Inconsistent responses of carabid beetles and spiders to land-use intensity and landscape complexity in Northwestern Europe

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.