Summary of results

The two transplantation experiments differed in their results. Transplants of laboratory germlings performed best in their native zones. Survivorship (%

survival, final density) and biomass production (yield, final biomass, indi-vidual biomass) were highest in zones of natural occurrence. These are fea-tures of the model of niche differentiation. Transplants of adult wild Fucus, in contrast, performed best in the lowermost zone of F. serratus, where they suf-fered least from desiccation, while survival rates on higher shore levels were lower and individual plant mass decreased sharply. These findings are con-cordant with the competitive hierarchy hypothesis.

A common feature of both experiments was the reduced performance of F.

serratus and F. vesiculosus when transplanted above their native zones, with no survival at all for F. serratus in the uppermost F. spiralis zone (at least in summer when desiccation stress is greatest). It seems clear that here, for both Fucus species, a physiologicallimit of desiccation tolerance was approached or exceeded. This is in concordance with both the competitive hierarchy and the niche differentiation models.

With only the information from transplantation experiments performed in this study, it is not possible to interpret the different outcomes of the two experi-ments as result of the different origin of the experimental algae (laboratory or field) or of usage of different life stages (germlings or adolescent/adult plants ).

3.

GRAZING 4

4 .1 Introduction

Along with physical factors and interspecific competition, herbivory is another important factor potentially controlling the distribution of intertidal algae (reviewed e.g. in Chapman 1995). Causes of zonation may be differ-ential grazing pressure along the tidal gradient Ce.g. Janke 1990; Worm &

Chapman in prep.) and selective grazing among macroalgae Ce.g. Watson &

Norton 1985, 1987; Barker & Chapman 1990; Denton & Chapman 1991). As is the case for competition, grazing frequently influences upper CCubit 1975 cited by Lubchenco 1980; Underwood 1980) or lower boundaries of intertidal macroalgae CLubeheneo 1980; Moreno & Jaramillo 1983). In some cases, competition in combination with grazing seemed to prevent the establishment of Fucus CLubeheneo 1982, 1986; Worm & Chapman in prep.).

The most conspicuous grazers in the NE intertidal zone of Helgoland are littorinids. Here, Littorina littorea and L. mariae play the most important role, while its congeners L. obtusata and L. saxatilis are only of minor abun-dance and importance CJanke 1986, 1990).

L. littorea feeds epilithically (Hawkins & Hartnoll 1983) and is a generalist, consuming a range of algae, but showing clear preferences when given a choice CLubeheneo 1978, 1982; Watson & Norton 1985). L. mariae may feed epilithically and epiphytically and is normally associated with Fucus CWatson

& Norton 1987). Despite the apparent similarity between both species of flat periwikle, L. obtusata and L. mariae, these snails differ in various important respects (Gallardo & Götting 1985; Reid 1990; Tatarenkov 1995), notably feeding mode and food preferences CWatson & Norton 1987; Norton et al.

1990). lt is therefore impossible, to extrapolate from the larger amount of published data for L. obtusata to L. mariae.

Preferecences for certain macroalgal species may not be the only critical fac-tor. Certain algal life stages may be preferred by herbivores, and life stages may differ in their susceptibility to grazing. "Preferential grazing of the juvenile stages has the potential to influence community structure and algal distribution in a way that selectivity at the adult phase cannot do, unless

grazing is abnormally intensive" (from Watson & Norton 1985). If grazing on juveniles Ieads to the death of the algae whereas adults are scarcely harmed by it, the early life stages, which often suffer high mortalities from other causes (Dean et al. 1989; Brawley & Johnson 1991; Kendrick 1994; Kendrick &

Walker 1995), can be regarded as "bottlenecks" stage also in this respect (Lubchenco 1983). This is especially true for species such as Fucus that do not have a ernst that may survive grazing or a heteromorphic life cycle where one morph represents an escape from grazing (Lubchenco & Cubit 1980).

Lubchenco (1983) found that germlings of F. vesiculosus were grazed by L.

Littorina but, depending on littorinid density, by chance could grow to sizes relatively safe from grazing darnage by L. littorea ("size escape").

In one of her experiments, Lubchenco offered small F vesiculosus germlings of 1-3 cm lengths and "medium Fucus" plants of 5-7 cm tagether to L.

littorea. The snails completely consumed the former leaving the latter.

The objective of the following experiments was to examine whether escape in size may also exist for Helgoland F. vesiculosus. Escape in size could also play a role in the lower intertidal zone where L. littorea still has mean densities of ca. 100fm2 (140fm2 at mid tidallevels; Dahms 1993). Hence, the experiment was repeated with germlings of F. serratus. I chose three germling size classes. The biggest was at the upper Iimit of the range of Lubchenco's small germlings (3 cm). In this way, I could test if an escape in size occurs already at an earlier stage and if there is some threshold or successively decreasing consumption with increasing germling size. The experiments were repeated with L. mariae as the grazer.

In another set of experiments, preferences among Fucus species were examined. As mentioned above, preferential grazing may be another cause of Fucus zonation pattems.

Work from the NW Atlantic has dealt with littorinid grazing on F. vesiculosus only (New England: Lubchenco 1978, 1982, 1983; Petraitis 1987), on F.

vesiculosus and F. spiralis (Nova Scotia: Chapman 1989; Barker & Chapman, 1990; Denton & Chapman 1991; Worm & Chapman in prep.), and on other species not found on Helgoland (F. distichus, F. evanescens). From European coasts there are data for all three Helgoland Fucus species from Watson &

Norton (1985, 1987), but not all (Helgoland) Fucus-Littorina combinations were included. While many data exist from grazing experiments with L.