Download 3.2 Forest insects and their habitat requirements

In Focus – Managing Forest in Europe
3.2
Forest insects and their habitat
requirements
Beat Wermelinger, Thibault Lachat, and Jörg Müller
Among insects, some well-known groups such as beetles, ants, butterflies,
and moths are very suitable as indicators of biodiversity, ecological
integrity, and management of forests
Insects form a hyperdiverse taxonomic category contributing to more than half of terrestrial
species diversity. As a rough estimate, some 30,000 insect species live in European forests.
They respond to the structural complexity of forests at different temporal and spatial scales
and are markedly influenced by natural and anthropogenic disturbances such as windthrow,
logging, and fragmentation. Insects are considered good indicators for the overall biodiversity
and integrity of forests because they meet many of the requirements defined for ecological
indicators: Many are relatively easy and cost-efficient to assess with standardized methods,
measurements are reliable due to the high abundance of insects, and they cover a wide range
of life histories, habitat requirements, and functional groups with important roles in forest
ecosystems (Ferris and Humphrey 1999; Maleque et al. 2006). Due to their short turnover
cycles, they are sensitive to and react rapidly to changes in their environment.
With few exceptions, taxonomic or functional groups rather than single species have
been used as indicators in forests. These include ants (Formicidae), butterflies and moths
(Lepidoptera), hover flies (Syrphidae), parasitic wasps (Terebrantes), and most of all beetles, in
particular ground beetles (Carabidae), longhorned beetles (Cerambycidae), saproxylic beetles
in general, and dung beetles (part of Scarabaeidae). Among the numerous environmental
factors known to affect species diversity, such as breeding substrate, food supply, or canopy
openness/insolation, the amount and quality of deadwood is the most important prerequisite
for saproxylic insects and has therefore been the most investigated. There is hardly any
quantitative information for other habitat requirements of forest insects.
Deadwood is a vital resource for thousands of saproxylic (wood-inhabiting)
organisms. Beetles alone comprise more than 1,400 saproxylic species in
Central Europe and are good indicators of biodiversity. Depending on the
forest type, deadwood volumes from 20 to 80 m3/ha are recommended to
support a rich assemblage of saproxylic species
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3.2 Forest insects and their habitat requirements
Deadwood is widely acknowledged as a crucial resource for many species and as an indicator
for forest naturalness. Saproxylic insects exploiting this resource are very susceptible to the
type and intensity of forest management and its effect on the quantity and quality of deadwood.
Saproxylic organisms depend, at least during some part of their life cycle, on dead or dying
wood or on organisms associated with these substrates (Speight 1989). Among the saproxylic
insects, the requirements of beetles have been most extensively studied (Figure 45). They
are considered excellent indicators of woodland biodiversity in general and have been found
to represent different habitat qualities. Most obviously, deadwood is a prime prerequisite.
The amount of deadwood and/or its diversity are positively correlated to the species richness
of saproxylic beetles. Longhorned beetles, a readily identified group, are particularly good
indicators not only for deadwood volume, but also for forest openness and flower supply
(Müller et al. 2008). This is because most of these species depend on different qualities of bark
and wood as a food resource during their larval development, but in their adult stage feed on
pollen of flowering plants to complete their maturation. The latter resource is mainly available
in insolated, open forest habitats or in forests with many gaps and borders. Open forests not
only harbour more longhorned beetles, but also more indicator species of beetles in general
(Lehnert et al. 2013). Stag beetles (Lucanidae) are considered to be a suitable indicator group
representing habitats with high amounts of deadwood in different climate conditions (Lachat
et al. 2012).
Figure 45. The lesser stag beetle (Dorcus parallelipipedus) is a good indicator for warm beech forests with medium and
high amounts of deadwood. Photo by B. Fecker.
Stipulating a single threshold value for deadwood does not capture the complexity of this
multifaceted substrate and its colonisers. This resource encompasses different tree species,
positions, decomposition stages, and diameters, and each insect species depends on specific
deadwood qualities. Generally, an increase in the total volume of deadwood coincides with an
increase in the diversity of this substrate (Similä et al. 2003).
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A comprehensive meta-analysis of literature data by Müller and Bütler (2010) provides a set
of thresholds for different forest types in Europe (see Box 26). Since these figures are based on
various systematic groups such as fungi, plants, insects, and birds, the threshold ranges for
the saproxylic beetles alone are narrower. Saproxylic beetles tend to have a requirement for
higher deadwood volumes than other groups of organisms. The required amount of deadwood
ranges from 40 to 70 m3/ha for lowland beech-oak forests, 35 to 140 m3/ha for montane-mixed
forests (beech-fir-spruce), and 24 to 70 m3/ha for boreo-alpine spruce-pine forests. The upper
end of these ranges is particularly significant for threatened species (Müller et al. 2008). From
these findings, recommendations for minimal deadwood volumes can be derived (see Box 26).
In particular, large dimensions of deadwood are generally scarce and therefore need to be
fostered. However, small diameter-limbs, due to their large variation of qualities, are also a
valuable substrate for beetle and fly fauna (Schiegg 2001). There are distinct indicator groups
for early or late successional stages in the forest development. For example, bark beetles (Curc.:
Scolytinae) and jewel beetles (Buprestidae) colonise fresh bark and wood, while longhorned
beetles utilize a large range of deadwood conditions (Wermelinger et al. 2002). Towards the
end of the decomposition process, rose chafers (Cetoniidae) colonise the soft and rotten wood.
Box 26. Recommended deadwood volumes for saproxylic beetles
Saproxylic beetles are one of the best-investigated indicator groups for forest
biodiversity. Their requirements for deadwood volumes were compiled by Müller and
Bütler (2010). Based on these values and considering their applicability in managed
forests, the following volumes of deadwood are suggested as guidelines for sustaining
a rich saproxylic beetle fauna:
Lowland beech-oak forests
Montane-mixed forests (beech-fir-spruce)
Boreo-alpine spruce-pine forests
Dead wood volumes
30–50 m3/ha
40–80 m3/ha
20–50 m3/ha
Habitat trees offering microhabitats such as dead branches, woodpecker cavities, bark losses,
or polypore fruiting bodies are vital for a wide range of specialised saproxylic beetles. The
number of such trees and their microhabitats is lower in managed forests than in old growth
forests (Brunet and Isacsson 2010; Müller et al. 2008). In particular, hollow trees harbour many
red-listed insect species with low dispersal capacities such as the hermit beetle (Osmoderma
eremita), one of the best-investigated saproxylic species and an excellent single species
indicator. Formerly distributed all over Europe, its populations are now very fragmented. This
species depends on hollow trees, mostly oak, and lives for generations in suitable single trees.
Modelling revealed that 20 stands with at least 10 hollow trees within a distance of 250 m are
needed to form a viable metapopulation (Ranius and Hedin 2004).
Another very important resource for a large number of fungicolous species are polypore fungi.
Many species are indicators for forests with a long deadwood tradition and high conservation
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3.2 Forest insects and their habitat requirements
value. In a recently compiled list of “Urwald relict beetle species” (i.e. species that have only
relict records, high deadwood requirements, declining populations, and are tied to habitat
tradition), 16 % are fungivores (Müller et al. 2005).
Large amounts of total deadwood volumes do not guarantee the long-term existence of any
species. The levels may still be too low for particular species, specific microhabitats may
be too scarce, or the connectivity of suitable habitats may be limiting. In managed forests,
late-decay stages of large dimension logs and microhabitats such as rot holes harbouring
many red-listed beetle species are lacking. Also, the dynamics of saproxylic populations at the
landscape level needs further research.
Bees, wasps, hover flies, butterflies, and moths are similar in that they are
potential pollinators but otherwise have a very broad range of ecologies.
These indicators not only reflect the differing food demands for their
larval development, but also the supply of flowers providing nectar and
pollen to the adult insects and thus represent the openness of a stand
Besides many pollen-feeding beetle species, bees and wasps (Hymenoptera), butterflies and
moths (Lepidoptera), and hover flies (Syrphidae) are the most relevant pollinators (Figure 46).
They pollinate herbaceous plants rather than forest trees, which are mostly wind-pollinated
in Europe. Hover flies represent a wide range of different ecologies and thus have different
requirements and functional roles. Their species diversity reflects the diversity of their habitats.
While their larvae have quite diverse biologies, most adult hover flies consume nectar or pollen
and pollinate flowers. Many syrphid species as well as bees and wasps are good indicators for
the openness or gappiness of forests (Lehnert et al. 2013; Bouget and Duelli 2004).
Figure 46. Hover flies (here Episyrphus
balteatus) are important pollinators of forest
ground vegetation. Their abundance is an
indicator for the canopy openness of forests.
Photo by B. Wermelinger.
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In Focus – Managing Forest in Europe
Likewise, most butterflies and moths, as adults, feed on nectar provided by flowers. Therefore,
they are reliable indicators for open and recently disturbed forests, former and present
management regimes, and forest fragmentation (Freese et al. 2006; Maleque et al. 2009). For
example, the density of the wood white (Leptidea sinapis) (a butterfly) varies with the amount
of shading (Warren 1985). Some species are typical for, and thus depend on, early successional
forest stages (coppicing) with understorey vegetation, such as the endangered butterflies
Euphydryas maturna and Lopinga achine (Freese at al. 2006; Streitberger et al. 2012). In North
American deciduous forests, families of moths (Arctiidae and Notodontidae) have proven to
be good indicators for general moth species richness (Summerville et al. 2004).
Insect predators and parasitoids consume mostly herbivorous insects.
They represent the different ecologies of their prey. Therefore, a high
species richness of these natural enemies indicates rich prey diversity
Predatory insects feed on other animal species, most often herbivores or detritivores.
Representing a higher trophic level, they integrate extensive ecological information of other
communities (Ferris and Humphrey 1999). Old growth forests usually have a higher diversity
and abundance of predators and detritivores (Schowalter 1995). Among the predators, ground
beetles, tiger beetles (Cicindelidae), and epigeic rove beetles (Staphylinidae) are considered
to be a good reflection of forest management intensity (Osawa et al. 2005; Pearson and Cassola
1992; Pontégnie et al. 2004). Other predatory groups such as ants can also serve as indicators
for forest management and resulting canopy structure (Dolek et al. 2009).
Box 27. Indicator species
Indicator species or species groups are characteristic for specific habitat features
and are often of high conservation value. Thus, for enhancing the quality of forest
biodiversity, management should aim at improving habitat quality in order to sustain or
increase the number of indicators rather than maximizing species number. In this way,
a broad range of ecological requirements and functional types of insects are favoured,
which is likely to also benefit rare specialists.
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