2.3. Roost networks

minimise energy expenditure concerning the warming of breeding roosts. The cavities selected are thus situated high in trees in order to limit the predation risk where the canopy foliage is

With increasing pressure for timber products in European forests, tree-dwelling bats must demonstrate sufficient plasticity in their behaviour in order to adapt to the challenge of a fluctuating availability of suitable roosts. Nyctalus noctula and N. leisleri, which preferentially use old trees of large diameters [29], can adapt to using young trees [12]. Myotis nattereri may target woodpecker hollows but is also known to utilise narrow fissures in small diameter trees such as Birch, Betula sp. However, this capacity to adapt can have its limits. In New Zealand, certain colonies of Chalinolobus tuberculatus appear no longer able to produce enough young to maintain colony numbers when usable cavities are lacking and food resources are reduced following forest exploitation [35]. Hence, management can affect the capacity of forests to produce the required roosts for maternity colonies. As a result roosts clearly become a limiting

Bat colonies select cavities of which the size and the type govern the structure and the numbers of the group occupying them. Consequently, pregnant females of Myotis bechsteinii may target small cavities high in trees limiting the risk of predation, and then select large cavities with a significant volume that can shelter larger groups at lower sites when they are lactating. In contrast, pregnant females of Plecotus auritus select a diversity of cavities in order to use smaller roosts after the birth of young (therefore limiting the size of the group) [31]. Most available cavities in forests have a reduced internal volume. Indeed, cavities of large volume take longer to form and are mainly found on old trees of large diameters [36, 37]. In order to respond to such ecological constraints, i.e. the limited number of tree microhabitats of suitable volume, forest-dwelling bat populations split into subgroups, demonstrating an exchange of individuals on a daily basis [38, 39]. Thus, groups divide and disperse each night (fission) then form new groups the following day, reorganised in accordance to the specific rules of each

Fission-fusion may limit the risk of parasitism [38, 40, 41], as tree cavities being confined spaces are conducive to the development of micro-organisms linked to wood degradation [19] and maintain an increase in humidity and heat favouring the development of bat parasites. In mammals, parasite density decreases with the frequent changing of rest sites [42]. For bats, this phenomenon outlines the importance of using a large number and diversity of cavities. The changing of roosts also allows bats to limit the risk of predation, as predators would no longer know which cavity exit to prowl [41]. Though, the frequency in which bats switch roosts is more dependent on meteorological conditions and the individuals' reproductive status [31, 43]. A disadvantage is that over time, these repeated fissions can lead to a loss of familiarity among individuals as life in a fission-fusion society imposes constant regroupings of different individuals [17, 22, 44, 45]. Myotis bechsteinii counteracts this problem with the oldest females organising the colonies around a few lineages or familiar groups that have maintained social links for more than 5 years [45]. In every example, fission-fusion behaviour

sparser favouring the insulation of roosts [34].

factor for bats.

66 Bats

2.2. Fission-fusion behaviour

species, often selecting a new roost (fusion) [38].

The turnover of roosts in relation to fission-fusion dynamics occurs, on average, every 2 or 3 days (Table 1). As a result, a substantial quantity of available cavities within the home range of a given bat colony is essential. The frequency in which bats change roost is largely influenced by the individuals' reproductive status, as outlined above. Females often change roosts when pregnant and tend to stay longer in the same cavity when lactating because maintaining higher temperatures, which is crucial, is energy-intensive for animals [22]. Accordingly, Nyctalus lasiopterus changed the cavity every 2.52 0.74 days before the birth of young but stayed 4.88 1.91 days in a cavity after birth [47]. Similarly, pregnant Barbastella barbastellus switched roosts every 2.6 1.6 days, whereas lactating females stayed in the same cavity for up to 9.4 1.8 days [27]. Conformably, reproductive females of the species Myotis bechsteinii, M. nattereri and Plecotus auritus stayed longer in their roosts than non-reproductive females [31].

Myotis bechsteinii may exploit more than 300 different cavities in the same year [31] and is able to use more than 15 different zones for roosting purposes [48]. This is in line with other species such as Myotis nattereri, Barbastella barbastellus and Plecotus auritus [13, 28, 31]. In New Zealand a colony of Chalinolobus tuberculatus used more than 300 cavities and re-used only 48% over the course of a year [49]. Conversely, Mystacina tuberculata selected a small number of roosts but was likely to stay for longer periods of time [50], demonstrating a much higher loyalty to its chosen cavities. Roosts should be voluminous in order to accommodate a large number of individuals (310 88.1 on average) [51]. However, for the majority of species, colonies do not


Table 1. Average duration of presence ( standard deviation) in one roost by a number of forest-dwelling bat species demonstrating a fission-fusion society, in New Zealand (1), North America (2) and in Europe (3).

exceed more than a few dozen individuals in one roost. This pattern was observed in the small European tree-dwelling bat species such as Myotis bechsteinii, Myotis nattereri, Barbastella barbastellus and Plecotus auritus [31]. On the other hand, Nyctalus noctula composes larger groups that occupy large cavities for greater periods of time (unpublished). These diverse behaviours thus imply the need for different conservation strategies. Bat colonies that exploit many cavities are less sensitive to the disappearance of roosts but species that occupy a tree cavity for longer lengths of time are obviously more affected by their disappearance [32, 50]. Thus, the forest manager is faced with the challenge of maintaining a high capacity to accommodate all species throughout his territory. In addition, depending on the forest, the number of cavities can vary considerably. If we consider only woodpecker hollows and fissures on live trees, the number of microhabitats per hectare in a production forest may be greater than 10, while other forests may offer only a very limited or even zero carrying capacity [2, 6, 17, 32]. In the latter, a loss or alteration of roosts constitutes a major limiting factor for bats.

associated with these tree species [62]. Native Quercus spp. have the highest number of dependent insect species including various orders of saproxylics as well as defoliators (Coleoptera, Lepidoptera, Heteroptera, etc.). They are followed by Betula spp. which can sometimes have a greater number of individuals present but have less taxonomic groups associated with Salix spp., Crataegus spp., Prunus spp., and Populus spp. The first conifer species is Pinus sylvestris before Fagus spp. and Picea abies [63]. Moreover, the more the forest habitat is diversified, the more the insects' emergence is spread over time. For example, defoliators commonly emerge at different times in accordance with shade-tolerant and shade-intolerant trees [19]. In addition, a diversification of accompanying tree species and a strong presence of forest gaps in mature conifer plantations can have a positive impact on bat activity [64, 65], with bats contributing significantly to the control of insect pests in forests [66]. Furthermore, a higher density of vegetation and a greater heterogeneity from the ground to the canopy appear to increase bat species richness. Myotis bechsteinii principally forages within the dense canopy [31, 67] whereas Myotis myotis is a specialist of bare ground-dwelling prey [68]. Indeed, a complex structure of the forest with dense foliage, depressions, protuberances, and other ecotones is favourable to the development of different thermal and hygrometric conditions, the source of high entomological production [23, 69]. This can result in a higher activity of hawkers and gleaners [59, 70–72]. The latter forage for prey by gleaning insects from the substrate in dense foliage, while the former requires open spaces, such as forest clearings, paths, corridors, and edges even though they mainly hunt insects dependent on foliage [59, 70, 71]. Hence, the more diverse a forest is in composition, structure, and stratification, the higher bat species abundance and species richness will be [2]. Finally, additional forest environments such as streams and ponds, which present drinking sites for all individuals, also favour the occur-

Bat Conservation Management in Exploited European Temperate Forests

http://dx.doi.org/10.5772/intechopen.73280

69

In Europe few bat species roost in dead trees. Nyctalus leisleri and N. lasiopterus are known to roost from time to time in cavities found on dead or dying trees [29, 73], and Barbastella barbastellus roosts regularly behind peeling bark, especially on snags [27, 28, 74] just like some Myotis and Pipistrellus species. The use of these ephemeral roosts is usually only part of a

Deadwood constitutes an important support for the development of wood decaying insects with nearly a third of all forest insects directly depending on it [19, 26, 75]. Foraging bats can indeed take advantage of the presence of fresh deadwood by targeting any emerging Coleoptera insects. It is often the case with conifer stands that have been cut and stacked which favour the rapid concentrations of bark beetles (Scolytidae), subsequently attracting opportunistic species of Nyctalus, Eptesicus and Pipistrellus (Figure 1) [76, 77]. More widely, the richness of bat species has been found to be positively correlated to the volume of deadwood, either lying or stand-

explained by deadwood-dwelling preys or by changes in the forest structure, due to openings created by dead trees that are favourable for edge-hawkers such as Pipistrellus spp., Eptesicus

/ha [78]. This relation can be

rence of forest-dwelling bats [6, 7].

network of trees surrounding optimal roosting sites [13].

ing, greatly increasing when deadwood quantity exceeded 25 m3

4. The role of deadwood
