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A new study from the University of Würzburg shows that a varied forest structure influences spider diversity. Creating forest clearings can, in some cases, significantly reduce this diversity.
In ecology, the principle holds that the more diverse and heterogeneous a habitat is, the more different species it supports. To promote species diversity in forests, clearings are therefore created for nature conservation purposes, or deadwood is deliberately left in place. For many species, such as birds, bats, or beetles, this structural diversity is indeed beneficial.
However, a new study by scientists at Julius-Maximilians-Universität Würzburg (JMU) reveals what appears to be a striking exception: For spiders, the supposedly monotonous, closed forest is the true refuge.
This finding is ecologically significant. As important predators, spiders regulate insect populations and make a substantial contribution to natural pest control. The fact that they are the only animal group studied so far to react negatively to forest changes shows that a complete picture requires looking beyond species boundaries.
The study was led by a team headed by Julia Rothacher and Jean-Léonard Stör, the first authors of the publication, who conducted the research as part of the nationwide BETA-FOR research project led by JMU Professor Jörg Müller. Researchers from Canada, Japan, and Taiwan also participated in the study, which has now been published in the Journal of Animal Ecology.
18,500 Spiders Under the Microscope
For its study, the research team examined eleven sites across Germany. The Würzburg University Forest served as a central pillar of the investigation, supplemented by areas in the Bavarian Forest and Hunsrück-Hochwald National Parks, in Saarland, and near Lübeck. The researchers had previously created distinct differences in the forest stands under study: While one half of the forest was allowed to remain as densely forested and dark as it originally was, they created partial gaps in the canopy and introduced various deadwood structures in the other half.
The key finding: “Surprisingly, it turned out that spiders – unlike many other species groups in native forests – do not benefit from this structural diversity,” explains Jean-Léonard Stör, who identified the animals as part of his master’s thesis. “In areas where gaps were deliberately created and deadwood was left in place, the number of spider species decreased by an average of five species per study plot,” adds doctoral candidate Julia Rothacher.
Specifically, this means:
Individuals: Scientists collected 18,540 adult spiders using 1,404 pitfall traps which were set up at the eleven sites.
Species richness: 206 species were recorded, representing about 20 percent of Germany’s spider fauna. These included regionally rare species such as the jumping spider Evarcha laetabunda in the Bavarian Forest.
The Forest: An Ecological Skyscraper
Although the structural diversity enhancing measures taken by the BETA-FOR research team led to significant differences in the composition of the spider community between the individual study areas, this was not sufficient to compensate for the losses at the local level, which overall led to lower spider diversity at the landscape level.
The result – which may seem surprising at first glance – can be explained quite simply: “A closed forest with an intact canopy is like an ecological skyscraper,” says Stör, describing the effect. “When gaps are created in the forest, species such as the flower crab spider (Misumena vatia) – which relies on open structures at ground level – do benefit; however, the top floor of this diverse, three-dimensional hunting ground is literally torn down.”
The researchers’ data confirms this: highly mobile wolf spiders quickly migrate into the new clearings. However, these are common generalists. The unfortunate losers are sedentary, sometimes highly specialized tree and trunk dwellers such as the Angular orbweaver (Araneus angulatus) or the shiny sac spider (Clubiona caerulescens). They are locally filtered out by the forest opening, and the entire spider community thus loses diversity.
Consequences for the Forest of the Future
The findings of the JMU researchers show that a one-sided focus on maximizing forest gaps can significantly alter the arachnid community. At the same time, however, Central European forests are characterized over large areas by closed canopy cover, which means that light- and gap-dependent species of many other organism groups are rare and endangered in native forests.
Taken together, this means for nature conservation that there can be no one-size-fits-all solutions; rather, the different needs of all species groups must be specifically taken into account in order to preserve biodiversity in forests.
Julia Rothacher, University of Würzburg, Phone: +49 931 31 84210, julia.rothacher@uni-wuerzburg.de
Jean-Léonard Stör, University of Würzburg (Tierökologie), Phone: +49 931 31-82495, jean-leonard.stoer@uni-wuerzburg.de
Temperate forest heterogeneity decreases local and landscape-scale spider diversity through habitat filtering despite increasing species turnover. Jean-Léonard Stör, Julia Rothacher, Marc Cadotte, Anne Chao, Maike Huszarik, Michael Junginger, Lisa Köstler-Albert, Oliver Mitesser, Akira S. Mori, Clara Wild, Jörg Müller. Journal of Animal Ecology, https://doi.org/10.1111/1365-2656.70297
Ambush hunters, such as the flower crab spider (Misumena vatia), rely on flowers for camouflage
Quelle: Louis Puille
Copyright: University of Würzburg
Web-building spiders, such as the Angular orbweaver (Araneus angulatus), need branches and twigs to ...
Quelle: Louis Puille
Copyright: University of Würzburg
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Ambush hunters, such as the flower crab spider (Misumena vatia), rely on flowers for camouflage
Quelle: Louis Puille
Copyright: University of Würzburg
Web-building spiders, such as the Angular orbweaver (Araneus angulatus), need branches and twigs to ...
Quelle: Louis Puille
Copyright: University of Würzburg
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