV Summary ofthe disturbance factors present at each site: factors ranked according to results of S.N.K. tests; + , factor present; - , factor absent; 0, factor present but to a lesser degree; for symbols see Table I.

Site

Sand

PI PO LP UP R

+ + +

Lashing by plants +

0

Gastropods _ 0 + +

Disturbance index 0 1.0 2.5 3.0 3.5

Fig. 4. Correlation between the disturbance index D, and the per cent cover of epifauna on the primary blade of Eckloniu (n = 50, r = - 0.852, P i 0.01). Vertical bars indicate + 1 SE.

213

EPIFAUNA OF ECKLONIA VARIATIONS

IN SPECIES COMPOSITION

A total of 37 taxa from seven phyla were recorded. The seven most common species were selected for further analysis. It was found earlier that species diversity was greater on crinkled than on smooth plants. The number of crinkled and smooth plants colonized by each species was analysed by the chi-square method (Table VI). The ascidian, Diplosoma rayzen’ MacDonald, as well as two species of spirorbid, were found more often on crinkled plants. In addition, both Campanularia and Spirorbid A were significantly more abundant on crinkled plants than on the smooth plants. There were also differences in the parts of the crinkled plants occupied. The two species of hydroids occurred more often in the troughs than on the ridges of these plants (x2 = 57.1, P < 0.05). Spirorbids were also found to prefer the troughs; all 51 individuals found on the five plants examined were in the troughs. The relationship between mean abundance of each of the common species and the length and bushiness of the plants that they colonized was also examined (Table VII). TABLEVI Utilization of crinkled and smooth plants by the epifaunal species. Percentage of plants colonized

Mean abundance x* SD

Species

Crinkled

Smooth

Campanularia Obelia Spirorbis A

41 66

33 66

0.2 0.0

Spirorbis B

54 68

20 20

5.0* 8.1*

Hippothoa Membranipora Tricellaria

52 25 45

26 46 30

2.5 0.7 0.7

x2

Crinkled 11.7 * 11.2

t

Smooth 6.5 + 3.1

12.6 f

8.6

15.1 f

3.0 f 2.4 f

2.3 1.8

1.7 * 1.2 2.9 + 2.7

3.3 k 11.9 + 4.0 +

2.7 9.1 2.3

1.9 f 1.3 12.4 k 11.1 4.2 + 1.1

- 1.83*

9.3

- 1.33

- 1.77* 1.01 - 1.08

0.16 - 0.26

TABLEVII Correlations between mean abundances per plant colonized with the plant characteristics, bushiness.

length, and

Species

Plant length

Plant bushiness

d.f.

Multiple correlation coefficient

Campanularia Obelia Spirorbis A Spirorbis B Hippothoa Membranipora Tricellaria

- 0.50** - 0.32* - 0.28

- 0.66** - 0.16 - 0.33;

26 48 28

0.69** 0.33* 0.35

- 0.33*

- 0.35*

34

0.38

- 0.50** - 0.37* - 0.01

- 0.47* 0.28

29 23 26

0.53* 0.41 0.02

- 0.01

214

WARRICK

J. FLETCHER

AND ROBERT

DISTRIBUTION

2. ,

W. DAY

ABUNDANCE

~ampanulario20

,

~

Membranipom

‘“t-dz!zl

‘“L-Jk!z3 Hippothoo

Tricellaria

Spirorbis

“l_.dLl

21-_d.3 Spirorbis

20

A

B 2

n 12345678910 DIVISION

the per cent frequency of colonizationof the zones along the primary blade ofEckloniu Fig. 5. Distribution, by each species named: one zone is equal to one-tenth of the primary blade length; abundance, the mean per cent cover of each species, when present, in each zone.

EPIFAUNA

215

OF ECKLONIA

Most of the species were correlated to one or both of the characters. The variation accounted for by these correlations differed among species; the abundance of Campanularia was negatively correlated with bushiness, this accounted for 43 y0 of its variability. This value was < 1% for Tricellaria. A similar range occurred in relationships between abundance and plant length. ZONATION

ALONG

THE ECKLONIA

BLADE

The settlement or survival of species may be affected by their relative positions on the plant. As the plants examined were of differing lengths, a classification system was devised whereby each plant was divided into ten equal divisions or zones. In this way, the distribution and abundance of the species could be compared in relation to their relative (rather than absolute) position along the frond. For all species, the frequency of zones which were colonized, increased distally (Fig. 5), presumably following the age gradient of the plant. As the age of the frond increases distally, there has been more time to settle and grow on these distal regions. Therefore, the sizes of the colonies may be larger on these distal zones as well as being colonized more frequently. While some species show this trend, the abundance of the two species of hydroid actually decrease distally. This result may be due to increased competition in this region. COMPETITION

Ascidians fared best in the observed interactions (i.e. overgrowths) winning 90% of the encounters with other taxa (where winning was defined as one individual or colony which had grown onto the feeding portion of another individual or colony). The bryozoan, Membranipora membranacea (Linnaeus) also had a high success rate (60x), but the hydroids and the spirorbids only won z 10% of their encounters and even in these cases the underlying species was not affected. Only the interactions between Membranipora and the hydroids warranted individual attention (Table VIII). Such interactions are hard to quantify (Stebbing, 1973), as Membranipora may completely surround the stem of the hydroid leaving feeding hydranths functioning (35 % of times) or the erect stems may be completely smothered, to the obvious detriment of the hydroid (60%). If these interactions were affecting the abundance of the hydroids, then on plants TABLE VIII Details

of competitive

interactions

between

Outcome Membranipora overgrows hydroids Hydroids overgrow Membranipora Membranipora growth stops/hydranth

remains

the hydroids

and Membranipora. No.

%

13 6 44

60 5 35

216

WARRICK

J. FLETCHER

AND ROBERT

W. DAY

without Membranipora, hydroids might be expected to be more abundant. This, however, was not the case (t = 0.5, n.s.), likewise, the distribution of hydroids along the primary blade was not altered by the presence or absence of Membranipora (Kolmogorov-Smirnov test, Kd = 0.06, n.s.). DISCUSSION

Differences in the abundance of species among areas may result from a large number of causes including settlement preferences of larvae (Ryland, 1959; Crisp & Williams, 1960; Bernstein dz Jung, 1979), from variations in the amount ofpredation (Paine, 1966) or disturbance (Osman, 1977; Sousa, 1979). No investigation of the settlement of larvae was made in this study. The results obtained indicate, however, that the levels of disturbance at the various sites can largely explain the differences in epifaunal cover observed. The Ecklonia blade itself is subject to continual erosion at its distal end (Fletcher, 1980). This imparts an intrinsic level of disturbance to this community, for the length of time available for colonization and growth of any potential epiphyte will be limited by the amount of time a region of the blade remains intact. Consequently, most of the epifaunal species were more frequently found on the older more distal regions where the time for colonization has been longer. The rates of growth and erosion did not differ among sites (Fletcher, 1980) but other types of disturbance were found to vary in their intensity between sites. When these factors were present, each was shown to reduce the cover of hydroids on plants. Not all epifaunal species, however, will be affected in a similar fashion. The hydroids with their delicate arborescent growth form, are removed from the plants by increased amounts of lashing. The stronger attachment, plus the armour - like structure of the bryozoans, may be the reason that Tricellaria colonies were not affected. Thin encrusting forms, such as Membranipora, should be affected even less and this may explain why these were found on plants at high density. Abrasion by sand was also found to be detrimental to the hydroids Obelia australis von Lendenfeld and Campanularia on Ecklonia. This could either be by clogging their feeding structures, as was found by Round et al. (1961) for the hydroid Sertularia and by Seed & Boaden (1977) for Gonothyraea and Dynamena, or by direct removal of the polyps by large sand particles. Each sampling site was subjected to a different combination of disturbance factors. An index of disturbance was calculated for each site, and larger values of this index were related to decreases in the abundance of epifauna. Presumably, organisms growing on plants with a large disturbance value will be consistently removed or damaged by one form of disturbance or another. A reduction in cover under high levels of disturbance was also found by Sousa (1979) for algae on intertidal boulders. With decreasing levels of disturbance, the amount of cover increases, possibly as the length of time available for growth between disturbance events is increased. Intermediate levels of disturbance have been found to produce the greatest species

EPIFAUNA

OF ECKLONL4

217

diversity (Osman, 1977; Connell, 1978; Lubchenco & Menge, 1978; Sousa, 1979). On Ecklonia, however, the greatest species diversity occurred on plants with the smallest value of dist~b~ce. One possible expl~ation is that the continual erosion of the primary blade never allows sufIicient time for dominant species (e.g. ascidians) to monopolize space and reduce diversity. The gross rate of erosion of the primary blade of Ecklonia at Portsea is x 4.5 cm a month, while the average length of the primary blade is 40 cm (Fletcher, 1980). A region of the primary blade is, therefore, available for colonization and growth for approximately nine months, which is long enough for a “mature” community of ascidians and sponges to dominate panels placed under the pier (Russ, 1980). Very few Eck~on~aplants reach this stage of development, indicating that either the lowest levels of disturbance encountered by these communities is sufficient to prevent domination by ascidians and sponges or that these species avoid settling on Ecklonia.

The selection of a suitable subs~at~ on which to settle is one of the major requirements of a sessile individual. Invertebrate larvae are described as being highly specific in their settlement responses (Crisp, 1964; Hayward & Harvey, 1974). In particular, the bryozoan Membranipora, appears to settle preferentially on algae and is seldom found on other substrata (Pinter, 1969; Ryland, 1959; Wing & ~enden~ng, 1971). The community of epifauna on Ecklonia at Portsea was similar to that found on the same species in New Zealand (Morton & Miller, 1968), and on L~jna~a in Great Britain (Stebbing, 1973). Furthermore, many species on EckZonia were missing from or were rare on the surrounding substrata. Thus, Membranipora, which was abundant on Ecklonia, was not found on pilings of ages l-9 yr (Harris, 1978), nor on settlement panels of 3 months to 3 yr (Russ, 1980). Most other species which were common on Ecklonia were either rare or early colonizers on Russ’s panels. This indicates that a number of species may be preferentially settling on, or surviving on Ecklonia. Larvae displaying such selection are probably responding to chemicals either adsorbed to the surface of the substratum (Crisp, 1965) or within the alga itself (Willies, 1965). Many epiphytic organisms display habitat selection on a much finer scale, settling preferentially on the younger parts of the fronds (Stebbing, 1972) or in the grooves of blades (Wisely, 1960; Wi~i~s, 1965; Ryland & Gordon, 1977). Such responses were seen as a mechanism to avoid inter- and intraspecilic competition (Ryland & Stebbing, 197 1; Stebbing, 1972) or to avoid areas where growth would be suboptimal (Hayward & Harvey, 1974). A preference was seen in this study by the hydroids and spirorbids for the troughs of the crinkled blades rather than the ridges or the smooth blades. Similar results were found for spirorbids on Macrocystis pyrifera (Bernstein & Jung, 1979) and Fucus serratus (Wisely, 1960), and for the hydroids on Ascophyllum nodosum (Williams, 1965). The depressions may be benificial to some species in that abrasion may be reduced or because feeding is enhanced due to the small eddy currents formed behind the ridges (Riedl & Forstner, 1968). Either enhanced settlement or survival may explain why these plants had a greater species diversity.

218

WARRICKJ.FLETCHERANDROBERTW.DAY

It has been shown that epifaunal species are not distributed at random along the length of the fronds of Fucus serrutus (Ryland, 1959; Hayward & Harvey, 1974; Boaden et al., 1975). On Ecklonia, while the abundance of most species increased distally, merely following the age gradient (Fig. 5), the abundance of the hydroids actually decreased towards the distal end. These species fared badly in overgrowth interactions both in this study and on Macrocystis (Bernstein & Jung, 1979) which suggests that their reduced abundance may be due to smothering by other species. There was, however, no difference in either their distribution or abundance on plants with or without their major “competitor” Membranipora. Hydroids have also been found to be more abundant on the basal regions of Fucus serrutus (Boaden et al., 1975) which has the opposite age gradient to Eckloniu. Thus the hydroids may prefer the basal regions of plants where turbulence is usually lower (Boaden et al., 1975) or they may be removed from the distal regions due to an increase in disturbance. In general, the structure of this community may be explained without the need to invoke niche diversification. Boaden et al. (1976) and O’Connor et al. (1979, 1980) believed that the species of a Fucus set-ram community were segregated due to interspecific competition, yet they have not done any manipulative experiments to test this theory. The results of this study revealed no clear segregation among species but instead found that, like the extensive work on rocky intertidal communities (Paine, 1966; Dayton, 1971,1973,1975; Sousa, 1979), on coral reefs (Connell, 1973, 1976, 1978) and on sublittoral boulders (Osman, 1977; Lieberman et al., 1979), the level of disturbance may be the ultimate factor determining community structure and diversity.

ACKNOWLEDGEMENTS

We are grateful to J. Goodie, D. Fletcher, and J. Harris for assistance in the field. We also thank Dr. A. J. Underwood, K. McGuinness and anonymous referees for helpful comments on the manuscript.

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