Page 1


Contents

Preamble

Index and List of Plates Skokholm Map Part I Contents Part I: An outline account of the vegetation of the islands of Skokholm, Skomer, Middleholm

and Grassholm Part II Contents and 'The Inanimate Environment: Soil, Wind and Salt' Title Page Part II: A comparative account of the environmental factors influencing the vegetation on the islands of Skokholm and Grassholm with special reference to the effects of the birds and

mammals 'The Interaction of Living Organisms: The Grazing Factor' Title Page Part IIb: The Interaction of Living Organisms: The Grazing Factor 'The Interaction of Living Organisms: Biotic Factors other than Grazing'. 'Treading, Burrowing, Manuring and Seed Distribution' Title Page Part IIc: The Interaction of Living Organisms: Biotic Factors other than Grazing. Treading, Burrowing, Manuring and Seed Distribution

Appendix Contents, Table Index, Plate Index Appendix


General Subject Index Part I An outline of the vegetation of the islands of Skokholm, Skomer, Middleholm and Grassholm Page I. II. III. IV. V. VI. VII. VIII. IX.

Introduction…………………………………………………………………………………………..……………1 Size, position and climate……………………………………………………………..……………..……….5 Geology and soil………………………………………………………………………………………………….7 Description of the major plant communities…………………………………………………………16 Exposure to spray-bearing winds………………………………………………………………………….53 Human settlement and introduction of grazing mammals……………………………………….62 The seabird population of the islands…………………………………………………………………..66 The biotic interrelationships……………………………………………………………………………….71 Summary of Part I……………………………………………………………………………………………..75

Part II A comparative account of the environmental factors influencing the vegetation on the islands of Skokholm and Grassholm with special reference to the effects of birds and mammals Page I. II. III. IV. V.

VI. VII. VIII. IX. X. XI.

Introduction………………………………………………………………………………………………………77 Edaphic factors………………………………………………………………………………………………….80 Exposure to spray-bearing winds………………………………………………………………………..115 Some of the effects of salt spray on the vegetation………………………………………………..157 The effect of grazing on the vegetation……………………………………………………………….188 a. Description of the grazed vegetation of Skokholm as it exists today compared with the ungrazed vegetation of Grassholm……………………………….188 b. Vegetation changes resulting from alterations in the intensity of grazing on Skokholm………………………………………………………………………………………..254 Treading………………………………………………………………………………………………………...314 Burrowing………..……………………………………………………………………………………………..326 Manuring………………………………………………………………………………………………………..364 Distribution of seeds by gulls…………………………………………………………………………….420 Summary of Part II…………………………………………………………………………………………..432 References………………………………………………………………………………………………………439


List of Plates (Only the most important plates are included in the text. For others see the appendix.) Text figures are not included in this list. Following page Ordinance Survey map of Skokholm Island……………………………………………………...Frontispiece

Part I 1. Map showing the position of the South Pembrokeshire Islands………………..………………4 2. Simplified map of the vegetation of Skokholm Island, October 1948………….…………..15 3. Distribution of the freshwater habitats of Skokholm………………………………………………39 4. Percentage cover histograms and soil profiles in seven freshwater communities on Skokholm……………………………………………………………………………………………………42 5. Vegetation map of Grassholm Island, June 1950…………………………………………………..44 6. “Fetch” diagram for the South Pembrokeshire Islands and three maps showing the direction of wind waves and swell around Skokholm shores……………………………..52 7. Simplified map of the chief breeding birds of Skokholm Island, summer 1949. (After Conder and Keighley)……………………………………………………………………………….65

Part II 8. Soil profiles in the boulder clay of the North and South Ponds on Skokholm to show types of soil and vegetation…………………………………………………………………….90 9. Representative belt transects showing the vegetation of cliffs suffering slight, intermediate and severe exposure to spray-bearing winds……………………………………..120 10. Morphological variation in Armeria maritima arising largely as a result of excessive exposure……………………………………………………………………………………………149 11. Soil profiles showing wind and water erosion in Armerietum………………………………..152 12. Illustrations to show the effect of grazing on Festuca rubra and Plantago maritima…..199 13. Morphological variation in Samolus valerandi dependent upon fluctuations in the environment………………………………………………………………………………………………201 14. Illustrations of the dwarf plants growing on 2 square inches of ground in a heavily grazed Plantago coronopus sward……………………………………………………………202 15. Morphological variation in Plantago coronopus var. vulgaris according to differences in the environment…………………………………………………………………………..202 16. Morphological variation in the large succulent variety of Plantago coronopus…………202 17. Morphological differences between grazed and ungrazed plants of Littorella uniflora in a terrestrial habitat………………………………………………………………206


List of Plates (2) Following page 18. Graphs showing the relative rates of leaf loss and leaf production in grazed and ungrazed plants of Littorella uniflora……………………………………………………………206 19. Morphological changes in Littorella uniflora brought about by the change from an aquatic ungrazed habitat to a dry grazed one…………………………………………..206 20. (i) and (ii). Maps of a rabbit-proof enclosure on the Head of Skokholm to show the change from Holceto-Armerietum to Festucetum rubrae after the exclusion of grazing.. (1948 – 1952 inclusive)………………………………………………………256 21. Maps of a rabbit proof enclosure on the Neck of Skokholm to show the change from Armeria maritima to Festuca rubra and Rumex acetosella on the exclusion of grazing.. (1948 – 1949)…………………………………………………………257 22. Maps of this same enclosure following the re-entry and exclusion of the rabbits, to show the change from a devastated area of almost bare ground to a closed Festucetum rubrae..(1950-1952 inclusive)…………………………………………..257 23. (i), (ii), and (iii). Maps of a rabbit-proof enclosure on the East bay cliffs of Skokholm to show the change from an open society of Armeria, Poa, Rumex and Silene through the succulent variety of Plantago coronopus to Festuca rubra after the exclusion of grazing; together with maps of a grazed control quadrat. (1950 – 52 incl.)…………………………………………………………….259 24. Re-estimations of a belt transect at the same season in five successive years to show a plant succession dominated in turn by Senecio jacobaea, Holcus lanatus, Rumex acetosa with R. acetosella and Holcus, in the last 2 years. Succession is controlled by the pressure of grazing………………………………………………263 25. Maps of a cliff edge quadrat in 1948 and 1952 to show the change from an open society of Armeria, Senecio and Stellaria to an almost closed society of Erodium maritimum………………………………………………………………………….268 26. Maps showing the increase in the ground cover of individual species after the elimination of grazing……………………………………………………………………………………….287 27. (i), (ii), and (iii). Maps to show the initial rise of Littorella uniflora on the elimination of grazing and its subsequent dying away beneath the more aggressive Agrostis stolonifera……………………………………………………………………………289 28. Belt transects through puffin colonies in Agrostidetum and Armerietum to show the effects of treading in producing a bare cliff edge strip………………………….315 29. Two soil profiles showing (a) The downward deflection of rhizomes of Carex arenaria as a result of light penetrating into a burrow, and (b) the distribution of Silene maritima in relation to treading and manuring on heavily burrowed slopes……………………………………………………………………………………318 30. Photographic views of heavily burrowed bird colonies to show hummocky growth, erosion and co……………………………………………………………………………………..333 31. Quadrats to show the characteristic vegetation of shearwater and puffin colonies in Armerietum and Agrostidetum…………………………………………………………333


List of Plates (3) Following page 32. Superimposed maps and profiles of an eroding Armeria hummock to show the amount of denudation between 1948 and 1951……………………………………………..337 33. Soil profiles to show the progressive degeneration of Armeria hummocks as a result of undermining burrows…………………………………………………………………….340 34. Drawings to show the rank growth forms induced in Silene, Cochlearia and Hypochaeris by guano………………………………………………………………………………..367 35. Photographic views of the succulent, ornithocoprophilous variety of Plantago coronopus in the Greater Black Backed Gull colony of an ungrazed stack…………………………………………………………………………………………………369 36. Quadrats to show the vegetation of a razorbill colony and that adjacent to 2 cliff face Herring Gulls’ nests………………………………………………………………………375 37. Quadrats to show the vegetation adjacent to gulls’ nests among bracken, on cliffs and on an inland rock outcrop………………………………………………………………375 38. Anomalous forms of Himanthalea lorea produced in a pool beneath the Grassholm gannetry where guano accumulates………………………………………………395


Part I An outline account of the vegetation of the islands of Skokholm, Skomer, Middleholm and Grassholm

Contents Page I. II. III.

Introduction……………………………………………….………………………………………..………..1 Size, position and climate of Skokholm, Skomer, Middleholm and Grassholm…..5 Geology and soil of the four islands………………………………………………………………….7 A. Geology…………………………………………………………………………………………………..7 B. Soil………………………………………………………………………………………………………..12 IV. Brief description of the major plant communities…………………………………………….16 A. Skokholm………………………………………………………………………………………………16 1. General observations………………………………………………………………………….16 2. Armerietum………………………………………………………………………………………16 3. Other cliff communities……………………………………………………………………..19 4. Grassland……………………………………………………………………………………….…24 5. Callunetum……………………………………………………………………………………….30 6. Pteridium……………………………………………………………………………………….…32 7. Senecio jacobaea societies…………………………………………………………………..36 8. Rumex societies…………………………………………………………………………………38 9. Freshwater and marsh communities…………………………………………………….39 B. Grassholm…………………………………………………………………………………………..…45 C. Skomer………………………………………………………………………………………………….47 D. Middlehom…………………………………………………………………………………………….50 V. Exposure of the South Pembrokeshire Islands to spray-bearing winds……………….53 A. Observations on the severity of the exposure suffered…………………………………53 B. The effects of desiccating winds and blown salt spray on the vegetation…………55 VI. A brief resume of the human settlement of the islands and the introduction of grazing mammals………………………………………………………………………………………….62 VII. Some facts relating to the seabird populations of the islands……………………………..66 VIII. A brief resume of the biotic interrelationships of the islands……………………………..71 IX. Summary…………………………………………………………………………………………………….75


Part II A comparative account of the environmental factors influencing the vegetation on the islands of Skokholm and Grassholm with special reference to the effects of the birds and mammals

The Inanimate Environment Soil. Wind. Salt.

The Interaction of Living Organisms The grazing factor.

Biotic factors other than grazing Treading. Burrowing. Manuring. Seed Distribution.


Part II A comparative account of the environmental factors influencing the vegetation on the islands of Skokholm and Grassholm with special reference to the effects of the birds and mammals

Contents (1) The Inanimate Environment Soil, Wind and Salt Page I. II.

III.

IV.

Introduction……………………………………………….………………………………………..………77 Edaphic factors…………………………………………………………………………………………….80 A. General observations……………………………………………………………………………….80 B. Distribution of brown earths, fault clays and boulder clays on Skokholm in relation to the plant cover. Mechanical analysis of soils………………………………………………………………………82 C. Organic matter and water retaining capacity of the soils of Skokholm and Grassholm……………………………………………………………………………………………..93 D. Carbonate content of the Skokholm soil………………………………………………….101 E. Exchangeable calcium or calcium oxide content of the Skokholm soil………..105 F. Sequioxide content of the Skokholm soil…………………………………………………108 G. pH of the island soils in relation to exposure and biotic pressure……………….110 Exposure to spray-bearing winds………………………………………………………………….115 A. The effect of exposure on the distribution of the major plant communities of Skokholm and Grassholm……………………………………………..115 B. The effect of exposure in modifying the rate of plant growth and morphological form………………..…………………………………………………………….136 1. The late commencement of spring growth and flowering induced by high wind strengths………………………………………………………………………136 2. Dwarfing and other morphological adaptations arising as a result of exposure…………………………………………………………………………………………137 3. Scarcity of woody species………………………………………………………………….140 4. Modification in the growth of Calluna as a result of wind action……………143 5. Modification in the leaf structure of Armeria induced by strong winds…..145 C. Wind and water erosion………………………………………………………………………..150 Some of the effects of salt spray on the vegetation………………………………………….157 A. Salt content of the soil in relation to the predominating type of vegetation, weather conditions and aspect………………………………………………………………157


Contents (2) Page

B. C. D. E.

1. Soil salt in relation to the distribution of the vegetation…………………..……157 2. Soil salt in relation to weather conditions……………………………………………162 3. Soil salt in relation to aspect………………………………………………………………163 Salinity of the ponds and streams……………………………………………………………167 Estimations of the amount of salt spray falling on a known area………………….171 Approximate estimations of the amount of salt retained on the foliage of different plants………………………………………………………………………………………174 The amount of salt taken up by plants in relation to exposure, root depth, succulence etc……………………………………………………………………………………….177 1. General observations and method of analysis……………………………………..177 2. Salt content of plant tissues in relation to exposure of the habitat………….178 3. Salt content of plant tissues in relation to root depth……………………………180 4. Succulence in plants in relation to the salt content of the environment and of the plant tissues……………..………………………………………………………182 5. Note on the location of salt within the plant………………………………………..186


Contents (3) Biotic effects – Grazing. Page V.

The effect of grazing on the vegetation………………………………………………………….188 A. Facts relating to the population of grazing mammals on Skokholm…………….188 1. History and population density…………………………………………………….……188 2. Notes on the differential grazing habits of the various herbivores present……………………………………………………………………………………………192 B. Description of the heavily grazed vegetation of Skokholm…………………………196 1. Classification of plants in relation to their tolerance of grazing………………196 a. ‘Rabbit-avoided’ species………………………………………………………………196 b. ‘Rabbit-resistant’ species……………………………….…………………………….196 c. Non-resistant palatable species which are relatively ill-equipped to withstand grazing………………………………………………………………………..208 2. Relative distribution of palatable and unpalatable plants………………………210 a. The distribution of the dominant plant types over the island as a whole………………………………………………………………………………………..210 b. Zonation of vegetation around the rabbit warrens…………………………..231 C. The influence of grazing on the number of species present and the proportion of these which will flower……………………………………………………...237 1. Observations relating to the local effect of rabbit activity in increasing the species number maritime localities – as opposed to the decrease recorded by other workers in more inland situations……………………………237 2. The effect of grazing on flower production…………………………………………243 D. Description of the ungrazed vegetation of Grassholm with special reference to the points in which it differs from the grazed vegetation of Skokholm..……248 E. Changes in the composition of the vegetation on Skokholm resulting from the cessation or reduction of grazing………………………………………………………..254 1. Observations on the controlling effect of grazing on the vegetation of the more maritime situations…………………..…………………………………….254 a. Rapid changes resulting from the artificial exclusion of animals……....254 b. More gradual changes resulting from natural fluctuations in the grazing intensity………………………………………………………………………….260 2. Observations on the controlling effect of grazing on the vegetation of more inland situations……….………………………………………………………...278 F. Changes in the composition of the vegetation on Skokholm resulting from an increase in grazing………………….…………………………………………………294 G. Diagrammatic representation of the plant successions which might be expected to occur as a result of alterations in the grazing and exposure factors on Skokholm.……………………………………………………………….……………306 1. Successions depending upon the alteration of grazing………………………….307 a. Conditions of severe exposure……………………………………………………..307 b. Conditions of intermediate exposure……………………………………………308 c. Sheltered conditions, (i) and (ii)…………………………………………………..309 2. Successions depending upon the alteration of exposure……………………….311 a. Conditions of heavy grazing…………………………………………………………311 b. In the absence of grazing…………………………………………………………….312 3. Hypothetical successions involving the islands as a whole (taking into account grazing and exposure)…………………………………………………………313


Contents (4) Biotic factors other than grazing. Page VI. VII.

Treading…………………………………………………………………………………..……………….314 Burrowing………………………………………………………………………………………………….326 A. Nature and frequency of burrowing…………………………………………………………326 B. Burrows as a factor in the initiation of small secondary plant successions…….329 C. Burrows as centres of erosion…………………………………………………………………334 D. The effect of burrows in the introduction of atmospheric conditions underground…………………………………………………………………………………………341 1. General observations………………………………………………………………………..341 2. Fluctuations of soil moisture content in the vicinity of burrows……………..346 3. The effect of burrows on soil temperatures……………………………………..….357 4. The effect on rhizomes on Carex arenaria of the penetration of light along burrows…………………………………………………………………………..361 VIII. Manuring…………………………………………………………………………………………………..364 A. Description of the characteristic coprophilous flora of different types of habitat………………………………………………………………………………………………….364 1. Vascular plants………………………………………………………………………………..364 a. Vegetation of the coastal bird colonies………………………………………….364 b. Vegetation of the inland bird colonies…………………………………………..376 2. Non-vascular plants of the bird colonies……………………………………………..385 a. Terrestrial algae………………………………………………………………………….385 b. Lichens……………………………………………………………………………………..387 c. Fungi………………………………………………………………………………………..388 d. Bryophytes………………………………………………………………………………..389 e. Note on the algae of brackish and tidal pools below the larger bird colonies………………………………………………………………………………392 B. Distribution and intensity of manuring…………………………………………………….396 1. Varying defaecation habits of the different birds………………………………….398 2. The effect of guano on the more important soil nutrients…………………….398 a. Observations on the composition of guano and allied debris in the bird colonies.……………………………………………………………………….398 b. The effect of manuring on soil nitrates……………………………………….…403 c. The effect of manuring on soil phosphates……………………………………409 d. The relation of guano to the exchangeable calcium and carbonates of the soil.…………………………………………………………………412 e. The effect of guano on soil organic matter…………………………………….415 IX. The distribution of seeds by gulls………………………………………………………………….420 X. Summary of Part II…………………………………………………………………………………….432 References…………………………………………………………………………………………………439


The Inanimate Environment

Soil, Wind & Salt


The Interaction of Living Organisms

The Grazing Factor


The Interaction of Living Organisms

Biotic Factors other than Grazing

Treading, Burrowing, Manuring and Seed Distribution


Appendix


Appendix Summarised index of tables Species lists No. of page or table List of vascular plants found on Skokholm……………………………………………………………………..i-iv List of bryophytes found on Skokholm……………………………………………………………………………..v List of lichens found on Skokholm………………………………………………………………………………….vi List of freshwater and terrestrial algae found on Skokholm…………………………………………….vii-ix List of marine algae of Skokholm and Grassholm………………………………………………………….x-xv List of terrestrial plants found on Grassholm…………………………………………………………………..xvi

Part I 1. Climatological data for St Ann’s Head. 2. Floristic analysis of coastal Armerietum 3. Floristic analysis of ‘inland’ Armerietum 4. Floristic analysis of Silenetum 5. Species lists for societies of Cochlearia, Matricaria and Atriplex 6. Species lists for societies of Heracleum, Plantago and Erodium 7. Species lists for societies of coastal Agrostis and Holcus 8. Floristic analysis of Holcetum 9. Floristic analysis of Festucetum 10. Floristic analysis of Agrostidetum 11. Floristic analysis of Callunetum 12. Floristic analysis of Pteridietum 13. Summary of structure of four types of Pteridietum and species list 14. Floristic analysis of society of Senecio 15. Floristic analysis of central marshland

Part II I. Edaphic factors 16. Mechanical analysis of soils 17. Organic matter and water retaining capacity in wet habitats 18. Moisture holding capacity of Armeria peat 19. Moisture holding capacity of ‘brown earth’ soils 20. Summary of soil organic matter and water retaining capacity in Armerietum, Agrostidetum and Pteridietum 21. Soil temperatures of organic and non organic soils 22. Summary of soil temperature ranges in organic and non organic soils 23. Carbonate content of soils 24. Exchangeable calcium content of soils in wet habitats 25. Soil content of sesquioxides of iron and aluminium 26. Increasing soil alkalinity with increasing proximity to sea 27. Comparison of soil acidity in inland and coastal communities 28. pH and nitrate contents of soils in relation to biotic pressure.


Index to tables (2) II. Exposure to strong winds 29. Relative heights of different species above sea level 30. Classification of dominant species according to exposure of the habitat 31. List of some of the commoner bryophytes in relation to their exposure tolerance 32. Relationship of the amount of eroding bare soil in different plant communities and the degree of wind action suffered

III. Exposure to salt spray 33. Soil chloride contents in relation to the exposure of the locality and the weather 34. Soil chloride contents in relation to local shelter and soil waterlogging 35. Chloride content of ponds and streams 36. Salometer readings – estimation of salt deposited on a given area 37. Amount of chloride present in plant tissues

IV. Grazing 38. Comparison of floristic composition of burrowed and unburrowed zones in Armerietum 39. Ditto 40. Comparison of floristic composition of Festucetum under differing intensities of grazing 41. Floristic analysis of heavily grazed and ungrazed Festucetum 42. Comparative counts of rabbit pellets in relation to changing flora 43. Floristic analysis in five successive years to show changes on the Neck 44. Figures showing change from Armerietum to Erodietum in four years 45. Changes in frequency and abundance of species on the Neck 46. Analysis showing the significance of the changes in 9 of the above species 47. Figures relating to a succession from Armerietum to Callunetum to Holcetum to Rumicetum and back to Holcetum 48. Figures relating to a succession from Armerietum to Heraclietum to Rumicetum to Holcetum 49. Degradation of Armerietum under heavy grazing and its subsequent recovery 50. Floristic analysis of 10 areas of Callunetum on Skokholm 51. Abundance and frequency figures for the chief species of Callunetum 52. Summary of the importance of the chief species in healthy, partly degenerate and degenerating Callunetum 53. Floristic composition of the transitional zone between advancing Pteridietum and retreating Callunetum


Index to tables (3) V. Treading 54. Principal path species in different communities 55. Relative abundance and frequency of the most important path species 56. Classification of Skokholm plants according to their tolerance of treading

VI. Burrowing 57. Course of secondary seres on bare soil at burrow entrances 58. Figures showing the course of erosion of old Armeria hummock during a three year period 59. Effect of burrow in promoting local drought in non-organic soil 60. Fluctuations of soil temperature adjacent to burrows 61. Ditto

VII. Manuring 62. Coprophilous flora of puffin colonies 63. Coprophilous flora in cliff colonies of razorbills and gulls 64. Coprophilous flora of inland gull colonies 65. Effects of gulls on the composition of Pteridietum 66. Nitrophilous soil algae if the bird colonies 67. Algae and lichens of the bird colonies 68. Coprophilous fungi 69. Bryophytes of the biotic communities 70. Correlation between the nitrate content of soils and the birds 71. Soil nitrate contents in a puffin colony after the departure of the birds in the autumn 72. Soil phosphate contents in the bird colonies 73. Agricultural standards for soil phosphates 74. Relation of exchangeable calcium in the soil to the deposition of guano 75. Modifying effect of guano on soil organic matter 76. Water retaining capacity of soil organic matter in relation to deposition of guano 77. Ditto

VIII. Seed distribution by gulls 78. Arable weeds of the gull feeding grounds


Appendix Index of Plates (For more important plates, see text) Part I 1. A. Climatic data for St. Ann’s Head. Mean temperatures, total hours of sunshine and average rainfall. 2. A. Map of Skokholm showing physical features and local place names. 3. A. Percentage cover histograms for freshwater habitats on Skokholm. 4. A. Four quadrats showing angiospermous and bryophyte vegetation of sheltered cliffs on Skokholm. 5. A. Four quadrats I typical vegetation on Grassholm, one showing relationship of gulls and Cochlearia.

Part II I. Edaphic factors 6. A. Diagrams to show the prevalent type of root systems in plants of the poorly aerated Skokholm boulder clays. III. Exposure to strong winds bearing salt 7. A. Graphs showing the chloride content of the water in different habitats. (Also diurnal fluctuations of temperature, pH and oxygen) IV. Grazing 8. A. Morphological variation in Badellia ranunculoides dependent upon fluctuations in the environment. 9. A. Morphological variation in Juncus bulbosus dependent upon fluctuations in the environment. 10. A. Young plants of the succulent variety of Plantago coronopus showing differences of root habitat in different environments. 11. A. Seedlings of the succulent variety of Plantago coronopus to show the prevalence of the erect habit. 12. A. Morphological changes in Littorella uniflora brought about by the change from a dry grazed habitat to a wet ungrazed one. 13. A. Old partly grazed plants of Littorella uniflora showing the more erect habit of moister situations. (Also germination) 14. A. Diagrams to show the size attained by leaves of the succulent variety of Plantago coronopus in the absence of grazing 15. A. Belt transect histogram for Grassholm Festucetum rubrae, passing from a Sedum society to a Plantago society. 16. A. Chart of a colony of succulent Plantago coronopus at the time of its appearance on the East Ba clifftop in May 1950. 17. A. Maps to show the typical dry phase and wet phase associates of Littorella uniflora.


Index of plates (2) V. Treading 18. Views of North Pond to show the change in the vegetation where gulls and waders congregate


Mary GIllham's PhD Thesis  

The Vegetation of the South Pembrokeshire Islands in relation to the environment

Mary GIllham's PhD Thesis  

The Vegetation of the South Pembrokeshire Islands in relation to the environment