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Letters
Light pollution as a biodiversity threat
Franz Ho¨ lker
1
, Christian Wolter
1
, Elizabeth K. Perkin
1,2
and Klement Tockner
1,2
1
Leibniz-Institute of Freshwater Ecology and Inland Fisheries, Mu
¨ggelseedamm 310, 12587 Berlin, Germany
2
Institute of Biology, Freie Universita
¨t Berlin, 14195 Berlin, Germany
In a recent TREE article, Sutherland and colleagues [1] used
horizon scanning to identify fifteen emerging issues in
biodiversity conservation. They discussed both threats
and opportunities for a broad range of issues, including
invasive species, synthetic meat, nanosilver and microplas-
tic pollution. We recognize that the article was not intended
to be comprehensive, but feel they overlooked an emerging
problem of great importance and urgency, namely that of
light pollution. Although the widespread use of artificial
light at night has enhanced the quality of human life and is
positively associated with security, wealth and modernity,
the rapid global increase of artificial light has fundamental-
ly transformed nightscapes over the past six decades, both in
quantity (6% increase per year, range: 0–20%) and quality
(i.e. color spectra) [2,3]. Despite these significant increases,
the impacts of artificial lighting on the biosphere, many of
which are expected to be negative, are seldom considered.
Most organisms, including humans, have evolved mo-
lecular circadian clocks controlled by natural day–night
cycles. These clocks play key roles in metabolism, growth
and behavior [4]. A substantial proportion of global biodi-
versity is nocturnal (30% of all vertebrates and >60% of all
invertebrates, Table A1), and for these organisms their
temporally differentiated niche has been promoted by
highly developed senses, often including specially adapted
eyesight. Circadian photoreceptors have been present in
the vertebrate retina for 500 million years, and a nocturnal
phase is thought to mark the early evolution of the mam-
mals ago. It was only after the extinction of the dinosaurs
that mammals radiated into the now relatively safe day
niche [5,6]. Although unraveling 500 million years of cir-
cadian habituation is a difficult task, it seems that, with
the exception of amphibians, the proportion of nocturnal
species appears greater in recent radiations than in more
ancient radiations (Figure 1). Nocturnality might therefore
have been an important step in the evolution of verte-
brates, and is currently threatened by the unforeseen
implications of the now widespread use of artificial light.
Light pollution threatens biodiversity through changed
night habits (such as reproduction and migration) of
insects, amphibians, fish, birds, bats and other animals
and it can disrupt plants by distorting their natural day–
night cycle [7]. For example, many insects actively congre-
gate around light sources until they die of exhaustion.
Light pollution can therefore harm insects by reducing
total biomass and population size, and by changing the
relative composition of populations, all of which can have
effects further up the food chain. Migratory fish and birds
can become confused by artificial lighting, resulting in
excessive energy loss and spatial impediments to migra-
tion, which in turn can result in phenological changes and
reduced migratory success. Daytime feeders might extend
their activity under illumination, thus increasing preda-
tion pressure on nocturnal species. For plants, artificial
light at night can cause early leaf out, late leaf loss and
extended growing periods, which could impact the compo-
sition of the floral community. Finally, it can be assumed
that a population’s genetic composition will be disturbed by
light-induced selection for non-light sensitive individuals.
Furthermore, light pollution is considered an important
driver behind the erosion of provisioning (for example, the
loss of light-sensitive species and genotypes), regulating (for
example, the decline of nocturnal pollinators such as moths
and bats) and cultural ecosystem services (for example, the
loss of aesthetic values such as the visibility of the Milky
Way) [2,3,8,9]. As the world grows ever-more illuminated,
many light-sensitive species will be lost, especiallyin or near
highly illuminated urban areas. However, some species, in
particular those with short generation times, may be able to
adapt to the new stressor through rapid evolution, as is
described for other human disturbances [10].
In summary, the loss of darkness has a potentially
important, albeit almost completely neglected, impact on
biodiversity and coupled natural–social systems. Thus, we
Update
[()TD$FIG]
0
20
40
60
80
100
0 50 100 150 200 250 300 350 400 450
Basal divergence time (million years)
% Nocturnal
Mammals
Bats
Amphibians
FishReptiles
Birds
Primates
TRENDS in Ecology & Evolution
Figure 1. Percentage of extant nocturnal species within different vertebrate classes
and orders. With the exception of amphibians, recent radiations have a higher
proportion of nocturnal species than more ancient radiations (sources: [11,12]).
This fact underlines the hypothesis that nocturnality is an important step in
vertebrate evolution. Because the highly permeable skin of amphibians makes
them susceptible to typical daytime stressors such as heat and light, the thresholds
to radiate into the day niche are probably higher for amphibians than for other
vertebrates. This reduced flexibility, in turn, could result in a higher vulnerability to
adverse effects from light pollution at night, and could contribute to the recent
amphibian declines.
Corresponding author: Ho
¨lker, F. (hoelker@igb-berlin.de).
681
see an urgent need to prioritize research, and to inform
policy development and strategic planning.
Acknowledgements
We are grateful to Michael Monaghan and Gernot Glo
¨ckner for helpful
comments and to Peter Kappeler and Christian Voigt for information on
primates and bats. This work was supported by the project ‘Verlust der
Nacht’ (funded by the Federal Ministry of Education and Research),
Milieu (FU Berlin) and the Senatsverwaltung fu¨ r Bildung, Wissenschaft
und Forschung, Berlin.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at doi:10.1016/j.tree.2010.
09.007.
References
1 Sutherland, W.J. et al. (2010) A horizon scan of global conservation
issues for 2010. Trends Ecol. Evol. 25, 1–7
2 Smith, M. (2009) Time to turn off the lights. Nature 457, 27
3Ho
¨lker, et al. (2010) The dark side of light - a transdisciplinary researc h
agenda for light pollution policy. Ecol. Soc.15
4 Dunlap, J.C. (1999) Molecular bases for circadian clocks. Cell 96, 271–
290
5 Menaker, M. et al. (1997) Evolution of circadian organization in
vertebrates. Braz. J. Med. Biol. Res. 30, 305–313
6 Bowmaker, J.K. (2008) Evolution of vertebrate visual pigments. Vision
Res. 48, 2022–2041
7 Rich, C. and Longcore, T., eds (2006) Ecological Consequences of
Artificial Night Lighting, Island Press
8 Carpenter, S.R. et al. (2009) Science for managing ecosystem services:
beyond the Millennium Ecosystem Assessment. Proc. Nat. Acad. Sci.
U. S. A. 106, 1305–1312
9 Potts, S.G. et al. (2010) Global pollinator declines: trends, impacts and
drivers. Trends Ecol. Evol. 25, 345–353
10 Hendry, A.P. et al. (2010) Evolutionary biology in biodiversity science,
conservation, and policy: a call to action. Evolution 64, 1517–1528
11 Alfaro, M.E. et al. (2009) Nine exceptional radiations plus high
turnover explain species diversity in jawed vertebrates. Proc. Nat.
Acad. Sci. U. S. A. 106, 13410–13414
12 Bininda-Emonds, O.R.P. et al. (2007) The delayed rise of present-day
mammals. Nature 446, 507–512
0169-5347/$ –see front matter ß2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tree.2010.09.007 Trends in Ecology and Evolution, December 2010,
Vol. 25, No. 12
Letters Response
Swarm intelligence in plant roots
Frantis
ˇek Balus
ˇka
1
, Simcha Lev-Yadun
2
and Stefano Mancuso
3
1
IZMB, University of Bonn, Kirschellee 1, 53115 Bonn, Germany
2
Department of Science Education - Biology, Faculty of Natural Sciences, University of Haifa - Oranim, Tivon 36006, Israel
3
LINV, Plant, Soil & Environmental Science, University of Firenze, Viale delle idee 30, 50019 Sesto Fiorentino (FI), Italy
Swarm intelligence in animals and humans has recently
been reviewed [1]. These authors posited that swarm
intelligence occurs when two or more individuals indepen-
dently, or at least partly independently, acquire informa-
tion that is processed through social interactions and is
used to solve a cognitive problem in a way that would be
impossible for isolated individuals. We propose at least one
example of swarm intelligence in plants: coordination of
individual roots in complex root systems.
Plants develop extremely complex root systems, which
colonize large soil areas. For example, calculations for one
winter rye plant revealed 13 815 672 roots with a surface
area of about 130 times that calculated for shoots [2].
Growing root apices show complex behaviour based on
‘intelligent’ decisions about their growth directions [3,4].
Moreover, growing roots show coordinated group behav-
iour that allows them to exploit the soil resources opti-
mally. There are three possible communication channels
for context-dependent information transfer among the
numerous root apices of the same plant. Firstly, neuro-
nal-like networks within plant tissues that support rapid
electrical and slower hydraulic and chemical information
transfer between the root apices [5,6]. Secondly, secreted
chemicals and released volatiles allow rapid communica-
tion between individual roots. Thirdly, there is a possibil-
ity that the electric fields generated by each growing root
[7] might allow electrical communication among roots.
These electric activities and electric fields show maximal
values [7,8] at the transition zone of growing root apices [3]
which behaves as a ‘brain-like’ command centre [6,9].
Roots may use swarm intelligence for their navigation,
coordination, cooperation, as well as for their ‘war-like’
aggressions [10]. It is important that every root has its own
identity provided by its unique sensory history accumu-
lated via its own command centre. Each root apex acts both
as a sensory organ and as a ‘brain-like’ command centre to
generate each unique plant/root-specific cognition and
behaviour [3,6,9]. Recent advances in the emerging field
of sensory plant ecology suggest that the sensory informa-
tion collected by one plant is shared with neighbouring
plants [11,12]. In the case of root apices, sensory informa-
tion appears to be processed collectively in the root system
to optimize root-mediated territorial activities [13–16].
Theserootapicessolvecognitiveproblemssuchaswhere
to grow and whether to grow at all, to fight or retreat in
a face of competitive roots and root systems [10] and to
enter symbiotic relationships with mycorrhiza fungi
(and Rhizobium bacteria in the case of some species)
[3–6,13–15]. So roots enjoy a rich ‘social’ life at the indi-
vidual plant level and they continuously solve problems
that could be called cognitive [4,13]. Swarm intelligence is
essential for the evolutionary success of roots and, conse-
quently, the whole plant. The accumulating data on the
Corresponding author: Balus
ˇka, F. (baluska@uni-bonn.de); Lev-Yadun, S.
(levyadun@research.haifa.ac.il); Mancuso, S. (stefano.mancuso@unifi.it).
Update Trends in Ecology and Evolution Vol.25 No.12
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