Of Dwarves and Giants How large herbivores shape arthropod communities on salt marshes Roel van Klink This PhD-project was carried out at the Community and Conservation Ecology group, which is part of the Centre for Ecological and Environmental Studies of the University of Groningen, The Netherlands. This project was funded by the Waddenfonds (Project WF200451) and carried out in cooperation with It Fryske Gea. The printing of this thesis was partially funded by the University of Groningen and the Faculty of Mathematics and Natural Science. Lay-out & figures: Dick Visser Cover: Bill Hauser (http://billhauser.deviantart.com) Photo credits: Chapter 1: Salt marsh of Westerhever, Germany (C. Rickert) Chapter 2: The birth of a conceptual framework, Herdershut, Schiermonnikoog, January 2010 (R. v. Klink) Chapter 3: Enoplognatha mordax, NFB (R. v. Klink) Chapter 4: Vegetation mosaics at the Hamburger Hallig, Germany (C. Rickert) Chapter 5: Compaction experiment at NFB, May 2011 (R. v. Klink) Chapter 6: Thymelicus lineola on Aster tripolium, NFB (R. v. Klink) Box I: Mine of Calycomyza humeralis in leaf of Aster tripolium (R. v. Klink) Box II: Setting up the experiment at NFB (R. v. Klink) Chapter 7: Meadow Pipits (Anthus pratensis) at NFB, 2011 (R. v. Klink) Box III: Colletes halophilus at Schiermonnikoog, 2010 (R. v. Klink) Chapter 8: Ballooning spiders at Noord Friesland Buitendijks, September 2011 (R. v. Klink) Appendix: Caterpillars of Aglais urticae on Urtica dioica, summerdike of NFB, September 2012 (R. v. Klink) References: Spittlebugs (Philaenus spumarius and Neophilaenus lineatus) in the compaction experiment at NFB (R. v. Klink) Summary: Whittleia retiella at the salt marsh of Westerhever, Germany (C. Rickert) Acknowledgements: Cattle at NFB (R. v. Klink) Printed by: CPI – Koninklijke Wöhrmann, Zutphen ISBN: 978-90-367-7084-2 ISBN: 978-90-367-7083-5 (electronic version) Of Dwarves and Giants How large herbivores shape arthropod communities on salt marshes Proefschrift ter verkrijging van de graad van doctor aan de Rijksuniversiteit Groningen op gezag van de rector magnificus prof. dr. E. Sterken en volgens besluit van het College voor Promoties. De openbare verdediging zal plaatsvinden op vrijdag 20 juni 2014 om 14.30 uur door Roel van Klink geboren op 12 oktober 1981 te Aalsmeer Promotores Prof. dr. J.P. Bakker Prof. dr. ir. M.F. Wallis de Vries Beoordelingscommissie Prof. dr. J.C. Biesmeijer Prof. dr. H. van Dyck Prof. dr. D. Bonte Contents Chapter 1 General introduction 7 Intermezzo: A short history of research on salt-marsh arthropods Chapter 2 Effects of large herbivores on grassland arthropod diversity 17 Chapter 3 Contrasting responses of the diversity of arthropod habitat specialists and generalists 41 to salt-marsh elevation and livestock grazing Chapter 4 Grazed vegetation mosaics do not maximize arthropod diversity: evidence from 57 salt marshes Chapter 5 Defoliation and soil compaction jointly drive grazing effects on plants and soil 77 arthropods on fine-textured soil Chapter 6 Effects of livestock species and density on flower-visiting insects in a salt marsh 101 Box I The herbivorous fauna of Aster tripolium on grazed and ungrazed salt marshes 113 Box II Contrasting effects of microclimate and defoliation on the fauna of five halophytes 123 Chapter 7 Foraging site choice and diet selection of Meadow Pipits Anthus pratensis breeding 133 on grazed salt marshes Box III Combining faunistic and ecological knowledge to optimise conservation management 147 for salt-marsh invertebrates Chapter 8 General synthesis 161 Epilogue: The little things that run the world Appendix The arthropod fauna of the Noorderleech area 177 References 183 Summary 206 Samenvatting 213 Acknowledgements 222 List of publications 225 Co-author affiliations 226 Chapter 1 General introduction Roel van Klink Chapter 1 ARTHROPOD CONSERVATION Arthropods are the most species-rich clade of eukaryotic life forms on earth, with an esti- mated number of species ranging between 3 and 7 million (Novotny et al. 2002, Hamilton et al. 2010). They play a key role in many ecological processes and have important effects on plant communities (De Deyn et al. 2003, Allan and Crawley 2011). Their central place in all terrestrial food webs makes them an important link between primary producers and vertebrate (top-) predators. Moreover, they perform myriad ecosystem services that human society depends upon, ranging from decomposition and crop pollination to ama- teur entomology and bird watching (Losey and Vaughan 2006, Prather et al. 2013). Wild invertebrates alone were estimated to account for a yearly economic value of $60 billion in the U.S. alone (Losey and Vaughan 2006), and domesticated bees accounted for another $19 billion (Calderone 2012). Hence, they constitute “the little things that run the world” (Wilson 1987), and ecosystems and human society would soon collapse without arthro- pods. Yet, the conservation effort directed towards invertebrates is only a fraction of the effort directed towards the conservation of vertebrates (Wilson 1987, Cardoso et al. 2011), and was thus dubbed “the awkward ‘kid sister’ to vertebrate conservation” (Dunn 2005). This fails to recognize, however, that due to their vast diversity and often strong speciali- sation, the majority of species that are endangered or have gone extinct due to human influence must be invertebrates (Dunn 2005). Moreover, it was shown that invertebrates are more prone to be endangered or to go (locally) extinct than either plants or vertebrates (Thomas et al. 2004). Fortunately, there is an increasing recognition of the conservation value of arthropods. Over the past decades, the effort directed at their conservation has increased considerably, resulting in numerous books (e.g. Samways 1994, 2005, New 2009), high profile publica- tions (e.g. Thomas et al. 2004, 2009, Biesmeijer et al. 2006), and two specialised journals (Journal of Insect conservation and Insect Conservation and Diversity) on this topic. How the conservation of arthropods should be achieved, though, requires a context specific approach. For each species to persist at a location, its requirements in terms of trophic and non–trophic resources must be met in a spatial and temporal context that can be used by the individuals making op the population (Southwood 1977). The conservation of arthropod species, and any other organism for that matter, should therefore be based on maintaining (or creating) habitats in which required trophic and non-trophic resources are sufficient to retain a viable population (Dennis 2003). The extreme diversity of arthropods entails an equally large variation in life history traits and, habitat requirements. Contrary to the better known plants and especially verte- brates, these traits and requirements are poorly known for the vast majority of arthropod species, even in the relatively well-studied fauna of Western Europe. To provide condi- tions meeting the requirements of all species in an ecosystem, and thus to preserve a maxi- mum diversity of arthropods, some form of nature management is often necessary. 8 General introduction GRAZING MANAGEMENT Most grassland types in Western Europe have a millennia long history of human land use and management (Barker 1985). Interestingly, this long-term extensive land use has led to an extremely high biodiversity (Wilson et al. 2012), and many plant and animal species now depend on the continued existence of these grasslands (Thomas 1993, Thomas et al. 1994). Typically, these grasslands stay in a plagioclimax state (sensu Tansley 1935) due to management actions, and are therefore prone to succession (in most cases to forest) if man- agement is ceased. In most cases, this succession will lead to a decline in plant species rich- ness due to an increasing dominance of tall-statured species (Grime 1973), which is deemed undesirable from a conservation perspective (Bakker 1989). Livestock grazing, as well as mowing or burning, can prevent this succession (Clements 1916), and can maintain plant species richness (Grime 1973). As an increasingly popular management tool to main- tain grassland biodiversity, grazing is currently used as management tool in over 500 proj- ects in the Netherlands alone (Alterra 2013). Grazing is often seen as preferable over burn- ing or mowing because large grazers usually display some selectivity in food choice, and can thereby create spatial heterogeneity (McNaughton 1984, Adler et al. 2001). Further- more, large, charismatic herbivores are seen as a more natural type of grassland manage- ment than mowing or burning in Western European landscapes (WallisDeVries 1998), and are seen as replacement for extinct herbivores such as the Tarpan and Aurochs (Bakker et al. 2004, Vera et al. 2006), although this view is heavily debated (Prins 1998, Svenning 2002, Louwe Kooijmans 2012). The effects of grazing on plant diversity have been synthesised on several occasions (Milchunas et al. 1988, Olff and Ritchie 1998). Generally, in productive systems grazing will increase plant diversity by reducing competitive displacement, whereas in dry, unpro- ductive systems, such as deserts, grazing will decrease plant diversity because of selective grazing for palatable species (Olff and Ritchie 1998, Proulx and Mazumder 1998, Bakker et al. 2006). For arthropods, few attempts at synthesis have been made (but see Morris 2000, Bell et al. 2001), and both positive (Joern 2005, Woodcock and Pywell 2009), negative (e.g. Kruess and Tscharntke 2002a, 2002b) and neutral