Comparison of three trap types in sampling saproxylic beetles living in hollow urban trees

Journal of Insect Conservation https://doi.org/10.1007/s10841-018-0115-3 ORIGINAL PAPER Comparison of three trap types in sampling saproxylic beetles living in hollow urban trees Elina Peuhu1 · Pia‑Maria Thomssen1 · Juha Siitonen2 Received: 13 July 2018 / Accepted: 28 November 2018 © The Author(s) 2018 Abstract Hollow trees are an important habitat for a large number of saproxylic invertebrates, many of which are rare or threatened. Large old trees occur frequently in cities, but the saproxylic fauna inhabiting these trees has been poorly studied. Sampling in urban areas includes the risk of trap failure due to human interference, which needs to be considered when designing sampling. The aim of our study was to find an efficient trap type for sampling saproxylic beetles in hollow urban trees. We compared the species richness and species composition of saproxylic beetle assemblages between trunk window, aluminium foil tray and pitfall traps placed inside hollow trees in the Helsinki metropolitan area, Finland. A total of 30 traps of each trap type were set in 15 trees. The traps caught a total of 4004 saproxylic beetle individuals belonging to 131 species. Trunk window and aluminium foil traps had similar assemblage and trapping efficiency, and were significantly more efficient than pitfall traps. However, pitfall traps caught certain species more efficiently than the other two trap types. Time spent separating insects from samples was the most laborious work stage. The time increased with increasing sample weight, i.e. the amount of wood mould in the trap. Trunk windows were the most efficient trap type also in terms of saproxylic species and individuals per handling time. We conclude that saproxylic beetle fauna living in hollow urban trees can be efficiently sampled with small trunk window traps or containers placed on the inner walls of hollows. Keywords Saproxylic Coleoptera · Tree cavities · Urban parks · Trunk window traps · Pitfall traps · Aluminium foil traps Introduction Hollow trees are an important habitat for a large number of saproxylic invertebrates. Many are strict habitat specialists living only in tree cavities (Speight 1989; Siitonen 2012a). Decaying wood regularly occurs in mature living trees as heartwood decay caused by heart rot fungi. Cavity formation and hollowing are normal life cycle stages for nearly all long-lived deciduous trees such as oak (Quercus spp.) and lime (Tilia spp.) (Schwarze et al. 2000; Alexander 2008). Numerous cavity-dwelling saproxylic species have become threatened (Speight 1989; Nieto and Alexander 2010) because the number of large old trees has decreased and * Elina Peuhu 1 Department of Forest Sciences, University of Helsinki, PO Box 27, 00014 Helsinki, Finland 2 Natural Resources Institute Finland (Luke), Latokartanonkaari 9, 00790 Helsinki, Finland continues to decline, both in Europe and globally (Gibbons et al. 2008; Lindenmayer et al. 2012). In most regions of Europe, old hollow trees occur mainly in various semi-natural habitats created and maintained by human activity, including pasture woodlands, hedgerows, avenues and parks (Oleksa et al. 2006; Dubois et al. 2009; Siitonen 2012b; Hartel et al. 2013). For example, the basal area of large (> 40 cm) trees is approximately three times greater in old manor parks in Estonia compared to mature forest patches in the surrounding landscape, and the proportion of broadleaved trees (Tilia spp., Quercus robur and Fraxinus excelsior) is also ca. three times greater than in the forests (Lõhmus and Liira 2013). Mature hollow trees can also be found in cities. As urban areas continue to expand, valuable habitat patches containing old trees may become surrounded by built-up environments. Even more importantly, most major cities maintain large tree populations (in the order of tens to hundreds of thousands of trees per city) by actively planting and tending trees in parks and along streets (Nowak et al. 2001; Sjöman et al. 2012). Some of these trees are sufficiently old to contain hollows. 13 Vol.:(0123456789) Journal of Insect Conservation Surveys of saproxylic beetles and dipterans in old parks in Central Europe (Franc 1997), Britain (Denton and Chandler 2005) and Northern Europe (Andersson 1999; Jonsell 2004, 2012; Sverdrup-Thygeson et al. 2010) have shown that rare and threatened species frequently occur in such habitats. In Britain, saproxylic beetle fauna in landscape parks that include relic trees are strikingly richer than fauna in parks without such continuity (Harding and Alexander 1994; Alexander 1998). However, man-made parks with mainly planted trees can also be important if they contain certain natural elements, if they are located close to potential source areas or if they are old enough (Siitonen 2012b). Hollow lime trees in old manor parks in southern Sweden host as many specialist and red-listed saproxylic beetle species as similar trees in open wood pastures or overgrown former wood pastures (Jonsell 2012). Despite hollow trees being valuable for biodiversity, urban hollow trees have received less attention. Thus, more knowledge on the conservation value of hollow trees in urban areas is needed, and such information can be used to guide management procedures and decisions. Quantitative sampling methods with known performance are needed for studying insect assemblages living in hollows. Sampling of invertebrate is usually carried out using traps or other methods especially designed for catching certain group of species (Leather 2005; Bouget and Nageleisen 2009). Window traps (Siitonen 1994; Bouget et al. 2008), trunk window traps (Kaila 1993; Sverdrup-Thygeson et al. 2009) and emergence or eclector traps (Lindhe and Lindelöw 2004) are the methods used the most for sampling saproxylic beetles. Special methods have been developed for surveying species living in tree hollows, including pitfall traps buried in the wood mould (Ranius and Jansson 2002), emergence traps closing the cavity opening (Gouix and Brustel 2011) and even vacuum cleaning of insect fragments from the bottom of deep cavities (Bußler and Müller 2014). Several previous papers have compared the efficiency of various sampling methods in sampling saproxylic beetles (Siitonen 1994; Økland 1996; Wikars et al. 2005; Hyvärinen et al. 2006; Alinvi et al. 2007). However, we are aware of only two previous studies that have compared the efficiency of various methods in surveying saproxylic beetle assemblages living in tree hollows (Ranius and Jansson 2002; Quinto et al. 2013). Some of the methods used in these and other studies (large window traps set outside tree hollows, emergence traps covering entire hollows) are clearly not suitable for studying trees in an urban environment. The high probability of vandalism directed at research equipment (Clarin et al. 2014) must be taken into account, especially when designing sampling in an urban environment. Hence, sampling any arthropod group in ur (...truncated)