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Invasion of the Signal Crayfish, Pacifastacus Leniusculus, in England: Implications for the Conservation of the White-Clawed Crayfish, Austropotamobius Pallipes

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Invasion of the Signal Crayfish, Pacifastacus Leniusculus, in England: Implications for the Conservation of the White-Clawed Crayfish, Austropotamobius Pallipes Invasion of the signal crayfish, Pacifastacus leniusculus, in England: implications for the conservation of the white-clawed crayfish, Austropotamobius pallipes Daniel David Adrian Chadwick University College London PhD – Environmental Science Page 1 of 249 I, Daniel David Adrian Chadwick, confirm that the work presented in this thesis is my own. Where information has been derived from other sources, I confirm that this has been indicated in the thesis. Page 2 of 249 Abstract The spread of invasive species is a key driver of UK native biodiversity loss. The UK’s native white-clawed crayfish, Austropotamobius pallipes, is in severe decline. The primary contemporary cause of this decline is the invasive non-native signal crayfish, Pacifastacus leniusculus, and associated ‘crayfish plague’ Aphanomyces astaci. In this thesis, I provide an updated distribution map of crayfish in England. This work shows that A. pallipes continues to significantly decline within England, whilst P. leniusculus continues to spread. Special Areas of Conservation were also analysed in the context of localised threats. At a regional scale, I explored the impacts of P. leniusculus on native ecological communities in headwaters, using both A. pallipes and crayfish-free rivers as controls. At the highest observed Catch-Per-Unit-Effort, populations of P. leniusculus severely depleted both invertebrate abundance and richness. I considered P. leniusculus population density and structure to be paramount in understanding its invasion ecology, but the literature was often based on biased sampling methods or semi-quantitative data. A novel technique, referred to as a ‘triple drawdown’, was developed and tested along a high density invaded river, with the intention of defining an exhaustive method of surveying P. leniusculus. Densities in excess of 110 m-2 crayfish dominated by young-of-year and juvenile cohorts were recorded. The conservation significance of these findings are considered. Finally, the impact of dense P. leniusculus populations was explored, using Gut Contents Analysis (GCA) and Stable Isotope Analysis (SIA). P. leniusculus exhibited high levels of cannibalism in both low and high density sites. Both SIA and GCA showed a diversification to include other invertebrate groups under high density pressure. As a whole, the thesis shows the importance of understanding the fundamental information of distribution, structure and density of P. leniusculus populations, when attempting to manage this highly damaging invasive species, and conserve A. pallipes. Page 3 of 249 Statement of originality with regards to post-graduate collaborations Collaborative fieldwork with postgraduate students was undertaken during the 2015 and 2016 field seasons. To ensure safety standards in the field, a minimum of two operatives were required at all times to operate the electrofishing equipment. The electrofishing data gathered in 2015 by myself and Lawrence Eagle comprised the core of his MSc thesis at UCL, however both parties agreed to share and use the raw data. All analysis of fish data presented in this thesis was performed solely by the author (Chapter 3), and all discursive ideas presented are the authors own. Collaborative field work was conducted in 2016 with Eleri Pritchard, who assisted with the drawdown method, and utilised the data in support of her MSc thesis at UCL. The drawdown method was acknowledged within her thesis as having been developed by myself and Paul Bradley. All analysis and discussion of ideas pertaining to the drawdown within the present thesis are the authors own (Chapter 4). Page 4 of 249 Acknowledgements The author is grateful to NERC, who funded this work through the London NERC Doctoral Training Partnership and through a grant at the NERC East Kilbride isotope facility, Glasgow. The author is grateful to Dr Carl Sayer (UCL), Dr Michael Chadwick (KCL) and Dr Jan Axmacher (UCL) for their roles in supervising this PhD. Finally, I would like to extend my thanks to the lab staff at both UCL and KCL for their expertise and assistance. I would like to thank my wonderful girlfriend Eleri, for all her help, support, and patience during this PhD. Being there with a kind word or a coffee has been a great help and comfort. I would like to thank my mum and dad, Susan and Paul, for encouraging me to develop a childhood fascination with my pond and wildlife into (hopefully!) a passionate career in ecology. I would like to thank my sister Ella for our sibling solidarity, and wish her luck finishing her PhD. And finally, I would like to thank Paul Bradley, for being a continued source of inspiration and sage advice. Page 5 of 249 UCL Thesis Impact Statement This thesis has developed a novel field sampling method capable of obtaining in-situ data on signal crayfish population size and structure, leading to density estimates far in excess of those previously recognised (Chapter 4). This method, which involves three successive de-waterings of the channel (a so-called “triple drawdown”) and subsequent crayfish collection, is applicable to small streams and is of immediate interest to aquatic practitioners concerned with invasive crayfish populations and threats to the native white-clawed crayfish. Importantly, application of the triple drawdown approach highlights current flaws in contemporary methods of sampling crayfish (especially trapping) and has paved the way forward to new crayfish sampling approaches. The data derived from a drawdown can be of wider use to persons modelling systems containing crayfish, as size classes and growth functions can be derived from it. In particular results from this work have been shared with the Environment Agency and Natural England, the governmental bodies responsible for licensing crayfish work and for implementing legislation. The drawdown method is currently under further study (through a London NERC DTP studentship to Eleri Pritchard) and has recently been published as a case study in the 2018 textbook: Freshwater Ecology and Conservation: Approaches and Techniques, edited by Jocelyne M. R. Hughes, by Oxford University Press. Through a collaborative internship with the Environment Agency and Natural England, the author has generated the most recent and comprehensive distribution dataset and associated set of maps of both native white-clawed crayfish and invasive signal crayfish in England. This output has been directly used for UK reporting to Europe under Article 17 of the EC Habitats Directive. Reporting population trends for both species on a European platform facilitates delivery of key conservation messages at a scale much larger than usually attainable by a doctoral study. A major impact here is highlighting the unfavourable status of native crayfish within England, which will hopefully stimulate better white-clawed crayfish conservation prioritisation. Finally, the database produced through the digitisation and collation of the Environment Agency and Natural England records, will form a central resource for the new ‘National Crayfish Strategy’ document for the Environment Agency in England, set to be implemented in January 2019. The strategy will outline how the Environment Agency will approach conservation of the white-clawed crayfish in the short to medium term. Page 6 of 249 Contents Invasion of the signal crayfish, Pacifastacus leniusculus, in England: implications for the conservation of the white-clawed crayfish, Austropotamobius pallipes .............................................. 1 Abstract ........................................................................................................................................................ 3 Chapter 1 – Introduction, theories and concepts ........................................................................................ 11 Invasive species in freshwater ecosystems ................................................................................................. 11 Invasive success of crayfish species ........................................................................................................... 12 Ecology of A. pallipes in the UK ............................................................................................................... 15 Ecology of P. leniusculus in the UK .......................................................................................................... 17 Morphological differences and identifying features of A. pallipes and P. leniusculus .............................. 18 Historical context of A. pallipes and P. leniusculus in the UK .................................................................. 21 Conservation challenges and opportunities for A. pallipes in the face of invasive crayfish ....................... 23 Overall thesis aims ..................................................................................................................................... 25 Chapter 2 – Status and distribution of crayfish species in England............................................................ 28 Introduction ................................................................................................................................................ 29 Review of past crayfish distribution in the UK through the Habitats Directive ......................................... 31 Establishment of the SAC network over the 1994-2000 Article 17 reporting period in the UK ............ 31 Summary of the general assessment for all species under the 2001-2006 Article 17 reporting period in the UK ...................................................................................................................................................
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