Hierarchical spatial structure and levels of resolution of intertidal grazing and their consequences on predictability and stability at small scales Thesis submitted in fulfilment of the requirements for the degree of DOCTOR OF PHILOSOPHY of Rhodes University By ELIECER RODRIGO DIAZ DIAZ June 2008 Abstract The aim of this research was to assess three hierarchical aspects of alga-grazer interactions in intertidal communities on a small scale: spatial heterogeneity, grazing effects and spatial stability in grazing effects. First, using semivariograms and cross-semivariograms I observed hierarchical spatial patterns in most algal groups and in grazers. However, these patterns varied with the level on the shore and between shores, suggesting that either human exploitation or wave exposure can be a source of variability. Second, grazing effects were studied using manipulative experiments at different levels on the shore. These revealed significant effects of grazing on the low shore and in tidal pools. Additionally, using a transect of grazer exclusions across the shore, I observed unexpected hierarchical patchiness in the strength of grazing, rather than zonation in its effects. This patchiness varied in time due to different biotic and abiotic factors. In a separate experiment, the effect of mesograzers effects were studied in the upper eulittoral zone under four conditions: burnt open rock (BOR), burnt pools (Bpool), non- burnt open rock (NBOR) and non-burnt pools (NBpool). Additionally, I tested spatial stability in the effects of grazing in consecutive years, using the same plots. I observed great spatial variability in the effects of grazing, but this variability was spatially stable in Bpools and NBOR, meaning deterministic and significant grazing effects in consecutive years on the same plots. Both the significance in grazing effects and spatial stability depended on the level of resolution (species, functional, biomass) at which the algal assemblage was evaluated, suggesting hierarchical variability. ii In order to be able to predict spatial variability in the effects of grazers in the upper eulittoral zone using biotic and abiotic micro- and macrofactors, a conceptual model was proposed, based on data from several multiple-regressions. This linked the interactions among three elements: idiosyncratic heterogeneity, micro and macrofactors. This suggests that spatial variability can be a product of these factors, while spatial stability can be caused by the same or different combinations of factors. In conclusion, grazing and other ecological phenomena must be studied hierarchically, not only through spatiotemporal scales, but also at different levels of resolution, as these also influence our perception of patterns. iii CONTENTS TITLE………………………………..…………………………………………………..I ABSTRACT……………………………………………………………………………..II CONTENTS………………………………………………………………………….....IV DEDICATION………………………………….…………………………..…………XII ACKNOWLEDGEMENTS……………………………………...…………………..XIII THANKS TO THE EXAMINERS...............................................................................XV CHAPTER I: GENERAL INTRODUCTION…………………………………………1 I. The context of grazing effects and their variability along the coast of South Africa……………………………………………………………………………………...2 II. Consequences of grazing effects among intertidal zones: differences in spatial patchiness ….......................................................................................................................7 III. Grazing effects and the problem of scale…………………………………………10 IV. Grazing patchiness to hierarchy theory: generating variability in grazing effects…………………………………………………………………………………….12 V. Stability and persistence of mosaics: persistence of grazing effects…………..……..15 VI. Hypotheses and structure of the thesis..............................................................……..17 VII. Specific hypotheses....................................................................................................18 CHAPTER II: THE EFFECTS OF GRAZING ON SPATIAL PATTERNS.….....20 INTRODUCTION…………………………………………………………………..…...21 iv MATERIALS AND METHODS………………………………………………………..29 I. Site…………………………….……………………………………………………..... 29 II. Data analysis………………………………………………………………….……….30 II.1. Detection of multiple scaling regions…………………………………….... 33 RESULTS………………………………………………………………………………..34 I. Low shore………………………………………………………………………..…….34 I.1. Spatial structure of algal functional groups……………………...…………..34 I.2. Spatial structure of grazers………………………………………………......34 I.3. Relationship between the spatial variability of algal groups and grazers on the low shore......................................................................................................................43 II. Mid Shore………………………………………………………………………….….52 II.1. Spatial structure of algae……………………………………………………52 II.2. Spatial structure of grazers………………….................................................52 II.3. Relationship between the spatial variability of algal groups and grazers on the midshore………….………………………………………………….….62 III. High shore……………………………………………………………………………70 III.1. Spatial structure of grazers…………………………………………….…...70 IV. Comparisons of density and biomass of grazers between sites…………......……….75 DISCUSSION…………………………………………………….……………………....77 I. Technique…………………………………………………….………………………...77 II. Spatial structure at different levels on the shore…………………….....……………...78 II.1. Low shore…………………………………………………………………...78 II.2. Mid shore…………………………………………………………………....83 v II.3. High shore……………………………………………………….....…….….85 III. Conclusions………………………………………………………..……………...….86 CHAPTER III: EFFECTS OF GRAZERS ALONG AND ACROSS THE SHORE UNDER EARLY SUCCESION ……………………………………………………..88 INTRODUCTION………………………...…………………………….………….......89 MATERIALS AND METHODS……..………………………………..……………....97 I. Study site……………..……………………………………………………………....97 II. Experiment 1: Randomized block design at different levels on the shore………......97 III. Experiment 2: Grazing effect across the shore using a transect………….……….100 IV. Statistical analyses………….……………………………………………………....101 IV.1. Experiment 1………………………………………………………….….101 IV.2. Experiment 2……………………………………………………………..102 IV.2.a. Heterogeneity and dependent scaling regions of grazing across the shore………………………………………………………………..…...103 RESULTS………………………………………………………………………………106 I. Experiment 1: experiments among zones……………………………………….……106 I.1. Variability of abiotic and biotic factors among levels on the shore…….….106 I.2. Grazing effects at the level of algal biomass, represented by concentration of chlorophyll a……………………………………………………………………108 I.3. Grazing effect at the level of algal functional groups……………………...110 I.3.a. Low shore……………………………………………………..….110 I.3.b. Mid shore…………………………………….…………………...113 vi I.3.c. High shore………………………………………………………...113 II. Experiment 2. Grazing effects across the shore………………………………….….116 II.1 Relationships among physical and biological factors……………………………...116 II.2. Grazing effects across the shore: effects on different algal functional groups…....119 II.3. Factors affecting grazing effects: a multiple regression approach……….………..119 II.4. Spatial structure of Ulva rigida in the exclusion treatments………………………128 II.5. Spatial structure of physical and biotic factors along the transect………………...128 II.6. Factors affecting effects of grazing for Ulva rigida, analysed using cross- semivariograms………………….....…………………………………………………....136 DISCUSSION…………………………………………………………………………..139 I. Spatial structure of grazing effects across the shore……………………………….....143 II. Relationship between grazing effect: both physical and biotic factors……….….….144 III. Conclusions…………………………………………………………………………146 CHAPTER IV: EFFECTS OF RESOLUTION, AND SPATIAL STABILITY OF GRAZING EFFECTS IN THE UPPER EULITTORAL ZONE……………….….149 INTRODUCTION…………………………………………………………………..….150 I. Integrative hypothesis approach...................................................................................157 MATERIALS AND METHODS……………………………………………………….160 I. Site…………………………………………………………………………………....160 II. Assessment of the mesograzing effect and set-up……………………….………..…160 II.1. Levels of resolution………………………………………….………….….162 II.2. Spatial stability between years……....…………………………………….164 vii III. Grazer abundance………………………………………………………………......165 III.1. Mesograzer abundance by habitat and manipulation ………………….....165 III.1.a. Spatial variability of mesograzers among blocks…………...…..166 III.1.b. Temporal variation of mesograzer abundance by habitat and manipulation during 2004 and 2005……………………………………166 III.2. Micrograzer analysis ……………………………………………………..166 IV. Statistical consideration………………………………………………………….....167 RESULTS………………………………………………………………………………169 I. Mesograzer effects on the algal assemblage at different levels of resolution………..169 I.1. The finest level of resolution: species level………………………………...169 I.1.a. Succesional patterns during 2004 and 2005.…………………..…169 I.2. The functional group level. Comparison of conditions in 2004…………....187 I.3. Coarsest level of resolution: chlorophyll a concentration. Comparison between experiments in 2004 and 2005………………………………………...197 II. Spatial stability or persistence of mesograzing effects between years……………....203 II.1. Spatial stasis or determinism………………………………………………203 II.2. Consistence in the significance of grazing……………………………...…205 III. Grazer community in the upper eulittoral zone of the south coast of Southern Africa………………………………………………………………………………...…206 III.1. Composition……………………………………………………………...206 III.2. Mesograzer abundance, variation