Contrasting Responses of Saproxylic Insects to Focal Habitat Resources: The Example of Longhorn Beetles and Hoverflies in Belgian Deciduous Forests

Abstract

Although both saproxylic longhorn beetles and hoverflies benefit from the presence of woody substrates for reproduction, they differ in their requirements for floral resources and for microbiotopes of overmature and senescent trees. This led us to expect contrasting responses between the two species groups in relation to these essential resources. We examined this prediction in 22 mature oak-and beech-dominated stands of southern Belgium by relating their species assemblages to local vegetation structure and composition, altitude and landscape composition. Stands were organised in pairs as a function of their overall dead wood supply. Free-hanging window traps, stump emergence traps and Malaise traps produced 30 longhorn beetle species (1637 individuals) and 106 hoverfly species (3020 individuals). Paired-comparisons controlling for annual variation in captures showed that, unlike saproxylic hoverflies, stands with dead wood hosted more species and individuals of longhorn beetles. Accordingly, the two species groups were found to be independent on ordination axes, responding to different sets of environmental conditions. While stands dominated by oaks with a high snag volume were highly favoured by longhorn beetles, saproxylic and threatened syrphids were limited to open-stands with large trees and a well-developed, species rich herb layer providing the floral resources required for their reproduction. Our results suggest that, when defining criteria to identify or restore important habitats for saproxylic insect conservation, variables related to different aspects of dead wood supply should not be the only criteria taken into account.

A comparative analysis of the evolution of imperfect mimicry

“…Finally, a novel phylogeny based on COI was constructed to control for evolutionary relationships between these species. Our findings, in combination with previous results, allow us to reject several key hypotheses for imperfect mimicry: (i) human ratings of mimetic fidelity are positively correlated with both morphometric measures and avian rankings, indicating that variation in mimetic fidelity is not simply an illusion based on human perception 3 , (ii) no species of syrphid maps out in multi-dimensional space as intermediate in appearance between several different hymenopteran model spe -model hypothesis 4 requires, and (iii) we demonstrate no evidence for a negative relationship between mimetic fidelity and abundance, which calls into question the "kin selection" 5 hypothesis. By contrast, a strong positive relationship between mimetic fidelity and body size supports the "relaxed selection" hypothesis 6,7 , suggesting that reduced predation pressure on less profitable prey species limits the selection for mimetic perfection.…”

“…Of course, if mimicry were poor, then one might intuitively expect that signal receivers would learn be able to distinguish mimics from their models and (depending on context) accept or reject them. In reality however, there are many examples of inaccurate or low fidelity mimics 2,4,5,7,9 . For example, the relative composition of key odour compounds of specific non-rewarding orchids differ markedly from the bees they have evolved to resemble 9 .…”

“…For example, the relative composition of key odour compounds of specific non-rewarding orchids differ markedly from the bees they have evolved to resemble 9 . Likewise, while mimetic spiders of the genus Cosmophasis bear some resemblance to ants, they are readily visually discriminated by the human observer 4 . Perhaps the best examples of imperfect mimics are found in hover flies (Diptera: Syrphidae), which are considered Batesian (harmless) mimics of hymenopteran models 1 but which appear to vary markedly across species in the degree of mimetic perfection 2,10 (see Figure S1).…”

“…Syrphids and hymenopterans tend to cluster with their respective orders, largely on the basis of relative antennae length (RELAntL), a feature that is thought to be used in discrimination by birds 13 . The "multi-model" hypothesis 4 would predict that mimics fall between several models, gaining greater benefit from multiple, weaker associations. Our analysis clearly indicates that there are no mimetic phenotypes in our sample that could be considered as falling morphologically between two or more distinct model phenotypes (Figure 1).…”

Effects of forest management on ground beetle diversity in alpine beech ( Fagus sylvatica L.) stands

“…Bare ground, defined as exposed soil deprived of vegetation, is not considered a suitable habitat (Fry and Lonsdale, 1991;Key, 2000) for many insect groups. Often, it is the result of the erosive action of rainwater, which is particularly strong on steeper slopes, that leads to a reduction of vegetation and microhabitat complexity (Fayt et al, 2006).…”