Introduction

Fucus serratus and Himanthalia elongata are important components of the intertidal flora on NE Atlantic coasts (Lüning 1990). They colonize the shore and maintain their populations by means of sexual reproduction by spores (Creed 1995; Creed et al. 1996a; Stengel et al. 1999). On the semi-exposed rocky shores of the Isle of Man, both species co-occur near the bottom of the littoral zone and have overlapping reproductive periods in the winter (Gibb 1937; Lewis 1964; Creed et al. 1996a). Reproductive effort, measured as the relative biomass allocated to the reproductive organs, is over 98% for H. elongata and between 38 and 50% for F. serratus (Norton 1991; Brenchley et al. 1996). Thus, mixed stands of juveniles of the two species are expected to be common on the shore, but mixed stands are rarer than monospecific stands of either species (Lewis 1964; Lüning 1990; Creed 1995).

The formation of monospecific and mixed stands relates to the release time of gametes and their dispersal (Amsler et al. 1992; Pearson et al. 1998). H. elongata and F. serratus may release gametes in calm water conditions, resulting in the formation of monospecific stands near to the parent plants, as found in other fucoid species (Brawley 1992; Creed 1995; Pearson and Brawley 1996). However, on semi-exposed rocky shores the dispersal distance of zygotes may be far greater than the 5 m of F. serratus (Arrontes 1993). Indeed, H. elongata juveniles are often observed in algal turfs situated far away from the parent plants (Stengel et al. 1999). Therefore, the scarcity of mixed stands of juveniles of the two species is surprising, and the occurrence of competitive interactions between germlings before they become visible is likely (Amsler et al. 1992; Vadas et al. 1992). Intraspecific competition determines the performance of both germlings and juveniles in pure stands of each species (Creed 1995; Creed et al. 1996a, 1997; Stengel et al. 1999). In a mixture of F. serratus and H. elongata germlings, interspecific competition also occurs (Norton 1986), although other conditions that may affect germling growth, such as the culture conditions, density and relative abundance of each species, were not reported.

In mixed stands, the growth of plants may be influenced by the morphology of competing species, since a plant’s morphology influences its ability to obtain growth resources (reviews by Carpenter 1990; Olson and Lubchenco 1990). F. serratus is different in morphology and growth pattern from H. elongata (Creed 1995; Brenchley et al. 1996, 1997, 1998). F. serratus grows mainly upwards (Knight and Parke 1950) but H. elongata has four different growth patterns during its life span. The germling expands outward for a few days after settlement to form the hat-like ‘syncytial stage’ (Moss 1969; Ramon 1973). It then grows upwards to the so-called ‘button stage’ and later expands laterally (‘mushroom stage’) in the vegetative stage (Gibb 1937; Creed 1995). Finally, H. elongata produces thong-like receptacles (‘receptacle stage’) from the center of the mushroom-like plant (Gibb 1937; Creed 1995). The self-sustaining receptacles again grow vertically, up to a length of 2 m (Brenchley et al. 1996, 1997). These are held more or less erect when submerged, but flop over when out of the water.

F. serratus and H. elongata are ideal species to test the following hypotheses:

  1. 1.

    the monospecific stands of juveniles of either species are the consequence of competitive exclusion between germlings of the two species; and

  2. 2.

    the differing growth patterns of the two species affect the outcome of interspecific competition.

To test these hypotheses, competition experiments were performed on both the germling and juvenile stages, in which H. elongata grows outwards. Here, juvenile plants are defined as the mushroom stage in H. elongata (ca. 2 cm in height and ca. 0.6 cm in diameter) and small (3–6 cm tall) plants of F. serratus lacking reproductive organs. Three experimental designs were applied: the additive, modified additive and replacement series designs. Additive and modified additive designs were used to examine the occurrence of interspecific competition and the relative intensity of intra- and interspecific competition at the germling stage. The replacement series design was used to investigate the relative importance of intra- and interspecific competition on the performance of plants at the juvenile stage. There has been criticism of the replacement series design (Silvertown and Lovett Doust 1993), but it remains useful for examining the effect of species proportion on the growth of plants, and the results can be analyzed statistically (Cousens 1996).

Materials and methods

Irradiance and competition between germlings in culture

Ten fertile females and one male plant of both F. serratus and H. elongata were collected from Port St. Mary ledges (54°0′N, 4°44′W) on 23 November 1999. Propagules of F. serratus and H. elongata were released as described by Creed et al. (1996b) and Gibb (1937), respectively, to provide suspensions of propagules. In all laboratory experiments, the effects of irradiance on the germlings were examined at 60 and 120 µmol m−2 s−1, which is the light saturation point of F. serratus germlings (McLachlan 1974). Temperature (10±1°C) and photoperiod (16:8 h L:D) were kept constant. For each treatment, two Petri dishes, each containing eight glass slides (2.5×2.0 cm) and 30 ml of autoclaved seawater, were prepared. After inoculating zygote suspensions, Petri dishes were left 24 h for settlement. From each Petri dish, four slides on which germlings were most evenly distributed were chosen. The culture medium was renewed weekly over 17 days. Each treatment was replicated four times.

Additive experimental design

The interspecific competition between H. elongata and F. serratus was examined in experiments in which the density of one species was held constant in all treatments and the density of a second species varied (Cousens 1991).

The density of the original zygote suspensions was 1,000 zygotes ml−1 for F. serratus and 700 zygotes ml−1 for H. elongata. First, 5 ml of F. serratus suspension was inoculated into four treatments and then three different amounts (5, 10 and 20 ml) of H. elongata suspension were added to each of three treatments. The settlement density of F. serratus was ca. 100 zygotes cm−2 in four treatments (monoculture and three different mixtures with H. elongata) and that of H. elongata was 0, 75, 150 and 300 zygotes cm−2.

Modified additive experimental design

Three suspensions of F. serratus zygotes with different concentrations (1,000, 2,500 and 5,000 zygotes ml−1) and one H. elongata suspension with 700 zygotes ml−1 were prepared. To make three different monospecific densities, 5 ml of each F. serratus zygote suspension and 5, 10 and 20 ml of the H. elongata zygote suspension were inoculated into six Petri dishes. Three mixed cultures were obtained by mixing the zygote suspensions of the two species. Thus, a total of nine treatments were made and the density of germlings in each treatment is shown in Table 1 and the experimental design in Table 2.

Table 1 Density of Fucus serratus and Himanthalia elongata (zygotes cm−2) in experimental treatments
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Table 2 Design of experiments (see Table 1 for explanation of treatment numbers) to examine intra- and interspecific competition between F. serratus and H. elongata
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Measurements and analyses

Initial settlement density of propagules was determined in four random 6 mm2 areas on each of four replicate slides. The mortality of germlings was estimated at the end of experiment, and the lengths of a total of 25 germlings were measured on each of four replicate slides for both species. The widths of H. elongata germlings were also measured because of its hat-like ‘syncytial stage’. Relative growth rates (RGR) for the lengths and widths of germlings were also calculated using the equation:

$$ {\text{RGR = (ln }}L_{t} {\text{ - ln }}L_{0} {\text{)/}}t $$

where L0 is the mean diameter of propagules (n=100) of each species, L t is the mean length and width of germlings after t days, and t is the number of days. The propagule diameters at the outset of experiments were 85.6±1.07 µm (mean±SE, n=100) for F. serratus and 286.8±5.66 µm (n=100) for H. elongata. To test the effects of competition on the performance of germlings, ANOVA tests were used (Sokal and Rohlf 1981; Zar 1984). Homogeneity of variance was tested using Cochran’s test. Where appropriate, data were transformed before analysis to meet the assumptions of parametric tests. The significance of the differences was evaluated using the Tukey HSD test.

Competition between juveniles on the shore

The study was carried out on the Port St. Mary shore, Isle of Man, where dense monospecific and rare mixed stands of Fucus spp. and H. elongata were found. The monospecific stands of H. elongata and F. serratus adults were found about 3–5 m away from experimental stands. Thus, Fucus juveniles are referred to as “Fucus serratus”, even though their identification was impossible at the beginning of experiment. However, we found by the middle of experimental period that F. serratus and F. vesiculosus were only mixed in the ratio of 85% to 15%. Initial lengths were 3–6 cm for F. serratus and 2–3 cm for H. elongata. The diameter of H. elongata at the ‘mushroom stage’ was 0.5–0.8 cm and the age of plants is probably less than 1 year (Knight and Parke 1950).

Experimental density and the proportions of the two species in mixtures were determined after examining the natural density and proportions within the experimental areas. In January 2000, juveniles of the two species were thinned to a density of 100 individuals per plot (10×10 cm) and five different proportions (100:0, 75:25, 50:50, 25:75 and 0:100) of F. serratus:H. elongata. Experiments were replicated four times for each treatment, and each plot was positioned where plants could not influence those plants in the other plots. When plants were thinned, both in monospecific and mixed stands, their spatial arrangement was manipulated so as to be as even as possible. The position of each plot was mapped and marked with a piece of fluorescent plastic tape. Algae surrounding the experimental plots were cleared fortnightly to prevent shading and ‘whiplash’ effects (Vadas et al. 1992) and the experiment ran for 176 days.

The biomass and mortality of the plants were monitored. Mortality was examined by counting the plants survived at 64, 149 and 176 days after density manipulation, and both initial and final biomass were measured for each plot. To estimate the initial biomass, lengths of F. serratus and button diameters of H. elongata were measured for 25 plants of each species chosen randomly within each plot, and then converted to dry weight by the use of linear regression equations. The equations were derived from 30 sacrificed juveniles of each species not used in the experiment.

$$ \begin{aligned} & F.{\text{ }}serratus{\text{: DW = - 0}}{\text{.046 + 0}}{\text{.022 }}L{\text{ (}}R^{2} {\text{ = 0}}{\text{.89}}{\text{, }}P{\text{ $<$ 0}}{\text{.001)}} \\ & H.{\text{ }}elongata{\text{: DW = - 0}}{\text{.005 + 0}}{\text{.032 }}D_{{\text{B}}} {\text{ (}}R^{2} {\text{ = 0}}{\text{.82}}{\text{, }}P{\text{ $<$ 0}}{\text{.001)}}{\text{,}} \\ \end{aligned} $$

where DW is dry weight, L is plant length, and DB is button diameter.

The average dry weight of individuals for each species in each plot was obtained as follows: with knowledge of the density, total starting biomass of each species in each plot could be estimated. At the end of the experiment, all remaining plants were collected and transported to the laboratory. The plants were separated into each species, counted, rinsed, and then weighed after drying to constant weight at 60°C.

Replacement series graphs

A replacement series graph was drawn by plotting final yield (biomass) of each component species of a replacement series experiment against its proportion in mixtures, as explained by Khan et al. (1975).

Crowding coefficients

Individual and relative crowding coefficients were calculated from initial and final mean dry weight of juveniles. Initial weight was calculated using the linear regression equations described above.

The individual crowding coefficient Ki (Khan et al. 1975) is calculated for species i as:

$$ K{\text{ }}_{{\text{i}}} {\text{ = (}}W_{{{\text{FMi}}}} {\text{/}}W_{{{\text{IMi}}}} {\text{)/ (}}W_{{{\text{FMo}}}} {\text{/}}W_{{{\text{IMo}}}} {\text{)}}{\text{, }} $$

where WFMi is the final and WIMi is the initial mean dry weight of plants in mixed stands, WFMo is the final and WIMo is the initial mean dry weight of plants in monospecific stands. Thus, the individual crowding coefficient is the ratio of the growth in mixed stands to the growth in monospecific stand on a plant basis. Ki<1 indicates depression of growth in mixed cultures, Ki>1 elevation of growth. If Ki and Kj>1, both species take advantage of growth in mixed stands: this is the theory of mutualism.

The relative crowding coefficients (Kij) were calculated as:

$$ K_{{{\text{ij}}}} {\text{ = }}K_{{\text{i}}} {\text{ / }}K_{{\text{j}}} $$

If Kij=1, the two species are equal in their interaction: if Kij>1, then species i takes more advantage than species j in mixed stands; if Kij<1, species j succeeds.

Results

Irradiance and competition between germlings in culture

Additive experimental design

The germination of F. serratus zygotes was about 98% and that of H. elongata was between 70 and 80%. After 17 days in culture, the mortality of germlings ranged from 3.06 to 4.14% for F. serratus and between 1.02 and 2.19% for H. elongata. There was no evidence to suggest that density-dependent mortality was taking place within the experimental period, because no differences in mortality were detected between treatments (P>0.05).

The growth of F. serratus (the target species) was influenced by the density of H. elongata and by irradiance. After 17 days, the growth rates of F. serratus ranged between 0.074 and 0.100 day−1 (mean lengths 299.98–474.03 µm) at 60 µmol m−2 s−1 and between 0.078 and 0.103 day−1 (mean lengths 321.12–499.99 µm) at 120 µmol m−2 s−1. At both irradiance levels, the growth rates of F. serratus declined significantly with increasing spore density of H. elongata (two-way ANOVA, F3,24=87.95, P<0.001) and there were significant differences in mean growth rates among all treatments (Tukey test). F. serratus grew significantly faster at 120 than at 60 µmol m−2 s−1 (two-way ANOVA, F1,24=9.16, P<0.01). However, no interaction was found between irradiance and density of H. elongata (P=0.71).

H. elongata grows first laterally then vertically after settlement. In the presence of F. serratus (100 zygotes cm−2), mean lengths of H. elongata germlings were between 551.3 and 606.6 µm at 60 µmol m−2 s−1 and between 572.6 and 627.7 µm at 120 µmol m−2 s−1, and their mean widths were between 487.6 and 545.6 µm at 60 µmol m−2 s−1 and between 505.9 and 561.1 µm at 120 µmol m−2 s−1. The relative growth rates of H. elongata were in the ranges of 0.038–0.046 day−1 for length and 0.031–0.039 day−1 for width by 17 days after settlement. The growth rates of H. elongata for both length and width declined significantly with increasing density (Table 3). The width-growth of H. elongata germlings increased with irradiance levels but its length-growth was not affected by irradiance.

Table 3 Results of two-way ANOVA and Tukey tests for the effects of density and irradiance on RGR of H. elongata in an additive experimental design
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Modified additive experimental design

In monoculture, the growth of both species was influenced by settlement density (Figs. 1 and 2). The relative growth rate was significantly greater at lower densities than at higher densities for both F. serratus (two-way ANOVA, F2,18=82.01, P<0.001) and H. elongata (Table 4). The growth rates for F. serratus were significantly different among density treatments (Tukey test), but irradiance levels did not significantly affect its growth (P>0.05). Also, no interaction was found between irradiance and density. In H. elongata, a Tukey test revealed that relative growth rates for germling length were significantly greater at lower densities (75 and 150 germlings cm−2) than at the highest density, and for width it was greater at 75 germlings cm−2 than at 150 or 300 germlings cm−2 (Table 4).

Fig. 1
figure 1

The effect of density on relative growth rates (RGR) of Fucus serratus in monocultures. Germlings grew for 17 days in culture at two levels of irradiance and at three densities (see Table 1). Bars show SE (n=4 replicates)

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Fig. 2
figure 2

The effects of density (see Table 1) and irradiance on RGR for length and width of Himanthalia elongata germlings grown in monoculture for 17 days. Bars show SE (n=4 replicates)

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Table 4 Results of two-way ANOVA and Tukey tests for the effects of density and irradiance on RGR of H. elongata germlings grown in monocultures in a modified additive experimental design
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In mixed culture, the growth of F. serratus declined more markedly than in monoculture, as the density of H. elongata increased (Fig. 3). Two-way ANOVA revealed that the relative growth rate of F. serratus was significantly lower at the higher densities of H. elongata (F2,18=78.85, P<0.001) but it was not influenced by irradiance (F1,18=0.18, P>0.05). There were significant differences in relative growth rate of F. serratus between F. serratus monocultures and mixtures with H. elongata (Tukey test).

Fig. 3
figure 3

RGR for the length of F. serratus at various densities of H. elongata (see Table 1 for explanation of codes). Bars show SE (n=4 replicates)

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The effect of F. serratus on the growth of H. elongata was examined by comparing the relative growth rates of H. elongata between monoculture and mixtures (Fig. 4). Both length and width of H. elongata declined as the density of F. serratus increased (Fig. 4), indicating that interspecific competition occurred between the two species. Relative growth rate of H. elongata in both length and width was significantly greater when germlings grew in monoculture, where interspecific competition was absent, compared to mixtures (Table 5).

Fig. 4
figure 4

RGR for the length and width of H. elongata at various densities of F. serratus (see Table 1 for explanation of codes). Bars show SE (n=4 replicates)

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Table 5 Results of two-way ANOVA and Tukey tests for the effects of density of F. serratus and irradiance on RGR of H. elongata germlings in a modified additive experimental design (see Table 1 for codes)
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Competition between juveniles on the shore

Mortality

The mortality of both species increased over time and ranged between 45.0 and 55.3% in F. serratus and 42.8 and 64.5% in H. elongata 176 days after manipulation. It increased over time in both species (Fig. 5) and there was no significant difference in mean mortality between F. serratus and H. elongata (F1,22=0.30, P=0.97).

Fig. 5
figure 5

Mortality of F. serratus and H. elongata juveniles in both monospecific and mixed stands over time. Density was constant (100 individuals per plot). Bars show SE (n=4 replicates)

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The mortality of juveniles significantly increased over time for both F. serratus (two-way ANOVA, F2,36=19.21, P<0.001) and H. elongata (Table 6). A Tukey test revealed that there was a significant difference in mortality between 64, 149 and 176 days in both species.

Table 6 Results of two-way ANOVA and Tukey tests for the effects of time and relative proportion on the mortality of H. elongata juveniles on the shore
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The relative proportions of the two species significantly influenced the mortality of H. elongata but not that of F. serratus (two-way ANOVA, F3,36=0.87, P=0.46). The mortality of H. elongata juveniles generally increased with increasing proportions of F. serratus, and it was highest at the mixtures with F. serratus of 50%. This indicates that the negative effect of F. serratus on H. elongata is more severe than the other way round.

Growth and yield

F. serratus grows from the tips of the fronds whereas H. elongata grows outward in the juvenile stage. H. elongata in monospecific stands were dome-shape at the end of experiment, and the length of plants grown in the centre of plots was greater than those at the periphery. This may reflect the fact that crowded plants have no space to expand laterally. Indeed, the H. elongata juveniles were so closely packed that no room was left, whereas the monospecific stands of F. serratus had plenty of space.

At the end of experiment, some F. serratus plants produced a few receptacles and one H. elongata had begun to produce a receptacle.

In the graph of the total yields from the replacement series experiments (Fig. 6), the shapes of the individual curves are indicative of competitive interaction. When no interaction occurs in mixtures of the two species, the yield of each species increases with increasing number of plants. Therefore, non-linear relationships between the number of plants and yield indicate competitive interactions. No linear relation between yield and plant number was found for either species (Fig. 6). For F. serratus, the curve is partially convex, while that of H. elongata is concave, indicating that F. serratus succeeds at the expense of H. elongata.

Fig. 6
figure 6

Replacement series graph for mean biomass of F. serratus and H. elongata at a density of 100 individuals per plot. Bars show SE (n=4 replicates)

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All individual crowding coefficients of F. serratus (KF) were greater than 1, whereas those of H. elongata (KH) were less than 1 (Table 7) in all three mixtures. These values indicate that the growth of F. serratus was elevated in mixtures, but that of H. elongata was suppressed, compared to when in monoculture. Relative crowding coefficients were >1 for KFH and <1 for KHF, confirming that F. serratus succeeded relative to H. elongata in all mixtures.

Table 7 Average ( n =4 replicates) individual and relative crowding coefficients showing the competitive interaction between F. serratus and H. elongata in three mixtures. K F and K H are individual crowding coefficients; K FH relative crowding coefficient of F. serratus on H. elongata; K HF relative crowding coefficient of H. elongata on F. serratus
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Discussion

The percentage germination of H. elongata is lower than recorded for any other fucoid algae (Gibb 1937; Moss 1969; Moss et al. 1973). Unlike other fucoids, the oospheres of H. elongata, if exposed to air for some time, cannot fertilize when they are returned to water. Therefore, liberation into water is essential for their development (Gibb 1937; Moss 1969). In the present study, even though the oospheres of H. elongata were released in water, their percentage germination was still lower than that of F. serratus. Thus, the causes of the lower germination in H. elongata are not clear. It may be a consequence of a different development pattern of zygotes compared to other Fucus species (Moss 1969; Ramon 1973) or the thick egg membrane that must be penetrated at fertilization, consuming considerable energy (Moss 1969). Whatever the cause, such differences in germination between the two species did not allow an assessment of the relative intensity of intra- and interspecific competition based on comparisons made at the same density in monoculture and mixtures (Underwood 1986, 1997).

Both intra- and interspecific competition are generally density-dependent (Ang and DeWreede 1992; Silvertown and Lovett Doust 1993; Creed et al. 1996b, 1997). In the present study, the growth of both F. serratus and H. elongata germlings declined with increasing density, irrespective of whether they were with cohorts or rival species. However, the intensity of competition may be influenced by the growth form of the plants (Carpenter 1990; Olson and Lubchenco 1990; Worm and Chapman 1998), and the growth pattern of H. elongata germlings differs from that of F. serratus (Gibb 1937; Ramon 1973; McLachlan 1974). After settlement, a Fucus germling differentiates into an apical part and a primary rhizoid, and grows mainly upwards (McLachlan 1974). H. elongata, on the other hand, has a prolonged pre-rhizoidal ‘syncytial stage’ in which it grows outwards for 5–7 days; before growing upwards for 3 months to become the visible ‘button stage’ (Gibb 1937; Ramon 1973). This initial difference in growth pattern may explain why the adverse effects of H. elongata germlings on the growth of F. serratus were stronger than that of cohorts. In mixtures, H. elongata outcompetes F. serratus because: (1) zygotes of H. elongata are bigger to begin with (Moss 1969; Moss et al. 1973; Ramon 1973); (2) its lateral expansion in the early ‘syncytial stage’ squeezes out or overshadows tiny adjacent germlings of F. serratus; and (3) it is larger than F. serratus at least until the ‘button stage’ (Choi, personal observation). At higher densities of H. elongata, the lower relative growth rate of F. serratus suggests that H. elongata germlings may competitively exclude F. serratus.

Competitive exclusion between F. serratus and H. elongata may occur not only at the germling stage, but also when they are juveniles. H. elongata at the ‘button stage’ switches its growth pattern from vertical to a lateral expansion, much as the cap of mushroom does (Gibb 1937; Kitching 1987; Creed 1995; Stengel et al. 1999), whereas F. serratus continues to grow upwards and begins to branch (Knight and Parke 1950; McLachlan 1974). In the ‘mushroom stage’, H. elongata forms a miniature canopy (height ca. 2–3 cm) with the ‘caps’ of the adjacent plants abutting to form a continuous layer. The shade from this low-level canopy may inhibit the growth of F. serratus juveniles beneath. The level of light under a canopy depends on the thickness and opacity of the canopy-forming plant (Hay 1986; Carpenter 1990). The caps of H. elongata are about 1.5 mm thick and almost opaque. Therefore, the level of irradiance below is almost zero. It is also a rigid, unmoving canopy, so there is no parting to allow sunflecks to penetrate. Thus, F. serratus juveniles would not survive beneath and H. elongata should come to dominate. Such canopy effects are well known, both for terrestrial plants and seaweeds (Begon et al. 1986; Vadas et al. 1992; Silvertown and Lovett Doust 1993; Chapman 1995).

In nature, however, H. elongata does not invariably eliminate all plantlets of F. serratus and some mixed stands of juveniles of the two species are found on the shore in the study area: F. serratus survives and grows in mixtures in sparse stands of H. elongata juveniles. In such stands, the F. serratus juveniles are taller than those of H. elongata because the former grows vertically continuously whereas the latter grows predominantly laterally at two stages in its development, so size-dependent competition occurs. Larger plants almost always out-compete smaller plants (Begon et al. 1986; Underwood 1986; Goldberg and Barton 1992; Silvertown and Lovett Doust 1993). In the shore experiment, mushroom-like H. elongata grew more slowly in mixed than in monospecific stands, and slower still with an increasing abundance of F. serratus. Thus, at this stage of development, F. serratus may outcompete H. elongata because it is taller thanks to its growth pattern.

As the shape of plants changes during their development, so may the resource for which they compete, and this could also be governed by the nature of the morphological changes that take place. For example, dense settlement of propagules may at first lead to competition for space, which would be intensified later when the incipient holdfasts begin to expand, were it not for self-thinning (Carpenter 1990; Amsler et al. 1992). Soon, in F. serratus the resulting crowded juveniles compete for light, so those that become taller quicker are more likely to succeed. Then, by branching they will form a light-monopolizing canopy, which consolidates their dominance. In H. elongata, the battle for space is sustained longer even if the original propagules were well spaced (Creed 1995). This becomes apparent when the buttons begin to expand. Unlike the erect flattened fronds of Fucus juveniles, the horizontally expanded thalli of H. elongata are not easily packed into a limited space, for they do not overlap or interleaf (Russell 1990; Creed 1995; Stengel et al. 1999). So space is at a premium and the buttons that do not expand early on are often squeezed out. The function of the cap is of course to collect light, but it defeats rival cohorts by stealing their space and preventing them from expanding. H. elongata may be unique in that its main battle for space is fought not at ground level, but several centimeters above.

Although both species also compete for light, the effect of irradiance on the performance of the two species may be different. In the present study, the growth of F. serratus germlings, but not those of H. elongata, was significantly lower at 60 µmol m−2 s−1 than at 120. This is probably because the light-saturation point for growth is only 13–25 µmol m−2 s−1 for H. elongata germlings (Moss et al. 1973) compared to 120 µmol m−2 s−1 for Fucus germlings (McLachlan 1974). Adaptation to low light intensity may enable H. elongata to grow in dense turfs of red algae (Stengel et al. 1999), where F. serratus juveniles are rarely found. Even under the F. serratus canopy, H. elongata juveniles are more abundant than those of F. serratus (Choi, personal observation).

Lateral expansion continues until the diameter of H. elongata is greater than 15 mm and receptacles begin to be produced from the centre of the ‘cap’ of the vegetative plant (Gibb 1937; Stengel et al. 1999). H. elongata again switches the growth pattern from lateral expansion to vertical extension (Gibb 1937; Kitching 1987; Russell 1990; Stengel et al. 1999). As the H. elongata receptacles develop, it is these receptacles that compete with the F. serratus, because the vegetative ‘mushroom’ is now merely an appendage to the holdfast. The receptacles of H. elongata grow far faster (2.7–16 mm day−1) than the vegetative fronds of F. serratus (<1.2 mm day−1) (Brenchley et al. 1998; Stengel et al. 1999), therefore H. elongata should prevail over F. serratus, although by this time the critical competitive battles have already been won.

The maintenance of monospecific stands of either species may be a consequence of interspecific competition at the germling and juvenile stage. H. elongata excludes F. serratus in the ‘syncytial stage’, ‘button stage’ and early ‘mushroom stage’ in mixed stands where the density of H. elongata is high. In contrast, the survivors of F. serratus in the competitive battles of germlings can outcompete those of H. elongata at the juvenile stage. By the procedures of alternative competitive exclusion resulting from their growth pattern, both species maintain their discrete monospecific stands and can co-occur at a similar shore height.