Could artificial night lighting be contributing to amphibian population collapse? Effects on
the growth of the European common frog Rana temporaria tadpoles.
Abstract
Severe declines in amphibian populations are reported globally. Several contributory causes
have been identified including disease, habitat loss and climate change. In this study, we
explored the potential impacts of artificial night lighting with Light Emitting Diodes (LEDs) on
the growth of the most common and widespread amphibian in Europe, Rana temporaria.
An experimental approach was used, with animals being group housed in groups of 5 in 40
study tanks, half of which were randomly allocated to receive artificial lighting at night. The
study continued for 83 days, the time at which complete metamorphosis had occurred in
some individuals. The growth of tadpoles was significantly reduced under artificial night
lighting, and this effect increased over time (interaction p < 0.005 for all outcomes
measured). At day 83, the raw differences equated to more than 10% of body size (mean
length 14.4% (0.26/1.81); minimum length 19.4% (0.28/1.44), maximum length 10.1%
(0.22/2.17)). Given that amphibians are most vulnerable to predation during larval stages,
and risks are also increased among individuals metamorphosing at small body sizes, night
lighting may have important consequences for amphibian conservation.
Introduction
The density and distribution of artificial lighting at night is rapidly increasing (Riegel 1973,
Holden 1992, Cinzano et al. 2001, Cinzano 2003, Hoelker et al. 2010). The implications for
individuals and populations are poorly explored for most taxa. However, it is known that
street lights elicit important behavioural and physiological effects in some animals including
bats (Rydell 2006, Stone et al. 2009, Mathews et al. 2015), moths (Svensson and Rydell
1998, Eisenbeis 2006, van Langevelde et al. 2011, Somers-Yeates et al. 2013), turtles
(Witherington 1992, Lorne and Salmon 2007), and migrating birds (Gauthreaux Jr et al.
2006)). Recently, it has been shown that the interactions of species within a community can
also be altered (Davies et al. 2012, Davies et al. 2013).
Amphibians are suffering global population declines, and many species are threatened with
extinction (Beebee and Griffiths 2005, Hof et al. 2011, Blaustein et al. 2012). Contributory
factors include infections with the parasitic fungus Batrachochytrium dendrobatidis and
Ranavirus, the loss of ponds and other wetland habitats due to urbanisation and agricultural
intensification, and climate change (Hof et al. 2011, Alton et al. 2012, Blaustein et al. 2012).
Increased exposure to ultraviolet B (UV-B) radiation, which is a particular concern at
latitudes most affected by ozone-layer depletion, has been linked experimentally with
developmental abnormalities in amphibians, and may also alter predation risk (Blaustein et
al. 1997, Alton et al. 2011). However, many behavioural and physiological processes in
amphibians are naturally triggered by diurnal and seasonal light cycles, so it has been
suggested that exposure to artificial light at night may be contributing to population
declines.
A variety of mechanisms has been proposed. In adults, navigation may involve lightdependent mechanisms, and therefore movement patterns may be disrupted by artificial
light (Phillips and Borland 1992, Phillips and Borland 1994). There are also reports of altered
feeding behaviour (Buchanan 1993), perturbed behaviours likely to influence breeding
success (Baker and Richardson 2006), and collision with road vehicles (Fahrig et al. 1995,
Mazerolle 2004, Baker and Richardson 2006). In larvae, a light-dependent magnetic
compass has been identified in common frogs Rana temporaria, which may imply that
orientation can be disrupted by lighting (Diego-Rasilla et al. 2013). In addition, the
disruption of normal patterns of light and dark larval has been linked with altered growth
patterns. Painted frogs Discoglossus pictus had reduced growth (Gutierrez et al. 1984), and
African Clawed Frogs Xenopus laevis metamorphosised at a smaller size (Delgado et al.
1987, Edwards and Pivorun 1991). These outcomes would be expected adversely to affect
survival probability due to increased vulnerability to predation (Werner 1986). Conversely,
exposure to continuous lighting increased the growth rates of Northern Leopard Frogs, Rana
pipiens (Eichler and Gray 1976) and no clear effect was found in European common frogs R.
temporaria (Laurila et al. 2001).
There is no published research that specifically assesses the impact of street lighting on the
development of amphibian larvae. We conducted an experimental study to test the impacts
on tadpole growth rates. The European common frog, Rana temporaria, was chosen as the
study species. This is the most abundant and widespread anuran Europe, but is thought to
be in severe population decline.
Methods
Light in the experimental room was derived from daylight simulation tubes programmed to
be active from dawn until dusk (civil twilight on the day of the experiment). The
temperature was also regulated to match the annual mean temperature on the relevant
study day. To account for any small variations in the intensity of light falling on different
areas of the benchtop, the experimental tanks were repositioned by random relocation
every 9 days. Twenty tanks received lighting during the day and were dark at night, and a
further 20 tanks were additionally subjected to light-emitting diode (LED) lighting at night.
The LED arrays were positioned directly above each study tank, and shielding was used to
prevent light spill to other tanks. The intensity of light at the water’s surface approximated
that found at ground level beneath LED streetlights on the University of Exeter Campus.
The animals used in the experiment were derived from a single batch of frogspawn taken
from the wild and maintained in the laboratory until hatching. Tadpoles were housed in
groups, and 5 individuals of similar size were randomly allocated to a study tank (25 x 15 x
19 cm filled to 10cm depth) at the start of the experiment. The mean tadpole length at the
start of the experiment was 1.22cm (mean within-tank SD 0.08, within-tank SD range 0.020.16). All tadpoles were provided with water derived from the same bulk tank. The water
was dechlorinated prior to use, seeded with pond-water, and allowed to develop natural
algae and plankton. A quarter of the tank water was replenished every second day.
Tadpoles were also provided with additional forage (algae-rich water, liquidised lettuce and
daphnia) as they matured. Each tank contained a refuge, in the form of a 5cm length of grey
uPVC piping, in which the animals could hide. Photographs were taken of each animal
individually every alternate day: the tadpoles were staged on a white weighing boat and
photographed in an extended position against a reference scale. They were then returned
to their original study tank. The lengths of the tadpoles were then measured using ImageJ
(Rasband et al.)
Preliminary inspection of the data indicated that the growth patterns of tadpoles followed a
curvilinear relationship with time. Because measurement error was proportionally higher at
small larval sizes, the analyses are based data collected from day 19 until the end of the
experiment (day 83). Across this period, the relationship between length and loge(day) was
linear. The experiment ended when the first tadpoles fully metamorphosised. This
endpoint was chosen because of the possibility that the growth of remaining individuals in
each tank would be affected by the removal of the most developed individual, and because
constant experimental conditions could not be provided across all tanks whilst also offering
metamorphosing individuals with the ability to climb out of the water.
Because it was not possible to distinguish individuals within each tank, the outcome data
are expressed at tank level. To allow for the possibility that individual animals might stage
their growth depending on the development stage of others within the tank, maximum and
minimum lengths per tank, and the standard deviation in length per tank were used as
outcome variables in addition to mean lengths. Tadpole sizes are expressed as deviations
from the mean length for the tank at the start of the experiment. The number of tadpoles
was not constant in every tank throughout the experiment because of natural mortality.
The potential effect of tadpole numbers per tank was therefore explored. The data were
analysed using R 3.1.1 (R Development Core Team 2013). The links between bat activity and
lighting regime were investigated using generalised linear mixed effects models (GLMERs) in
lme4 (Bates et al. 2011) with normal error structures and with tank specified as a random
effect to account for the repeated measures. Model fit was assessed by visual inspection of
the residuals and assessment of AIC values. Comparisons were made between models with
random intercepts, and those with random intercepts and random slopes. The former were
found to provide the best fit. Selection of fixed factors followed a backwards stepwise
procedure, beginning with models that included treatment, number of tadpoles and
loge(study day), and the interactions between treatment and each of the other two
variables. Tests of significance were based on likelihood ratio tests comparing alternative
models (fitted using Maximum Likelihood (ML)) (Zuur et al. 2009). Plots were produced
using the ‘effects’ package, and confidence intervals are based on the fixed effects only (Fox
2003).
The project was approved by the Ethical Review Committee of the College of Life and
Environmental Sciences, University of Exeter; and it was agreed with the Home Office
Inspector that the conditions provided fell below the threshold of the Animals (Scientific
Procedures) Act.
Results
In total 989 tank-day observations were available for analysis (including 3,471 individual
tadpole measurements). The interaction of lighting treatment and day was a highly
significant predictor of tadpole length (F = 15.00, df = 1, p < 0.001; Fig. 1). The number of
tadpoles in the tank was not linked with average length, nor was there an interaction with
lighting. The sizes of the smallest and largest tadpoles recorded for each tank were also
strongly influenced by lighting (loge(day)* treatment F = 7.78, df = 1, p = 0.005, Fig 2; F =
7.61, df = 1, p = 0.006, Fig. 3 respectively). In both of these cases, the number of tadpoles
was also a significant predictor but there were no interactive effects with lighting. The raw
size differences at the end of the experiment represented at least 10% of body size for each
of the measured outcomes (mean length 14.4% (0.26/1.81); minimum length 19.4%
(0.28/1.44), maximum length 10.1% (0.22/2.17). None of the predictor variables was linked
with the standard deviation in tadpole size.
Discussion
This study provides the first experimental evidence that amphibian larval growth is reduced
by LED night lighting. The mechanism is not clear, but we observed that animals in the
experimental tanks appeared to spend more time hiding in their refuges at night than did
animals in control tanks. It is therefore possible that their foraging opportunities are more
limited. It has also been shown that lighting can affect the metabolic rate of amphibians.
Comparisons have been made of the physiological functions of terrestrial adults of A.
maculatum kept on a 16L:8D and 8L:16D photoperiod. Those maintained on a 16L:8D
photoperiod had significantly higher pulmonary, cutaneous, and total rates of O2
consumption and higher cutaneous and total rates of CO2 production (Whitford and
Hutchison 1965). It has therefore been hypothesized that artificially increasing the length of
photophase through night lighting may disrupt normal cyclical changes in metabolic rates,
changing the energy demands of salamanders (Wise and Buchanan 2006). The production
of melatonin, a hormone responsible for many aspects of photoperiodic behaviour and
physiology (Vanecek 1998), can also be disrupted in larval amphibians by exposure at night
to as little as 1 minute of artificial light (Lee et al. 1997). However, previous research failed
to find clear evidence of a link between photoperiod and growth in common frogs (Laurila et
al. 2001)
Increasing time to metamorphosis, and also smaller size at metamorphosis, are linked with
increased predation risk in amphibians. Given the link between lighting and reduced larval
growth identified in this project, further work is required as a matter of urgency to
understand the implications of streetlight and amenity lighting for amphibian development.
Data collected within this project could be used to investigate the timing of hind leg
development, but additional work using study designs appropriate for the maintenance of
air-breathing adults is also required to assess the effects on full metamorphic conversion.
This should be complemented by field research to investigate whether the presence of
vegetation and/or water turbidity can mitigate the effects of artificial night lighting.
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Fig. 1. Estimated effect of artificial night lighting on mean growth of tadpoles (tank average length
compared with size on day 1).
Fig 2. Estimated effect of artificial night lighting on minimum tadpole growth (relative to minimum
size recorded on day 1).
Fig 3. Estimated effect of artificial night lighting on maximum tadpole growth (relative to minimum
size recorded on day 1).