RESEARCH ARTICLE Rewilding Europe’s large grazer community: how functionally diverse are the diets of European bison, cattle, and horses? Joris P. G. M. Cromsigt1,2,3 , Yvonne J. M. Kemp4,5, Esther Rodriguez5, Hubert Kivit5 Trophic rewilding is the introduction of species to restore top–down trophic interactions and associated trophic cascades to promote self-regulating biodiverse ecosystems. A core example of trophic rewilding is the restoration of large mammalian grazer communities to restore or maintain biodiverse open to half-open landscapes. Across Europe, cattle and horse breeds are being introduced as substitutes of the extinct aurochs (Bos primigenius)andtarpan(Equus ferus). More recently, European bison (Bison bonasus) is being introduced because it is supposed to fill a niche that pure grazers such as cattle and horses leave empty, especially in terms of reducing woody encroachment. But how functionally diverse are the diets of these three species? We investigated this question in the Kraansvlak pilot; a trophic rewilding project in the Netherlands where European bison, horses, and cattle have been introduced in spatially heterogeneous landscapes of forest, shrub land, and grassland. We present 4 years of data from direct observations on the diet use of all three species. Whereas cattle and bison included a significant proportion of woody plants in their diet throughout the year, horses strictly grazed. However, cattle and bison differed clearly in terms of the woody plant part they used (bark vs. twigs), and we discuss how this may affect the way they influence vegetation structure. Finally, we discuss the implications of our study for the increasing number of trophic rewilding initiatives in Europe. Key words: Bison bonasus, Bos taurus, Equus ferus, grazing, refugee species concept, substitute species, woody encroachment introduced the concept of trophic rewilding and defined it as Implications for Practice “species introductions to restore top-down trophic interactions • Trophic rewilding with European bison, horses, and cat- and associated trophic cascades to promote self-regulating bio- tle can work without supplementary feeding, also in rela- diverse ecosystems.” A key example of trophic rewilding is the tively small nature reserves. restoration of the process of grazing by large mammalian her- • The three species found sufficient natural food sources bivores (Naundrup & Svenning 2015) to restore or maintain throughout all seasons and their diets consisted predomi- open to half-open, structurally diverse, landscapes (Olff et al. nately of grass (≥ 80–90% of diet). • European bison and cattle, but not horses, included a 1999; Smit & Putman 2010). Woody encroachment of such significant proportion of woody plants in their diet (20%) landscapes is seen as a serious threat to biodiversity (Ostermann throughout the year. 1998; Cremene et al. 2005; Henle et al. 2008; Bergmeier et al. • In winter and early spring, European bison debarked trees, 2010). Consequently, free-ranging cattle and horse breeds are whereas cattle browsed twigs, which suggests that they being introduced across Europe and elsewhere in the context of may have a different impact on woody communities. ecological restoration (Van Wieren 1995; Wallis De Vries et al. • The foraging behavior of European bison did not reflect 1998; Finck et al. 2002; Smit et al. 2015). that of a strict forest species, but rather that of a species that prefers a mixture of grassland and wooded habitats. Author contributions: JPGMC, HK conceived and designed the research; all authors performed the experiments; JPGMC analyzed the data; all authors wrote and edited the manuscript. Introduction 1Department of Wildlife, Fish, and Environmental Studies, Swedish University of Agricultural Sciences, 90183 Umeå, Sweden In many parts of the world, the concept of rewilding (Reardon 2Centre for African Conservation Ecology, Department of Zoology, Nelson Mandela Metropolitan University, Port Elizabeth, South Africa 2014; Svenning et al. 2016) is increasingly influencing con- 3Address correspondence to J. P. G. M. Cromsigt, email [email protected] servation and land-use agendas (Bauer et al. 2009; Navarro & 4ARK Nature, Molenveldlaan 43, 6523 RJ, Nijmegen, The Netherlands Pereira 2012; Ceau¸su et al. 2015; Pereira & Navarro 2015; Jep- 5PWN Waterleidingbedrijf Noord-Holland, Postbus 2113, 1990 AC, Velserbroek, The Netherlands son 2016). Navarro and Pereira (2012) defined rewilding as “passive management of ecological succession with the goal © 2017 Society for Ecological Restoration doi: 10.1111/rec.12661 of restoring natural ecosystem processes and reducing human Supporting information at: control of landscapes.” More recently, Svenning et al. (2016) http://onlinelibrary.wiley.com/doi/10.1111/rec.12661/suppinfo Restoration Ecology 1 Rewilding Europe’s large grazer community Following the rewilding philosophy, these introductions must This is a crucial question since there is now an increasing push be seen in the light of the relatively recent extinctions of toward restoring herbivore communities that include all three Europe’s wild large grazers; the aurochs (Bos primigenius)and European large grazers; cattle, horses, and European bison. The the tarpan or Eurasian wild horse (Equus ferus) (Naundrup main argument for restoring these communities is the func- & Svenning 2015). Both species were originally distributed tional complementarity of the three species; that is, that cattle, across Europe and further east into Eurasia but the last aurochs horses, and bison differ in their diet use and thus impacts on died in Jaktorów forest in Poland in 1627 (Szafer 1968) while the landscape (see e.g., Van de Vlasakker 2014). Unfortunately, tarpan survived until the first decades of the twentieth cen- we currently lack strong empirical evidence to support such an tury (Vereshchagin & Baryshnikov 1993). Although there is argument. an ongoing debate about the historical importance of these We present some of the first results of a unique trophic large grazers for creating and maintaining more open land- rewilding project in the Netherlands (the Kraansvlak pilot), scapes in northwestern Europe (Svenning 2002), recent evi- where European bison, horses, and cattle have been introduced dence suggests that they may have played this role during the in a spatially heterogeneous landscape of forest, shrub land, last interglacial, 132,000–110,000 years bp, before they were and grassland. Within the confines of the fenced reserves, all hunted to extinction or functionally insignificant densities by three species are free-ranging year-round and do not receive humans (Sandom et al. 2014). Aurochs and tarpan, however, any supplemental feeding, even during winter. In contrast to survived in their domesticated forms. Domestic cattle and horses many other rewilding initiatives (Svenning et al. 2016), the are, therefore, increasingly seen as functional equivalents, or project has had intense scientific monitoring since its start in ecological replacements (Griffiths et al. 2010; Svenning et al. 2007. Here, we present data on the diet composition of all three 2016), of aurochs and tarpan (Naundrup & Svenning 2015), species, collected over a 4-year period through a large number and there are now more and more efforts focused on rewild- of observations of individual bites. ing horses (Linnartz & Meissner 2014) and cattle (Vermeulen 2015). However, when thinking about rewilding Europe’s large grazer community, one species should not be ignored, the Euro- Methods pean bison (Bison bonasus). Although European bison went extinct in the wild in 1927, it was reintroduced in the wild Study Area in Białowiezȧ forest, Poland, in 1952 following a successful Our study took place between 2008 and 2012 in National breeding program in zoos (Pucek 2004). Since then, the pop- Park Zuid-Kennemerland (NPZK), a coastal dune reserve in ulation has grown considerably and European bison have been the Netherlands between IJmuiden and Zandvoort. The area reintroduced in several areas in mostly Eastern Europe, but experiences a temperate maritime seasonal climate. Average increasingly in western and southern Europe as well (Kerley monthly temperature is 3–4∘C in January and 17–18∘Cin et al. 2012). Apart from species conservation, several of the July, and less than 10 days per year temperatures drop below recent introductions are implemented as trophic rewilding ini- 0∘C for the whole 24 hours (average during 1981–2010, http:// tiatives; e.g., in Denmark (Brandtberg & Dabelsteen 2013) and www.klimaatatlas.nl). Average annual precipitation is around Spain (Burton 2011). The suggestion is that bison, as a mixed 850–900 mm, and during an average winter, the ground is cov- feeder from forested environments (Hofmann 1989; Krasinska´ ered by snow for less than 10 days per year (average during &Krasinski´ 2007), fills a niche that pure grazers such as cattle 1981–2010, http://www.klimaatatlas.nl). We focused on two and horses leave empty, especially in terms of reducing woody areas within the NPZK that are separated by a main road; the encroachment. Kennemerduinen area (52∘25′1.41′′N; 4∘34′54.53′′E), consist- The classic view of European bison as a typical forest ing of circa 2,069 ha, and the Kraansvlak area (52∘23′17.03′′N; species was recently challenged by Kerley et al. (2012), who 4∘34′13.11′′E), consisting of around 220 ha at the time of this suggested that bison is a refugee species that evolved on study. The Kennemerduinen is accessible