Download Master thesis for master Environmental Biology at Utrecht University

Master thesis for master Environmental Biology at Utrecht University.
Written at Wageningen University.
May 2013 – June 2013
10th June, 2013
The Influence of Artificial Night Lighting on Bats
Author: Manon Ravesteijn
UU: 3344541
WUR: 900917681040
Supervisor: Dr. Roy van Grunsven
Nature Conservation and Plant Ecology
Wageningen University
Preface
This thesis has been written as the completion of my master education Environmental
Biology at the University of Utrecht. During my master I did two research projects, which
were both about companion animals. I therefore looked for a thesis subject that would have
a clear ecological component, to complement my earlier applied research topics. I was
pleased when I found a ongoing study in Wageningen about the influence of artificial night
lighting on a variety of species. I chose to focus on bats, since they are very interesting
animals and of conservational concern. Even though I can see bats very often from my own
living room, I did not have much knowledge about bats before the start of this thesis. As a
result I have learned a lot during the writing process. Not just about the process itself, but
also that bats are even more diverse and interesting than I had imagined.
I would like to thank my supervisor Roy van Grunsven for his willingness to supervise me and
his comments on the manuscript. On a more personal note I would like to thank my
boyfriend and family for their support and not disturbing me when I wanted to write.
All in all I really enjoyed working on my thesis and I hope it will contribute to research into
bat conservation and increase the awareness of the effects of artificial night lighting on
biodiversity in general.
Manon Ravesteijn
Veenendaal, June 2013
2
Contents
Page
-
Abstract
4
-
Laymen’s summary
4
-
Introduction
5
-
Biology of Bats
7
-
Influence of Artificial light on bats
11
-
Impact reducing measures
15
-
Conclusions
17
-
References
19
3
Abstract
This thesis summarizes studies on the influence of artificial night lighting on bats. It reviews
normal bat biology, the influences of light on behaviour and possible mitigation strategies.
Artificial light can have both positive and negative effects, depending on the species. Some
species use lights to forage, since many types of lamps attract insects. On the other hand,
many bat species avoid lights when commuting, foraging or when selecting a roost site,
possibly resulting in a lower fitness when forced to take alternative routes or roosts. Several
measures have been developed to reduce disturbance of bats by lights. The type of light,
light colour, light intensity and the timing of artificial lighting can all be adjusted so that bats
are less affected. Planting screening vegetation can also reduce light spillage in the
surrounding environment of the lights. To promote and sustain a diverse bat population a
diverse environment is needed. Therefore, urban areas populated by bats should also
contain darker places, such as unlit parks, to support bats that are disadvantaged in light
circumstances.
Laymen’s summary
Many bat species are threatened and therefore protected by conservational laws.
Conservation has mainly focused on providing suitable roosts, resting places, but foraging
areas and the routes between roost and feeding area have to be suitable for bats as well.
The last century has known a dramatic increase in the amount of electrical lighting used to
illuminate the night. Many roads, city centres and sport parks are lighted for at least a part
of the night. This has effects on night animals, which are optimally adapted to dark
circumstances. Bats are one of these animal orders that are influenced by artificial night
lighting. Some bat species are attracted to lights to hunt on insects that have gathered
around the lamp, but other species avoid the lights. This can lead to lower foraging and
reproduction success, resulting in declining populations. Conservation of bats should
therefore also focus on minimizing disturbance from artificial lighting. Depending on the
specific species, this can be achieved by changing the type or colour of the lamp, reducing
light intensity, imposing light curfews and planting screening vegetation around the lighted
area. When it is an option not to light a area at all, this is naturally the preferred choice.
4
Introduction
Most of the land area that was covered by natural vegetation has been converted for
human use, restricting most of animal life to smaller nature fragments surrounded by
agricultural land or cities (Reis et al., 2012). This process has consequences for the animal
populations, such as a reduced habitat, diminished food supplies and increased inbreeding
(Reis et al., 2003). For most organisms, habitat destruction and modification have been
gradual, where the local population is declining and the impacts are perceived only when
species disappear from a significant part of the original habitat.
One of the ways in which humans have altered the environment is by artificial
illumination of the night. There has been a sharp increase in artificial lighting in the past
century (Frank, 1988), and an increasing part of the world is affected by electrical lighting.
The term ‘light pollution’ usually refers to ‘astronomical light pollution’, where stars and
other celestial bodies have become invisible because of light that is either directed or
reflected upward (Longcore & Rich, 2004). It has been calculated that approximately 18.7%
of the earths terrestrial surface is subjected to nocturnal sky brightness that is polluted by
astronomical standards (Cinzano et al., 2001). These illumination levels might affect
ecosystems, but light sources that do not contribute to sky glow, or barely, may also have
ecological consequences. This ensures that ‘ecological light pollution’ afflicts a greater
proportion of the earth than astronomical light pollution alone (Longcore & Rich, 2004).
Sources of ecological light pollution contain sky glow, lighted buildings, street/security
lighting, lights on vehicles, flares on offshore oil platforms and lights on underwater research
vessels, all of which potentially disrupt ecosystems (Longcore & Rich, 2004).
Ecological light pollution has evident effects on behaviour and population ecology of
animals. These effects follow from changes in (dis)orientation and attraction or repulsion
from the altered light conditions, which in turn may affect foraging, migration, reproduction
and communication (Longcore & Rich, 2004; Hölker et al., 2010). Increased orientation at
night can result in diurnal animals extending their activity into the night. This can be in using
the artificial light for foraging (birds and reptiles) or communication (territorial singing in
birds) (Longcore & Rich, 2004). On the other hand, animals that are adapted to a dark
environment may find ecological light pollution disorienting. A considerable proportion of
global biodiversity is nocturnal (30% of all vertebrates and > 60% of all invertebrates), and
their differentiated niche has promoted highly developed senses (Hölker et al., 2010). The
most famous example of disorientated animals are the hatchlings of sea turtles, which are
no longer ‘directed’ away from the dark dunes towards the sea (Salmon, 2003; Salmon,
2006). Reproduction and communication can also be altered by night lighting. Some species
use light as a way of communication (glow-worm), which is interrupted if the environment is
also light, and birds prefer nest sites away from night lighting (Longcore & Rich, 2004). The
change of behaviour in response to night lighting by individual animals or species will
eventually influence community structure. If the darkest natural conditions do not occur any
longer, the species exploiting dark conditions will be disadvantaged, possibly resulting in
local extinction (Hölker et al., 2010). This changed community structure could then affect
ecosystem characteristics (Longcore & Rich, 2004).
Since most bats are nocturnal, they may be largely influenced by artificial night
lighting. Their eyes are adapted to low light levels, resulting in impaired vision in high light
levels, and therefore orientation ability tends to be better in lower light levels (Fure, 2006).
This can influence bats on their commuting routes when the routes are suddenly lighted,
5
having a major impact on bat activity (Stone et al., 2009). Other possible influences are bats
following their prey into the light (Svensson & Rydell, 1998; Rydell, 2006), or bats shunning
the lights to avoid their predators (Reith, 1982; Russo et al., 2011). Despite possible negative
effects on bats by artificial night lighting, some species do very well in cities and other urban
areas (Coleman & Barclay, 2011).
Since many bat species are threatened it is important to know in what ways artificial
night lighting effects individual bats and bat populations. Almost all European bat species are
protected by conservation laws (Dietz et al., 2011), and knowledge about the influence of
night lighting can be applied in regulations for road lighting and other sources of artificial
illumination. Therefore this thesis concerns the direct and indirect influences of artificial
night lighting on bats. The biology of bats will firstly be discussed, followed by the influence
of artificial lighting on several bat behaviours, and lastly possible mitigation strategies will be
considered.
6
Biology of bats
Bat diversity
About 25% of mammal species are represented by the order of bats (Chiroptera).
These are divided in two suborders: the Macrochiroptera and the Microcheroptera.
Macrochiroptera, also known as Old World fruit bats, are mostly large and mainly
nonecholocating. They feed on fruits, flowers and nectar, and are all tropical or subtropical.
Microcheroptera are usually small, use echolocation and feed predominantly on insects.
They occur in almost all terrestrial habitats, from forests to deserts, but are most abundant
in the tropics. The plant-feeding and carnivorous Microcheroptera are exclusively
(sub)tropical, but the insectivorous Microcheroptera also occur throughout the temperate
regions (Rydell, 2006).
Bats could appear to be a uniform group of mammals, but no other mammal order
occupies as many ecological niches as bats. The largest part of bats feed on insects, often
using echolocation. Other species complement an insectivorous diet with amphibians,
reptiles and small mammals. The second largest group of bats feeds on fruits. They can eat a
variety of fruits, depending on the availability in each season, and sometimes even feed on
certain leaves when fruit is unavailable. Other species have specialized in hunting reptiles,
amphibians, birds and even fish. The specialization of vampire bats is probably one of the
reasons bats have bad reputation: they make a very small wound in the skin of large animals
and feed on the blood. A final interesting food source are nectar and pollen. Some very
specialized species are feeding exclusively on nectar and pollen from flowers, and because
they can flutter in front of the flower when feeding they are also called ’hummingbirds of
the night’ (Dietz et al., 2011).
Insectivorous echolocating bats have to be small to maneuver rapidly in the air when
hunting for insects (Rydell, 2006). In spite of the need to be small, size and wing morphology
varies widely between species. Together with the type of echolocation used (see next
section) are these the main determinants of flight speed, agility, and prey detection
capabilities, establishing to a large extent habitat use and diet (Norberg & Fenton, 1988;
Rydell, 2006). In general, large bats and long-winged bats fly faster and are less
maneuverable than small or broad-winged bats (Rydell, 2006). Furthermore do fast-flying
bats use a form of echolocation suitable for a long prey detection range, but unsuitable for
detecting small prey. Consequently, they feed predominantly on larger insects such as moths
(Barclay, 1986).
Echolocation
Echolocation might by one of the best-known features of bats. Many bats, especially
those hunting for flying insects, use echolocation when foraging to localize and identify prey
(Schnitzler & Kalko, 2001). Bats also use echolocation for orientation, so to define their
position relative to the echo-producing objects. Most bats echolocate by producing the very
high pitched vocal signals in their larynges and analyze the returning echoes (Schnitzler &
Kalko, 2001; Fenton, 2013). The distance to the echo-producing object can be determined
accurately in this way, but the direction can not. However, bats have developed a complex
shape of the ear to use a direction specific frequency filter to decide from which direction
the echo is coming. The pinna is built in such a way that the incoming sound is modulated
into a direction specific frequency filter, by interference between different routes to the
7
eardrum (Dietz et al., 2011). In this way the vibrations of some frequencies are enhanced,
and others reduced.
Bats use a wide variety of species-specific signal types, differing in frequency
structure and duration. In addition, the signal structure varies depending on the
echolocation task confronting the bat: signals that are emitted when bats search for prey
differ from signals that are emitted when they approach prey (Schnitzler & Kalko, 2001). The
frequency of the signal determines the recognisability of objects, since higher frequency
signals allow to see smaller objects. The downside of the high frequencies is a high
atmospheric absorption. Therefore high frequencies have a very small detection range, so it
is most useful in dense vegetation or when closing in on prey (Dietz et al., 2011). Another
parameter of echolocation is bandwidth. The advantage of a narrow band is the high
sensitivity and the opportunity for specialization, but a broad band can give information on
the direction of the object and its surface (Schnitzler & Kalko, 2001). The last parameter is
the duration of the signal. Long signals heighten the chance to locate an interesting echo,
and increase the signal-noise ratio. However, with long signals there is a chance to become
overwhelmed by all the returning echoes. So a long signal is only useful in an open space
with few returning echoes, while short signals are better suited for cluttered spaces (Dietz et
al., 2011).
Vision
The human retina contains two types of photoreceptor cells: rods and cones. Cones
function best in bright light and register colour and detail, while rods work in low light
conditions and detect basic motion and basic visual information. So the rods are more
sensitive to and functional in faint light than cones. Therefore it is not surprising that most
bats have no cones at all and some other nocturnal animals just have a few (Fure, 2006).
However, cone-like photoreceptors are present in some insectivorous bats, which are
presumably early emerging species benefitting optimally from the available light at emerging
(Fure, 2006). Some bats with these cone-like structures have dichromatic colour vision, but
other species only have type of cone and are colour blind (Müller et al., 2007). Dietrich and
Dodt (1970) calculated that the light-gathering power of the mouse-eared bat (Myotis
myotis) is four to five times that of man. This suggests that bats can readily use visual cues at
dusk, when they normally emerge from their roosts, and probably also under nocturnal
conditions (Ellins & Masterson, 1974). Since the retina of bats constitutes mainly of rods,
their visual sensitivity typically weakens when light levels increase towards daylight levels.
(Bradbury & Nottebohm, 1969) verified behaviourally that little brown bats (Myotis
lucifugus) avoid obstacles better when the light conditions resembled dusk, than when they
resembled daylight. Even though their eyes generally work well under low illumination, the
sensitivity to light levels and brightness discrimination varies substantially between different
bat species (Eklöf, 2005).
So bats do not only use echolocation for orientation and navigation, but visional cues
can also be used, especially over larger distances (Schnitzler & Kalko, 2001). Therefore bats
do not echolocate often during migration, but rely mainly on vision (Johnson et al., 2004).
Another study, looking at the opsin genes, suggested a functional role of vision in the little
brown bat. They speculated that this echolocating bat may be able to use visual cues to
orientate, navigate and forage at night, to discriminate colours in moonlight and starlight
conditions, or to avoid predation (Zhao et al., 2009). Ruczyński et al. (2011) showed that the
brown long-eared bat (Plecotus auritus) was more effective in finding conspicuous roost
8
entrances when light was provided. They suggested that those entrances were better visible
due to high contrast between the light bark and dark wood and that visual cues could play a
role in preselection of roost sites for this species.
Vision can also be used in foraging. The northern bat (Eptesicus nilssonii) uses visual
cues, at least in the initial search phase, to complement echolocation when foraging for
stationary targets in clutter (Eklöf et al., 2002). The brown long-eared bat uses vision not
only for roost (pre)selection, but also for foraging. The bats preferred foraging situations
with both sonar cues and visual cues, and the visual information was considered more
important than the sonar information (Eklöf & Jones, 2003).
Nocturnal activity and foraging
Bats usually hunt during the night, and mainly use their hearing. They can use the
buzzing sound of wings, the rustling of leaves on the ground or the echoes from
echolocation to locate their prey (Dietz et al., 2011). Bats are rarely seen hunting during the
day, even when really hungry, and multiple hypotheses have been developed for why they
are exclusively nocturnally active. The first hypothesis is that it is an adaptation to avoid
predators. Bats are quite helpless against birds of prey, so avoiding them by becoming active
when it is dark and they are hard to see seems an useful adaptation. In support of this
hypothesis is the observation of diurnal activity by bats on an island without predatory birds
(Russo et al., 2011). Other hypotheses are that they can find more prey at night because
there is no competition from diurnal birds, or that they will get overheated by absorption of
sunlight by their uninsulated wings (Speakman & Hays, 1992). Although this last option is
more likely a result from nocturnal living than a reason for it.
The hunting methods of bats are very diverse. Aerial hawking bats hunt in open
areas, where barely any disturbance from background echoes is present. Most insectivorous
European bats are aerial hawkers (Dietz et al., 2011). When hunting close to vegetation,
echoes from the vegetation will overlap the echoes coming from the prey, making it more
difficult to localize prey. Horseshoe bats (Rhinolophidae) have solved this problem by
specifically recognizing the wing-strokes of insects, and vesper bats (Vespertilionidae) by
increasing the differentiation of different echoes by using very short signals and broadening
their bandwidth (Dietz et al., 2011). Other bats also hunt on non-flying arthropods and use
passive localization of their prey. They catch prey by flying close to the ground and landing
when they hear a rustling insect (Dietz et al., 2011). Several bat species can hunt over open
water and catch insects on the surface. These species often have relatively large feet with
long toes, so they can catch the prey with their feet before bringing it to their mouth to eat
(Dietz et al., 2011).
Bats are opportunistic selective feeders in the sense that their morphology, hearing
and foraging tactics limits the diversity of potential prey, but they will eat whatever is
available and palatable (Dietz et al., 2011). The long fingered bat (Myotis capaccinii) not only
eats insects from the water surface, but also hunts on small fish swimming close to the
surface when they are present (Aihartza et al., 2003). Despite the high specialization within a
certain biotope, most species can employ multiple foraging tactics, which allows them to
hunt in diverse areas. The whiskered bat (Myotis mystacinus) for example has been shown to
forage in both riparian zones as well as mixed woodlands (Buckley et al., 2013).
Bats do not rely solely on hearing when searching for prey. As mentioned before, also
vision can play a part in the initial search phases for prey, especially when foraging in clutter,
but other sensory cues are exploited as well. Page et al. (2012) assessed the ability of bats to
9
assess prey cues during capture, handling, and consumption, when confronted with
conflicting information about prey quality. They used advertisement call from a preferred
prey item, while offering palatable, poisonous, and chemically manipulated frogs as prey.
Their study suggests that echolocation and chemical cues obtained at close range improve or
complement information obtained from acoustic cues at long range.
Roosting
The use of caves as roosts may have been one of the key developments in the
evolution of bats (Dietz et al., 2011). Even now, many species still use caves as shelter, using
the darkness as a protection against predators. Some of these cave dwelling species
expanded their habitat by colonizing artificial ‘caves’, such as roof spaces. Not all species are
roosting in (artificial) caves however. Some bat species use small crevices in rock, buildings
or trees, instead of large spaces. They prefer small spaces that are just wide enough to fit in.
Many bat species (also) use tree cavities as roosts, which are much more abundant and
easier to find than scarce caves, especially for migrating species (Dietz et al., 2011).
Bats are generally very quick in finding roosts when released in an unfamiliar
environment. It is likely they are constantly paying attention to cavities and crevices that
would be suitable as shelters. Bats can create a search image based on experience, which
makes it easier to find suitable roost sites. Some species, which are not so good at detecting
new roost sites themselves, tend to follow smaller species to roost sites, or use calls of other
species (Dietz et al., 2011).
Many species of bats change roost frequently (Lewis, 1995). This happens mostly
when roost sites are highly available but less permanent, such as trees. Switching may result
from trade-offs between advantages from staying at a roost site, like greater site familiarity
and maintenance of social relationships, versus switching, like decreased commuting costs,
choosing roost with best microclimate, lower probability of predator detection, and lower
ectoparasite levels (Lewis, 1995). Even though female whiskered bats did not utilise roosts
where environmental conditions frequently change, they used more than one roost during
the breeding period. The most likely scenario in that study is that bats switched to roosts
close to their core foraging areas, since bats roosted in close proximity to their foraging
areas and roosts were occupied by a relatively small numbers of bats (Buckley et al., 2013).
Reproduction
Mating season for temperate species is in September and October. Even though
spermatogenesis in most bats peaks in summer, mating and development of accessory
organs are delayed until autumn (Gustafson, 1979). Migrating females mostly find their
partners on their migrating route, or in the winter roosts and sedentary species find their
mates often in the winter roosts as well (Dietz et al., 2011). After mating, females hibernate
with the spermatozoa stored in their reproductive tracts for about 5 months until spring
(Kawamoto, 2003). After awakening in spring, fertilization and implantation take place and
female bats form maternity colonies (Oxberry, 1979; Kawamoto, 2003). The highly
synchronized parturition within a colony is probably caused by the rising temperature that
influences all females in a similar way. This also has the ecological advantage of lowering
predation, since young animals are only present for a very small amount of time (Dietz et al.,
2011). The timing of lactation in summer matches with peak food availability, which is likely
to maximize reproductive success (Kawamoto, 2003).
10
Influence of artificial light on bats
Activity and foraging
Artificial light sources often lure insects, leading to an accumulation of insects around
the light. Three main kinds of lamps are used as streetlights: mercury vapour lamps, emitting
bluish white light including ultraviolet, high pressure sodium vapour lamps, emitting orange
light including some ultraviolet, and low pressure sodium vapour lamps, emitting nearly
monochromatic yellow light without ultraviolet. The mercury vapour lamp and to a lesser
extent the high pressure sodium lamp therefore mainly attract insects (Rydell, 2006). Bats
are attracted to the lights because of the insect accumulation and come to forage in villages
as well as (sub)urban areas (Rydell, 1992). Not only does evidence suggests that streetlights
increase the feeding efficiency of these bats (Svensson & Rydell, 1998), but it may also
supply food on a spatially more predictable basis than other habitats (Rydell, 2006).
During broadband bat monitoring in which nine species were observed, four species
foraged around streetlights, but five other species were only observed away from the lights
(Rydell, 1992). Apparently, artificial lights do not enhance foraging success in all bat species.
Their research also suggested that only the fast-flying species using long-range echolocation
were likely to be found foraging around streetlights while the slow-flying, manoeuvrable
species did not feed around the lights at all (Rydell, 1992).
Another study surveyed bat activity at eight sites in Adelaide. Five parkland sites and
three sports parks with the capacity for artificial lighting were monitored weekly for bat
activity. Gould’s wattled bat (Chalinolobus gouldii) and Mormopterus species were the only
species observed at the three sport sites when the floodlights were either on or off, while
the white-striped freetail bat (Tadarida australis), chocolate wattled bat (Chalinolobus
morio) and Vespadelus species were only present when the lights were off. Unlike the
previous study, in which artificial lights were considered important foraging areas for
insectivorous bats in urban environments, this research stresses that the dark areas were
more important for bat diversity in urban parklands. The chocolate wattled bat was
completely absent from lighted areas and avoided illuminated parks, but it was recorded at
those same parks when the lights were off (Scanlon & Petit, 2008).
Stone et al. (2009, 2012) conducted experiments with both high pressure sodium
lights and light-emitting diode lights (LED lights) along hedgerows in use as bat commuting
routes. Both kinds of light had no influence on the activity of the common (Pipistrellus
pipistrellus) and soprano (Pipistrellus pygmaeus) pipistrelles and Nyctalus and Eptesicus
species. However, activity by the lesser horseshoe bat (Rhinolophus hipposideros) was lower
during lit treatment by high pressure sodium lights, demonstrating that these lights had a
significant negative effect on bat activity (Stone et al., 2009). LED lights reduced commuting
activity for the lesser horseshoe bat and activity for Myotis species. Lesser horseshoe bat
activity was significantly lower during high than during medium light intensity treatment, but
Myotis species showed no difference in response to differences in light intensity. Despite the
differences in response to light intensity, the activity in both the lesser horseshoe bat and
Myotis species were significantly lower during all light treatments than during the control,
even for very low light levels (3.6 lux, low light treatment) (Stone et al., 2012). Their results
also indicated that there was no evidence of short-term habituation during lit nights and that
bats did not avoid the lit side of the hedge by flying down the darker unlit side (Stone et al.,
2009; Stone et al., 2012).
11
Linear habitat features such as hedgerows are important for many bat species, but
the reliance on those features makes them susceptible to habitat fragmentation (Stone et
al., 2009). The results imply that light pollution may fragment the network of commuting
routes, causing bats to alter their commuting behaviour. Commuting bats may respond to
light disturbance of their commuting route in four ways: (1) flying high above or around the
lights, (2) flying on the unlit side of the hedge, (3) choosing an alternative route, and/or (4)
returning to the roost (Stone et al., 2009). At each site, several bats were observed flying
alternative commuting routes when the hedgerows were lit, and it seems likely that most of
the bats selected an alternative route in response to lighting their preferred route. The
fitness consequences of switching commuting routes depends on the availability, length, and
quality of alternative routes. If they are suboptimal in terms of quality or distance to foraging
areas, this could have conservational consequences. Alternative routes could provide less
shelter against predation or increase exposure to wind, creating higher costs to foraging
(Stone et al., 2009). When no alternative routes are available it may lead to the
disappearance of the colony.
Another way in which artificial lighting can influence bats is by creating different
circumstances for interspecific competition. The lesser horseshoe bat has known a dramatic
decline over the past decades, even in the better preserved Swiss highlands. Most of these
valleys were still harbouring large populations of the bat until the 1960s, but are nowadays
virtually abandoned, even though landscape, roosts and insect fauna have not suffered
dramatic alterations (Arlettaz et al., 2000). The common pipistrelle bat has however
massively expanded, also in the Swiss valleys, probably due to adapting to hunting around
streetlights. Since both species feed on the same prey, it might be possible that the
pipistrelle bat has gained a competitive advantage over the horseshoe bat when street lights
were installed, since the lesser horseshoe bat does avoid lights. Street lights cannot sustain
insect populations per se, but attract insects from the surrounding natural environment
(Rydell, 1992). In consequence, attraction of the available prey towards the artificial lights
could deplete food in the surrounding zones, optimizing foraging circumstances for the
common pipistrelle, but leading to a depleted foraging area for the lesser horseshoe bat
(Arlettaz et al., 2000). Whether this is actually what is happening has not been determined
yet, but studies on the insect distribution could shed light on this subject.
A recent study investigated the effects of artificial lighting in the desert on the flight
behaviour of two aerial insectivorous bat species: a synanthropic bat, common in urban
environments, and a desert-dwelling species. Whereas the synanthropic bat foraged under
both light and dark conditions, the desert-dwelling species only foraged in the dark (Polak et
al., 2011). The activity of the desert-dwelling bat decreased in the light and it only crossed
the lighted area at commuting speed rather than foraging speed. Thus the non-desert
synanthropic species may have a competitive advantage over the native desert species
under artificially lit conditions and may outcompete it for aerial insect prey (Polak et al.,
2011).
Roosting
Artificial light may not only have an influence during active periods, but can also
affect the commencement of activity. Downs et al. (2003) looked at the influence of
different light colours and intensities on the emergence behaviour of the soprano pipistrelle.
At both of the roosts they studied, most bats emerged when lights were off and fewest
when the white light was switched on. In concordance with this study, Stone et al. (2009)
12
found that commencement of activity by the lesser horseshoe bat was later when the roost
entrances were lit, than during unlit nights. Since this bat mainly forages at dusk, in line with
peak abundance of prey, delayed activity resulting from light disturbance could lead to a
lower food intake (Stone et al., 2009).
When bats roost in buildings, some roost entrances may become lit to a certain
degree. A study was done in order to assess the effect of direct lighting on house-dwelling
bats. Colonies of the greater horseshoe bat (Rhinolophus ferrumequinum), Geoffroy's bat
(Myotis emarginatus) and the lesser mouse-eared bat were observed in illuminated and nonilluminated buildings in close proximity to each other. The difference in emergence activity
between bats in the illuminated and non-illuminated buildings were substantial. Almost all
bats from the non-illuminated buildings left their roost in the first 30 minutes after dusk,
while the onset of emergence in the illuminated buildings was delayed by two hours,
basically until the light was switched off (Boldogh et al., 2007). Some bats never totally left
the site when the lights were on, but re-entered the roost repeatedly. A clear and
unfortunate example of the effect of illumination of roosts was when the largest colony of
Geoffroy's bat abandoned their roost after lights had been installed by the local council. The
floodlights poured light through the wide roost exit and illuminated the complete loft
(Boldogh et al., 2007).
Tourism can also play a role in lighting and disturbing bat roosts. A small fruit bat in
Madagascar roosts in caves that can be visited by tourists. They showed the greatest
response to visits when visitors came close and when they were directly illuminated by the
lights (Cardiff et al., 2012). Since many bat species roost in caves, this response to tourist
visits may also apply to other species.
Reproduction
Not many research has been performed yet to the affects of artificial lighting on
reproduction. However, the subject is not entirely unstudied. Boldogh et al. (2007) not only
investigated the effect of lighting on the onset and timing of nocturnal emergence, but also
measured the body mass and forearm length of juvenile bats. The forearm length of juvenile
bats was shorter in the illuminated colonies than in the non-illuminated colonies and this
difference disappeared by mid-September. They used the differences in forearm length for a
rough estimation of disparity in age between the young from illuminated and nonilluminated colonies, which was estimated at at least seven to ten days. One observation
clearly indicated that birth had been delayed in the illuminated buildings, since at one stage
well developed young were found in the dark roost, while the illuminated roost contained
only pregnant females and neonates (Boldogh et al., 2007). The body mass of juveniles was
also lower in the illuminated buildings and this difference did persist into late summer. As
mentioned before, bats from illuminated roosts emerge later and miss the peak of prey
availability. This lower availability of prey to lactating females leads directly to a lower body
mass in juveniles. This study showed that the body mass remained lower even after
weaning, so the juvenile bats could probably not compensate for that early disadvantage.
Since hibernation success depends on the achieved body mass, the illumination of roosts
may reduce the hibernation success of the juvenile bats (Boldogh et al., 2007).
An alternative way of artificial lighting to affect reproduction is by the creation of
artificial long days. As mentioned before, the spermatogenesis in male bats occurs in
summer, when the photoperiod is long, but mating takes place in autumn when the days
become shorter (Kawamoto, 2003). Generally, seasonally breeding mammals are categorized
13
as long-day or short-day breeders, depending on which day length stimulates reproduction.
In the male pallid bat (Antrozous pallidus) it has been demonstrated that exposure to a
short-day photoperiod accelerated the testicular regression and development of accessory
sex glands, while exposure to a long-day photoperiod stimulated spermatogenesis and
resulted in undeveloped accessory sex glands (Beasley & Zucker, 1984). When artificial
lighting conditions reach a point where bats may constantly be living under artificially longday conditions, this could affect the reproductive cycle. When the accessory sex gland
development is inhibited by artificial long days, males will not be prepared to mate in the
mating season, possibly resulting in declining populations.
14
Impact reducing measures
Light properties
The colour of lights can be adapted to minimize the disturbing impact of artificial
lights. Downs et al. (2003) investigated the extent to which light of different colours
modified bat emergence behaviour. At both of the observed roosts of the soprano
pipistrelle, most bats emerged without a light treatment, an intermediate number of bats
during red or blue light treatment, and the least bats with white light treatment. In one of
the roosts there was no significant difference between the amount of emerged bats
between the no light and red light conditions, indicating that red light might be less
disturbing for bats.
The impact can also be minimized by using a low pressure sodium lamp instead of
high pressure sodium lamps or mercury lamps, or by equipping mercury lamps with UV
filters (Fure, 2006). These measures decrease the negative impact on the light avoiding
species, but when bat conservation concerns the species that forage around lamps the high
pressure sodium and mercury lamps are more suitable since they attract insects. A way to
reduce disturbance from light spillage is by specifically directing the light where it is needed.
This can be implemented by installing hoods over the lamp and by reducing the height of the
lighting columns (Fure, 2006).
Time and intensity of lighting
An obvious mitigation strategy would be to lower the light intensity, assuming there
is a light threshold below which the negative effects on bats are negligible. However, the
study by Stone et al. (2012) showed that, at least for the lesser horseshoe bat, this threshold
would have to be below 3.6 lux. This is about the dark limit of dusk, so dimming light
intensity to these levels would probably not enhance the public perception of safety any
more (Anon, 2009).
Light curfews could be imposed as conditions for permission for the lighting of
buildings or sports areas. When areas are only lit when needed, during the short days of
winter when bats are in hibernation, it may solve a large part of the problem. The lighting of
buildings should be limited to special occasions (Fure, 2006). When it is unavoidable to
illuminate buildings with roosts or places where commuting bats pass by, the lights could be
switched off during bat emergence time as well as peak bat activity to reduce disturbance.
Roads and other lighted pathways that cross important foraging areas for bats should
contain unlit stretches to allow bats to transverse the road and prevent the isolation of bat
colonies (Fure, 2006).
Screening vegetation
Planting of screening vegetation could help to minimize the ecological light pollution
around a lighted area considerably. The light pollution on a site of conservation importance
in Redbridge got worse when the surrounding bushes, which would harbour burglars, were
removed (Fure, 2006). This illustrates that it is best not to remove boundary planting when it
is already present, and planting new or additional screening vegetation is desirable.
Overhanging tree canopies near water create dark shadows and should also be retained for
bats. They can use these large shadows as an initial foraging site in the early evening when
the light levels are still relatively high. In urban areas the loss may be even larger when these
15
trees are removed, because not only the dark shadows disappear, but light penetration
throughout the night becomes higher (Fure, 2006).
Other authors have also shown the importance of vegetation for bats. For example,
in the design of a urban microbat flyway for commuting bats the significance of tree
canopies was emphasized (White, 2011). Other authors have pointed out that until now
most conservation efforts focused on protecting or providing roosts, while protection of
commuting routes and foraging areas is just as important. For the brown longeared bat this
means that a greater species diversity in the cover of the understory layer and hedgerows
have to be preserved (Murphy et al., 2012).
16
Conclusion
This thesis concerned the effects of artificial night lighting on bats. It appears that
artificial lighting has positive effects for certain species, mostly the fast-flying species, by
creating new and predictable foraging opportunities (Rydell, 1992; Rydell, 2006). However,
multiple negative effects of artificial lighting have been observed as well. Many species do
not use the lights to forage and many species actively avoid lights when commuting or
foraging (Scanlon & Petit, 2008; Stone et al., 2009; Polak et al., 2011; Stone et al., 2012).
Switching commuting routes or otherwise actively avoiding lights could have fitness
consequences, but this depends on the availability, length, and quality of the alternative
routes. If they are suboptimal in terms of quality or distance to foraging areas, this could
lead to population declines and have conservational consequences (Stone et al., 2009).
Other influences of light on behaviour include commencement of activity and
reproduction success. Multiple bat species have been observed to emerge later from their
roosts when the roost entrance was lit (Downs et al., 2003; Boldogh et al., 2007). Since peak
prey availability is mostly around dusk, it could be detrimental for food intake when bats
emerge later to forage. Not only do they emerge later, but also do fewer bats emerge under
lit circumstances (Downs et al., 2003; Boldogh et al., 2007; Stone et al., 2009). Boldogh et al.
(2007) discovered that birth was delayed by seven to ten days in roosts in illuminated
buildings compared to non-illuminated buildings. The body mass of the juveniles was lower
in the lighted buildings as well, probably as a direct result from the lower food intake by the
mother emerging later from a lit roost. The juvenile body mass remained lower at the end of
summer, making it likely that those juveniles have reduced hibernation success.
Since artificial lighting does thus appear to have many negative effects on species of
conservational concern mitigation strategies have to be developed. Several possibilities have
been considered already. The type of lights that are installed can be adapted to minimize bat
disturbance. Low pressure sodium lamps are considered to be less intruding, and red light
disturbed emergence behaviour the least (Downs et al., 2003; Fure, 2006). Other possibilities
include lowering the light intensity, depending on the species involved, and imposing light
curfews (Fure, 2006). Planting or maintaining screening vegetation around a lighted area can
also help to reduce light pollution in the surrounding area substantially (Fure, 2006).
However, there is a clear need for more research on mitigation strategies, especially focused
on the effects on multiple species.
To promote and sustain bat diversity in urban environments the focus of
conservation should be on the species that are disadvantaged in the light and urban
environment. Some bats profit from the foraging opportunities in open areas with artificial
lights, while others shun these areas when the lights are on and are present at nights the
lights are off. Urban conservation managers should provide a diverse environment, including
dark places such as unlit parks, to attract and maintain a diverse bat population (Scanlon &
Petit, 2008). To support bat species in relatively undisturbed areas it is preferred to minimize
artificial night lighting only to places where it is necessary and to have the surrounding area
shielded from diffusing light.
It would be a shame to possibly diminish populations of several bat species, just
because we, as humans, have the wish and possibilities to artificially make night as bright as
day. When planning on lighting anything in a bat populated area the disturbance for that
particular species will have to be determined, and measures should be taken to minimize the
impact on bat behaviour. Naturally, when it is possible to refrain from placing lights at all,
17
this is the most enviable solution, but this will often not be an option. For now the focus
should be on unravelling all the effects of artificial night lighting on bats and to develop
suitable mitigation strategies for every negative effect.
18
References
AIHARTZA, J.R., GOITI, U., ALMENAR, D. and GARIN, I., 2003. Evidences of piscivory by Myotis
capaccinii (Bonaparte, 1837) in Southern Iberian Peninsula. Acta Chiropterologica, 5(2), pp. 193-198.
ANON, 2009. Artificial Light in the Environment. Richmond, Surrey: Royal Commission on
Environmental Pollution.
ARLETTAZ, R., GODAT, S. and MEYER, H., 2000. Competition for food by expanding pipistrelle bat
populations (Pipistrellus pipistrellus) might contribute to the decline of lesser horseshoe bats
(Rhinolophus hipposideros). Biological Conservation, 93(1), pp. 55-60.
BARCLAY, R.M.R., 1986. The echolocation calls of hoary (Lasiurus cinereus) and silver- haired (
Lasionycteris noctivagans) bats as adaptations for long- versus short-range foraging strategies and
the consequences for prey selection. Canadian journal of zoology, 64(12), pp. 2700-2705.
BEASLEY, L.J. and ZUCKER, I., 1984. Photoperiod influences the annual reproductive cycle of the male
pallid bat (Antrozous pallidus). Journal of reproduction and fertility, 70(2), pp. 567-573.
BOLDOGH, S., DOBROSI, D. and SAMU, P., 2007. The effects of the illumination of buildings on housedwelling bats and its conservation consequences. Acta Chiropterologica, 9(2), pp. 527-534.
BRADBURY, J.W. and NOTTEBOHM, F., 1969. The use of vision by the little brown bat, Myotis
lucifugus, under controlled conditions. Animal Behaviour, 17(PART 3), pp. 480-485.
BUCKLEY, D.J., LUNDY, M.G., BOSTON, E.S.M., SCOTT, D.D., GAGER, Y., PRODÖHL, P., MARNELL, F.,
MONTGOMERY, W.I. and TEELING, E.C., 2013. The spatial ecology of the whiskered bat (Myotis
mystacinus) at the western extreme of its range provides evidence of regional adaptation.
Mammalian Biology, 78(3), pp. 198-204.
CARDIFF, S.G., RATRIMOMANARIVO, F.H. and GOODMAN, S.M., 2012. The effect of tourist visits on
the behavior of rousettus madagascariensis (Chiroptera: Pteropodidae) in the Caves of Ankarana,
Northern Madagascar. Acta Chiropterologica, 14(2), pp. 479-490.
CINZANO, P., FALCHI, F. and ELVIDGE, C.D., 2001. The first World Atlas of the artificial night sky
brightness. Monthly Notices of the Royal Astronomical Society, 328(3), pp. 689-707.
COLEMAN, J.L. and BARCLAY, R.M.R., 2011. Influence of urbanization on demography of little brown
bats (Myotis lucifugus) in the prairies of north america. PLoS ONE, 6(5),.
DIETZ, C., VON HELVERSEN, O. and NILL, D., 2011. Vleermuizen. 1 edn. Utrecht: De Fontein|Tirion.
DOWNS, N.C., BEATON, V., GUEST, J., POLANSKI, J., ROBINSON, S.L. and RACEY, P.A., 2003. The
effects of illuminating the roost entrance on the emergence behaviour of Pipistrellus pygmaeus.
Biological Conservation, 111(2), pp. 247-252.
DIETRICH, C. E. & DODT, E., 1970. Structural and some physiological findings on the retina of the bat
Myotis myotis. In: WIRTH, A., eds, Symposium on Electroretinography. Pisa: Pacini, pp. 120-132.
EKLÖF, J., SVENSSON, A.M. and RYDELL, J., 2002. Northern bats, Eptesicus nilssonii, use vision but not
flutter-detection when searching for prey in clutter. Oikos, 99(2), pp. 347-351.
19
EKLÖF, J. and JONES, G., 2003. Use of vision in prey detection by brown long-eared bats, Plecotus
auritus. Animal Behaviour, 66(5), pp. 949-953.
EKLÖF, J., 2005. Vision in echolocating bats, University of Göteborg, Sweden.
ELLINS, S.R. and MASTERSON, F.A., 1974. Brightness discrimination thresholds in the bat, Eptesicus
fuscus. Brain, behavior and evolution, 9(4), pp. 248-263.
FENTON, M.B., 2013. Questions, ideas and tools: lessons from bat echolocation. Animal Behaviour, .
FRANK, K.D., 1988. Impact of outdoor lighting on moths. Journal of the Lepidopterists' Society, 42(2),
pp. 63-93.
FURE, A., 2006. Bats and lighting. The London Naturalist, 85, pp. 1-20.
GUSTAFSON, A.W., 1979. Male reproductive patterns in hibernating bats. Journal of reproduction and
fertility, 56(1), pp. 317-331.
HÖLKER, F., WOLTER, C., PERKIN, E.K., TOCKNER, K., 2010. Light pollution as a biodiversity threat.
Trends in Ecology and Evolution, 25(12) , pp. 681-682.
JOHNSON, G.D., PERLIK, M.K., ERICKSON, W.P. and STRICKLAND, M.D., 2004. Bat activity,
composition, and collision mortality at a large wind plant in Minnesota. Wildlife Society Bulletin,
32(4), pp. 1278-1288.
KAWAMOTO, K., 2003. Endocrine Control of the Reproductive Activity in Hibernating Bats. Zoological
Science, 20(9), pp. 1057-1069.
LEWIS, S.E., 1995. Roost fidelity of bats: A review. Journal of mammalogy, 76(2), pp. 481-496.
LONGCORE, T. and RICH, C., 2004. Ecological light pollution. Frontiers in Ecology and the
Environment, 2(4), pp. 191-198.
MÜLLER, B., Goodman, S.M., Peichl, L., 2007. Cone Photoreceptor Diversity in the Retinas of Fruit
Bats (Megachiroptera). Brain, Behaviour and Evolution, 70, pp. 90–104.
MURPHY, S.E., GREENAWAY, F. and HILL, D.A., 2012. Patterns of habitat use by female brown longeared bats presage negative impacts of woodland conservation management. Journal of zoology,
288(3), pp. 177-183.
NORBERG, U.M. and FENTON, M.B., 1988. Carnivorous bats? Biological Journal of the Linnean
Society, 33(4), pp. 383-394.
OXBERRY, B.A., 1979. Female reproductive patterns in hibernating bats. Journal of reproduction and
fertility, 56(1), pp. 359-367.
PAGE, R.A., SCHNELLE, T., KALKO, E.K.V., BUNGE, T. and BERNAL, X.E., 2012. Sequential assessment of
prey through the use of multiple sensory cues by an eavesdropping bat. Naturwissenschaften, 99(6),
pp. 505-509.
20
POLAK, T., KORINE, C., YAIR, S. and HOLDERIED, M.W., 2011. Differential effects of artificial lighting
on flight and foraging behaviour of two sympatric bat species in a desert. Journal of zoology, 285(1),
pp. 21-27.
REIS, N.R., BARBIERI, M.L.S., LIMA, I.P. and PERACCHI, A.L., 2003. O que é melhor para manter a
riqueza de espécies de morcegos (Mammalia, Chiroptera): um fragmento florestal grande ou vários
fragmentos de pequeno tamanho? Revista Brasileira de Zoologia, 20(2), pp. 225-230.
REIS, N.R., GALLO, P.H., PERACCHI, A.L., LIMA, I.P. and FREGONEZI, M.N., 2012. Sensitivity of
populations of bats (Mammalia: Chiroptera) in relation to human development in northern Paraná,
southern Brazil. Brazilian Journal of Biology, 72(3), pp. 511-518.
REITH, C.C., 1982. Insectivorous bats fly in shadows to avoid moonlight. Journal of Mammalogy,
63(4), pp. 685-688.
RUCZYŃSKI, I., SZARLIK, A. and SIEMERS, B.M., 2011. Conspicuous visual cues can help bats to find
tree cavities. Acta Chiropterologica, 13(2), pp. 385-389.
RUSSO, D., MAGLIO, G., RAINHO, A., MEYER, C.F.J. and PALMEIRIM, J.M., 2011. Out of the dark:
Diurnal activity in the bat Hipposideros ruber on São Tomé island (West Africa). Mammalian Biology,
76(6), pp. 701-708.
RYDELL, J., 1992. Exploitation of insects around streetlamps by bats in Sweden. Functional Ecology,
6(6), pp. 744-750.
RYDELL, J., 2006. Bats and Their Insect Prey at Streetlights. In: C. RICH and T. LONGCORE, eds,
Ecological Consequences of Artificial Night Lighting. Washington: Island Press, pp. 43-60.
SALMON, M., 2006. Protecting sea turtles from artificial night lighting at Florida’s oceanic beaches.
In: C. RICH and T. LONGCORE, eds, Ecological consequences of artificial night lighting. Washington:
Island Press, pp. 141-168.
SALMON, M., 2003. Artificial night lighting and sea turtles. Biologist, 50(4), pp. 163-168.
SCANLON, A.T. and PETIT, S., 2008. Effects of site, time, weather and light on urban bat activity and
richness: Considerations for survey effort. Wildlife Research, 35(8), pp. 821-834.
SCHNITZLER, H.-. and KALKO, E.K.V., 2001. Echolocation by insect-eating bats. Bioscience, 51(7), pp.
557-569.
SPEAKMAN, J.R. and HAYS, G.C., 1992. Albedo and transmittance of short-wave radiation for bat
wings. Journal of thermal biology, 17(6), pp. 317-321.
STONE, E.L., JONES, G. and HARRIS, S., 2009. Street Lighting Disturbs Commuting Bats. Current
Biology, 19(13), pp. 1123-1127.
STONE, E.L., JONES, G. and HARRIS, S., 2012. Conserving energy at a cost to biodiversity? Impacts of
LED lighting on bats. Global Change Biology, 18(8), pp. 2458-2465.
SVENSSON, A.M. and RYDELL, J., 1998. Mercury vapour lamps interfere with the bat defence of
tympanate moths (Operophtera spp.; Geometridae). Animal Behaviour, 55(1), pp. 223-226.
21
WHITE, A.W., 2011. Factors in the design of an urban microbat flyway. Australian Zoologist, 35(SPEC.
ISSUE), pp. 464-470.
ZHAO, H., XU, D., ZHOU, Y., FLANDERS, J. and ZHANG, S., 2009. Evolution of opsin genes reveals a
functional role of vision in the echolocating little brown bat (Myotis lucifugus). Biochemical
systematics and ecology, 37(3), pp. 154-161.
22