Where have all the beetles gone? Long‐term study reveals carabid species decline in a nature reserve in Northern Germany

Abstract

1. The drastic insect decline has received increasing attention in scientific as well as in public media. Long-term studies of insect diversity trends are still rare, even though such studies are highly important to assess extent, drivers and potential consequences of insect loss in ecosystems.2. To gain insights into carabid diversity trends of ancient and sustainably managed woodlands, we analysed data of carabid beetles from a trapping study that has been run for 24 years in an old nature reserve of Northern Germany, the L€ uneburg Heath. We examined temporal changes in several diversity measures (e.g. biomass, species richness, functional diversity and phylogenetic diversity) and tested diverse species traits as predictor variables for species occurrence.3. In contrast to recently published long-term studies of insect diversity, we did not observe a decline in biomass, but in species richness and phylogenetic diversity in carabids at our study site. Additionally, hibernation stage predicted the occurrence probability of carabids: Species hibernating as imagines or both imagines and larvae and breeding in spring showed strongest declines.4. We assume the detected trends to be the result of external effects such as climate change and the application of pesticides in the surrounding. Our results suggest that the drivers for the insect decline and the responses are multifaceted. This highlights the importance of long-term studies with identification of the catches to, at best, species level to support the understanding of mechanisms driving changes in insect diversity and abundance.

Declining abundance of beetles, moths and caddisflies in the Netherlands

“…Here, the less abundant and smaller species showed stronger declines than common or larger species, giving rise to a much lower decline rate in biomass as compared to the numerical declines. These results imply that the declines in insect biomass, although indicative to diversity loss, may not always show a one‐to‐one correspondence to numerical declines (Homburg et al ., ).…”

Factors contributing to the decline of an endangered flightless longhorn beetle: A 20‐year study

“…Our result of both decreasing abundance and number of I. fuliginator populations is in accordance with studies on many other insect taxa, such as butterflies, moths, wild bees and beetles in agricultural landscapes (Goulson et al ., ; Potts et al ., , Brooks et al ., ; Fox et al ., ; Homburg et al ., ). Some of these studies compared recent surveys with data from older (or even historical) surveys (e.g.…”

“…Another frequently used method to examine potential drivers of insect declines is the space-for-time approach, in which the insect diversity in heavily impacted habitats is compared with diversity in less impacted or assumingly undisturbed (control) habitats at the same time (Habel et al, 2019). Still other studies are based on data of long-term monitoring projects (Conrad et al, 2006;Hallmann et al, 2017;Homburg et al, 2019). The different approaches complement each other and demonstrate that drivers of insect decline are manifold, acting in different combinations and different magnitudes at different sites, and influence different species to a different extent (Thomas, 2016).…”

“…The main drivers of insect species declines are habitat degradation and loss, habitat conversion to intensive agriculture and expanding urbanisation, pollution by synthetic pesticides and fertilisers, introduced non-native species and pathogens, and climate change (Dirzo et al, 2014;Hallmann et al, 2017;Homburg et al, 2019;Sanchez-Bayo & Wyckhuys, 2019). Depending on habitat type and region, single or multiple factors in different combinations may impact native insect communities Correspondence: Dénes Schmera, MTA, Centre for Ecological Research, Balaton Limnological Institute, Klebelsberg Kuno 3, 8237 Tihany, Hungary.…”

Increase of insular exotic arthropod diversity is a fundamental dimension of the current biodiversity crisis

“…Despite their importance, insect biodiversity and population abundances tend to be overlooked by the public, policy‐makers and local and national authorities (Cardoso et al ., 2011). Yet, recently, insect population declines have made the news, with multiple reports describing what appear to be dramatic losses in richness, abundance and biomass (Halmann et al ., 2017, 2019, 2020; Habel et al ., 2019; Homburg et al ., 2019; Seibold et al ., 2019; Wagner, 2020). There is a recent call for urgent measures to be taken to avoid massive insect declines (e.g.…”