Fallows benefit beetle conservation in a traditionally managed grassland landscape
Introduction
Human activity has caused severe losses of biodiversity at regional and global scales (Ceballos et al., 2015, Dirzo et al., 2014). Declines have been documented in various taxa, but the massive loss of insects has received widespread attention in recent years only (Wagner et al., 2021). Long-term studies indicated declines of 76% in the biomass of flying insects over 27 years in nature reservations of western Germany (Hallmann et al., 2017), or of 67% in the biomass of grassland arthropods over 9 years in three grassland regions across Germany (Seibold et al., 2019). Global losses of insect abundance and biomass have been estimated at 9% per decade (van Klink et al., 2020). Insect declines are particularly pronounced in agricultural landscapes, for which there is strong empirical evidence (e.g. Hallmann et al., 2020; van Strien et al., 2019; Vogel, 2017). However, not all insect taxa are equally affected (e.g. Brooks et al., 2012; Crossley et al., 2020; Powney et al., 2019), and the specific drivers of decline are not yet clear (Wagner et al., 2021). Nevertheless, there is a widely accepted consensus that the main drivers of biodiversity loss are habitat degradation including agricultural intensification, pollution, and anthropogenic climate change (Gossner et al., 2016, Habel et al., 2019a; Raven et al., 2021; Sánchez-Bayo and Wyckhuys, 2019).
In landscapes dominated by semi-natural grassland, which often harbour species-rich insect communities, agricultural intensification is the most important driver of biodiversity decline (Habel et al., 2019b, Humbert et al., 2021, Johansen et al., 2019). The conversion of traditionally managed hay meadows into highly productive silage grassland has strong detrimental effects on the species richness of plants, insects, and birds (Manning et al., 2015, Woodcock et al., 2021). More specifically, increases in mowing frequency, high grazing intensity, and nitrogen fertilisation were found to cause reduced plant and insect diversity (Kruess and Tscharntke, 2002a, Midolo et al., 2019, van Klink et al., 2019). In addition, increasing land use intensity on grasslands results in multitrophic homogenisation of species assemblages, favouring generalists at the expense of specialist species (Birkhofer et al., 2017, Börschig et al., 2013, Chisté et al., 2018, Gossner et al., 2016).
Concerning grassland management, the effects of delayed mowing, low-intensity grazing, and mosaic (i.e., patch-wise) management have been repeatedly investigated (e.g. Bonari et al., 2017; Buri et al., 2016; Cizek et al., 2012; Humbert et al., 2010; Lyons et al., 2017). Abandoned, long-term grassland fallows with natural vegetation cover may also play an important role in grassland conservation, but are prone to secondary succession (Kahmen et al., 2002; Meichtry‐Stier et al., 2018; Schmidt et al., 2008). Thus, research on grassland fallows has produced contradictory results, as abandonment may increase or decrease plant and insect diversity (Öckinger et al., 2006, Queiroz et al., 2014, Valkó et al., 2018). Though many conservationists believe that ceasing grassland management is detrimental, long-term fallows may comprise a promising tool for insect conservation, benefitting for example butterflies and other taxa (Fiedler et al., 2017, Görn et al., 2014, Toivonen et al., 2015). Beetles (Coleoptera), especially Carabidae, have been widely used to investigate effects of grassland management (Batáry et al., 2007, Grandchamp et al., 2005, Woodcock et al., 2010). They include primary and secondary consumers, generalists and specialists, as well as species with different mobility or size, and are therefore suitable for investigating effects of management (Anderson et al., 2020, Batáry et al., 2007, Helden et al., 2020, Kruess and Tscharntke, 2002b).
We here investigate effects of meadow intensification, ranging from long-term grassland fallows though to silage grassland, within the Westerwald mountain range in western Germany, using beetles as model taxon. We test the hypotheses that (1) beetle taxonomic diversity is lower on more intensively used grassland sites compared to low-intensity meadows and fallows, (2) species assemblages differ strongly between grassland management types, and (3) the species’ biotope and diet preferences, weighted by their relative abundance, differ between managed and abandoned grassland. Since the Westerwald mountain range is still a traditional landscape harbouring a substantial proportion of high value grassland that has never been intensified, our main goal is to identify the best strategy to preserve species-rich communities.
Section snippets
Study area and design
The study was performed in the Westerwald mountain range, which belongs to the Rhenish Slate Mountains, in mid-western Germany (N 50° 39′ 40.1″, E 8° 6′ 5.3″). This region is characterised by plateaus and hills that reach a maximum altitude of 656 m above sea level. The climate is oceanic, rather humid, and cool with an annual mean temperature of 6.5 °C and an annual precipitation of > 1000 mm per year (Sabel and Fischer, 1987). The combination of these climatic conditions with the widespread
Data base
In total, 483 different species of beetles in 28,365 individuals were recorded (Table A.1, supplementary material), 70 of which were either marked as c.f. record (40) or determined to genus (25) or higher clades (5) only. The highest number of individuals was captured by pitfall trapping (21,995; Table A.6a, supplementary material), followed by pan trapping (4891; Table A.6b, supplementary material) and finally suction sampling (1479; Table A.6c, supplementary material). The overall most
Discussion
Based on the combination of different sampling methods covering ground-dwelling, flower-visiting, and vegetation-dwelling beetles, we compiled a comprehensive data set entailing 483 species. Nineteen of the recorded species are included in the red list of Germany, underlining the high importance of the Westerwald mountain range for grassland biodiversity. The three endangered species are hygrophilic wetland specialists. E. uliginosus is dependent on high water levels and low-intensity
Conclusions and conservation implications
Our study revealed highly consistent effects of grassland management across different metrics of species richness and diversity. While our results document strong effects of on-site management regime on local beetle diversity and assemblage structure, other environmental parameters and surrounding land cover were clearly of subordinate importance. Our results thus support the notion that patterns of diversity depend strongly on the specific landscape type under investigation. In traditional
Funding
This study was financially supported by the Ministry for Environment, Energy, Nutrition and Forest Rheinland-Pfalz, Germany, through the ‘Aktion Grün’ initiative.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We are indebted to the local farmers Thomas Bader, Anton Boerkamp, Karl-Martin Gros, Manfred Milnikel, Gregor Theisen, Benjamin Türk, and Joachim Uhr for the permission to conduct our study on their properties. We thank two anonymous reviewers for their constructive criticism. For help and advice, we thank Ulli Bange, Roland Busch, Melina Frenzel, Domenica Kaiser, Jonas Köhler, Markus Kunz, Wolfram Remmers, Daronja Trense, Thomas Wagner, and Anne Wörsdörfer.
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ORCID-ID: https://orcid.org/0000-0003-2188-5234.