DOI: 10.5061/dryad.6m905qg4x
title: Inconsistent responses of carabid beetles and spiders to land-use intensity and landscape complexity in Northwestern Europe

description: Reconciling biodiversity conservation with agricultural production
 requires a better understanding of how key ecosystem service providing
 species respond to agricultural intensification. Carabid beetles and
 spiders represent two widespread guilds providing biocontrol services.
 Here we surveyed carabid beetles and spiders in 66 winter wheat fields in
 four Northwestern European countries and analyzed how the activity density
 and diversity of carabid beetles and spiders were related to crop yield
 (proxy for land-use intensity), percentage cropland (proxy for landscape
 complexity) and soil organic carbon content, and whether these patterns
 differed between dominant and non-dominant species. Less than 17% of
 carabid or spider species were classified as dominant, which accounted for
 more than 90% of individuals respectively. We found that carabids and
 spiders were generally related to different aspects of agricultural
 intensification. Carabid species richness was positively related with crop
 yield and evenness was negatively related to crop cover. The activity
 density of non-dominant carabids was positively related with soil organic
 carbon content. Meanwhile, spider species richness and non-dominant spider
 species richness and activity density were all negatively related to
 percentage cropland. Our results show that practices targeted to enhance
 one functionally important guild may not promote another key guild, which
 helps explain why conservation measures to enhance natural enemies
 generally do not ultimately enhance pest regulation. Dominant and
 non-dominant species of both guilds showed mostly similar responses
 suggesting that management practices to enhance service provisioning by a
 certain guild can also enhance the overall diversity of that particular
 guild.
description: We used data on natural enemy communities in 66 paired winter
 wheat fields in four Northwestern European countries (Germany, the
 Netherlands, Sweden and United Kingdom) to investigate the response of
 natural enemy communities to landscape complexity, local land-use
 intensity and soil organic matter content, and specifically examined
 whether and how responses differ between dominant and non-dominant
 species. We focused on carabid beetles and spiders as they represent the
 two groups of natural enemies in arable fields in Northwestern European
 and widely used as bioindicators (Lang et al., 1999; Borchard et al.,
 2014). We used pitfall traps to collect carabids and spiders in field
 pairs that covered a gradient in land-use intensity and landscape
 complexity, with fields within pairs having contrasting soil organic
 carbon content.  Pitfall traps (polypropylene beakers
 155 mm high and 95 mm across) were used to survey ground-dwelling
 arthropods during the wheat flowering season (late May to early June). We
 placed one pitfall trap in the center of each treatment subplot at least
 10 m from the field edge and filled it with 200 mL of a mixed solution of
 2/3 water and 1/3 glycol and a drop of detergent to lower surface tension.
 A square aluminum plate was placed approximately 10 cm above each pitfall
 trap to prevent flooding by rain. Pitfall traps were opened for 10 days.
 All of the collected arthropods were stored in 70% ethanol solution for
 later identification. For the purpose of our study, the two most abundant
 species groups, carabid beetles (<em>Carabidae</em>) and adult
 spiders (<em>Araneae</em>), were selected as our bioindicators
 and they were counted and identified to species level using standard keys
 (Hackston, 2020; Nentwig et al., 2021). We determined the diet preference
 of each carabid beetle species based on Larochelle (1990) and the hunting
 strategy of all observed spider species based on Cardoso et al. (2011)
 following Gallé et al. (2019). Furthermore, because the arthropod
 communities will inevitably differ in composition between countries, we
 classified the carabids or spiders as <em>nationally</em>
 dominant and non-dominant species based on whether species made up
 respectively more or less than 5% of the total number of individuals
 caught of each species group in a country following Kleijn et al.
 (2015).