In Light of Energy: Influences of Light Pollution on Linked Stream-Riparian Invertebrate Communities THESIS Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University By Lars Alan Meyer Graduate Program in Environment and Natural Resources The Ohio State University 2012 Committee: Professor Mažeika S.P. Sullivan, Advisor Professor Mary M. Gardiner Professor Paul G. Rodewald
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In Light of Energy: Influences of Light Pollution on Linked Stream-Riparian Invertebrate Communities
THESIS
Presented in Partial Fulfillment of the Requirements for the Degree Master of Science in the Graduate School of The Ohio State University
By
Lars Alan Meyer
Graduate Program in Environment and Natural Resources
The Ohio State University
2012
Committee:
Professor Mažeika S.P. Sullivan, Advisor
Professor Mary M. Gardiner
Professor Paul G. Rodewald
Copyrighted by
Lars Alan Meyer
2012
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Abstract
The world’s human population is expected to expand to nine billion by the year
2050, with 70% projected to be living in cities. As urban populations grow, cities are
producing an ever-increasing intensity of ecological light pollution (ELP). At the
individual and population levels, artificial night lighting has been shown to influence
predator-prey relationships, migration patterns, and reproductive success of many aquatic
and terrestrial species. With few exceptions, the effects of ELP on communities and
ecosystems remain unexplored. My research investigated the potential influences of ELP
on stream-riparian invertebrate communities and trophic dynamics, as well as the
reciprocal aquatic-terrestrial exchanges that are critical to ecosystem function. From June
2010 to June 2011, I conducted bimonthly surveys of aquatic emergent insects, terrestrial
arthropods, and riparian spiders of the family Tetragnathidae at nine Columbus, OH
high 2 - 4 lux). In August 2011, I experimentally increased light levels at the low and
moderate treatment reaches to ~12 lux. I quantified invertebrate biomass, family
richness, density (individuals m-2) of aquatic and terrestrial invertebrates, and measured
reciprocal stream-terrestrial invertebrate fluxes. Using stable isotopes of carbon (δ13C)
and nitrogen (δ15N), I estimated trophic position, variability in trophic position, food-
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chain length, and contribution of aquatic (i.e., epilithic algae) vs. terrestrial (i.e., leaf litter
detritus) carbon.
I found that light strongly influenced invertebrate family richness, biomass, and
density for discrete time periods over the course of the year. The experimental addition
of light resulted in a ~42% decrease in tetragnathid spider density, a ~54% decrease in
aquatic emergent insect biomass, a ~ 16% decrease in aquatic emergent insect family
richness, and a ~38% decrease in density of terrestrial arthropods entering stream.
Trophic position and variability in trophic position for the stream-riparian invertebrate
community, as well as, the families Tetragnathidae, Formicidae, and Chaoboridae
showed a strong positive relationship with ELP. The experimental addition of light
resulted in a ~2 trophic position increase in food-chain length and a two-fold increase in
variability in trophic position. Artificial light was also related to the contribution of
aquatic vs. terrestrial C at both the invertebrate community and family levels, such that
the contribution of aquatic C was lowest at moderate ELP and greatest at high ELP.
Collectively, these results are among the first to show the ecological consequences of
ELP at both community and ecosystem levels of biological organization.
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Acknowledgements
I would like to thank my faculty advisor, Professor Mazeika Sullivan, for his expert
guidance and invaluable support. Thanks to my committee members Professor Mary
Gardiner and Professor Paul Rodewald for valuable input during the initial and final
stages of this project. I also thank the School of Environment and Natural Resources
faculty and staff for the much needed top quality professional support generously
provided. I convey my appreciation to the research personnel in the Stream and River
Ecology Laboratory, SENR for their dedicated hard work in the field and laboratory
especially Paradzayi Tagwireyi, Brittany Gunther, Jeremy Alberts, Leslie Rieck, Adam
Kautza, and Xiaoxue Yang. I thank my two wonderful children Markus and Caroline for
their support in the field, laboratory, and most importantly at home.
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Vita
June 1983 .......................................................Hillsdale High School, Hillsdale, MI
July 1983- July 2003………………………. United States Navy, Active Duty.
July 2003…………………………………….United States Navy, Retired.
2008................................................................B.S. in Environment and Natural Resources The Ohio State University 2008 to present ..............................................Graduate Research and Teaching Associate,
School of Environment and Natural Resources, The Ohio State University
Fields of Study
Major Field: Environment and Natural Resources (Fisheries and Wildlife)
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Table of Contents
Abstract……………………………………………………………………………………ii
Acknowledgements…………………………………………………………………….…iv
Vita…………………………………………………………………………………..…….v
List of Tables……………………………………………………………………………..vi
List of Figures…………………………………………………………………………....vii
Chapter 1: Background and Literature
Review………………………………….……………………………………………..…1.
Chapter 2: Bright lights, big city: influences of ecological light pollution on reciprocal
Aquatic Chaoboridae Impalers, ectoparasites Mostly active at night Benthos (ubiquitous)Chironomidae Detritivore, filter-feeder, gatherer, Mostly active at night Benthos (ubiquitous)
fungivore, parasiteCeratopoginidae Impaler, ectoparasite Benthos (ubiquitous)Heptaginiidae Herbivore, scavenger, predator Mostly active at night Under stream cobbles, sandy riversHydopsychidae Algivore, detritivore Light indifferent Benthic net spiners (cobble) Baetidae Algivore, detritivore Mostly active at night Cool swift running streams
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Figure 1.1. Representation of reciprocal food-web linkages (e.g., energy flows as represented by arrows) in a stream-riparian ecosystem (from Sullivan and Rodewald 2012). Also see Baxter et al. (2005).
As the River Continuum Model (Vannote 1980) predicts, natural low-order
deciduous forested stream canopy limits primary production of autochthonous carbon
(i.e., epilithic algae). Stream secondary production relies heavily on the input of
allochthonous carbon from terrestrial sources (i.e., leaf litter detritus). Invertebrates
entering the stream can also provide a significant proportion of energy to aquatic food
webs. For example, Nakano and Murakami (2001) found that trout, char, and salmon
from stream substrate, base2 = detrital leaf litter collected from water surface, α is an
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estimate of nitrogen derived from autochthonous sources = (δ13Csc – δ13Cbase2) / (δ13Cbase1
– δ13Cbase2), and Δn = isotopic enrichment value for each trophic level. The following
assumptions are often made when using this model: (1) periphyton is the major
contributing autochthonous primary producer and detrital leaf litter is the primary
allochthonous energy source (n =2); (2) Trophic enrichment values for prey are 1.0‰ for
δ13C and 3.4‰ δ15N (Post 2002).
Isotope analysis experiments, such as the systematic addition of 15N, allow an
accurate estimate of subsidy pathway (Peterson 1999). A 15N tracer addition experiment
conducted in a Sonoran desert stream revealed that orb-weaving spiders living along the
stream edge obtained 100% of their C and 39% of their N from instream sources and
ground dwelling hunting spiders obtained 68% of their C and 25% of their N instream
sources (Sanzone et al., 2003). Natural abundance studies are also a useful tool to
determine energy flow and trophic linkages. Collier et al. (2002) showed that aquatic
insects provided approximately 60% of C assimilated by riparian spiders alongside two
New Zealand streams and Iwata et al. (2003) observed that 54% of riparian spider diet
was made up of aquatic prey in twenty six deciduous forested streams in Japan.
Artificial Night Lighting
ecological light pollution (ELP)
Artificial light (e.g., streetlamp, vehicle headlight) that alters the natural patterns of light and dark in ecosystems (ELP, sensu Longcore and Rich) 2004).
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Over the past few decades, artificial night lighting, such as, roadway, security
lighting, and other urban light sources, has dramatically increased (Smith 2009, Holker
2010). Approximately one fifth of the earth’s terrestrial surface is exposed to ecological
light pollution (ELP, Cinzano 2001). This trend is likely to increase given that the
world’s urban population is expected to increase from 50% to 70% by 2050 (U.N. 2010).
In the U.S., ~30% of outdoor electrical light is wasted as light pollution (California
Energy Commission 2005). Certain urban areas (i.e., shopping mall parking lots) may
(Falchi 2011). However, only recently have the ecological implications of artificial night
lighting received serious attention (Perkin et al. 2011).
As 30% of vertebrates and 60% of all invertebrates are nocturnal (Kyba 2011),
artificial night lighting carries serious implications as a threat to diversity and for changes
in ecosystem function (Holker et. al., 2010). To date, ELP has largely been explored
relative to individuals or populations of both aquatic and terrestrial taxa. For example,
ELP has been shown to influence mating success, predator-prey relations, and migration
of many organisms including birds, bats, fish amphibians, zooplankton, amphipods
(Table 1.2).
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Table 1.2. Terrestrial and aquatic biotic response to artificial night lighting.
Only very recently has artificial light been investigated relative to higher levels of
biological organization. Davies et al. (2012) found an increased number of ground-
dwelling arthropod predators and scavengers near street lights. Artificial night lighting
has also been shown to attract aquatic emergent insects, thereby disrupting their dispersal
patterns and in some cases serving as ecological traps leading to direct mortality and
increased predation (Schlaepfer et al., 2002, Horvath 2009). Studies conducted by Yoon
et al. (2010) implicated artificial light sources as the main cause of extinctions of local
populations of the giant water bug (Lethocerus deyrolli).
Spectral composition (i.e., intensity, polarization, frequency) of light can be an
important influence on the biological function of insects. Kyba et al. (2011) showed that
urban light (i.e., skyglow) reflecting from natural surfaces (i.e., cloud cover) can
Taxon Response to artificial light CitationBats Juvenile bat health negatively affected. Buldough et al. 2007
60% reduction in feeding buzzes of the pond bat (Myotis dasycneme ) Kuiper et al. 2008Flight activity reduced Lesser horseshoe bat (Rhinolophus hipposideros ) Stone et al. 2009
Birds Increased the foraging success of nocturnal wading birds( e.g., ringed plover [Charadrius hiaticula ], grey plover [Pluvialis squatarola] ) Santos et al. 2010
Early egg lay date in song birds (Cyanistes caeruleus, Parus major, Turdus merula) Kempenaers 2010Early morning singing behavior in American Robins (Turdus migratorus) Miller 2005
Frogs Fewer mating calls and decreased activity in green frog (Rana clanitans melanota) Baker and Richardson 2006
Freshwater shrimp Initiated untimely diel migration in the water column. Gal et al. 1999Daphnia Initiated untimely diel migration in the water column. Moore et al. 2000Zooplankton Initiated untimely diel migration in the water column. Hansson et al., 2007
Fish Increased egg developmnet and hatching time of perch (Perca fluviatilis) Bruning et al. 2011and roach (Rutilus rutilus) .
Invertebrates Strongly effected by artificial night lighting at individual and community levels Longcore and Rich 2004Kyba 2011
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consistently produce light levels equivalent to areas naturally lit by the full moon. Some
nocturnal insects (i.e., Scarabacus zambesianus) are known to use polarized light to
navigate during the night where they align themselves at ~90° to the naturally polarized
light of the moon. Robertson et al. (2010) showed that natural light reflected from
artificial surfaces (i.e., window glass, road surfaces, plastic sheeting) polarizes and has
properties similar to light reflecting from the water surface ( Horvath et al. 2008).
Polarotactic aquatic emergent insects (e.g., Ephemeroptera, Trichoptera, Diptera) are
drawn to these surfaces where they fail to mate and often become prey for terrestrial
predators such as birds (Horvath et al., 2008). Horvath (2008) estimated that the
quantity of plastic sheeting used for a 10-hectare strawberry farm could trap and kill
approximately one ton of aquatic emergent insects a day.
Ali et al. (1984) experimentally showed a differential attraction response to
artificial light by midges (i.e., Chironomidae), where color of light (i.e., white, yellow,
blue, green, red) and species were significant factors in determining attraction to artificial
light. Spectral composition has the potential to alter insect community diversity and
composition. For example, Langevelde et al. (2011) reported a size-biased flight to light
behavior, where smaller moth species were more abundant at artificial light sources than
were larger species and this increased the potential of selective mortality.
In a laboratory experiment where artificial light sources were used to test the
control of light on activity of benthic macroinvertebrates, Bishop (1969) found that
photoperiod and intensity of artificial light over the model stream had strict control on
invertebrate activity as measured by drift biomass. Overall, benthic stream drift was
15
suppressed by artificial light. Benthic taxa showed a differential response to artificial
light where Limnephiliidae showed no response to light and Phasganophora,
Ephemerella, and Stenonema activity was restricted to the dark periods. Activity was
suppressed at light intensities between (0.01 – 0.1 lux).
Terrestrial arthropod predators such as spiders are also susceptible to the effects
of artificial night lighting. Nocturnal spiders, such as orb-web weaving spiders of the
family Araneidae, capture prey at higher rates when building webs in well-lit locations
(Heiling, 1999). The collective effects of artificial night lighting on aquatic and
terrestrial invertebrates might be expected to not only shift patterns of invertebrate
community diversity, but to exert strong effects on broader ecosystem function by
restructuring important aquatic-terrestrial linkages (i.e., reciprocal flow).
Summary and Objectives
The ecological perturbations caused by urbanization are widespread and increasing.
Although we are becoming increasingly aware of the biotic responses at the individual
and population levels to ecological light pollution in both aquatic and terrestrial
ecosystems, consequences to communities and ecosystems are poorly understood.
Because exchanges of material and energy between aquatic and terrestrial systems are
critical for broader ecosystem function, it is crucial to determine the effects of artificial
light on aquatic-terrestrial linkages. The overarching goal of this study was to better
understand the effects of artificial night lighting on the dynamics of linked stream-
riparian ecosystems. In particular, this thesis addresses the influence of ecological light
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pollution on reciprocal stream-riparian invertebrate fluxes (Chapter 2) and the trophic
dynamics of linked stream-riparian invertebrate food webs (trophic structure, food-chain
length, contribution of aquatic carbon to aquatic and terrestrial invertebrate consumers;
Chapter 3). This work will expand understanding of the ecological consequences of
artificial night lighting on linked stream-riparian ecosystems. In turn, it is my hope that
results from this work will inform conservation initiatives related to biodiversity of urban
stream ecosystems.
17
Chapter 2: Bright lights, big city: influences of ecological light pollution on reciprocal
stream-riparian fluxes
(submitted for review to Ecological Applications – June 2012)
Authors: Lars A. Meyer and S. Mažeika P. Sullivan, School of Environment & Natural
Resources, The Ohio State University, 2021 Coffey Rd., Columbus, OH 43210, USA
Corresponding author: Lars A. Meyer, School of Environment & Natural Resources,
The Ohio State University, 2021 Coffey Rd., Columbus, OH 43210, USA. Email:
2012). We provide initial evidence that ecological light pollution can significantly
influence stream-riparian invertebrate community characteristics, cross-boundary
invertebrate fluxes, and riparian spiders that rely on these fluxes.
Our results supported our expectation that high ELP levels would associate with a
greater relative input of terrestrial arthropods entering the stream. Although this pattern
was limited to spring and early summer (Figures 2.1d and f), evidence from the
experimental increase in light supported this trend (Figure 2.2d). Artificial light has been
shown to disrupt nocturnal navigation and migration in some arthropods by masking the
physical properties (i.e., polarization) of the moon’s naturally reflected light (Kyba et. al.,
2011a) and is widely known to attract phototaxic insects (Ali and Lord 1980). An
increase in artificial light reflecting off the water’s surface may thusly alter the magnitude
of terrestrial arthropods entering the stream. The reversal of the effect of ELP on
terrestrial-to-aquatic arthropod input between fall and spring/summer (i.e., terrestrial-to-
aquatic input at high ELP sites was significantly reduced during the fall) may be due to
decreased activity (i.e., spiders, amphibians) or migration (i.e., birds) of key predators in
the terrestrial component of the food web.
We found aquatic emergent insect density and richness were significantly
increased in moderate ELP sites in October. However, increasing light levels to 12 lux
resulted in decreases in both family richness (Figure 2.2b) and biomass (Figure 2.2c). At
the site scale, a lower terrestrial-to-aquatic arthropod flux (Figure 2.2d) may lead to prey-
switching by stream fish from terrestrial arthropods on the water surface to benthic
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invertebrates, thereby reducing aquatic emergent insect density (Baxter et al., 2004).
Additionally, given that the common invertivore fish species found in the study system
(e.g., creek chub [Semotilus atromaculatus], green sunfish [Lepomis cyanellus]) are
visual predators, artificial lighting may increase predation efficiency and extend
predation hours beyond the natural daylight hours (Santos et al., 2010). Although
increases in ELP may provide conditions more favorable for visual predators throughout
the year, this may be particularly consequential in autumn when light reaching the stream
increases due to tree canopy senescence. Emigration by aquatic emergent insects away
from the stream channel towards artificial light can result in ecological traps (Ali and
Lord 1980), with potential consequences to community structure at the broader, stream
scale.
Even at low light levels (Figure 2.2g), our results were not consistent with the
scenario in which the forest feeds stream food webs during the summer, and that
conversely, the stream fuels terrestrial food webs from fall to spring (Power 2001).
Measurable differences in net flux estimates among light levels did provide evidence that
artificial light may shift the balance of invertebrate feedback loops between the stream
and the riparian zone. However, the precise nature of these changes will require further
investigation.
Tetragnathid spider density showed a strong negative response to high ELP in all
months in which they were active (April thru October) as well as to the experimental
increase in light (Figure 2.2a). Light-induced increases in the activity of terrestrial
predators (birds and other invertebrates) likely contribute to reduced tetragnathid density.
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For example, Davies et al. (2012) found an increase in the number of predators and
scavengers in ground-dwelling invertebrate communities under street lights. High light
levels may also increase predator capture rates by inhibiting the ability of spiders to
remain hidden. Additionally, high light levels have been shown to reduce the efficacy of
the ventrum spots used to lure prey in some spiders (Chuang et. al., 2008), potentially
forcing emigration to less lit areas.
Conclusions
To our knowledge, our findings provide the first evidence that artificial night
lighting alters ecosystem function. In addition, this study documents shifts in community
characteristics (e.g., biomass, density, diversity), supporting recent work that ELP affects
higher levels of biological organization (Davies et al. 2012). As the world’s populations
continue to urbanize, the potential for ELP to influence communities and ecosystems at
broader spatial scales also increases. Additional research that further explores the effects
of ELP in its many forms (e.g., point source, atmospheric reflection, polarization,
spectrum frequency, intensity, length of exposure period) in both aquatic and terrestrial
environments will be critical. In particular, we advocate for research that addresses the
influence of artificial night lighting on biodiversity, food webs, ecological networks, and
ecosystem function.
30
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Table 2.1. Repeated measures analysis of variance for bimonthly aquatic-terrestrial
invertebrate responses to ecological light pollution for study reaches in the Columbus
= detrital leaf litter collected from water surface, λ = trophic position of periphyton and
detrital leaf litter, n = number of primary food sources (i.e., 2), and Δn = isotopic
52
enrichment value for each trophic level. In our study catchment, periphyton is the major
contributing autochthonous primary producer and detrital leaf litter is the primary
allochthonous energy source (n = 2). We corrected prey isotope values for trophic
enrichment using widely-accepted values of 1 and 3.4‰ for δ13C and δ15N, respectively
(Post 2002). We calculated FCL of the aquatic invertebrate community, terrestrial
invertebrate community, and whole stream-riparian invertebrate community as trophic
positionmax – trophic positionmin. We used the standard deviation of trophic position to
represent VTP.
We used general linear models (GLMs) to test for the influence of ELP on TP and
the contribution of aquatic C to consumers for (1) the whole stream-riparian invertebrate
community; (2) the terrestrial invertebrate community; (3) the aquatic insect community;
and (4) Tetragnathidae (common riparian predator), Formicidae (common terrestrial
consumer), and Chaoboridae (common aquatic consumer). We included canopy cover as
a covariate in the GLMs as the influence of canopy cover on aquatic insects and
terrestrial arthropods is well known (Progar and Moldenke 2009, Riley et al. 2009). In all
GLMs, ‘reach’ (nested within ‘ELP’) was included as a random variable. ‘ELP’ was
included as a fixed variable for all models; ‘canopy’ was included as a covariate for TP
and contribution of aquatic C models.
Canopy was excluded as a covariate in models where degrees of freedom were lost when
evaluating FCL (i.e., range of TP) to avoid pseudoreplication affects.
Where statistically significant main effects were detected, linear contrasts were run
between ELP levels.
53
We used regression analysis to explore potential relationships between δ13C of
periphyton and 13C of Formicidae , Chaoboridae, and Tetragnathidae. We used paired t-
tests to test for differences in TP and contribution of aquatic C between August 2010
(pre-experimental light addition) and August 2011 (post light addition). Given multiple
GLM ‘tests’, the Bonferroni adjustment for α was α /k = 0.05/12 = 0.004, where k is the
number of tests/treatments (Wright 1992). An α of 0.05 was used for all other tests. All
statistical analyses were performed using JMP 9.0 Statistical Discovery Software (SAS
Institute, Inc., Cary, NC).
Results
Isotopic signatures of aquatic emergent insects were highly variable across the
invertebrate community, with δ15N values ranging from 4.08 ̶ 9.6‰, 2.02 ̶ 9.93‰, and
4.35 ̶ 12.69‰ at low, moderate, and high ELP sites, respectively. The δ13C values
ranged from -28.72 – -15.01‰, -27.61‰ ̶ -15.10‰, and -29.27‰ ̶ -15.04‰ at low,
moderate, and high ELP sites, respectively (Table 2).
Food web structure suggested shifts among low, moderate, and high light levels.
For instance, we noted the absence of Hydroptilidae, Heptaginidae, Empididae at high
ELP sites. Tetraganthidae, Formicidae, and Chaoboridae exhibited a trend towards
increasing 15N enrichment at higher light levels (Figure 3.1). FCL for the entire stream-
riparian invertebrate community ranged from 4.1 to 9.7. We observed greater FCL in the
terrestrial community, with Tetragnathidae typically occupying the highest trophic
position irrespective of light level.
54
GLMs indicated that TP and the contribution of aquatic C to consumers was
significantly different among ELP levels for multiple invertebrate descriptors (Table 3.3).
We found community-wide trophic position increased with an increase in ELP (χ2 =
31.71, p < 0.001). Linear contrasts showed that TP at high ELP (5.27) was different than
at low (1.88) and moderate (3.34) ELP sites (Figures 3.2a, 3.2b; p < 0.05). At the family
level, Tetragnathidae exhibited a ~4 TP increase from low to high ELP sites (Figure
3.2c); Formicidae, a ~3.5 TP increase (Figure 3.2d); and Chaoboridae a ~4.25 TP
increase (Figure 3.2e).
The contribution of aquatic C to invertebrate consumers was significantly
different across light levels (Table 3.3), where the greatest contribution of aquatic C
tended to be at high light levels (p < 0.05, Figure 3.3). For the invertebrate community
(Figure 3.3a), this pattern appeared largely driven by a few key taxa (Figure 3.3c, 3.3d,
3.3e).
The influence of ‘Site’ was also significant in many GLMs, although this was
largely constrained to whole community and family-level measures. Stream canopy was
not a significant factor for the majority of models, although it was significant for the
contribution of aquatic C to Tetragnathidae (χ2 = 3.72, p < 0.001; Table 3.3).
For the entire stream-riparian invertebrate community, VTP increased with high ELP,
whereby VTP at low (1.28) and moderate (1.56) ELP sites were different than at high
ELP sites (3.06) (Figure 3.4a, 3.4b). At the family level, Tetragnathidae, Formicidae, and
Chaoboridae exhibited increases in VTP at high ELP. We observed a similar pattern for
55
community-wide FCL (χ2 = 21.94, p = 0.003), where FCL at low (5.65) and moderate
(5.17) ELP sites was different than at high ELP sites (10.55; Figure 3.5a, 3.5b).
Responses to the experimental addition of light were variable (Table 3.4). We
observed an overall decrease in community-wide TP of ~1 at the experimental reaches
(t = -1.94, p = 0.062; Figure 3.6a); the pattern was largely driven by the aquatic
invertebrate community (Figure 3.6b). The terrestrial predator Tetragnathidae (t = -
0.694, p = 0.263) decreased by ~0.5 TP (Figure 3.6c), the terrestrial consumer
Formicidae (t = - 1.25, p = 0.156) decreased by ~1 TP (Figure 3.6d), and the aquatic
emergent insect Ceratopoginidae (t = - 1.49, p = 0.106) decreased by ~1 TP (Figure
3.6e). VTP increased by ~0.5 and FCL increased by 3 (Figure 3.6f). We found no
difference in the contribution of aquatic C with the increase of light to ~12 lux (Figure
3.6g).
Discussion
Previous research has shown that ELP affects multiple biotic characteristics at the
individual and population levels (i.e., migration, predator-prey relationships, mating
success; Longcore and Rich 2004, Eisenbeis and Hänel 2009, Kyba 2011), but less is
known relative to the potential impacts of ELP on community- and ecosystem-level
processes (but see Moore et al. 2000, Davies 2012). In small urban streams, we found
that ELP was associated with higher TP as well as greater VTP of linked stream-riparian
invertebrate communities. Initial evidence also indicated that the contribution of aquatic
C to invertebrate consumers was greater at sites characterized by higher levels of ELP.
56
An experimental addition of light resulted in a community-wide decrease in FCL,
confirming similar observational results and indicating that even short-term (~30-40
days) exposure to high artificial light levels may be consequential to stream ecosystem
function.
Trophic structure
Biotic interactions between adjacent ecosystems via reciprocal resource fluxes are
common (Polis et al. 1992, Baxter et al. 2005, Marczak et al. 2007) and can
fundamentally alter food-web structure (Paetzold 2006, Burdon and Harding 2008). For
example, Wesner (2010) showed that seasonal differences of aquatic emergent insects
can alter the trophic structure of riparian invertebrate food webs by changing the
proportion of aquatic prey subsidy vs. in situ prey in riparian invertebrate communities.
Our results indicate that the effects of ELP on stream-riparian trophic structure are
profound. In the current study, we observed increased enrichment of δ15N in the aquatic-
terrestrial invertebrate community at sites with higher ELP, whereby the magnitude of
δ15N aquatic insect enrichment was greater than the magnitude of δ15N terrestrial insect
enrichment (Figure 3.1) (but note that this result was not supported by the experimental
addition of light, Figure 3.6). Supporting this observation and consistent with our
hypotheses, we also found that TP of the aquatic-terrestrial invertebrate community was
greater at higher ELP levels (Figure 3.2a, 3.2b).
Altered community structure is likely driving changes in trophic structure and
position. In a companion study in the same study system, we found increased terrestrial
57
arthropod family richness and density, increased aquatic-to-terrestrial flux, and increased
Tetragnathidae density associated with higher ELP levels (Meyer and Sullivan, Chapter
2). Davies et al. (2012) found that ground-dwelling terrestrial invertebrate community
structure was affected by proximity to street lighting, such that communities closest to
stream lights were dominated by more predatory and scavenging individuals. Changes in
community structure as a consequence to artificial lighting have also been observed in
bats, whereby increased food concentrations of insects attracted to light sources is
advantageous to faster-flying species (Blake et al. 1994, Rydell and Baagoe 1996).
Similarly, positive phototaxic responses by ovipositing aquatic insects drawn to the area
by artificial light may result in a more diverse aquatic invertebrate community (Horvath
2004, Kriska et al. 2009), subsequently leading to increases in trophic complexity and
higher TP of consumers. In our study, Tetragnathidae increased by ~4 TP from low to
high ELP (Figure 2c), Formicidae exihibited an increase of ~3 TP (Figure 3.2d) and
Chaoboridae an increase of ~4 TP (Figure 3.2d).
Food-chain length
Food-chain length is a fundamental property of food webs (Post 2002, Sabo et al.
2009). Although productivity and disturbance have traditionally been offered as
important determinants of food chain length, empirical evidence increasingly suggests the
strong role of ecosystem size (Post et al. 2000, Takimoto et al. 2008, Sabo et al. 2010).
The role of disturbance has also been investigated, with contrasting results. Some
investigators (Parker & Huryn 2006, Marty et al. 2009) have found a negative
58
disturbance-FCL relationship, whereas others have found disturbance to be less
influential in regulating FCL (Takimoto et al. 2008). Takimoto et al. (2012) developed
and analyzed a metacommunity model of intraguild predation (IGP) and reported that the
model found increasing basal productivity, decreasing disturbance, and increasing
ecosystem size all increase FCL when local IGP is weak. As we had expected, we found
that FCL increase (~2x) for the aquatic-terrestrial invertebrate community at high ELP
sites (Figure 3.5a, 3.5b), yet ecosystem size was constrained. Although more research
would be required to accurately estimate IGP in our study system, this may be an
important factor. For example, in situations where the IGP link is strong (e.g., predators
limiting other predators), increases in the richness of prey species would be expected to
increase, thereby leading to greater FCL (Polis 1992, Power and Dietrich 2002, Holomuzi
2010) – similar to our observations at high ELP sites. Increased functional diversity
could also contribute to greater FCL at high ELP sites (Figure 3.6a) with an additional
intermediate predator. Alternately increased abundance of preferred prey may increase
dietary specialization and reduce omnivory resulting with increased FCL (Post et al.
2000). Although our findings indicate that ELP can play a significant role in regulating
FCL, the precise mechanisms will require further investigation. It is likely that a
complex interplay between ELP and biotic interactions is important in determining FCL.
Contribution of aquatic C to invertebrate consumers
Low-order streams are traditionally thought to be fueled by terrestrial organic matter
(Vannote et al. 1980). However, a spate of recent research (Walsh et al. 2005, Meyer et
59
al. 2005, Brown et al., 2009, Davies et al. 2010) has highlighted the consequences of
urbanization to streams. For example, O’Brien (2010) has shown that urbanization can
increase instream primary productivity via a combination of shifts in light availability,
nutrient delivery and hydrology, potentially making urban streams more autotrophic than
their more natural counterparts. Others (e.g., Villanueva et al. 2010) have reported that
light levels can significantly affect the structure and function of periphyton communities,
although higher light levels do not always promote greater in-stream productivity. We
found that ELP influenced the reliance on aquatic C of both aquatic and terrestrial
invertebrate communities, such that the contribution of aquatic C was greatest to the
whole invertebrate community as well as to individual consumer families at high ELP
sites (Figure 3.3a, 3.3b). However, at moderate ELP sites, there was a decrease in
periphyton utilization by the terrestrial community as well as by Tetragnathidae,
Formicidae, and Chaoboridae.
Collectively, these findings suggest shifts in feeding strategies whereby at low
and high ELP sites, grazing dominated but that at moderate ELP sites, detritivory was
dominant. Grazing aquatic insects responding to the seasonal increase in aquatic primary
production (i.e. periphyton) might be expected to lead to an increase in secondary
production of grazing aquatic insects, thereby increasing the contribution of aquatic C to
tetragnathid spider and other riparian consumers. Sabo and Power (2002), for example,
experimentally reduced aquatic emergent insect prey and observed concomitant decreases
in terrestrial predator abundance (i.e., lizards) The separation of δ13C between the riparian
predators and aquatic emergent insects was much lower in the moderate ELP (0.36‰
60
δ13C) as compared to the low ELP (2.50‰ δ13C) and high ELP (1.83 ‰δ13C), further
indicating that benthic algivory was replaced by detritovory at moderate ELP sites, in
spite of comparable riparian vegetation. An experimental study conducted by Bishop
(1969) showed an artificial light threshold of (0.1 – 1 lux) suppressed benthic insect (i.e.,
Ephemerella, Stenonema, Phasganophora) activity (as measured by aquatic insects in the
drift) where Limnephilidae showed an indifference to light. Bishop (1969) also described
a severe reduction (54%) in the density of invertebrate herbivores at the artificial light
treatment due to predation. While predator-prey relationships were likely at play in our
study, additional research will be required to identify the exact mechanisms. However, if
benthic detritivores do indeed replace grazers at moderate light levels, decreases in the
contribution of aquatic C to consumers would likely propagate throughout the system,
and likewise for grazers at low and light level sites.
In the experimental component of the study, we observed no difference in the
contribution to aquatic C to invertebrates with the addition of artificial light. This may
indicate that any predatory advantage may be suppressed by a much greater risk of
predation (e.g., by fish). Community-level feeding strategy is known to respond not only
to season (Miyasaka and Genkai-Kato 2010), but also to subtle changes in environmental
conditions (i.e., stream bed microhabitat, ambient light related to stream canopy), thus the
difference in the contribution of aquatic C among ELP treatments may also be a
consequence of shifts in microhabitat preferences by benthic grazing arthropods driven
by changes in ambient light levels.
61
Wesner (2010) showed that seasonal differences of aquatic emergent insects can
alter the trophic structure of riparian invertebrate food webs by changing the proportion
of aquatic prey subsidy vs. in situ prey in riparian invertebrate communities. The effects
of ELP on stream-riparian trophic linkage have implications for further disruption of
aquatic-terrestrial net energy flux. Potential mechanisms driving this response may be a
positive phototaxic response by ovipositing aquatic insects drawn to the area by artificial
light, providing a potentially more diverse community. Grazing aquatic insects
responding to the seasonal increase in aquatic primary production (i.e., periphyton) leads
to an increase in secondary production of grazing aquatic insects. Thereby, increases the
contribution of aquatic C to tetragnathid spider and other riparian consumers. Sabo and
Power (2002), for example, experimentally reduced aquatic emergent insect prey and
observed concomitant decreases in terrestrial predator abundance (i.e., lizards). Although
the results were not conclusive, our experimental light addition indicated that both
aquatic and terrestrial invertebrate communities responded with an increased reliance on
aquatic C. As light addition was over a short time period (~45 days), this suggests this
response occurs over the longer time scale related to insect productive cycles and
community diversity.
Conclusions
Our study provides evidence that ELP alters food-web complexity by increasing trophic
position, variability in trophic position, and FCL of stream-riparian invertebrate
communities. In general, we found stronger associations between artificial lighting and
62
the aquatic insect community. We also observed a shift in the contribution of aquatic vs.
terrestrial C to invertebrate consumers among light levels. Collectively, these results are
among the first evidence to point to ecosystem-level responses to artificial night lighting.
Globally, there remain few areas that are not affected by skyglow (i.e.,
brightening of the natural sky beyond background levels), as even wild areas such
national parks are in close proximity to urban areas (Albers and Duriscoe 2001) and
artificial night lighting may be increasing by around 6% per year (Hölker et al 2010).
The implications of this research are therefore broad, providing initial evidence that both
ecosystem structure and function may be significantly altered across large spatial scales.
Currently, information on environmental consequences of ecological light pollution is not
adequate for the potential scope and scale of the problem. Within urban settings, the
effects of artificial night lighting appear to be particularly severe, and our results indicate
that efforts to reduce both short-term and permanent ambient lighting should be
incorporated into management and conservation of urban stream systems. Looking
forward, we suggest that future research address the impacts of artificial night lighting at
broader spatial and temporal scales and across a range of ecosystems.
Acknowledgements
We extend our thanks to B. Gunther, L. Rieck, P. Tagwireyi, and Xiaoxue Yang for their
assistance in the laboratory and the field. This research was funded by The Ohio State
University and MacIntyre Stennis funds.
63
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Appendix A: Study reaches in the Slate Run sub-catchment of the Scioto River
Nine ecological light pollution (ELP) study reaches in the Slate Run sub-catchment of the
Scioto River, Columbus Metropolitan Area, Columbus, OH.
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Appendix B: Stream characteristics for study reaches in the Columbus
Metropolitan Area
Stream characteristics for study reaches in the Columbus Metropolitan Area. % Canopy
refers to the mean annual tree canopy cover over the stream. D50 is the median sediment
Appendix D: Terrestrial arthropod families captured in pan traps Terrestrial arthropod families captured in pan traps. The majority of riparian arthropods
came from families in the orders Diptera (23), Coleoptera (18), Arachnida (16), and
Hymenoptera (9). On the whole, riparian invertebrate communities displayed greater
evenness compared to aquatic emergent insects.
Arthropod family Study reaches Arthropod family Study reaches Arthropod family Study reachesfound (%) found % found (%)