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Scanned by Scan2net BERICHTE aus dem INSTITUT FÜR MEERESKUNDE an der CHRISTIAN-ALBRECHTS-UNIVERSITÄT ·KIEL NR. 293 Factcrs causing the zonation of three Fucus species (Phaeophyta) in the intertidal zone of Helgeland (German Bight, North Sea) Testing the validity of Keddy's 'competitive hierarchy model' "'.".~~~ / ........r,:"' ~ < ; -. ~-""} '"-''' '""""- "-"''' by Ralf Karez Kiel 1997 Kopien dieser Arbeit können bezogen werden durch: Institut für Meereskunde an der Universität Kiel -Bibliothek- Düsternbrooker Weg 20 D-241 05 Kiel Germany ISSN 0341-8561 Diese Arbeit wurde von der Mathematisch­ Naturwissenschaftlichen Fakultät der Universität Kiel 1996 als Dissertation angenommen. CONTENTS Glossary ................................................................................................................................ 1 Summary .............................................................................................................................. 3 Zusammenfassung .............................................................................................................. 5 1 General Introduction .................................................................................................... 7 1.1 Description of the study site ................................................................................ 2 2 2 Competition................................................................................................................... 27 2.1 Introduction ........................................................................................................... 27 2.2 Material & methods ............................................................................................... 37 2.2.1 Replacement series experiment in the field ............................................ 3 7 2.2.2 Replacement series experiment in the laboratory .................................. 48 2.3 Results ..................................................................................................................... 49 2.3 .1 Replacement series experiment in the field ............................................ 4 9 2.3.2 Replacement series experiment in the laboratory .................................. 68 2. 4 Summary of results ................................................................................................ 8 0 3 Niches ............................................................................................................................. 81 3.1 Introduction ........................................................................................................... 81 3.2 Material & methods ............................................................................................... 82 3 .2.1 Transplants of laboratory germlings ........................................................ 8 2 3.2.2 Transplants of adult field plants ............................................................... 84 3.3 Results ..................................................................................................................... 85 3.3.1 Transplants of laboratory germlings ........................................................ 85 3.3.2 Transplants of adult field plants ............................................................... 89 3.4 Summary of results ................................................................................................ 91 4 Grazing .......................................................................................................................... 93 4.1 Introduction ........................................................................................................... 9 3 4.2 Material & methods ............................................................................................... 96 4.2.1 Feeding preference among Fucus germling size classes ...................... 9 8 4.2.2 Feeding preference among Fucus species .............................................. 99 4.2.3 Littorinid densities in the Helgoland intertidal .................................... 101 ,.-- 4.3 Results .................................................................................................................. 102 4.3.1 Feedingpreference among Fucus germling size classes .................... 102 4.3.2 Feeding preference among Fucus species ........................................... 104 4.3.3 Littorinid densities in the Helgoland intertidal.. ................................... 111 4.4 Summary of results ............................................................................................... 112 5 General Discussion .................................................................................................... 115 Ackno\vledgements ........................................................................................................ 143 References ........................................................................................................................ 145 Appendix GLOSSARY ANOVA analysis of variance Dl,D2,D3 initial Fucus germling densities in competition experiments, Dl=10,000, D2=50,000, D3=100,000 shootsfm2 df degrees of freedom dw dry weight EAD 'extended additive design' for comparisons of competition experiment treatments fw fresh weight homo­ homogeneity of variances among groups, a prerequisite for scedasticity ANOVA MASD 'mixed additive/substitutive design' for comparisons of competition experiment treatments MD 'Manly's differences', pairing of controls with treatments in analysis of grazing experiments (after Manly 1993) in which differences between food types are calculated MR 'modified Roa' pairing of controls with treatments after Manly (1993) in which correlation between foods is preserved NE-Intertidal rocky intertidal flat on Helgoland ("NO-Felswatt") p Fucus spiralis PR 'pure Roa' pairing of controls with treatments in analysis of grazing experiments in which correlation between foods is lost (after Roa 1992) s Fucus serratus SD standard deviation TK Tukey-Kramer test for multiple comparisons V Fucus vesiculosus glossary 1 SUMMARY The main objective of my study was to test the validity of the 'competitive hierarchy hypothesis' of Keddy (1989a) for a marine site. According to the competitive hierarchy model, species are arranged along resource gradients so that the competitively dominant species occupies the end of the gradient most favourable for growth. For all species along the gradient, the benign end of the gradient is physiologically optimal. Hence, fundamental niches include the fundamental niches of competitively superior species on the gradient. Species' competitive abilities decrease with increasing distance from the benign end of the gradient. The corollary is that species' fundamental niche breadths and abilities to tolerate reduced resource concentrations decrease with distance from the less benign end of the gradient. Another assumption of Keddy's model is that competitive ability is an inherent characteristic of a species and mostly independent of environmental conditions. The competitive hierarchy model is thus a possible explanation for a growing body of evidence that upper limits of species' zones along the shore gradient are mainly set by physiological tolerances, whereas lower limits mostly are set by competitive interactions. Fucus serratus, F. vesiculosus and F. spiralis occur in progressively higher intertidal zones on Helgoland (North Sea). I tested the following predictions of Keddy's hypothesis along the gradient of desiccation stress: 1. the competitive hierarchy is: F. serratus > F. vesiculosus > F. spiralis. 2. the fundamental niche breadths decrease in the order F. spiralis > F. vesiculosus > F. serratus. 3. Ranks of competitive ability are consistent under different environmental conditions. Modified replacement series experiments (de Wit 1960) set up in situ at 3 densities showed the following competitive asymmetries: F. vesiculosus > F. serratus > F. spiralis. Additional replacement series experi­ ments at 3 densities in aerated 3 1 beakers in the laboratory showed the com­ petitive dominances: F. spiralis ~ F. vesiculosus > F. serratus. Hence, com­ petitive dominances depended on environmental conditions. Transplant experiments designed to elucidate fundamental niche breadths showed that each species grew best within its native zone and not at the benign (lower) end of the shore gradient when laboratory raised germlings were used. This is in contrast to the competitive hierarchy model and supports the summary 3 model of niche differentiation. Transplants of adult wild plants, in contrast, were in concordance with Keddy's model: survival of each species was highest, when transplanted to the presumably most benign zone of F. serratus and successively lower when transplanted to the zones above. I conclude that the competitive hierarchy hypothesis from Keddy cannot explain the zonation of Fucus spp. in the intertidal zone of Helgoland. I tested a further factor that might have influence on the zonation of Fucus spp. along the shore gradient: preferential grazing of Littorina spp. In both choice and no choice experiments in the laboratory, only very small amounts of F. serratus were eaten by Littorina littorea or by L. mariae, the macro­ grazer species with highest abundances in the Helgoland intertidal zone. F. spiralis was highly consumed by both littorinid snails, whereas F. vesiculosus was consumed at an intermediate Ievel. In the Helgoland intertidal zone, densities of L. littorea are highest in the mid­ intertidal zone
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