Chapter 17

Importance of Primary Forests

for the Conservation of Saproxylic Insects

Thibault Lachat and Joerg Müller

Abstract Primary forests represent the ultimate intact habitat for saproxylic insects.
However, their extent has been considerably reduced over the past centuries, and
those remaining are very heterogeneously distributed. Primary forests are still locally
abundant in tropical and boreal zones but are rare in temperate zones. Consequently,
many saproxylic insects that were adapted to typical characteristics of primary
forests, such as large amounts of dead wood or overmature and senescent trees,
might have become extinct regionally due to habitat loss. The remaining primary
forests therefore function as refuges for those saproxylic species that cannot survive
in managed forests because of their high ecological requirements. Here we identify
six characteristics of primary forests important for saproxylic insects that differen-
tiate these forests greatly from managed forests, namely, absence of habitat frag-
mentation, continuity, natural disturbance regimes, dead-wood amount and quality,
tree species composition and habitat trees. These six characteristics highlight the
importance of primary forests for the conservation of saproxylic insects in all three
main climatic domains (tropical, boreal and temperate). As primary forests are rare in
northern temperate zones and are being dramatically lost in boreal and tropical
zones, we propose that they should be strictly conserved independently of their
climatic zone. Furthermore, we recommend that studies in primary forests intensify
to provide reference data for integrating primary forest characteristics into managed
forests to improve the conservation of saproxylic species.

T. Lachat (*)
Bern University of Applied Sciences BFH, School of Agricultural, Forest and Food Sciences
HAFL, Zollikofen, Switzerland
Swiss Federal Institute for Forest, Snow and Landscape Research WSL, Birmensdorf,
Switzerland
e-mail: thibault.lachat@bfh.ch
J. Müller
Nationalparkverwaltung Bayerischer Wald, Grafenau, Germany
Feldstation Fabrikschleichach, Lehrstuhl für Tierökologie und Tropenbiologie (Zoologie III),
Biozentrum Universität Würzburg, Rauhenebrach, Germany

This is a U.S. government work and its text is not subject to copyright protection in the 581
United States; however, its text may be subject to foreign copyright protection 2018
M. D. Ulyshen (ed.), Saproxylic Insects, Zoological Monographs 1,
https://doi.org/10.1007/978-3-319-75937-1_17
582 T. Lachat and J. Müller

17.1 Introduction

As one of the last remnants of intact nature in terrestrial ecosystems, primary forests
have been a focus of conservation efforts for decades. They are an irreplaceable
habitat for thousands of species, including saproxylic insects (Bengtsson et al. 2000).
Such forests serve as a refuge and reservoir for species that cannot survive in
managed forests. Those saproxylic species that can survive in managed forests
even under very unnatural conditions, e.g. in Douglas-fir plantations in Europe
(Goßner and Ammer 2006) or teak plantations in tropical Africa (Lachat et al.
2007), deliver important ecosystem services, such as biomass decomposition. By
contrast, specialized species cannot cope with forest degradation, especially when it
impacts their livelihood as they have higher ecological requirements (Grove 2002b),
namely, large amounts of dead wood, dead or old trees of large dimension, habitat
continuity or natural dynamics such as wildfires or windthrows.
As the anthropogenic pressure on natural forest ecosystems increases, the impor-
tance of forests with negligible human influence becomes more apparent. These
irreplaceable primary forests (also known as primaeval or virgin forests or even
called ancient or old-growth forests if human impact cannot be excluded) worldwide
have unique qualities that significantly contribute to biodiversity conservation,
climate change mitigation and sustainable livelihoods (Foley et al. 2007; Luyssaert
et al. 2008; Körner 2017). Yet most of the world’s forests have been influenced in the
last centuries by logging, clearing or land-use change. For example, approximately
half of the tropical forest that was present at the beginning of the twentieth century
has already disappeared, with peak deforestation in the 1980s and 1990s (Wright
2005). In Central Europe, where the main deforestation occurred in the eighteenth
century or earlier, primary forests are now scarce (Williams 2002).
Depending on the definition, the status of natural primary forests can be very
exclusive. The irrevocable loss of their status is reflected in their definition by the
Food and Agriculture Organization of the United Nations (FAO 2016), i.e. a “nat-
urally regenerated forest of native species, where there are no clearly visible indica-
tions of human activities and the ecological processes are not significantly
disturbed”. This definition does not completely exclude human intervention in the
distant past, which cannot always be known. However, the FAO definition specifies
that significant human intervention has not been known to occur or was long ago
enough to have allowed the natural species composition and processes to become
re-established (FAO 2016). Such recovery processes can last centuries depending on
the intensity of the disturbance and on the surrounding landscape (Chazdon 2003).
A key factor of primary forests is the absence of human activities that could
disturb ecological processes. However, some forests that experienced human activ-
ities more than 500 years ago qualify as primary forests. For example, recent
archaeological research in the Amazon basin revealed pre-Columbian earthworks
and settlements throughout the area (Piperno et al. 2015). Lasting legacies of these
past human disturbances include earthworks, soil modification and introduction of
species. However, given the spatial and temporal pattern of the data, the human
17 Importance of Primary Forests for the Conservation of Saproxylic Insects 583

impact on the Amazonian forest is not evident (Bush et al. 2015). It is therefore
conceivable that large forest areas recovering from forest management and set aside
as forest reserves during the past decades could be considered as primary forests in
few hundreds of years, as long as the protection status is maintained.
Another definition of primary forest is based on the intactness of the ecosystem as
a forest landscape of conservation value. Intactness is related to ecosystem integrity
and resilience to natural disturbances and to other forest ecosystem functions, such
as biodiversity (Potapov et al. 2017). A seamless mosaic of forests and associated
natural treeless ecosystems with a minimum area of 500 km2 that exhibits no
remotely detected signs of human activity or habitat fragmentation and is large
enough to maintain all native biological diversity is classified as an intact forest
landscape (Potapov et al. 2017). Considering this concept, the size and the location
of a primary forest also influence its status.
In the following, we will first provide an overview of the state of primary forests
in three main climatic domains: tropical, temperate and boreal. We will then focus on
important characteristics of primary forests that might influence saproxylic insects as
compared to managed forests. Finally, we will provide implications for conservation
and make recommendations for the role of primary forest in the conservation of
saproxylic insects.

17.2 What Remains of Primary Forests and Consequences

for Conservation

Globally, the history of primary forest loss mirrors the development of human
settlements and population density. For example, the Roman Empire exploited
forests early on in a way that left only degraded shrub vegetation cover around the
Mediterranean Sea (Hughes 2011). Particularly the high demand for wood for war
vessels thousands of years ago caused the loss of primary forests around the sea
(Ehrlich et al. 1977). By contrast, other forest areas, e.g. in the tropics and boreal
zones, were only little affected up to a few hundred years ago due to limited access
and unfavourable conditions for human life, such as tropical diseases and short
vegetation periods. Under the favourable conditions in Central Europe, forest deg-
radation occurred in a cascade of pressure, starting with a clearance period up to
800 AD. After this, humans started to convert remaining forest fragments through
silvicultural practices of coppicing, pasture establishment and the like. A shift to
modern forestry starting in the late 1700s followed agricultural concepts of enhanc-
ing wood production, mainly through even-aged forestry and a focus on fast-
growing conifers (Bürgi and Schuler 2003; Kirby and Watkins 2015; McGrath
et al. 2015). In eastern North America, most of temperate deciduous forests were
cleared for agriculture during the settlement from the 1600s to the mid-1800s.
Consequently, in Europe and North America, less than 1% of all temperate
584 T. Lachat and J. Müller

deciduous forests remain free of deforestation or other intensive use (Reich and
Frelich 2002).
Boreal forests were managed later. Studies on fire regimes highlighted the
predominance of climate-driven fire regimes in Fennoscandia up to 1600s, followed
by an increased fire frequency due to anthropogenic influence up to 1800s (slash-
and-burn cultivation and forest pasture burning) (Niklasson and Drakenberg 2001;
Storaunet et al. 2013). Later, fires were suppressed due to increased value of timber
resources (Rolstad et al. 2017). Similar patterns have been indicated for North
American and Russian boreal forests (Drobyshev et al. 2004; Wallenius et al. 2011).
In the tropics, with exception of pre-Columbian farming in the Amazon, a number
of forests experienced such shifts even later, from primary forest to forest fragmen-
tation, followed by logging and plantations, e.g. of oil palms (Hughes 2017). This
latter process lasted less than a century but led to a dramatic and rapid loss of the
most diverse forest ecosystems (Williams 2002). In line with this history, primary
forests are scarce or absent in landscapes intensively and extensively impacted by
humans.
The cumulative loss of forest worldwide over the last 5000 years is estimated at
1.8 billion hectares. This represents an average net loss of 360,000 hectares per year
(Williams 2002) or in other words equivalent to nearly 50% of the total forest area
today. In 2015, primary forests accounted for about one-third of the world’s forests
(Table 17.1). These data are the most comprehensive statistics available today, but
many of the countries rely on forest proxies (e.g. in national parks or protected areas)
to estimate the extent of primary forest (FAO 2016). Half of the world’s primary
forests are found only in Brazil, Canada and the Russian Federation. The forests in
these three countries well represent the distribution of the remaining primary forest
and highlight the role of tropical and boreal regions for the conservation of primary
forests, as shown by Mackey et al. (2015), who found that 50% of the intact forest
landscape (primary forest in contiguous blocks >500 km2) occurs in boreal zone,
46% occurs in equatorial areas and 3% occurs in warm temperate climatic zones.
The global trend for primary forests is still negative, with an annual change of
0.1%. This decline is mainly due to the decreasing area of primary forests in the
tropical climatic domain (South America, 0.32%; Africa, 0.45%). Primary
forests in the boreal and temperate domains are slightly increasing. This is not a

Table 17.1 Surface area of natural forests and primary forests remaining today and the proportion
of natural forests that is regarded as primary forest (FAO 2016)
Natural forest (106 ha) Primary forest (106 ha) Proportion (%)
Africa 624 135 0.22
Asia 593 117 0.20
Europe 1015 277 0.27
North and central America 751 320 0.43
Oceania 174 27 0.16
South America 827 400 0.48
Worldwide 3999 1277 0.32
17 Importance of Primary Forests for the Conservation of Saproxylic Insects 585

real increase in surface area, but this increase is mostly due to the reclassification of
old-growth forest into primary forests following the definition of the FAO which
does not exclude human impact in the past (FAO 2016). Between 1990 and 2015,
primary forest area actually declined by 2.5% globally and by 10% in the tropics
(Morales-Hidalgo et al. 2015).

17.3 Differences Between Primary and Managed Forests

for Saproxylic Insects

Several studies could highlight differences regarding species richness, abundance or

species composition between primary and managed forests. Grove (2002a) provided
an overview of studies published on the relationship between saproxylic insects and
forest management. Additional studies, published since 2000, are listed in
Table 17.2. Generally, saproxylic insects show a positive relationship with respect
to “old-growthness” of the forest stands, independently of the climate zone and
forest type (Table 17.2). In this chapter, we further discuss how characteristics of
primary forests that change through forest management and land-use changes could
affect saproxylic insects. The topics are organized based on the spatial or temporal
scale at which a characteristic affects the insects.

17.3.1 Absence of Habitat Fragmentation

One major effect of the exclusion of human activities in forest ecosystems is the
absence of fragmentation driven by anthropogenic disturbances. According to the
Convention on Biodiversity (CBD), forest fragmentation refers to any processes that
result in the conversion of formerly continuous forest into patches of forest separated
by non-forested lands (Fig. 17.1). It can be argued that for saproxylic species
dependent on primary forests, even the conversion of continuous primary forest
into secondary forest rather than non-forest can qualify as habitat fragmentation. The
process of forest fragmentation generally starts with forest degradation driven by
land-use changes and results in habitat loss, increased edge effects and isolation of
populations of forest species (Laurance and Bierregaard 1997). Habitat fragmenta-
tion therefore has two consequences, namely, reduction of forest habitat and reduc-
tion of connectivity. Because these two consequences of fragmentation are often
interdependent, very few studies have been able to fully disentangle the importance
of habitat area and connectivity for saproxylic species (Komonen and Müller 2018).
It is important to note that an improvement in connectivity usually leads to an
increase in the amount of habitat. However, an increase in the habitat amount does
not necessarily lead to improved connectivity (Komonen and Müller 2018).
Table 17.2 Chronology of studies comparing saproxylic insects between reference (primary, old-growth forests) and managed forests published since 2000
586

Reference Species Assemblages

Country Taxa Forest type forests Managed forests Method Abundance richness composition Source
Finland Coleoptera Boreal Old- Mature managed Flight-intercept + + + Martikainen
forests growth forests traps et al. (2000)
forests
Australia Coleoptera Lowland Old- Selectively Flight-intercept + + + Grove
tropical growth logged, regrowth traps (2002b)
forests forests rainforests
Japan Coleoptera: Humid Old- Secondary forests Yellow/white col- 0 + + Maeto et al.
Disteniidae, warm- tem- growth + conifer lision traps (2002)
Cerambycidae perate forests plantations (baited)
forests
Brazil Wood-feeding Highland Primary Plantation + sec- Active searching 0 + Bandeira
termites evergreen forests ondary forests along transect et al. (2003)
forests
Mexico Coleoptera: Tropical Primary Secondary forests Manual extraction 0 + 0 Castillo and
Passalidae evergreen forests from logs Lobo (2004)
forests
Benin Wood-feeding Semi- Natural Teak plantations Active searching + + + Attignon
termites deciduous forests along transect et al. (2005)
tropical
forests
Benin Coleoptera Semi- Natural Teak and fuel- Emergence traps 0 + + Lachat et al.
deciduous forests wood plantations (2006)
tropical
forests
Canada Coleoptera: Hemlock- Old- Mature managed Flight-intercept 0 + Zeran et al.
Nitidulidae hardwood growth forests traps + trunk- (2006)
forests forests window traps
T. Lachat and J. Müller
 17

Colombia Coleoptera: Andean Old Andean alder Manual extraction + + NR Kattan et al.
Passalidae tropical growth plantations from logs (2010)
forests forests
Italy Coleoptera Deciduous Old- Conservation-ori- Rearing + vinegar NR + + Persiani et al.
forests growth ented managed pitfall traps + (2010)
(oak-beech) forests forests flight-intercept
traps
Brazil Wood-feeding Semi-arid Primary Disturbed Active searching + + + Vasconcellos
termites forests forests woodlands along transect et al. (2010)
USA Coleoptera Eastern Old- Secondary forests Emergence + 0 + Ferro et al.
deciduous growth chambers (2012)
forests forests
Singapore Wood-feeding Hill dip- Old- Selectively Active searching NR 0 + Bourguignon
termites terocarp growth logged forests + along transect et al. (2017)
forests forests secondary forests
Symbols indicate reported relationships with respect to reference forests (primary, old-growth forests): +, positive relationship; 0, no relationship; , negative
relationship; NR, not reported
Importance of Primary Forests for the Conservation of Saproxylic Insects
587
588 T. Lachat and J. Müller

Fig. 17.1 Habitat fragmentation and degradation of primary forests generally leads to habitat loss
(schematic view) (Picture: J. Müller)

Not all saproxylic species react similarly to forest fragmentation, regardless of

whether the consequence is the loss of habitat area or the loss of connectivity. Many
saproxylic insect species are able to track dead wood, which is a spatio-temporally
dynamic habitat (Seibold et al. 2017).Their dispersal ability might have been
underestimated; many species are able to disperse over long distances as they are
naturally adapted to search for their ephemeral resources (Komonen and Müller
2018). Even flightless species can disperse farther than expected, such as Morimus
asper (Sulzer), which colonizes fresh dead wood and disperses over 400 m
(de Gasperis et al. 2016). Furthermore, secondary users of dead wood are not
affected by isolation caused by salvage logging (Saint-Germain et al. 2013), and
common saproxylic beetle species are not affected by fragmentation in a forest-
dominated landscape (Ranius et al. 2015). However, it is important to recognize that
common saproxylic species in modified landscapes may be the ones best able to cope
with such landscapes, and they may be a subset of species from common species in
primary forests. Bouget et al. (2015) did not find any consistent differences in the
flight ability of saproxylic beetles in woodlots of different sizes or isolation. These
observations were confirmed in experimental approaches controlling the amount of
dead wood in areas of differing isolation, which indicates that the amount of dead
wood is independent of the spatial arrangement (Seibold et al. 2017). On the other
hand, Schiegg (2000) found a correlation between species richness and composition
of saproxylic Diptera and Coleoptera and found connectivity between logs at a
small scale (150 m). Brunet and Isacsson (2009) detected a lower dispersal capacity
of red-listed saproxylic beetles, which underlines the higher sensitivity of such
species to fragmentation. The effects of fragmentation on saproxylic insects with
different ecological traits, such as mobility, body size and habitat requirements, are
therefore contrasting, and no general conclusions can be drawn. Furthermore,
extinction of very sensitive species in the past owing to habitat fragmentation and
loss might have selected species communities that react less sensitively to changes in
their habitat compared to the original species assemblages.
The main effect of fragmentation on saproxylic insects in primary forest might be
more indirect. Fragmentation influences forest dynamics, which in turn has an
impact on the amount and quality of dead wood. Natural disturbances that affect
forests at the landscape scale, such as fire and windthrow, are driven by the size,
17 Importance of Primary Forests for the Conservation of Saproxylic Insects 589

shape and position of the surrounding landscape (Saunders et al. 1991). Larger areas
of intact primary forest should therefore be maintained to enable natural dynamics
such as the minimum size of 500 km2 set by the concept of intact forest landscape
(Potapov et al. 2017). According to Carbiener (1996), only surface areas of several
thousand hectares can harbour all forest development phases, including the variabil-
ity of natural disturbances and the associated fauna and flora.

17.3.2 Habitat Continuity

Another characteristic of primary forests is the continuous supply of high amounts of

dead wood and senescent trees over several centuries or longer (Whitehead 1997).
This habitat continuity focuses on the need for species to breed continuously in a
stand (Eckelt et al. 2018; Müller et al. 2005). Habitat continuity can be an important
driver that influences the presence of saproxylic insects in forests (Buse 2012;
Herrault et al. 2016). Many so-called old-growth forests harbour amounts of dead
wood and habitat structures similar to those of primary forests but have encountered
in the past an interruption in the supply of dead wood because of management and
wood harvesting (Brunet and Isacsson 2009). As a consequence, some saproxylic
insect species have disappeared and are still absent today, even though the present
structure would be favourable for them (Müller et al. 2005; Gossner et al. 2013).
Particularly specialized insect species with low mobility, such as some associated
with tree hollows, might be sensitive to an interruption of habitat continuity because
they are not able to find an alternative habitat within the reach in their distribution
range at the right time (Ranius and Hedin 2001). Consequently, not only spatial
continuity but also temporal continuity of dead wood and senescent structures
available in primary forests represent key factors for the conservation of saproxylic
species with high habitat requirements and low mobility.

17.3.3 Natural Disturbance Regimes

The intensity, frequency and severity of natural disturbances in primary forests have
a major effect on the quantity and quality of dead wood available for saproxylic
insects. The combination of small gap dynamics associated with the breakdown of a
single tree (Yamamoto 2000) and the disturbance of several square kilometres of
forest over millennia has shaped forest biomes that differ in composition and
structure throughout the world, which in turn has shaped the communities of species
(Gauthier et al. 2015) (Fig. 17.2). Through co-evolution and selection, forest species
are preadapted to the natural prevailing disturbance regime typical for their respec-
tive forests biome (McPeek and Holt 1992). Consequently, saproxylic insects are
prone to react sensitively to natural disturbances that produce dead wood in primary
forests (Grove and Stork 1999). For example, in natural boreal forests, where natural
590 T. Lachat and J. Müller

Fig. 17.2 Schematic view of the surface area affected by natural disturbances in relation to their
recurrence periods [after Spies and Turner (1999)] (Pictures: T. Lachat, J. Müller)

disturbances such as fire or storms can extend over several thousands of hectares
(Boulanger et al. 2010) and can recur, saproxylic insects are adapted to the resulting
mosaic structure and are therefore less sensitive to fragmentation than species in
tropical ecosystems (Langor et al. 2008). Because of the distribution of their
ephemeral habitat at the landscape scale, these species are therefore generally
good dispersers and are able to colonize newly formed habitats (Grove 2002b;
Messier et al. 2003). It is important to note that secondary users of dead wood also
benefit from the large quantities of dead wood created by natural disturbances on
large areas (Nappi et al. 2010). In temperate forests, such as primary beech forests
dominated by gap dynamics, most species will be adapted to single-tree replacement
dynamics, which lower the importance of heliophilous species because of the lack of
large open disturbed areas (Lachat et al. 2016).
The major dynamics of forests strongly depends on the climate zone and on the
soil in which they grow. On steep mountain slopes, avalanches and rockslides create
bare soil and open patches in forests, which regularly open the avenue for establish-
ment of pioneer tree species, e.g. species of the genera Pinus and Betula. In
floodplains, the rearrangement of rivers creates a diverse composition of tree species
mainly triggered by their inundation tolerance. In temperate broad-leaved forests,
major dynamics are gaps caused by fallen veteran trees and summer storms. In boreal
forests, insects and fires with stand-replacing dynamics dominate the natural forest
landscape. In the tropics, forests are naturally affected by hurricanes, gap dynamics
and fires (Chazdon 2003). In wet Australian eucalypt forests, for example, infrequent
fires lead to stand-replacing or partial stand-replacing dynamics sometimes across
17 Importance of Primary Forests for the Conservation of Saproxylic Insects 591

tens of thousands of hectares, leading to long-lasting pulses of very high volumes of

standing dead trees and dead wood. In drier eucalypt woodlands, by contrast, more
frequent but lower-intensity fires maintain a semiopen structure with lower, but more
constant, amounts of dead wood (Fox and Fox 2003).
Currently, most of these inherent dynamics of primary forests are interrupted by
channelling rivers, suppression of fires, pest species management and forest man-
agement measures aimed at lowering the sensitivity of stands to natural disturbances
(Kurz et al. 2008; Seidl et al. 2014). The mitigation or suppression of the effect of
natural disturbance might have dramatic effects on saproxylic species and can
compromise a large proportion of the regional species pool. This is especially the
case for those adapted to habitats created by disturbances such as the large inputs of
dead wood created by windthrows or regeneration of broad-leaved trees in boreal
forests after fire (Martikainen 2001).

17.3.4 Dead Wood

The main resource for saproxylic insects—dead wood—might be very heteroge-

neously distributed in space and time in all forests regardless of disturbance of
management history (Christensen et al. 2005; Ylisirniö et al. 2009; Amanzadeh
et al. 2013). Because of the natural dynamics in primary forests (from
competitive thinning in younger forests to tree death in older forests), the average
amount of dead wood is generally higher than in managed forests (Siitonen et al.
2000; Grove 2002b; Christensen et al. 2005; Lachat et al. 2007). However, in
contrast to the most intensively managed forests, where fallen woody biomass is
generally and mostly exported out of the harvested forest stands, the quantity of
fallen coarse wood might temporarily increase in many forests after logging, espe-
cially when legacy debris and logging slash are left on the site (Keller et al. 2004). As
Spies et al.(1988) showed in northwestern North America, this results in a fairly
typical “U-shaped” distribution of log volume among different stand ages, with
lower wood volume reported from forests of intermediate age than in recently
harvested stands. Consequently, a large amount of dead wood is not always an
indicator of naturalness, as it has been shown for European temperate forests
(MCPFE 2007). Gerwing (2002) found that in the Brazilian Amazon, the amount
of dead wood increased in relation to the intensity of logging. A study in Malaysian
Borneo has shown that up to 64% of the above-ground biomass in selectively logged
forests could consist of dead wood (Pfeifer et al. 2015). Such observations might be
reinforced in tropical forests without major natural disturbances leading to low
amount of natural coarse woody debris because of the rapid decay of the woody
biomass. Saproxylic species might therefore benefit from this artificial high amount
of coarse woody debris. However, especially in the tropics, the continuity over time
is not guaranteed, because dead wood will decompose before a new coarse woody
debris will be created.
592 T. Lachat and J. Müller

Grove and Stork (1999) designed a framework for future research on the effect of
logging on saproxylic species in tropical forests that considers the peculiarities of the
logging practices. Short-term results highlighted the changes in the species richness
and composition of saproxylic beetles, but long-term studies are needed to reveal the
effect of repeated logging on saproxylic species (Grove 2002b), especially for
species that depend on large old trees or dead wood of large dimensions (Seibold
and Thorn 2018) (Fig. 17.3). Another peculiarity of tropical forests is the presence of
wood-feeding termites. Several studies have revealed changes in termite species
richness and abundance along a gradient of logging intensity from primary forests to
forest cleared for agricultural production (Bandeira and Vasconcellos 2002; Jones
et al. 2003; Ewers et al. 2015). However, soil-feeding termites seem to be more
sensitive to logging than wood-feeding termites (Eggleton et al. 1997). Therefore, it
has been recommended to let dead wood decay in situ after a forest disturbance to
mitigate the loss of termite species (Jones et al. 2003).
Natural dynamics at a small scale, e.g. death or breakdown of a single large
senescent tree, and at a large scale, e.g. windthrow, fire or bark-beetle infestation, are
the main drivers that determine the amount of dead wood in primary forests.
Nevertheless, dead wood production in the absence of a major disturbance is not
limited to the senescent phase, e.g. breakdown of single old trees. Already during
regeneration phases, significant amounts of dead wood can be created through
exclusion of young trees by competition (Peet and Christensen 1987). This gives
rise to a continuous production of dead wood of different types and quality through-
out the forest and during the entire forest development phase (Saniga and Schütz
2002; Larrieu et al. 2014). Under natural condition in forests dominated by gap
dynamics, regeneration is characterized by remnants of large old trees that slowly die
and continuously produce coarse woody debris (Larrieu et al. 2014). In forests
dominated by stand-replacing disturbance such as fire, dead wood from competitive
thinning during regeneration is of small diameter and short-lived which might limit
its value for most saproxylic species. However, in boreal forests, pioneer deciduous
trees typically available after fire might harbour rare and threatened saproxylic insect
species (Siitonen and Martikainen 1994).
The continuous production of dead wood in the various forest development
phases enables the accumulation of dead wood with a high diversity of positions,
decay stages, diameters, tree species and sun exposure (Stokland et al. 2012). As the
diversity of dead wood also has an important effect on saproxylic beetles (Brin et al.
2009), not only the dead-wood amount but particularly the dead-wood diversity
should be promoted for the conservation of saproxylic beetles (Seibold and Thorn
2018).
Different inventory methods can influence the amount of dead wood recorded
(Vidal et al. 2016). Furthermore, both the scale and the number of sampling plots
considerably influence the results of the dead-wood inventory in the field, especially
because dead wood is generally heterogeneously distributed. Therefore, caution
should be used when comparing dead-wood amounts of different studies. In a
non-exhaustive literature search for minimal and maximal amounts of dead wood
in different primary or natural forests, we established the ratio of dead wood to living
17 Importance of Primary Forests for the Conservation of Saproxylic Insects 593

Fig. 17.3 Saproxylic beetle species associated with primary forest and their natural dynamics: (a)
Pytho kolwensis Sahlberg lives in boreal virgin spruce forests and needs high volumes of dead wood
(73–111 m3/ha) and a long habitat continuity (Siitonen and Saaristo 2000) (Picture: J. Müller). (b)
Melanophila acuminata DeGeer is a pyrophilous species that detects infrared radiation emitted by
forest fires. This species requires burned trees to reproduce (Picture: H. Bussler). (c) Rhysodes
sulcatus (F.) has disappeared from most European countries because of the suppression of primary
beech forests since the Neolithic (Speight 1989) (Picture: J. Müller). (d) Goliathus goliatus Drury is
one of the heaviest beetles worldwide. This species from West and Central Africa lives in savanna
and woodland and needs large logs at the end of the decay process for its development (Picture:
G. Goergen)
594 T. Lachat and J. Müller

Table 17.3 Overview of the amount of dead wood in natural forests in different climatic domains
(not always primary)
Proportion
Deadwood Deadwood dead wood/
Climatic lower range upper range living trees
domain (m3/ha) (m3/ha) (%) Source
Boreal or 56 389 13–89 Korpel (1997) and Herrmann et al.
coniferous (2012)
forests
Temperate 32 345 4–70 Tabaku (1999), Kucbel et al.
forests 1600a (2012) and Woldendorp et al.
(2004)
Tropical 30 126 6–18 Lachat et al. (2007), Harmon et al.
forests (1995), Chambers et al. (2000) and
Gerwing (2002)
a
Extreme value from tall open Eucalyptus forests in Tasmania

trees for three main climatic domains (Table 17.3). The lower range was relatively
consistent over the three climatic domains (30 to 56 m3/ha). The upper range in
temperate and boreal/coniferous forests reached about 350 m3/ha. Extreme values
were registered in tall open forests dominated by Eucalyptus in Tasmania where the
volume of coarse woody debris can reach 750 to 1600 m3/ha for a basal area
reaching 150 m2/ha (!) (Woldendorp et al. 2004). In tropical forests, the upper
range was about 150 m3/ha. These relatively low quantities in the tropics might be
due to the absence of studies of dead wood in large-scale disturbed primary forests
and to the high rate of decomposition owing to high temperature and humidity.

17.3.5 Tree Species Composition

In all climatic domains, tree species composition of primary forests is expected to

represent the natural tree species community and to harbour typical saproxylic insect
communities associated with native tree species. Forest management and tree selec-
tion affect the composition of tree species, which in turn can influence saproxylic
insect communities. Changes in tree species composition at a high taxonomic level,
such as the transition from broad-leaved or mixed forest to coniferous forest, are
mostly caused by human activities (Nilsson 1992). In boreal forests, logging and
forest fire suppression leads to impoverished forests by actively or indirectly (fire
suppression) eliminating deciduous trees such as aspen (Populus tremula L.)
(Vanha-Majamaa et al. 2007). In Central Europe, forest ecosystems have dramati-
cally changed during the conversion of broad-leaved forests to coniferous forests for
wood production (Kirby and Watkins 2015). The simplification of tree species
composition at this level (broad-leaved/mixed forest to coniferous forest) has dra-
matic consequences for saproxylic species associated with deciduous trees because
few species are able to develop on both of these tree types and because the
17 Importance of Primary Forests for the Conservation of Saproxylic Insects 595

proportion of monophagous saproxylic species on a specific tree genus can comprise

up to 40% of the community (Jonsell et al. 1998). This is especially true for species
associated with early decomposition stages of wood (Jonsell et al. 1998).
In tropical forests, logging and especially reduced-impact logging might have a
positive effect on the tree species community (Imai et al. 2012). Indeed, Hall et al.
(2003) reported little difference in tree species composition and diversity between
highly selectively logged and unlogged plots in Central Africa. On the other hand,
more intensive logging in tropical forests might dramatically affect tree diversity,
leading to about 25% of tree species loss (Saiful and Latiff 2014). The consequences
of such loss in tropical forests are poorly understood because of the high diversity of
tree species. However, even though alpha diversity might increase in tropical forests
managed by reduced-impact logging, beta-diversity of trees would decrease as well
as the prevalence of old trees and all the microhabitats that they offer (see 3.5 Habitat
Trees).

17.3.6 Habitat Trees

Habitat trees are characterized by the tree-related microhabitats (TreMs) they carry
(Larrieu et al. 2018). A TreM is defined as a distinct, well-delineated structure that
occurs on living or standing dead trees and that constitutes a particular and essential
substrate or life site for species or communities during at least a part of their life cycle
to develop, feed, shelter or breed (Larrieu et al. 2018). The majority of TreMs can be
considered saproxylic structures caused by biotic or abiotic impacts, such as bark
lesion, cavities and breakage, which expose sapwood and heartwood. Because such
structures have mostly disappeared from managed forests, many of the species
associated with TreMs have become rare and threatened and require special conser-
vation efforts for their survival (Ranius 2002b). The lack of TreMs is especially
critical for highly specialized saproxylic insects, which cannot find an alternative
habitat in the vicinity if needed (Gouix and Brustel 2012).
Tree cavities are one of the best-studied tree-related microhabitats (Fig. 17.4).
Tree cavities can be created by primary excavators such as woodpeckers. World-
wide, more than 350 bird species are considered as primary excavators, whereas
about 1900 bird species nest in tree cavities (van der Hoek et al. 2017). In absence of
primary cavity excavator such as in Australia, cavity-dependent species rely on
decay processes following bark or wood injury (e.g. fire, rock fall or branch
breakage) and induced by fungi and insects. Species involved in excavation of
cavities can be considered as ecosystem engineers.
Hollow trees are considered keystone structures in managed forest (Müller et al.
2014) because they can harbour the full range of dead-wood decomposition stages.
While rare in managed forests, they are a common character in many primary forests,
e.g. in Fagus orientalis L. forests in Iran, one-third of all mature trees have a cavity
(Müller et al. 2016). The effect of reduction of these trees on wildlife has been shown
by various studies focusing on cavity-nesting birds, such as parrots in Amazonia
596 T. Lachat and J. Müller

Fig. 17.4 Habitat trees with cavities offer very valuable, long-lasting and complex microhabitats
for saproxylic insects, with mould (dry, humid), fungi, heartwood and sapwood, and different decay
stages (Photograph: T. Lachat)

(de Labra-Hernández and Renton 2016), and on bird and mammal cavity users in
New Guinea (Warakai et al. 2013), where at least three times more cavities were
detected in primary forests than in secondary forests. Tree cavities can remain for
decades to centuries and evolve towards larger cavities with mould (Gibbons and
Lindenmayer 2002; Lindenmayer et al. 2012; Stokland et al. 2012). Such cavities
can then harbour saproxylic insects of high conservation value, whose larval devel-
opment requires constant conditions for several years (Dajoz 2000; Gouix and
Brustel 2012). A loss of such structures is expected to threaten specialized saproxylic
insect species even though the mechanism of colonization and extinction of such
species are still poorly understood (Ranius 2002a).
At the scale of a single tree, the older or larger a tree is, the higher is the
probability that it carries TreMs (Koch et al. 2008; Ranius et al. 2009). Even though
no evidence has been found for a higher number of TreMs in late development stages
in natural temperate forests compared with early phases of the silvicultural cycle
(Larrieu et al. 2014), ecologically more valuable TreMs, such as large mould cavities
which are slow to develop, will be more abundant in overmature and senescent
phases dominated by very old trees.
For all forest types, logging considerably reduces the lifespan of trees. Indepen-
dent of the diameter or age at which a tree is harvested (e.g. at small diameters in
boreal forests and at large diameters in tropical forests), logging prevents trees from a
natural death and from ageing—the major process characteristic of primary forest
17 Importance of Primary Forests for the Conservation of Saproxylic Insects 597

ecosystems. Consequently, managed forests lack the natural number of old trees and
associated TreMs. One way to mitigate this negative effect of logging on species
dependent on specific structures is to retain single habitat trees with evidence of
TreMs (Whitford and Williams 2001). Beyond their role as a habitat, habitat trees are
also important producers of dead wood. During the senescence process, they ensure
a slow input of dead wood in the form of, e.g. dead branches in the canopy and a
dead part of the trunk, until they completely die and remain as a snag or fallen
dead tree.

17.4 Implications for Conservation

Saproxylic species represent a significant proportion of the biodiversity in primary

forest in all climatic domains. For some species groups, this proportion can reach up
to 60%. For example, 56% of all forest beetles are associated with dead wood in
Central Europe (Köhler 2000), 35% in boreal forests in Finland, 33% in lowland
tropical forests of Sulawesi (Hanski and Hammond 1995) and 60% in Eucalyptus
forests of Tasmania (Grove, personal communication). Overall, about 25% of all
forest species are considered saproxylic. Many studies have shown the role of
old-growth forests for the conservation of saproxylic species (Martikainen et al.
2000), and even more have revealed the impact of forest management on this
sensitive species group (Paillet et al. 2010). However, few studies have focused on
saproxylic insects in primary forests (Grove 2001; Lachat et al. 2006; Lachat et al.
2016; Müller et al. 2016) because (1) most research is conducted in areas with few
remnants of primary forests; (2) studies of primary forests are logistically demand-
ing, mostly because of the remoteness of the remaining primary forests; (3) the
uniqueness of remnants of primary forest considerably limits the possibility of
researching beyond case studies; and (4) the taxonomic knowledge on saproxylic
insects from primary forest is insufficient (especially in the tropics).
Saproxylic insects are a highly threatened taxonomic group (Davies et al. 2008).
The threat goes back to the Neolithic clearances and concomitant woodland man-
agement practices and has been intensified in the past centuries. In Europe and
Northern America, two centuries of intensive commercial forestry and agricultural
management practices have strongly impacted natural forest ecosystems and their
associated species (Speight 1989). In tropical forests, the status of many saproxylic
species remains undocumented (Grove 2002b), but the situation for saproxylic fauna
is likely to be similar to that in Europe (Ghazoul and Hill 1999). In all climatic
domains, the loss of intact forests contributes directly to the biodiversity extinction
crisis (Mackey et al. 2015).
Nowadays, different conservation measures for saproxylic species such as the
retention of key structural habitat elements (habitat trees, snags or lying dead wood)
can be integrated in commercially used forests (Kraus and Krumm 2013).
Conservation-oriented forest management can therefore play a very important func-
tion for the conservation of saproxylic insects. Unfortunately, many of the resources
598 T. Lachat and J. Müller

required both quantitatively and qualitatively by saproxylic species cannot be

integrated completely into forest management schemes because the ecological
requirements of highly demanding species cannot be fulfilled in managed forests.
In landscapes where primary forests have mostly disappeared like in Europe or
North America, segregative conservation instruments such as protected forest areas
are needed for the conservation of saproxylics. Such approach often leads to tiny
relict forest reserves in a matrix dominated by intensive forestry or agriculture. Such
reserves, however, represent the backbone of the saproxylics conservation. For
regions with large areas of primary forest coverage or where there is a shorter history
of forest loss or intensive use, applying conservation principles at the landscape scale
is probably more likely to succeed. In this scenario, large reserves are important.
Because such large reserves are mostly isolated, smaller reserves help to fill the gaps
and to improve the migration of individuals from population sources.

17.5 Conclusions

For the conservation of saproxylic insect species, all types of primary forest should
be strictly protected as these forests represent the last intact biome on Earth where
these species can develop under natural conditions. The conservation priority might
be higher in regions with high endemism and high annual rates of primary forest
destruction, such as in the tropics, even though the rate of decline appears to be
slowing (Morales-Hidalgo et al. 2015). Strict protection is also needed in regions
with very low remaining proportions of primary forest, such as Central Europe,
where only 0.2% of the deciduous forests are considered to be in a natural state
(Hannah et al. 1995). Species associated with primary forest in Central Europe are
therefore also highly threatened, and their destruction would be fatal for the species
that have been maintained to date. Besides their role as refuges for saproxylic species
(Bengtsson et al. 2000), primary forests also represent references for managed
forests. Dead-wood amount, density of habitat trees, proportion and area of canopy
gaps and species assemblages in primary forests should be considered when setting
goals for the management of near-natural forests (Lachat et al. 2016). For this, more
research in primary forests worldwide is urgently needed to better understand their
complex ecology to stimulate sustainable forest management.

Acknowledgments We thank Sarah Hildebrand for her precious help on boreal forests and Karen
Brune for editing the manuscript. We also gratefully acknowledge the reviewers of this chapter.

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