Negative effects of forest gaps on dung removal in a full‐factorial experiment

Abstract

1. Ecosystem functioning may directly or indirectly-via change in biodiversityrespond to land use. Dung removal is an important ecosystem function central for the decomposition of mammal faeces, including secondary seed dispersal and improved soil quality. Removal usually increases with dung beetle diversity and biomass. In forests, dung removal can vary with structural variables that are, however, often interrelated, making experiments necessary to understand the role of single variables on ecosystem functions. How gaps and deadwood, two main outcomes of forest management influence dung removal, is unknown.2. We tested if dung removal responds to gap creation and deadwood provisioning or if treatment effects are mediated via responses of dung beetles. We expected lower removal rates in gaps due to lower dung beetle biomass and diversity.3. We sampled dung beetles and measured dung removal in a highly-replicated fullfactorial forest experiment established at 29 sites in three regions of Germany (treatments: Gap, Gap + Deadwood, Deadwood, Control). All gaps were experimentally created and had a diameter of around 30 m. 4. Dung beetle diversity, biomass and dung removal were each lower in gaps than in controls. Dung removal decreased from 61.9% in controls to 48.5% in gaps, irrespective of whether or not the gap had deadwood. This treatment effect was primarily driven by dung beetle biomass but not diversity. Furthermore, dung removal was reduced to 56.9% in the deadwood treatment. 5. Our findings are not consistent with complementarity effects of different dung beetle species linked to biodiversity-ecosystem functioning relationships that have been shown in several ecosystems. In contrast, identity effects can be pronounced: gaps reduced the abundance of a large-bodied key forest species (Anoplotrupes stercorosus), without compensatory recruitment of open land species. While gaps and deadwood are important for many forest organisms, dung beetles and dung removal respond negatively. Our results exemplify how

Insect decline in forests depends on species’ traits and may be mitigated by management

“…Nevertheless, our evidence is correlative. For establishing causality, experiments 61 that combine insect monitoring with the manipulation of single forest properties (such as tree species composition or harvesting) while keeping others constant would be desirable albeit logistically challenging. Because site selection in the Biodiversity Exploratories was stratified to include a wide gradient in land-use intensities 41 , we can rule out a site selection bias arising when deliberately choosing monitoring sites with high species diversity 62 .…”

Forest gaps increase true bug diversity by recruiting open land species

“…These nine sites reflect the dominant tree species with five even-aged beech sites and four uneven-aged beech sites. The experiment (Staab et al 2022 ) investigates the effects of creating gaps with and without deadwood and it is based on four treatments per site (Fig. 1 ):…”

“…Some animal species that colonize gaps may be species that live mainly in the adjacent closed forests (Staab et al 2022 ), whereas other colonizing species might be gap specialists or species from open habitats, as is also true for most plants in the gap (Liira and Paal 2013 ). For example, bird communities inhabiting early successional stages of forest are primarily obligate granivores or insectivores, compared with birds occupying closed forest which are specialist feeders, particularly frugivores (Bowman et al 1990 ).…”

“…Animal community assembly in forest gaps is affected by plant successional patterns, with most studies focusing on vertebrates (Gitzen and West 2002 ; Moorman et al 2007 ; Pollock et al 2020 ). For insects, there are fewer studies (Richards and Windsor 2007 ; Eckerter et al 2021 ; Staab et al 2022 ), and there is some evidence for butterflies that newly created forest gaps are first colonized by species with better abilities for dispersal (Viljur and Teder 2018 ). Gaps can also affect species not specialized on herbaceous vegetation.…”

“…In this study, we report some of the earliest results of a forest gap experiment (see Staab et al 2022 ), where gap formation and deadwood provisioning were experimentally manipulated independently, in a 2 × 2 full-factorial design. Specifically, we used true bug communities sampled for two consecutive years to test the following hypotheses: (1) gap treatments (i.e., with open canopy) will harbor a higher number of individuals and species of true bugs compared with closed-canopy treatments; (2) the community composition of true bugs in gaps will be distinct and not just a random subset of the species pool in surrounding forest; and (3) species colonizing gaps are species that generally prefer open habitats, such as grasslands (Supplementary material: Fig.…”