Chapter 1 An Introduction to the Diversity, Ecology, and Conservation of Saproxylic Insects Michael D. Ulyshen and Jan Šobotník Thousands upon thousands of lives would cease and their races become extinct were it not for the occasional death of such a jungle giant as this. Beebe (1925) Abstract Much like flowering plants set the stage for an explosion of herbivore and pollinator diversity, the origin of dead wood in early Devonian forests (~400 mya) was followed by an incredible diversification of life, giving rise to some of the most successful morphological adaptations and symbioses on Earth. Approximately one third of all forest insect species worldwide depend directly or indirectly on dying or dead wood (i.e., saproxylic), with major functional groups including wood feeders, fungus feeders, saprophages, and predators. Although beetles and flies dominate saproxylic insect communities worldwide, other orders are represented by a wide variety of species as well, and the composition of these assemblages varies biogeographically. Most notably, termites (Blattodea) and the subsocial beetle family Passalidae are both largely restricted to the tropics where they play a major role in the decomposition process. The large body of European research linking declines of saproxylic insect diversity to reductions in the amount of dead wood and old trees across the landscape serves as a cautionary tale for researchers and land managers working in other parts of the world. The conservation of saproxylic insects everywhere can be promoted by efforts to provide an adequate amount and variety of dead wood and old trees across space and time. The preservation of old-growth forests is also critically important as they support relict populations of the most sensitive species. There is a strong need for research outside the boreal and temperate M. D. Ulyshen (*) USDA Forest Service, Southern Research Station, Athens, GA, USA e-mail: [email protected] J. Šobotník Faculty of Forestry and Wood Sciences, Czech University of Life Sciences, Prague, Czech Republic This is a U.S. government work and its text is not subject to copyright protection in the 1 United States; however, its text may be subject to foreign copyright protection 2018 M. D. Ulyshen (ed.), Saproxylic Insects, Zoological Monographs 1, https://doi.org/10.1007/978-3-319-75937-1_1 2 M. D. Ulyshen and J. Šobotník zones to develop a more global appreciation for the diversity and ecology of saproxylic insects and to inform management strategies for conserving these organ- isms in subtropical and tropical forests. Although William Beebe made his observations of “the little people of bark and wood” in a South American rainforest, he could have reached this same conclusion from studying a recently fallen tree in any of the world’s forests. Indeed, approxi- mately one third of all forest insect species are saproxylic, meaning they depend, either directly or indirectly, on dying or dead wood. Directly dependent species are those that consume parts of woody stems (i.e., bark, phloem, or wood) for food, whereas indirectly dependent species include those that feed on other wood- dependent species (e.g., wood-rotting fungi or other saproxylic organisms), require dead wood for nesting purposes, etc. In addition to the strictly dependent saproxylic species, a very large number of other invertebrates are known to benefit from the presence of dead wood in forests but do not require it. Such taxa include a wide variety of litter-dwelling invertebrates that concentrate near logs, hibernating insects that overwinter within the shelter provided by dead wood, soil feeders that encounter richer food sources underneath fallen wood, and predators that respond opportunis- tically to an abundance of prey in and around woody debris. While dying and dead wood represent some of the most critical resources to life in forests worldwide, only in the past few decades have researchers turned their attention to the importance of dead wood to biodiversity, with most work taking place in boreal or temperate forests. Moreover, although saproxylic insects are thought to provide some key ecosystem services, research on the value of these organisms to the resiliency and productivity of forests is only just beginning. Early research on saproxylic insects focused primarily on the small fraction of saproxylic insects that pose a threat to forest health and reduce the marketability of timber products or are pests of wooden structures. In temperate zones, wood-boring beetles have received the most attention (Hickin 1963), whereas termites have long been the focus of study in the tropics. This difference in focus among regions reflects the uneven distribution of social insects, especially termites and ants, which gener- ally become more abundant and diverse toward equator (King et al. 2013). Whereas termites are absent from boreal forests and are represented by just a handful of species in temperate forests, the number of species present at tropical forest locations ranges from several dozen to well over a hundred (Krishna et al. 2013). As a consequence of these patterns, the literature on the diversity and ecology of saproxylic insects is largely fragmented along taxonomic and geographic lines. Efforts to unite these bodies of knowledge are needed to achieve a more holistic understanding of saproxylic insects including how their diversity and functions may vary geographically. This book represents an effort by researchers around the world to summarize the current state of knowledge about the diversity, ecology, and conservation of insect 1 An Introduction to the Diversity, Ecology, and Conservation of... 3 life in dying and dead wood. Because a comprehensive coverage of this vast topic is not possible within the covers of a single book, our main aims here include (1) uniting bodies of literature on social and nonsocial saproxylic insects that have, until now, existed in relative isolation from one another; (2) addressing a number of neglected topics including some less-studied orders or families of saproxylic insects, insects associated with highly decomposed woody substrates, and the diversity and ecology of wood-dwelling insect assemblages in tropical forests; and (3) providing the most up-to-date coverage of topics of particular ecological importance or interest to those making forest management decisions. Before further introducing the chapters that follow, it is worth taking a moment to consider the history of insect life in dead wood. Below we briefly discuss the origins and properties of woody plants, the origins and diversification of wood-dwelling insects, and a timeline of research on saproxylic insects. 1.1 Origins and Properties of Wood 1.1.1 Origins Fossils of a small shrub-like plant similar to the genus Psilophyton from the early Devonian (~407 mya) represent the earliest known records of wood (Gerrienne et al. 2011). Although the evolution of wood is thought to have initially been driven by hydraulic constraints (Gerrienne et al. 2011), wood also proved to be an effective solution to the challenge of maintaining an upright growth form arising from competition for sunlight among plants (Cichan and Taylor 1990). The earliest known arborescent plants appeared in the middle Devonian, approximately 380 mil- lion years ago (Willis and McElwain 2002). By the late Devonian and Carboniferous, much of the Earth’s land surface was covered in dense forests. These early forests consisted of a mixture of spore-producing and early seed-producing tree species (Stein et al. 2007;WillisandMcElwain2002). The spore producers included lycopsids (lycopods), sphenopsids (giant horsetails), filicopsids (ferns), cladoxylopsids, and progymnosperms (extinct), whereas the early seed producers included pteridosperms (seed ferns, extinct) and Cordaites (extinct). Pteridosperms would later give rise to the gymnosperms (cycads, ginkgos, and conifers) and angiosperms (flowering plants) that dominate modern forests (Frohlich and Chase 2007). Vascular cambium arose multiple times in the evolution of plants (Cichan and Taylor 1990), and the tree lineages listed above differed in how wood was arranged within their stems. Many of these early trees possessed unifacial vascular cambium and only produced small amounts of secondary xylem. One highly successful but now-extinct genus of trees from the period, for instance, was Lepidodendron,a lycopod that grew up to 35 m tall and over a meter in trunk diameter (Scheckler 2001; Willis and McElwain 2002). Because the stems of Lepidodendron possessed unifacial cambium and produced little wood, most of their support came from an extremely thickened cylinder of bark. In contrast, other early trees, like most forest 4 M. D. Ulyshen and J. Šobotník Fig. 1.1 Timeline showing major events in the history of saproxylic life (many of the dates given here represent median values from ranges reported in the literature). Superscripts refer to the following references: 1: Misof et al. (2014), 2: Gerrienne et al. (2011), 3: Willis and McElwain (2002), 4: Cichan and Taylor (1982), 5: Floudas et al. (2012), 6, 8, 11, 13, 15: Grimaldi and Engel (2005), 7: Bertone and Wiegmann (2009), 9: Wang et al. (2014), 10: Bell et al. (2010), 12: Scholtz and Chown (1995), 14: Hulcr et al. (2015) trees living today, possessed bifacial cambium which produces secondary phloem in addition to secondary xylem. Although easily taken for granted in modern forests, the appearance of trees possessing bifacial vascular cambium was a major step in the evolution