3Large-Scale Field Experiment on ALAN Impacts on Aquatic-Terrestrial Ecosystem Flux and Riparian Ecosystem Linkages

Study Background and Objectives

• Artificial Light at Night (ALAN): Recognized as a widespread environmental alteration affecting ecosystem functioning, animal movement, reproduction, physiology, and behavior.

• Freshwater and Riparian Linkages: These ecosystems are connected by reciprocal fluxes of energy and matter. Terrestrially derived carbon and nutrients support aquatic metabolism, while emerging aquatic insects and amphibians provide subsidies for terrestrial consumers.

• Research Hypothesis: ALAN can change the movement patterns of freshwater animals into adjacent riparian areas, potentially affecting consumers that rely on these aquatic subsidies.

• Study Novelty: This work is among the first to experimentally manipulate ALAN using a large-scale field experiment to study its effects on ecosystem-linking processes, disentangling light pollution from other stressors like urbanization or chemical pollution.

Experimental Site and Infrastructure

• Location: Westhavelland Nature Park, Germany, located within a $750\,\text{km}^2$ International Dark-Sky Reserve. The specific area is characterized by an agricultural drainage ditch system.

• Experimental Design: A Before-After Control-Impact (BACI) design was utilized at two identically managed grassland sites (Control and Treatment) separated by a Euclidean distance of $\sim 600\,\text{m}$ ($800\,\text{m}$ along the ditch).

• Streetlight Specifications:

  • Twelve streetlights per site arranged in three parallel rows at distances of $3\,\text{m}$, $23\,\text{m}$, and $43\,\text{m}$ from the water.

  • Streetlights were $4.75\,\text{m}$ high and spaced $20\,\text{m}$ apart.

  • Lamps: $70\text{-W}$ high-pressure sodium lamps (OSRAM VIALOX NAV-T Super 4Y).

• Illuminance Levels:

  • Maximum illuminance: $\sim 50\,\text{lux}$.

  • Minimum between rows: $\sim 1\,\text{lux}$.

  • Minimum between adjacent lamps in a row: $\sim 10\,\text{lux}$.

• Timeline:

  • Monitoring began in May 2012.

  • Lit phase at the Treatment site began on July 25, 2012.

  • Study concluded in July 2013.

Sampling Methodology and Taxonomic Identification

• Aquatic Insect Emergence: Measured using four floating pyramidal emergence traps per site ($0.85 \times 0.85\,\text{m}$, $300\,\mu\text{m}$ mesh) placed in the ditch adjacent to the bank, directly in front of streetlamps.

• Flying Insects: Collected using 12 air eclector traps per site, each consists of two perpendicular acrylic panels ($204 \times 500 \times 3\,\text{mm}$) mounted $0.5\,\text{m}$ below each lamp.

• Ground-dwelling Arthropods: Collected using 24 pitfall traps per site (tubes of $15\,\text{cm}$ diameter) placed at varying distances from the ditch.

• Taxonomic Resolution:

  • 2012: Spiders and harvestmen identified to species or genus; others to family or order.

  • 2013: Species-level identification for all wolf spiders and ground beetles.

• Data Standardization: Abundance was standardized to Catch Per Unit Effort (CPUE), defined as the number of individuals caught per hour of trap operation.

Effects on Aquatic Insect Emergence

• BACI-acute Comparison (2012 internal): Found no significant difference in emergence between phases and sites.

• BACI-chronic Comparison (2012 vs. 2013): Indicated significantly higher insect abundance at the Treatment site during illumination ($F(1, 30) = 13.89$, p < 0.001).

• Specific Growth: CPUE at the lit Treatment site in 2013 was 2-fold higher than the Control site and 3-fold higher than its own baseline (2012) level ($t(1, 30) = 5.01$, p < 0.001).

• Mechanism: The authors suggest ALAN may reduce predation pressure from invertivorous fish (like Gasterosteus aculeatus or Rhodeus amarus) because they must balance feeding efficiency with avoiding piscivorous visual predators (like Esox lucius) newly active under light. ALAN may also act as an orientation cue for egg-laying insects via polarized light reflections.

Flying Insect Attraction and Community Shifts

• Attraction Levels (BACI-acute 2012):

  • Aquatic flying insect CPUE at the lit Treatment site was 76-fold higher than the Control site and 70-fold higher than its own pre-illumination state.

  • Terrestrial flying insect CPUE was 34-fold higher than the Control site.

• Attraction Levels (BACI-chronic 2013):

  • Aquatic insect CPUE was 267-fold higher than the Control site and 918-fold higher than pre-illumination levels at the Treatment site ($F(1, 20) = 4.75$, p < 0.05).

• Proportion of Aquatic Insects: In 2013, aquatic insects comprised $75\%$ of the total catch at the Treatment site, compared to $54\%$ at the Control site and $58\%$ at the Treatment site prior to illumination.

• Ecological Implication: ALAN effectively acts as an "ecological trap," disrupting dispersal and potentially "sealing off" surrounding terrestrial ecosystems by concentrating insects near the light sources and reducing habitat connectivity.

Changes in Riparian Ground-Dwelling Communities

• General Trends: Contrary to expectations of a general increase in predators due to increased prey, the total number of ground-dwelling predators and scavengers was lower at the lit site, possibly due to decreased activity (less movement needed for food) or specific repulsion.

• Taxa Increasing Under ALAN:

  • Trochosa sp. (Wolf spider): CPUE was $2.5\text{-fold}$ higher at the lit site compared to the Control ($t(1, 17) = 10.94$, p < 0.001).

  • Pachygnatha clercki (Long-jawed spider): CPUE was $2.9\text{-fold}$ higher at the lit site ($t(1, 17) = 3.76$, p < 0.01). These spiders are night-active visual hunters with a tapetum in secondary eyes, making them efficient at low light levels.

  • Opiliones (Harvestmen): CPUE was $1.9\text{-fold}$ higher at the lit site compared to the Control ($t(1, 17) = 5.56$, p < 0.001).

• Taxa Decreasing Under ALAN:

  • Carabidae (Ground beetles): CPUE was $7.4\text{-fold}$ lower at the Treatment site than the Control ($t(1, 17) = 21.18$, p < 0.001).

  • Staphylinidae (Rove beetles): CPUE was $4.7\text{-fold}$ lower at the Treatment site in 2013 compared to the Control.

  • Pirata piraticus (Wolf spider): CPUE was $1.4\text{-fold}$ higher at the Control site relative to the lit site.

Alteration of Circadian Activity Patterns

• Extension of Activity: Predators including Pachygnatha clercki, Trochosa sp., and the scavenger Silpha sp. (carrion beetle) either increased nocturnal activity or extended their activity into the day at the lit Treatment site.

• Trochosa sp. Specifics: CPUE was 7-fold higher at night than during the day at the Treatment site, whereas at the Control site it was only 4-fold higher ($F(1, 21) = 98.15$, p < 0.001).

• Carabidae Specifics: CPUE at the Treatment site was 5-fold lower at night than during the day, showing a drastic decrease in nocturnal activity under ALAN exposure.

• Pirata piraticus Specifics: Showed 12-fold higher CPUE at night than day at the Control site ($t(1, 21) = 7.60$, p < 0.001), but no difference between day and night was observed at the Treatment site.

Environmental and Planning Significance

• Impact of LED Technology: The study utilized high-pressure sodium lamps; however, the global shift toward LED lamps (which peak in short-wavelength white-blue light) is expected to have even greater disruptive effects on nocturnal invertebrates.

• Landscape Planning: The authors argue that artificial light should be considered a relevant stressor in urban and landscape planning (Schroer and Hölker, 2016).

• Mitigation Recommendations: Installation of streetlights directly adjacent to riverbanks should be avoided or designed with riparian buffers to preserve the movement and dispersal of aquatic and terrestrial organisms.

1. Abstract Zusammenfassung, Ziele

   • Diese Studie untersucht die Auswirkungen von künstlichem Licht in der Nacht (ALAN) auf Ökosysteme, insbesondere die Bewegungsmuster von Süßwassertieren in angrenzende Uferbereiche.

2. Introduction eine allgemeine Kontext in die Studie

   • Künstliches Licht wird als bedeutende Umweltveränderung erkannt, deren Einfluss auf das Verhalten und die Physiologie von Lebewesen erforscht wird.

3.1. Methods

• Das Feldexperiment fand im Westhavelland Nature Park, Deutschland, statt, da dieser Ort innerhalb eines internationalen Dunkel-Schutzgebietes liegt und eine geeignete landwirtschaftliche Entwässerungsgrabenstruktur aufweist.

3.2. Methods

• Ein statistischer Test, der angewandt wurde, ist die BACI (Before-After Control-Impact) Analyse.

4.3. Results

• Ein bodenbewohnendes Taxon, das häufiger auf der unbeleuchteten Feldseite vertreten war, ist die Gattung Trochosa (Wolfspinnen), die unter ALAN signifikant an Abundanz zunahm.

5.4. Discussion

• Die Ergebnisse von Manfrin et al. (2017) zeigen teilweise Übereinstimmung mit anderen Studien, die ebenfalls den Einfluss von ALAN auf die Tiergemeinschaften untersuchen.

5.5. Discussion

• Schwierigkeiten der Studie umfassen die begrenzte Anzahl von Standorten und die Variation in Umweltbedingungen, die nicht vollständig kontrolliert werden konnten.

1. Conclusion

   • Die Autoren schließen, dass künstliches Licht weitreichende Folgen für die ökologische Interaktion und das Management von Lebensräumen haben kann, insbesondere in städtischen und bewirtschafteten Landschaften.