The data is for the publication:Jia-Yun Zou, Marc W. Cadotte, Claus Bässler, Roland Brandl, Petr Baldrian, Werner Borken, Elisa Stengel, Ya-Huang Luo, Jörg Müller, Sebastian Seibold*. 2023. Wood decomposition is increased by insect diversity, selection effects, and interactions between insects and microbes. Ecology*corresponding author: sebastian.seibold@tu-dresden.de
Bark beetles are currently causing unprecedented damage to European and North American forests.Their population dynamics rarely have been studied in a hypothesis-driven manner incorporating exogenous biotic variables.We propose a conceptual framework to reveal the drivers of bark beetle populations.This approach can be equally applied to other eruptive insect pests. Tree-killing bark beetles are the most economically important insects in conifer forests worldwide. However, despite >200 years of research, the drivers of population eruptions and crashes are still not fully understood and the existing knowledge is thus insufficient to face the challenges posed by the Anthropocene. We critically analyze potential biotic and abiotic drivers of population dynamics of an exemplary species, the European spruce bark beetle (ESBB) (Ips typographus) and present a multivariate approach that integrates the many drivers governing this bark beetle system. We call for hypothesis-driven, large-scale collaborative research efforts to improve our understanding of the population dynamics of this and other bark beetle pests. Our approach can serve as a blueprint for tackling other eruptive forest insects. Tree-killing bark beetles are the most economically important insects in conifer forests worldwide. However, despite >200 years of research, the drivers of population eruptions and crashes are still not fully understood and the existing knowledge is thus insufficient to face the challenges posed by the Anthropocene. We critically analyze potential biotic and abiotic drivers of population dynamics of an exemplary species, the European spruce bark beetle (ESBB) (Ips typographus) and present a multivariate approach that integrates the many drivers governing this bark beetle system. We call for hypothesis-driven, large-scale collaborative research efforts to improve our understanding of the population dynamics of this and other bark beetle pests. Our approach can serve as a blueprint for tackling other eruptive forest insects. The abundance of an organism is determined by a variety of factors related to intra- and interspecific biotic interactions as well as abiotic conditions [1.Thompson J.N. The Coevolutionary Process. University of Chicago Press, 1994Crossref Google Scholar]. In forest ecology, researchers have been fascinated and challenged by the diversity of drivers that govern the eruptive population dynamics of foliage-feeding moths (various Lepidoptera families) and tree-killing bark beetles (see Glossary) (Coleoptera: Scolytinae), including the influence of host trees, symbionts, natural enemies, and competitors (Table 1) as well as climate and land use [2.Weed A.S. et al.Population dynamics of bark beetles.in: Vega F.E. Hofstetter R.W. Bark Beetles: Biology and Ecology of Native and Invasive Species. Academic Press, 2015: 157-176Crossref Scopus (12) Google Scholar, 3.Lindgren B.S. Raffa K.F. Evolution of tree killing in bark beetles (Coleoptera: Curculionidae): trade-offs between the maddening crowds and a sticky situation.Can. Entomol. 2013; 145: 471-495Crossref Scopus (56) Google Scholar, 4.Safranyik L. Carroll A.L. The biology and epidemiology of the mountain pine beetle in lodgepole pine forests.in: Safranyik L. Wilson W.R. The Mountain Pine Beetle: A Synthesis of Biology, Management, and Impacts on Lodgepole Pine. Canadian Forest Service, 2006: 3-66Google Scholar, 5.Berryman A.A. The theory and classification of outbreaks.in: Barbosa P. Schultz J.C. Insect Outbreaks. Academic Press, 1987: 3-30Crossref Google Scholar, 6.Myers J.H. Corry J.S. Population cycles in forest lepidoptera: ecology, evolution and impacts of changing climates.Annu. Rev. Ecol. Evol. Syst. 2013; 44: 565-592Crossref Scopus (65) Google Scholar, 7.Myers J.H. Population cycles: generalities, exceptions and remaining mysteries.Proc. Biol. Sci. 2018; 28520172841Crossref PubMed Scopus (33) Google Scholar]. Given that the joint effect of these biotic and abiotic drivers as well as their interactions are still not well understood for many of these insects (Table 1) and their tree hosts [2.Weed A.S. et al.Population dynamics of bark beetles.in: Vega F.E. Hofstetter R.W. Bark Beetles: Biology and Ecology of Native and Invasive Species. Academic Press, 2015: 157-176Crossref Scopus (12) Google Scholar, 4.Safranyik L. Carroll A.L. The biology and epidemiology of the mountain pine beetle in lodgepole pine forests.in: Safranyik L. Wilson W.R. The Mountain Pine Beetle: A Synthesis of Biology, Management, and Impacts on Lodgepole Pine. Canadian Forest Service, 2006: 3-66Google Scholar, 5.Berryman A.A. The theory and classification of outbreaks.in: Barbosa P. Schultz J.C. Insect Outbreaks. Academic Press, 1987: 3-30Crossref Google Scholar, 7.Myers J.H. Population cycles: generalities, exceptions and remaining mysteries.Proc. Biol. Sci. 2018; 28520172841Crossref PubMed Scopus (33) Google Scholar, 8.Kausrud K. et al.Population dynamics in changing environments: the case of an eruptive forest pest species.Biol. Rev. 2012; 87: 34-51Crossref PubMed Scopus (94) Google Scholar, 9.Raffa K.F. et al.Cross-scale drivers of natural disturbances prone to anthropogenic amplification: the dynamics of bark beetle eruptions.Bioscience. 2008; 58: 501-517Crossref Scopus (1131) Google Scholar], it is questionable whether we are prepared to deal with the challenges our forests face in the Anthropocene (i.e., climatic changes and intensification of forest management) [10.Crutzen P.J. The "Anthropocene".in: Ehlers E. Krafft T. Earth System Science in the Anthropocene. Springer, 2006: 13-18Crossref Scopus (440) Google Scholar].Table 1Exemplary Insect Species That Exhibit Population Outbreaks and Their Biotic Regulators (after 3.Lindgren B.S. Raffa K.F. Evolution of tree killing in bark beetles (Coleoptera: Curculionidae): trade-offs between the maddening crowds and a sticky situation.Can. Entomol. 2013; 145: 471-495Crossref Scopus (56) Google Scholar, 4.Safranyik L. Carroll A.L. The biology and epidemiology of the mountain pine beetle in lodgepole pine forests.in: Safranyik L. Wilson W.R. The Mountain Pine Beetle: A Synthesis of Biology, Management, and Impacts on Lodgepole Pine. Canadian Forest Service, 2006: 3-66Google Scholar, 5.Berryman A.A. The theory and classification of outbreaks.in: Barbosa P. Schultz J.C. Insect Outbreaks. Academic Press, 1987: 3-30Crossref Google Scholar, 6.Myers J.H. Corry J.S. Population cycles in forest lepidoptera: ecology, evolution and impacts of changing climates.Annu. Rev. Ecol. Evol. Syst. 2013; 44: 565-592Crossref Scopus (65) Google Scholar, 7.Myers J.H. Population cycles: generalities, exceptions and remaining mysteries.Proc. Biol. Sci. 2018; 28520172841Crossref PubMed Scopus (33) Google Scholar, 14.Amman G.D. Mountain pine beetle (Coleoptera: Scolytidae) mortality in three types of infestations.Environ. Entomol. 1984; 13: 184-191Crossref Google Scholar, 22.Nealis V. Comparative ecology of conifer-feeding spruce budworms (Lepidoptera: Tortricidae).Can. Entomol. 2016; 148: S33-S57Crossref Scopus (41) Google Scholar, 26.Hofstetter R.W. et al.Symbiotic associations of bark beetles.in: Vega F.E. Hofstetter R.W. Bark Beetles: Biology and Ecology of Native and Invasive Species. Academic Press, 2015: 209-245Crossref Scopus (76) Google Scholar, 34.Wegensteiner R. et al.Natural enemies of bark beetles: predators, parasitoids, pathogens, and nematodes.in: Vega F.E. Hofstetter R.W. Bark Beetles: Biology and Ecology of Native and Invasive Species. Academic Press, 2015: 247-304Crossref Scopus (54) Google Scholar)Main regulatory biotic factoraThe role of other biotic factors and particularly their interaction with the changing abiotic environment [10] are not well understood in any of these systems. acting inInsectTree hostNon-outbreak populationsOutbreak populationsSpruce budworms, Choristoneura spp.ConifersNatural enemies (predators, specialist parasitoids)Food quality, natural enemies (generalist parasitoids)Gypsy moth, Lymantria disparDeciduous treesNatural enemies (predators, parasites)Food depletion, natural enemies (pathogens)European pine sawfly, Neodiprion sertiferPinus spp.Natural enemies (predators)Food depletion, natural enemies (pathogens)European woodwasp, Sirex noctilioConifersHost resistance, natural enemies (parasites)Food depletion, natural enemies (parasites)Australian psyllid, Cardiaspina albitexturaEucalyptus spp.Natural enemies (predators, parasites)Food depletionMountain pine beetle, Dendroctonus ponderosaePinus spp.Host resistance, predators, interspecific competitionHost resistanceSouthern pine beetle, Dendroctonus frontalisPinus spp.Host resistanceHost resistance, natural enemies, mite-associated antagonistic fungiESBB, Ips typographusPicea spp.Host resistance, inter- and intraspecific competitionHost resistance, intraspecific competitiona The role of other biotic factors and particularly their interaction with the changing abiotic environment 10.Crutzen P.J. The "Anthropocene".in: Ehlers E. Krafft T. Earth System Science in the Anthropocene. Springer, 2006: 13-18Crossref Scopus (440) Google Scholar are not well understood in any of these systems. Open table in a new tab Studies on the population dynamics of eruptive forest insects, and particularly bark beetles, currently focus on variables that can be easily measured over large geographic and temporal scales, like insect and tree host abundance, tree host connectivity, and abiotic climatic factors [2.Weed A.S. et al.Population dynamics of bark beetles.in: Vega F.E. Hofstetter R.W. Bark Beetles: Biology and Ecology of Native and Invasive Species. Academic Press, 2015: 157-176Crossref Scopus (12) Google Scholar, 4.Safranyik L. Carroll A.L. The biology and epidemiology of the mountain pine beetle in lodgepole pine forests.in: Safranyik L. Wilson W.R. The Mountain Pine Beetle: A Synthesis of Biology, Management, and Impacts on Lodgepole Pine. Canadian Forest Service, 2006: 3-66Google Scholar, 8.Kausrud K. et al.Population dynamics in changing environments: the case of an eruptive forest pest species.Biol. Rev. 2012; 87: 34-51Crossref PubMed Scopus (94) Google Scholar, 9.Raffa K.F. et al.Cross-scale drivers of natural disturbances prone to anthropogenic amplification: the dynamics of bark beetle eruptions.Bioscience. 2008; 58: 501-517Crossref Scopus (1131) Google Scholar, 11.Marini L. et al.Climate drivers of bark beetle outbreak dynamics in Norway spruce forests.Ecography. 2017; 40: 1426-1435Crossref Scopus (134) Google Scholar]. By contrast, other biotic factors are more difficult to measure at large scales and thus there is a lack of understanding of their roles in regulating insect abundances. Support for their importance comes from small-scale studies on the effects of antagonistic symbionts [12.Hofstetter R.W. et al.Antagonisms, mutualisms and commensalisms affect outbreak dynamics of the southern pine beetle.Oecologia. 2006; 147: 679-691Crossref PubMed Scopus (122) Google Scholar, 13.Myers J. Cory J. Ecology and evolution of pathogens in natural populations of Lepidoptera.Evol. Appl. 2015; 9: 231-247Crossref PubMed Scopus (48) Google Scholar], natural enemies [6.Myers J.H. Corry J.S. Population cycles in forest lepidoptera: ecology, evolution and impacts of changing climates.Annu. Rev. Ecol. Evol. Syst. 2013; 44: 565-592Crossref Scopus (65) Google Scholar, 7.Myers J.H. Population cycles: generalities, exceptions and remaining mysteries.Proc. Biol. Sci. 2018; 28520172841Crossref PubMed Scopus (33) Google Scholar, 14.Amman G.D. Mountain pine beetle (Coleoptera: Scolytidae) mortality in three types of infestations.Environ. Entomol. 1984; 13: 184-191Crossref Google Scholar, 15.Berryman A.A. What causes population cycles of forest Lepidoptera?.Trends Ecol. Evol. 1996; 11: 28-32Abstract Full Text PDF PubMed Scopus (200) Google Scholar, 16.Linit M. Stephen F. Parasite and predator component of within-tree southern pine beetle (Coleoptera: Scolytidae) mortality.Can. Entomol. 1983; 115: 679-688Crossref Scopus (42) Google Scholar, 17.Moore G.E. Southern pine beetle mortality in North Carolina caused by parasites and predators.Environ. Entomol. 1972; 1: 58-65Crossref Google Scholar], and insect genotype [18.Wallin K.F. Raffa K.F. Feedback between individual host selection behavior and population dynamics in an eruptive herbivore.Ecol. Monogr. 2004; 74: 101-116Crossref Scopus (105) Google Scholar, 19.Salle A. Raffa K.F. Interactions among intraspecific competition, emergence patterns, and host selection behaviour in Ips pini (Coleoptera: Scolytinae).Ecol. Entomol. 2007; 32: 162-171Crossref Scopus (23) Google Scholar] on the population dynamics of bark beetles, moths, and other eruptive forest insects (Table 1). Moreover, studies usually focus on examining the factors driving outbreaks but largely neglect the equally important causes of population collapse. For example, in cases with abundant but healthy host trees, collapse is often attributed to the absence of factors known to facilitate outbreaks (e.g., poor tree health [20.Marini L. et al.Population dynamics of the spruce bark beetle: a long-term study.Oikos. 2013; 122: 1768-1776Crossref Scopus (65) Google Scholar, 21.Stadelmann G. et al.Effects of salvage logging and sanitation felling on bark beetle (Ips typographus L.) infestations.For. Ecol. Manag. 2013; 305: 273-281Crossref Scopus (67) Google Scholar]). This is an oversimplification, however, because factors regulating non-outbreak populations are typically different from the ones regulating outbreak populations (Table 1; [4.Safranyik L. Carroll A.L. The biology and epidemiology of the mountain pine beetle in lodgepole pine forests.in: Safranyik L. Wilson W.R. The Mountain Pine Beetle: A Synthesis of Biology, Management, and Impacts on Lodgepole Pine. Canadian Forest Service, 2006: 3-66Google Scholar, 5.Berryman A.A. The theory and classification of outbreaks.in: Barbosa P. Schultz J.C. Insect Outbreaks. Academic Press, 1987: 3-30Crossref Google Scholar, 7.Myers J.H. Population cycles: generalities, exceptions and remaining mysteries.Proc. Biol. Sci. 2018; 28520172841Crossref PubMed Scopus (33) Google Scholar, 8.Kausrud K. et al.Population dynamics in changing environments: the case of an eruptive forest pest species.Biol. Rev. 2012; 87: 34-51Crossref PubMed Scopus (94) Google Scholar, 9.Raffa K.F. et al.Cross-scale drivers of natural disturbances prone to anthropogenic amplification: the dynamics of bark beetle eruptions.Bioscience. 2008; 58: 501-517Crossref Scopus (1131) Google Scholar, 14.Amman G.D. Mountain pine beetle (Coleoptera: Scolytidae) mortality in three types of infestations.Environ. Entomol. 1984; 13: 184-191Crossref Google Scholar, 22.Nealis V. Comparative ecology of conifer-feeding spruce budworms (Lepidoptera: Tortricidae).Can. Entomol. 2016; 148: S33-S57Crossref Scopus (41) Google Scholar, 23.Boone C.K. et al.Efficacy of tree defense physiology varies with bark beetle population density: a basis for positive feedback in eruptive species.Can. J. For. Res. 2011; 41: 1174-1188Crossref Scopus (211) Google Scholar]). Clearly, there is a severe lack of knowledge on the role of most biotic factors for the population dynamics of forest insects. Here, we systematically review these knowledge gaps for bark beetles, using the ESBB, Ips typographus (L.), as a model (Box 1). The importance of addressing these knowledge gaps is illustrated in a study by Marini et al. [11.Marini L. et al.Climate drivers of bark beetle outbreak dynamics in Norway spruce forests.Ecography. 2017; 40: 1426-1435Crossref Scopus (134) Google Scholar], who examined 17 ESBB populations over 30 years. They found that while the abundance of storm-felled trees and climate (Figure 1, I, II) were major determinants of local outbreaks in ESBBs, 65% of the variation in beetle population size remained unexplained. This unexplained variation might in part be due to variation in forest management between the different sites considered in the model [11.Marini L. et al.Climate drivers of bark beetle outbreak dynamics in Norway spruce forests.Ecography. 2017; 40: 1426-1435Crossref Scopus (134) Google Scholar]. However, some models on Dendroctonus bark beetles and forest moth species suggest that including biotic variables (i.e., competition, natural enemies, or phenotype) can reduce the unexplained to sometimes less than 30% [15.Berryman A.A. What causes population cycles of forest Lepidoptera?.Trends Ecol. Evol. 1996; 11: 28-32Abstract Full Text PDF PubMed Scopus (200) Google Scholar, 24.Friedenberg N.A. et al.Temperature extremes, density dependence, and southern pine beetle (Coleoptera: Curculionidae) population dynamics in east Texas.Environ. Entomol. 2008; 37: 650-659Crossref PubMed Google Scholar, 25.Candau J-N. Fleming R.A. Landscape-scale spatial distribution of spruce budworm defoliation in relation to bioclimatic conditions.Can. J. For. Res. 2005; 35: 2218-2232Crossref Scopus (60) Google Scholar]. Although biotic variables such as intraspecific and interspecific competition, natural enemies, pathogens, symbionts, host tree resistance, and frequency of insect phenotypes and/or genotypes (Figure 1, IV) are rarely recorded and included in population dynamics models (in particular in ESBB; Box 1), it is clear that they can be invariably applied to all eruptive insects. Biotic variables should vary only in their relative effects (and probably their interactive effects) on the focal insect species [4.Safranyik L. Carroll A.L. The biology and epidemiology of the mountain pine beetle in lodgepole pine forests.in: Safranyik L. Wilson W.R. The Mountain Pine Beetle: A Synthesis of Biology, Management, and Impacts on Lodgepole Pine. Canadian Forest Service, 2006: 3-66Google Scholar, 5.Berryman A.A. The theory and classification of outbreaks.in: Barbosa P. Schultz J.C. Insect Outbreaks. Academic Press, 1987: 3-30Crossref Google Scholar, 6.Myers J.H. Corry J.S. Population cycles in forest lepidoptera: ecology, evolution and impacts of changing climates.Annu. Rev. Ecol. Evol. Syst. 2013; 44: 565-592Crossref Scopus (65) Google Scholar, 10.Crutzen P.J. The "Anthropocene".in: Ehlers E. Krafft T. Earth System Science in the Anthropocene. Springer, 2006: 13-18Crossref Scopus (440) Google Scholar, 22.Nealis V. Comparative ecology of conifer-feeding spruce budworms (Lepidoptera: Tortricidae).Can. Entomol. 2016; 148: S33-S57Crossref Scopus (41) Google Scholar, 26.Hofstetter R.W. et al.Symbiotic associations of bark beetles.in: Vega F.E. Hofstetter R.W. Bark Beetles: Biology and Ecology of Native and Invasive Species. Academic Press, 2015: 209-245Crossref Scopus (76) Google Scholar]. Host tree resistance, for example, has little influence on the population dynamics of forest moth species, whereas it strongly affects tree-killing bark beetles [2.Weed A.S. et al.Population dynamics of bark beetles.in: Vega F.E. Hofstetter R.W. Bark Beetles: Biology and Ecology of Native and Invasive Species. Academic Press, 2015: 157-176Crossref Scopus (12) Google Scholar, 4.Safranyik L. Carroll A.L. The biology and epidemiology of the mountain pine beetle in lodgepole pine forests.in: Safranyik L. Wilson W.R. The Mountain Pine Beetle: A Synthesis of Biology, Management, and Impacts on Lodgepole Pine. Canadian Forest Service, 2006: 3-66Google Scholar, 7.Myers J.H. Population cycles: generalities, exceptions and remaining mysteries.Proc. Biol. Sci. 2018; 28520172841Crossref PubMed Scopus (33) Google Scholar, 9.Raffa K.F. et al.Cross-scale drivers of natural disturbances prone to anthropogenic amplification: the dynamics of bark beetle eruptions.Bioscience. 2008; 58: 501-517Crossref Scopus (1131) Google Scholar].Box 1The ESBB–Norway Spruce SystemThe ESBB, Ips typographus (L.), is a 5-mm beetle endemic to spruce forests across Eurasia (Figure I). The ESBB is the economically most important insect in Palearctic spruce forests and at the same time a keystone species from an ecological point of view [30.Hlásny T. et al.Living with Bark Beetles: Impacts, Outlook and Management Options. From Science to Policy 8. European Forest Institute, 2019Google Scholar]. The beetle is associated with a diverse and dynamic community of bacterial and fungal symbionts suggested to contribute to the exhaustion of tree defenses [43.Zhao T. et al.Fungal associates of the tree-killing bark beetle, Ips typographus, vary in virulence, ability to degrade conifer phenolics and influence bark beetle tunneling behavior.Fungal Ecol. 2019; 38: 71-79Crossref Scopus (26) Google Scholar, 53.Lieutier F. et al.Stimulation of tree defenses by ophiostomatoid fungi can explain attack success of bark beetles on conifers.Ann. For. Sci. 2009; 66: 801Crossref Scopus (108) Google Scholar], the detoxification of tree defenses [36.Wadke N. et al.The bark-beetle-associated fungus, Endoconidiophora polonica, utilizes the phenolic defense compounds of its host as a carbon source.Plant Physiol. 2016; 171: 914-931PubMed Google Scholar, 37.Lah L. et al.A genomic comparison of putative pathogenicity-related gene families in five members of the Ophiostomatales with different lifestyles.Fungal Biol. 2017; 121: 234-252Crossref PubMed Scopus (6) Google Scholar, 54.Boone C.K. et al.Bacteria associated with a tree-killing insect reduce concentrations of plant defense compounds.J. Chem. Ecol. 2013; 39: 1003-1006Crossref PubMed Scopus (123) Google Scholar], and nutrient provisioning [40.Six D.L. The bark beetle holobiont: why microbes matter.J. Chem. Ecol. 2013; 39: 989-1002Crossref PubMed Scopus (71) Google Scholar]. Intraspecific competition is probably one of the major drivers of ESBB population dynamics [42.Toffin E. et al.Colonization of weakened trees by mass-attacking bark beetles: no penalty for pioneers, scattered initial distributions and final regular patterns.R. Soc. Open Sci. 2018; 5170454Crossref PubMed Scopus (9) Google Scholar]. However, such competition is relaxed in the population build-up phase due to large numbers of dead or weakened trees. Interspecific competition with other bark beetle species and wood borers is little studied in ESBBs (but see [27.Thalenhorst W. Grundzüge der Populationsdynamik des grossen Fichtenborkenkäfers Ips typographus L.Schr. Forstl. Fak. Univ. Gott. 1958; 21: 1-126Google Scholar, 55.Byers J. Avoidance of competition by spruce bark beetles, Ips typographus and Pityogenes chalcographus.Cell. Mol. Life Sci. 1993; 49: 272-275Crossref Scopus (41) Google Scholar]) but is known to have substantial impact in other bark beetle species [3.Lindgren B.S. Raffa K.F. Evolution of tree killing in bark beetles (Coleoptera: Curculionidae): trade-offs between the maddening crowds and a sticky situation.Can. Entomol. 2013; 145: 471-495Crossref Scopus (56) Google Scholar]. It is unknown to what degree natural enemies (e.g., predatory beetles, parasitoids, woodpeckers, nematodes) and pathogens (e.g., entomopathogenic fungi, viruses) affect ESBB populations [34.Wegensteiner R. et al.Natural enemies of bark beetles: predators, parasitoids, pathogens, and nematodes.in: Vega F.E. Hofstetter R.W. Bark Beetles: Biology and Ecology of Native and Invasive Species. Academic Press, 2015: 247-304Crossref Scopus (54) Google Scholar] because existing studies are contradictory. Phoretic mites, some of which feed on and transmit fungal spores, seem particularly important [26.Hofstetter R.W. et al.Symbiotic associations of bark beetles.in: Vega F.E. Hofstetter R.W. Bark Beetles: Biology and Ecology of Native and Invasive Species. Academic Press, 2015: 209-245Crossref Scopus (76) Google Scholar, 38.Linnakoski R. et al.Seasonal succession of fungi associated with Ips typographus beetles and their phoretic mites in an outbreak region of Finland.PLoS One. 2016; 11e0155622Crossref PubMed Scopus (26) Google Scholar], but interactions between beetles, mites, and fungi are unstudied.The usual hosts of ESBBs are windthrown or standing but weakened spruce trees (primarily Norway spruce, Picea abies). Trees defend themselves with anatomical (e.g., stone cells) and chemical (e.g., terpenoid oleoresins) defenses [56.Krokene P. Conifer defense and resistance to bark beetles.in: Vega F.E. Hofstetter R.W. Bark Beetles: Biology and Ecology of Native and Invasive Species. Academic Press, 2015: 177-207Crossref Scopus (77) Google Scholar] and healthy trees with vigorous defenses can be overwhelmed only by pheromone-coordinated mass attack during population outbreaks [27.Thalenhorst W. Grundzüge der Populationsdynamik des grossen Fichtenborkenkäfers Ips typographus L.Schr. Forstl. Fak. Univ. Gott. 1958; 21: 1-126Google Scholar, 57.Wermelinger B. Ecology and management of the spruce bark beetle Ips typographus – a review of recent research.For. Ecol. Manag. 2004; 202: 67-82Crossref Scopus (507) Google Scholar]. There is evidence, however, that high intraspecific competition in healthy trees often results in low reproduction and thus can dampen population growth [27.Thalenhorst W. Grundzüge der Populationsdynamik des grossen Fichtenborkenkäfers Ips typographus L.Schr. Forstl. Fak. Univ. Gott. 1958; 21: 1-126Google Scholar, 28.Anderbrant O. et al.Intraspecific competition affecting parents and offspring in the bark beetle Ips typographus.Oikos. 1985; 45: 89-98Crossref Scopus (157) Google Scholar, 29.Botterweg P. The effect of attack density on size, fat content and emergence of the spruce bark beetle Ips typographus L.1.J. Appl. Entomol. 1983; 96: 47-55Google Scholar, 57.Wermelinger B. Ecology and management of the spruce bark beetle Ips typographus – a review of recent research.For. Ecol. Manag. 2004; 202: 67-82Crossref Scopus (507) Google Scholar].Intensification of forest management in Europe has resulted in unnaturally high densities of spruce. ESBB populations build up more frequently and more severely in these homogeneous spruce stands, especially if trees are weakened by climate change or other stressors. Higher temperatures and severe drought, for example, can reduce the efficacy of tree defenses and thus allow beetles to overwhelm trees at lower attack densities [27.Thalenhorst W. Grundzüge der Populationsdynamik des grossen Fichtenborkenkäfers Ips typographus L.Schr. Forstl. Fak. Univ. Gott. 1958; 21: 1-126Google Scholar, 57.Wermelinger B. Ecology and management of the spruce bark beetle Ips typographus – a review of recent research.For. Ecol. Manag. 2004; 202: 67-82Crossref Scopus (507) Google Scholar]. Non-outbreak bark beetle populations are therefore expected, and already observed, to undergo more frequent and severe population build-ups and outbreaks in the Anthropocene (Figure II) [30.Hlásny T. et al.Living with Bark Beetles: Impacts, Outlook and Management Options. From Science to Policy 8. European Forest Institute, 2019Google Scholar]. ESBB outbreaks in Central Europe alone caused annual losses of 14.5 million m3 of wood between 2002 and 2010 and currently windstorms in combination with the 2018 summer drought result in beetle damage that is unprecedented (40 million m3 in Europe, 18 million m3 in the Czech Republic alone) [30.Hlásny T. et al.Living with Bark Beetles: Impacts, Outlook and Management Options. From Science to Policy 8. European Forest Institute, 2019Google Scholar]. These outbreaks have strong negative consequences on ecosystem services like provisioning of clean water and timber, and the regulation of climate and carbon storage, but paradoxically they typically facilitate local biodiversity [31.Thom D. Seidl R. Natural disturbance impacts on ecosystem services and biodiversity in temperate and boreal forests.Biol. Rev. 2016; 91: 760-781Crossref PubMed Scopus (302) Google Scholar].Figure IIResponses of Bark Beetle Populations in the Anthropocene.Show full captionNon-outbreak (A), build-up, and outbreak (B) bark beetle population dynamics.View Large Image Figure ViewerDownload Hi-res image Download (PPT)Figure 1Overview of Variables Affecting Eruptive Forest Insects and Their Known or Unknown Effects in the European Spruce Bark Beetle (ESBB), (Ips typographus) System.Show full captionBoxes 3–11 represent measurable variables and arrows stand for single hypotheses describing the direct effect of one variable on another, which should be accounted for and tested in observational and experimental studies. (I) Major climatic variables affected by climate change at a macro- and regional scale. (II) Most important variables relating to properties of individual host trees and trees at a landscape scale. (III) Main population phases (non-outbreak, build-up, outbreak, collapse) of an eruptive insect species. (IV) Major biotic variables associated with an eruptive insect species plus intraspecific effects (phenotype, genotype, and intraspecific competition). Arrows are exemplary for the ESBB system. An arrow from one of the boxes in group I, II, or IV to one of the boxes in group III would indicate a direct effect on the population phases of the beetle. Arrows connecting multiple boxes and eventually pointing to one of the four population phases would indicate an indirect effect. The gray arrows represent hypotheses that have yet to be tested and thus mirror gaps in our knowledge of the ESBB. The absence of an arrow between boxes implies that there is probably no effect of one variable on another in the ESBB.View Large Image Figure ViewerDownload Hi-res
Abstract Dead wood is a habitat for numerous fungal species, many of which are important agents of decomposition. Previous studies suggested that wood‐inhabiting fungal communities are affected by climate, availability of dead wood in the surrounding landscape and characteristics of the colonized dead‐wood object (e.g. host tree species). These findings indicate that different filters structure fungal communities at different scales, but how these factors individually drive fungal fruiting diversity on dead‐wood objects is unknown. We conducted an orthogonal experiment comprising 180 plots (0.1 ha) in a random block design and measured fungal fruit body richness and community composition on 720 dead‐wood objects over the first 4 years of succession. The experiment allowed us to disentangle the effects of the host (beech and fir; logs and branches) and the environment (microclimate: sunny and shady plots; local dead wood: amount and heterogeneity of dead wood added to plot). Variance partitioning revealed that the host was more important than the environment for the diversity of wood‐inhabiting fungi. A more detailed model revealed that host tree species had the highest independent effect on richness and community composition of fruiting species of fungi. Host size had significant but low independent effects on richness and community composition of fruiting species. Canopy openness significantly affected the community composition of fruiting species. By contrast, neither local amount nor heterogeneity of dead wood significantly affected the fungal diversity measures. Synthesis . Our study identified host tree species as a more important driver of the diversity of wood‐inhabiting fungi than the environment, which suggests a host‐centred filter of this diversity in the early phase of the decomposition process. For the conservation of wood‐inhabiting fungi, a high variety of host species in various microclimates is more important than the availability of dead wood at the stand level.
Abstract Understanding species richness variation among local communities is one of the central topics in ecology, but the complex interplay of regional processes, environmental filtering, and local processes hampers generalization on the importance of different processes. Here, we aim to unravel drivers of spider community assembly in temperate forests by analyzing two independent data sets covering gradients in elevation and forest succession. We test the following four hypotheses: (H1) spider assemblages within a region are limited by dispersal, (H2) local environment has a dominant influence on species composition and (H3) resources, and (H4) biotic interactions both affect species richness patterns. In a comprehensive approach, we studied species richness, abundance, taxonomic composition, and trait‐phylogenetic dissimilarity of assemblages. The decrease in taxonomic similarity with increasing spatial distance was very weak, failing to support H1. Functional clustering of species in general and with canopy openness strongly supported H2. Moreover, this hypothesis was supported by a positive correlation between environmental and taxonomic similarity and by an increase in abundance with canopy openness. Resource determination of species richness (H3) could be confirmed only by the decrease of species richness with canopy cover. Finally, decreasing species richness with functional clustering indicating effects of biotic interactions (H4) could only be found in one analysis and only in one data set. In conclusion, our findings indicate that spider assemblages within a region are mainly determined by local environmental conditions, while resource availability, biotic interactions and dispersal play a minor role. Our approach shows that both the analysis of different aspects of species diversity and replication of community studies are necessary to identify the complex interplay of processes forming local assemblages.
Abstract The Earth's ecosystems are affected by a complex interplay of biotic and abiotic factors. While global temperatures increase, associated changes in the fruiting behaviour of fungi remain unknown. Here, we analyse 6.1 million fungal fruit body (mushroom) records and show that the major terrestrial biomes exhibit similarities and differences in fruiting events. We observed one main fruiting peak in most years across all biomes. However, in boreal and temperate biomes, there was a substantial number of years with a second peak, indicating spring and autumn fruiting. Distinct fruiting peaks are spatially synchronized in boreal and temperate biomes, but less defined and longer in the humid tropics. The timing and duration of fungal fruiting were significantly related to temperature mean and variability. Temperature‐dependent aboveground fungal fruiting behaviour, which is arguably also representative of belowground processes, suggests that the observed biome‐specific differences in fungal phenology will change in space and time when global temperatures continue to increase.
Abstract Organisms have evolved a fascinating variety of strategies and organs for successful reproduction. Fruit bodies are the reproductive organ of fungi and vary considerably in size and shape among species. Our understanding of the mechanisms underlying the differences in fruit body size among species is still limited. Fruit bodies of saprotrophic fungi are smaller than those of mutualistic ectomycorrhizal fungi. If differences in fruit body size are determined by carbon acquisition, then mean reproductive traits of saprotrophic and ectomycorrhizal fungi assemblages should vary differently along gradients of resource availability as carbon acquisition seems more unpredictable and costly for saprotrophs than for ectomycorrhizal fungi. Here, we used 48 local inventories of fungal fruit bodies (plot size: 0.02 ha each) sampled along a gradient of resource availability (growing stock) across 3 years in the Bavarian Forest National Park in Germany to investigate regional and local factors that might influence the distribution of species with different reproductive traits, particularly fruit body size. As predicted, mean fruit body size of local assemblages of saprotrophic fungi was smaller than expected from the distribution of traits of the regional species pool across central and northern Europe, whereas that of ectomycorrhizal fungi did not differ from random expectation. Furthermore and also as expected, mean fruit body size of assemblages of saprotrophic fungi was significantly smaller than for assemblages of ectomycorrhizal species. However, mean fruit body sizes of not only saprotrophic species but also ectomycorrhizal species increased with resource availability, and the mean number of fruit bodies of both assemblages decreased. Our results indicate that the differences in carbon acquisition between saprotrophs and ectomycorrhizal species lead to differences in basic reproductive strategies, with implications for the breadth of their distribution. However, the differences in resource acquisition cannot explain detailed species distribution patterns at a finer, local scale based on their reproductive traits.
