Sampling saproxylic beetles: lessons from a 10-year monitoring study Petri Martikainen a, * , Lauri Kaila b a Faculty of Forest Sciences, University of Joensuu, P.O. Box 111, FIN-80101 Joensuu, Finland b Finnish Museum of Natural History, FIN-00014 University of Helsinki, Helsinki, Finland Received 14 October 2003; received in revised form 4 February 2004; accepted 10 February 2004 Abstract We sampled saproxylic beetles using trunk window traps in two birch-dominated forests in Finland during 1990–1999. The sampling scheme, 10 traps attached to living fruiting bodies of Fomes fomentarius growing on dead birches, remained unchanged in both forests throughout the study period. Beetles belonging to 32 selected families were identified every year, whereas all species were identified during the last 4 years. Total number of identified individuals was 40,294 and number of species 583, of which 258 were saproxylic (dependent on dead wood). Species richness of rare and threatened saproxylic beetles in the samples varied a lot between the years and did not fluctuate synchronously between the forests. Variation between years was smaller when all saproxylic species were pooled together and some abundant species fluctuated synchronously in the two forests. Similarity indices and DCA- ordination did not generally suggest decreasing similarity between samples with increasing temporal isolation. Incidence-based similarities of common saproxylic species within and between forests and years were high (means 0.7–0.8), whereas those of rare saproxylics were roughly 50% smaller and much more variable. More than 75% of the common saproxylic species found during the entire 10-year period were detected already after 3 years of sampling but accumulation of rare and threatened species was much slower. Our results suggest that: (1) occurrence of rare and threatened species in samples is much less predictable than that of common species and, e.g. reserve selection based on rarities should be made cautiously; (2) estimation of total number of threatened species in a forest is very difficult, because such species accumulate slowly in the samples; (3) samples from different years can be comparable in certain cases. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: Long-term insect survey; Sampling methodology; Species accumulation; Temporal variability; Threatened Coleoptera 1. Introduction Saproxylic Coleoptera – beetles dependent on dead wood (Speight, 1989) – have been a subject of intensive study during the past 10–20 years in northern Europe (V€ ais€ anen et al., 1993; Kaila et al., 1994; Siitonen and Martikainen, 1994; Økland et al., 1995; Jonsell et al., 1998; Martikainen et al., 2000; Simil€ a et al., 2002; Sverdrup-Thygeson and Ims, 2002) and recently also in other areas (North-America, Central Europe, Australia, etc.) (Rauh and Schmitt, 1991; Hammond, 1997; Grove, 2002). This is mainly because saproxylic beetles have been particularly affected by forest management and make up one of the largest groups of red-listed species in many countries (Berg et al., 1995; Jonsell et al., 1998; Rassi et al., 2001). The high species richness of forest beetles, however, makes the study of saproxylic beetles challenging. Adequate sampling of rare and threatened species is especially difficult, since such species form only a small fraction of the total sample even though they may make up a considerable proportion of the whole fauna (Muona, 1999; Martikainen and Kouki, 2003). Furthermore, laborious identification of individuals and other practical constraints make it difficult to collect and process as large samples as would be desirable. Thus, sample sizes in forest beetle inventories are always compromises and usually rather small. Normally sam- pling is restricted to a relatively short time period, typ- ically to one season only. Larger samples can be attained either by increasing the number of traps, or by contin- uing sampling for several years. Long-term studies to show how much variation there is within a site between * Corresponding author. Tel.: +358-13-251-5244; fax: +358-13-251- 4444. E-mail address: petri.martikainen@joensuu.fi (P. Martikainen). 0006-3207/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2004.02.009 Biological Conservation 120 (2004) 171–181 www.elsevier.com/locate/biocon BIOLOGICAL CONSERVATION