British and Irish Butterflies: An Island Perspective

By Roger L H Dennis and Peter B Hardy

Islands are special places; they can be havens for unique plants and animals and refuges for wildlife. This book investigates the biogeography of butterfly species over the British islands, particularly the factors that influence their presence on the islands and that have made each island's butterfly fauna distinctive. The book contains a full log of records of species on the islands and much supporting information. The first three chapters set the scene, illustrating the basics of island biogeography theory, their changing circumstances during the current Holocene interglacial, and studies of natural history of British butterflies that mark the islands as the most intensively studied region for wildlife in the world. The book advances by increasing resolution downscale from a European continental perspective, through patterns and changes on the British mainland, a comparison of the two dominant islands of Britain and Ireland, to a close inspection of the dynamics of species on the multitude of offshore islands. Detailed investigations include contrasts in species' richness on the islands and then of the incidences of each species. Case studies highlight the continual turnover of species on islands. Attention is then given to evolutionary changes since the time that glaciers enveloped Europe. A powerful message is conveyed for the maintenance of butterfly species on the smaller British islands now experiencing population losses at a rate unprecedented since the spread of the last ice sheets: the incontrovertible importance of maintaining populations of species on nearby mainland sources for islands as pools for future migrants.

• Language: English
• Publisher: CAB International
• Release date: Jul 13, 2018
• ISBN: 9781786395078

Roger L H Dennis

- Roger Dennis has spent 50 years researching into butterfly ecology, biogeography and conservation during which he has produced over 220 publications. His previous book on a 'Resource-based View for Conservation' received the British Ecological Society's Book of the Year in 2012. A Fellow of the Linnean Society and the Royal Entomological Society, he is currently an Honorary Research Fellow at the NERC's Centre for Ecology and Hydrology, Wallingford, and an Honorary Professor at Staffordshire University and at Oxford Brookes University.

Book preview

BRITISH AND IRISH BUTTERFLIES: AN ISLAND PERSPECTIVE

Supporting material is available at http://www.cabi.org/openresources/95061.

BRITISH AND IRISH BUTTERFLIES: AN ISLAND PERSPECTIVE

Roger L.H. Dennis

and

Peter B. Hardy

[Publishing company:

CABI

Nosworthy Way

Wallingford

Oxfordshire

OX10 8DE, UK]

CABI is a trading name of CAB International

© Roger L.H. Dennis and Peter B. Hardy 2018. All rights reserved. No part of this publication may be reproduced in any form or by any means, electronically, mechanically, by photocopying, recording or otherwise, without the prior permission of the copyright owners.

A catalogue record for this book is available from the British Library, London, UK.

Library of Congress Cataloging-in-Publication Data

Names: Dennis, Roger L.H., author. | Hardy, Peter B., author.

Title: British and Irish butterflies : an island perspective / Roger L.H. Dennis & Peter B. Hardy.

Description: Boston, MA : CABI, [2018] | Includes bibliographical references and index.

Identifiers: LCCN 2017050570 (print) | LCCN 2017051602 (ebook) | ISBN 9781786394989 (ePDF) | ISBN 9781786395078 (ePub) | ISBN 9781786395061 (hbk : alk. paper)

Subjects: LCSH: Butterflies--British Isles. | Butterflies--Great Britain. | Butterflies--Ireland. | Biogeography--British Isles. | Biogeography--Great Britain. | Biogeography--Ireland.

Classification: LCC QL555.B65 (ebook) | LCC QL555.B65 D46 2018 (print) | DDC 595.78/90941--dc23

LC record available at https://lccn.loc.gov/2017050570

ISBN-13: 978 1 78639 506 1

Commissioning editor: Ward Cooper

Editorial assistant: Emma McCann

Production editor: James Bishop

Typeset by SPi, Pondicherry, India

Printed and bound in the UK by CPI Group (UK) Ltd, Croydon, CR0 4YY

Contents

Dedication

List of Figures

List of Plates

Preface

Acknowledgements

1. Basics of Island Biogeography

The Basic Model of Island Biogeography

A Broader View of Island Faunas

An Integrative Island Biogeography Model

Population and Ecological Changes on Islands

Evolutionary Transformations and Novel Species

Island Species as Unique Entities

2. The British Islands: a Geological and Historical Outline

How Many British Islands Are There?

The British Islands: Composition, Context and Complexity

Glacial–Interglacial Cycles: Ice Landscapes

Shifting Sea Levels, Coastal Physiognomy and Island Formation

Climate Change During Island Formation

Post-Glacial Transformations to Island Landscapes

Historical Imperatives

3. Island Butterflies: Understanding and Interpreting Records

Habitat and Biotope

Habitat and Movement

Dispersal and Migration: an Island Viewpoint

Species Records and What They Imply

Bias and Mistaken Records

Introductions

British and Irish Butterfly Databanks and Sources

Selecting Islands for Study

4. British and Irish Butterfly Species: Links and Gradients

How Unusual Are British Butterflies?

Island Environment–Species Gradients

A Recording History of Gains and Losses on the British Mainland

Predicting British Species

Ranges, Distributions and Species Diversity: Detecting Deeper Causes

Range Changes in Recent Times

Sources and Range Oscillations of Long-distance Migrants

5. Island Associations and Species Affinities: Geographical Influences

European Islands: the Wider Context

Comparing Butterfly Richness Profiles of Britain and Ireland

Ireland’s Unusual Case of the Cryptic Wood White

The Influence of Geography on Island Species Richness

Geography of Archipelagos: the Isles of Scilly

Island Assemblages of Butterfly Species

Butterfly Associations on British and Irish Islands

6. Assessing the Status of Individual Species on Islands: from Geography to Ecology

Using Geography to Predict Species Richness

Predicting the Incidence of a Butterfly Species on an Offshore Island: the Grayling on Grassholm

Predicting Incidences of Butterfly Species on the British and Irish Offshore Islands

The Drumlin Islands of Strangford Lough

Colonizing Islands: Modelling Individual Probabilities of Success and Failure

How Well Does Island Butterfly Incidence Reflect Butterfly Ecology?

Using Species’ Ecological Profiles to Examine Inter-island Relationships

7. Changing Patterns of Species Incidences on Islands

Ecological Bases for Stable and Changing Island Faunas

Historical Signals in Geographical and Ecological Data

Order and Change in Island Faunas

Turnover and Trends: the Case of Hilbre and the Farne Islands

Desert Islands: the Perpetual Turnover of Adventives (Itinerants)

Big Changes on Big Islands: Uncertain Visits and Problematic Extinctions on the Isle of Man

Rare Migrants: the Future Colonists?

8. British and Irish Islands: an Evolutionary Vantage

Pulses of Loss and Gain: Return Periods and Recurrence Intervals

Outlets of Change: Ecological Release and Marginality

Glacial Refuges and the Post-glacial Invasion and Colonization Process of Britain and Ireland

Butterfly Space–Time Responses to Changing Conditions in Post-Devensian Britain and Ireland

Shrinking Habitats and Ecological Islands in Holocene Britain

Variation: Genes, Phenes and Plastic Traits

Models of Differentiation and Adaptation

Adjusting to Living on Small Islands

9. Island Studies: a Glance Back and the View Ahead

Islands: a Pattern of Intricate and Endless Changes

Lessons from Island Studies: the Need to Conserve Sources

Directions for Future Research of Island Butterflies

Plates

1. Island Landscapes

2. Images of British Butterfly Species

Appendices: Records of Butterflies from British and Irish Islands

1. Checklist of British and Irish Butterflies

2. Butterflies Occurring on the British and Irish Islands as Adventives (Accidental and Deliberate Introductions)

3. List of Butterflies on British and Irish Offshore Islands

4. Sources of Personal Observations on Island Records

5. Regional Atlases of British and Irish Butterflies

6. Migration Records and Measures for Butterfly Species Resident on the British and Irish Islands

7. Migration Capacity and Colonization Ability Measures for British Butterflies

8. A Brief Guide to Island Studies: Observations and Analysis

9. Predictions for Resident Butterfly Species in the British Isles

10. Factors Affecting Butterfly Species Richness on the Offshore British and Irish Islands

11. Summary of Predictions for Species Incidences on British and Irish Offshore Islands

12. Butterflies Recorded on Strangford Lough Islands, Northern Ireland, During the Butterfly Conservation UK Big Butterfly Counts

13. Probabilities of Individual Butterfly Transfers to Islands and Locating a Resource on the Island

14. Species Richness and Species Occurrence Predictions for the Isle of Man

15. Modelling Evolutionary Changes on a Fossil Island: the Great Orme’s Head, North Wales

Glossary of Terms

Bibliography

Index

Online Supplementary Appendices

Supporting material is available at

http://www.cabi.org/openresources/95061.

Supplementary Appendices Chapter 3

3.1a  A copy of Dennis and Shreeve (1996) Butterflies on British and Irish Offshore Islands: Ecology and Biogeography. Gem Publishing Company, Wallingford, Oxon

3.1b The main data file for British and Irish offshore islands

3.2   Basic ecological and life history data used to build the indices for migration capacity and colonization ability

Supplementary Appendices Chapter 4

4.1   Contemporary Geography Study of British Butterflies: Data

4.2   Contemporary Geography Study of British Butterflies: Analyses

Supplementary Appendices Chapter 5

5.1a The European Islands Data File: Recent Sources

5.1b The European Islands Data File: Butterfly Records

5.1c The European Islands Data File: Geographical Data

5.2   Comparison of British and Irish Species Distributions

Supplementary Appendices Chapter 6

6.1a Species Incidences on Offshore Islands: Logit Regression Analyses

6.1b Species Incidences on Offshore Islands: Discriminant Function Analyses

6.2   Species Richness and Incidences on Offshore Islands: Predictions

Supplementary Appendix Chapter 7

7.1   Records and Data for the Isle of Man

This work is dedicated to all the butterfly recorders who have kindly contributed to the records and observations for the British and Irish islands, the Isle of Man and the Channel Islands.

List of Figures

Text figures

Appendix Figures

List of Plates

Island landscapes

Images of British butterflies

Preface

Initially, our reason for producing this book was to provide updated records of the original work on islands by Dennis and Shreeve (1996), together with a revision of the original text to present a brief summary of research undertaken during the past 20 years. However, it soon became obvious that the original work was outdated and that readers would appreciate a wider perspective on the biogeography of the butterflies found on British and Irish islands. Also, it was felt that readers would benefit from a less technical introduction to the subject than that originally produced by the first author. Thus, the present book has a different title and, with a completely new text and wider focus, is a very different work from that of 1996. It has involved a great deal more preparation, as not only have many more appendices of additional data and other information been created, including supplementary appendices of data and analyses built for open access use, but also a much greater number and variety of illustrations have been included. Furthermore, more unusual terms are described in an extensive glossary. One thing has not changed; the prime objective remains the same: to encourage thorough observations on butterflies on the islands and to provide material for further evolutionary and ecological research to underpin the conservation of these beautiful creatures. The book, then, provides a brief overview of the fast developing research on the butterflies of the islands over the past 20 years.

In producing the book, we are aware that we are catering for different levels of expertise and knowledge on island biogeography as well as a number of other allied subject areas. Thus, the first three chapters introduce basic topics in island biogeography, past environments and aspects of butterfly biology and study. Chapters 4 to 6 drop down in scale from a broad view of butterfly geography over Britain and the European islands to finer issues affecting butterfly species richness and species incidences on the smaller offshore islands. Chapters 7 and 8 investigate changes in the faunas on islands, especially turnover and evolutionary changes, with an increasing emphasis on butterfly ecology in the changing landscapes of the Holocene. A short final chapter identifies the key features emerging from our review, summarizing the basic issues of island butterfly faunas, and then looks ahead to a new era of studies on the islands.

It will be helpful to provide a brief guide to the use of the book. It may be considered to fall into two basic sections: first, the text comprising the nine chapters; and second, a set of 15 appendices which present records for the islands and some details on more complex issues. In addition, a set of 13 supplementary appendices online provide working files of records and detailed results of analyses to facilitate research. In this way, we hope we have been able to cater for readers having different levels of expertise, interest and objectives. For those less familiar with British butterflies, we have also provided plates of all the species likely to be seen on the islands; these are not provided as an identification guide (we do not illustrate sexual dimorphism or geographical variation) but to display the extraordinary variety and beauty of these creatures; thus, to encourage enthusiasm and more concern for their plight in these days of outright habitat destruction. We have used scientific names for species, but those more familiar with (or who prefer) vernacular names will find these listed in Appendix 1 and attached to each plate.

Island biogeography is not a simple subject but covers a wide variety of academic disciplines, each having its own jargon. To facilitate reading the text, more unusual terms are noted in bold, which is an indication that they appear in the glossary. Terms applying to statistical techniques are noted in bold italics in the glossary and, to help matters further, in Appendix Appendix 8 a brief overview is given not only of essential issues in surveying islands but also the rudiments (objectives) of statistical techniques applied in the work. Although we have kept statistical jargon to a minimum, we realize that some techniques are difficult for non-specialists to understand; thus, more technical results have been stored in supplementary appendices. The reference section is extensive, the reason being to avail readers of the original sources in the 1996 booklet of information on islands, as well as sources to key publications. In addition, for easy reference, we provide the original booklet as a supplementary appendix.

In the original work of 1996, the first author was responsible for the list of records, by searching through the journals, and the text; the analysis was shared with Professor Tim G. Shreeve. In producing the present book, the work has broadly been divided as follows: Peter Hardy has undertaken the arduous process of locating all new records and of carefully building all the databases available as part of this book, including the supplementary appendices of records for general use and the scanned copy of the 1996 booklet. The island records for Britain and Ireland have been built on to the database published in Dennis and Shreeve (1996). As of 19 January 2017, records of butterflies were obtained by Peter Hardy for 393 of over 900 islands identified as being of more than 10 ha (many smaller ones too) around the coasts of Ireland, Man and the British Isles, 177 more than in 1996 (216). At the same time, 3865 species records for islands were recorded, 1614 more than the 2251 noted in 1996. The supplementary appendices also contain a file of butterflies on European islands which Peter has revised from the work of Dennis et al. (2008); nomenclature has been updated to match that in Kudrna et al. (2015) as far as possible. Many of the butterfly photographs in the book, a key attraction, have also been taken by Peter. RLHD’s contribution has been to carry out new analyses and to produce the text, glossary, references, figures and appendices of analyses and results. Of the figures, Peter has produced five splendid illustrations using Alan Morton’s DMAP programme (www.dmap.co.uk/).

In any work of such complexity there must be cautionary notes; several are expressed in the text. For several reasons beyond PBH’s control (access to some databases, inevitable failure to locate all websites with butterfly data), it is unlikely that the work has picked up all the records available by the beginning of data analysis (1 February 2017). Furthermore, some analyses of the records are flawed if only because of biased recording, as explained in Chapter 3; how the results may be affected by biased recording is discussed at appropriate points in the text. It has to be said that, despite the detail in this book, we envisage this work as little more than a beginning for the island biogeography of butterflies on the British and Irish islands, a platform for more significant research as well as for the simple joy of discovery. With the development of techniques for tracing movements in butterflies and studies in molecular genetics (use of unique DNA signatures), we expect the knowledge of dispersal in butterflies, and of their evolutionary transformations, to be revolutionized over the next few decades. In the previous work of 1996 it proved difficult to distinguish events in ecological and geological time; that difficulty is in the process of being erased, as evident in our ability now to identify cryptic species and introgression among closely related species, revealed by the remarkable findings for both Irish Leptidea juvernica and British Aricia species.

Our hope is that this book, which focuses on broad geographical and ecological issues, will provide a guide to priorities in future research and the conservation of butterflies on British and Irish islands, the Isle of Man and the Channel Islands.

Roger L.H. Dennis and Peter B. Hardy

Acknowledgements

No academic book is entirely the product of an author’s work and this book is no exception. It has depended on the generosity of many colleagues and friends for illustration, PDF files of papers, and on ideas generated by many researchers over decades. Moreover, this particular work has depended on the immense generosity of a great many butterfly enthusiasts responsible for accumulating records on the islands. We extend our grateful thanks to all the butterfly recorders who have kindly sent us their records directly or indirectly through the county and regional recorders for Butterfly Conservation UK, and who have contributed to other schemes (e.g. Big Butterfly Count).

The records have mostly been obtained from the regional coordinators for Butterfly Conservation UK’s Butterflies for the New Millennium atlas; such is their contribution that we have listed them in a dedicated Appendix 4. The credit for these records, together with specific details of the records, lies in the original data files held by Butterfly Conservation UK, sourced by one of us (PBH) from the regional recorders; we are indebted to them all for their generosity and as such dedicate this book to those who have done the groundwork in building up the data used in this work. The contribution from Ian Rippey, who has taken great pains to accumulate records and to direct them to us, as in 1996, has been particularly important for our coverage of offshore Irish islands. Records for a detailed study of the Isle of Man and its satellites have kindly been supplied by Gail Jeffcoate, the late Dr Stephen Jeffcoate, and Garry Curtis. Selective data for UK Butterfly Monitoring Sites and the Wider Countryside Butterfly Survey have been kindly provided by Professor Tom Brereton of Butterfly Conservation UK. We thank Professor Chris Thomas, Dr Rob Wilson and Andrew Graham (regional recorder for North Wales) for data for the plot of species richness on the Great Orme’s Head, North Wales. This book also provides an up-to-date supplementary appendix (Microsoft Excel file) of species for European islands; our grateful thanks to Dr Leonardo Dapporto for adding many new records for the Italian islands and to Dr Enrique Garcia-Barros for tracking down records for islands off the Galician coast. A number of colleagues have also kindly assisted us in gathering records on butterfly movements for Appendix 6; our grateful thanks to them for their experience and observations, as to all those who have contributed to this work.

We have benefited immensely from work carried out by other researchers and extend our thanks to all whose research is cited in the book whether as information, ideas, or illustrations. Professor Chris Thomas and Professor Jim Mallet kindly consented to the publication of their fascinating map of DNA variation in Aricia. The book includes some original work and we thank those who have helped to produce this novel material, especially Professor Richard J. Tilley. The book also summarizes work that the first author has produced on butterflies of the British Isles with other authors since 1996; on this aspect he would like to point to the outstanding original contributions to this research of his close friends and colleagues, Dr Leonardo Dapporto of Università degli Studi di Firenze, Italy, and to Professor Tim H. Sparks of Coventry University, UK, to whom he is indebted. He would also like to thank Professor Tim G. Shreeve of Oxford Brookes University, UK, for his support in producing the original book on islands in 1996 and the many publications over the years.

A number of colleagues have greatly assisted our interpretation of historical events. We convey our grateful thanks to Scott Renshaw and the British Geological Society for providing details of the borehole taken at Llandudno in 1971 by M.J.C. Nutt, and to Professor Emeritus Michael J. Tooley for a copy of his meticulous monograph on sea-level changes in the Irish Sea basin; also to Dr Philippa Tomlinson for details of Devensian and Holocene geology of the Isle of Man.

We are entirely indebted to Gail Jeffcoate, Julie Williams (Chief Executive) and Natalie Ngo (Media Officer and Assistant editor of Butterfly) of Butterfly Conservation UK for tracking down colleagues with images of butterflies for the book; our grateful thanks to them and all these generous contributors. Butterfly images were kindly provided by Patrick Clement, Bob Eade, Peter Eeles, Tony Hoare, Mark and Neil Hulme, Nigel Kiteley, Iain H. Leach, Tim Melling, Ian Rippey and Andy Seely; their stunning contributions are also noted in the legends to the plates. Some landscape images are also included; our grateful thanks to all contributors. The image of Bawden Rocks, Cornwall in a storm, was one of the last ever taken by an old friend (the late Dr W. Gerry Tremewan, editor of the Entomologist’s Gazette for over 50 years and world expert of Burnet moths) shortly before he died. Gail Jeffcoate, again, kindly provided the appealing image of the Calf of Man and Matthew Oates contributed the photograph of the Isle of Wight. Wildlife film maker John Aitchison and his family generously trudged the length of Colonsay to photograph a special oak wood for us where the late Tom Dunn first found the Purple Hairstreak in 1965. A long-standing friend, Professor Andrew S. Pullin, provided the photograph of the Great Orme for the back cover, an image that impresses how close the Great Orme’s Head is to becoming an island again. Steven R. Williams, Records Officer for Hilbre Bird Observatory, provided the stunning image of Hilbre from Middle Eye; Sam Whitfield, those for Flat Holm and Steep Holm during his time as Warden of Flat Holm; and Stuart Murray the glorious images of Ailsa Craig, Bass Rock, Grassholm, South Bishop Rock and Ramsay (the front cover image). Two long-standing friends, Professor Tim H. Sparks and Dr Andrew Ayres, ensured many of the above contacts, the former via Professor Mike P. Harris; Tim’s daughter, doctoral student Alexandra Sparks, also provided a suite of photographs of St Kilda, all so evocative of the lost community there that it was a difficult task to select the final image! Permission to reproduce the image of Strangford Lough was provided by Mike Hartwell (Northern-eye Co. Ltd, File 776312424) and that for Lindisfarne by Graeme Peacock (D 0280; www.graeme-peacock.com). The copyright of all images credited to contributors and publishers remain with the named contributors.

Readers will find that the original book on this subject (Dennis and Shreeve, 1996) is available as a Supplementary Appendix. For its inclusion, we would like to thank Ian Johnson (Pemberley Books) and Geoffrey Morton (originally of Gem Publishing Company). In producing the book every effort has been made to contact holders of copyright to obtain permission to reproduce copyright material. However, if any have been inadvertently overlooked, the publishers will be pleased to make the necessary arrangements at the first opportunity. We would like to take the opportunity here of thanking all those publishing companies that gave permission to publish material in this book. Appropriate credit is given where reproductions are made. Copyright of illustrations is retained by authors contributing them to the book.

This work would not appear without the experience and encouragement of CABI’s publishing staff: Emma McCann (Editorial Assistant), Susan Philcox (Knowledge Business Intellectual Property Manager), James Bishop (Production Editor), Victoria Davies (Marketing Assistant), Leigh-Ann Bard (Marketing Manager) and particularly Ward Cooper (Commissioning Editor); our grateful thanks to Ward for taking on the project, and to all the CABI staff for all their help and encouragement. We are especially grateful to Val Porter (Copy Editor) for hunting down the errors in the text. If the book is error-free, it is entirely owing to her painstaking scrutiny of the work.

Roger L.H. Dennis and Peter B. Hardy

1  Basics of Island Biogeography

It is one thing to make recordings or collections of butterflies from islands. Collecting has a long history but even so records for butterflies on British and Irish islands were, until recently, scattered in the journal literature (Dennis and Shreeve, 1996). However, it is another matter to make sense of the records. It was not until the early 1960s that sound scientific principles were established for island faunas. The present chapter provides an introduction to modern concepts in island biogeography, factors that affect the presence of butterflies, and other organisms, on British and Irish islands.

The Basic Model of Island Biogeography

In the 1960s, one of the most important breakthroughs in the ecological sciences was achieved; it was the publication of The Theory of Island Biogeography by Robert H. MacArthur and Edmund O. Wilson (1963, 1967). Although it was previously well understood that numbers of species on islands relate to island area and isolation, they demonstrated that the number of species could well reflect a dynamic equilibrium between two ongoing processes: (i) the immigration (viz. colonization) of species to an island; and (ii) their extinction ( Fig. 1.1a); as such, it is referred to as the equilibrium theory. It was a theory already foreseen by Eugene Gordon Munroe (1948) working on the butterflies of the West Indies. This was an astonishingly important breakthrough because, then, even mobile organisms such as butterflies were thought to have long occupied islands – including those comprising Britain and Ireland – on which they were found; as such, overseas transfers, apart from by known long-distance seasonal migrants (e.g. Vanessa cardui, Vanessa atalanta), were considered to be rare events. This dichotomy in perception must now seem very strange to modern readers; after all, if one species can with facility make the journey in numbers, then surely all others must have some capability of achieving the journey! In fact, this very belief was one of the stumbling blocks for challenging the long-held view that British butterflies survived the last major glaciation (Devensian, 20 k years (20 ka) BP) (Dennis, 1977). Yet, even then, sufficient evidence existed in the amateur journals that very small butterflies, including many sedentary butterflies, were capable of sea crossings (Dennis and Shreeve, 1996).

Fig. 1.1. Equilibrium island biogeography theory. (a) The equilibrium state in numbers of species for a single island based on the rate of immigration and extinction. I, immigration rate; E, extinction rate; Sˆ, equilibrium number of species on island; T, Turnover rate; P, size of the mainland source pool for the island. (b) The approach to an equilibrium for two distinct kinds of island: (i) ocean island; (ii) land bridge island. Once the number of species on the land bridge fauna have reached equilibrium, this is known as the relaxation fauna. The red arrows indicate the period of delay before the first species is lost (land bridge island) and gained (‘oceanic’ island). Oceanic islands here are used in the sense of a newly created island starting with zero species. Note how equilibrium is unlikely ever to match the number of species on the source mainland.

The mathematical model for a single island off a mainland source beautifully illustrates the basis for equilibrium in species numbers on islands (Schoener, 2010); a ‘steady state’ in numbers of species occurs where the gross extinction rate in species equals the immigration rate of new species. In Fig. 1.1, the rates are typically shown to be curvilinear, owing to the limit in the number of species at mainland sources: as numbers on an island grow, so does the probability of extinction; similarly, the probability of immigration and colonization will decline, as there are fewer new species left in the transfer pool. This observation is supported by ecological observations, for instance by: (i) the saturation of the species community with an increase in numbers of island species, leading to increased interactions (i.e. competition, predation, parasitization), thus smaller populations and extinctions (Wilson, 1969); and (ii) declining immigration rates as poorer dispersing species trail behind better dispersers (MacArthur and Wilson, 1967). Moreover, poorer dispersers are likely to be more vulnerable to extinction, owing to the link-up of colonization ability and migration capacity (Gilpin and Armstrong, 1981) (see also Chapter 3).

Equilibrium theory of numbers of species on islands, then, envisages a perpetual turnover in species; both immigration of new species and extinction of current species will have a characteristic mean rate and variance (unless conditions change significantly, i.e. through climate warming). It is perhaps useful to imagine the basic process taking place at an individual level. Consider that an island is being continually bombarded by individual butterflies and is simultaneously experiencing a continual loss of individuals (at all stages of development) (Simberloff and Wilson, 1971). Every so often an individual of a new species will arrive on the island and colonize it, or the population of a species will crash to zero and become extinct. Together, these processes describe the relative turnover rate (T) for an island’s species:

where t0, t1, t2 ... tm are time intervals (years), ne is the number of species extinctions on an island, ni the number of novel colonizations on an island, and N the total number of species at different times. T can be made into a percentage by multiplying by 100. Later, attention will be given to two types of island: (i) an oceanic island; and (ii) a land bridge island. Although Britain and Ireland do not have typical oceanic islands, they do have islands that simulate the conditions on a typical oceanic island, with an initial ‘start-from-scratch’ condition of zero species as in the case of Hawaiian islands emerging as volcanic mounds over an oceanic mantle ‘hot spot’ (Funk and Wagner, 1995). In the case of some British islands a number emerged offshore with deglacial isostatic rebound (see Chapter 2) of ice-laden land, their fauna wiped out by glacial tabula rasa and/or marine inundation during the last major ice advance (c. 20 ka BP). In the case of these two types of islands, very different patterns of colonization emerge, continuous gains towards an equilibrium with an ‘oceanic’ island and continuous losses to an equilibrium in the case of a vicariant land bridge island ( Fig. 1.1b). These conditions describe extremes of what is found in nature; it will become evident that Britain displays more complex scenarios.

The intriguing picture of equilibrium theory is that large and small islands, near-to-source and isolated islands, have very different equilibria ( Fig. 1.2); the curve for extinctions is higher for small islands than large islands, and the curve of immigration for isolated islands is lower than that for near islands. This has consequences for both the number of species and absolute turnover rates: (i) near islands (having the same area) and large islands (experiencing the same