|Nuutinen, Visa -|
|Butt, Kevin -|
|Jauhiainen, Lauri -|
|Siren, Taisto -|
Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 23, 2014
Publication Date: February 6, 2014
Citation: Nuutinen, V., Butt, K.R., Jauhiainen, L., Shipitalo, M.J., Siren, T. 2014. Dew-worms in white nights: High latitude light constrains earthworm (Lumbricus terrestris) behaviour at the soil surface. Soil Biology and Biochemistry. 72:66-74. Interpretive Summary: Earthworms are oftentimes referred to as ecosystem engineers because of how profoundly they can affect soil properties. In agricultural fields they can affect how rapidly crop residues breakdown and release nutrients necessary for plant growth. Additionally, by ingesting soil and plant residues and constructing burrows they help form stable soil aggregates and pores that can make the soil a better environment for root growth while promoting movement of water through the soil and making it less susceptible to erosion. Earthworms, however, are not native to many regions of the world, including the parts of North America that were covered by glaciers during the last ice age. Nevertheless, earthworms are now commonly found in this region because they were accidently introduced when European farmers settled these areas. Some species of earthworms must come to the soil surface at night to feed and reproduce. Their activity can be limited in near-arctic regions where suitable temperature and moisture conditions mainly occur in the summer when there are few hours of darkness. In this study we investigated how a particular species of earthworm, Lumbricus terrestris, commonly referred to as a dew worm or nightcrawler, behaves when exposed to natural, near-arctic, light conditions. The activity of dew worms from Finland, southern England, and Ohio were compared. We found that the Finnish earthworms came to the soil surface and mated more frequently than earthworms from the other countries, suggesting that they have adapted to the high light conditions found in the far north during summer. Despite coming from more southernly latitudes, the Ohio dew worms were more active than the English earthworms. This suggests the Ohio earthworms may have initially come from more northernly regions in Europe and are still adapted to being active during short summer nights. Knowledge of how light affects earthworms activity and how they can adapt to light conditions when introduced to a new area from regions with either shorter or longer nights will improve our understanding of how earthworms affect soil development and influence processes that are important to crop growth.
Technical Abstract: Soil is an effective barrier to light penetration that limits the direct influence of light on belowground organisms. Variation in aboveground light conditions, however, is important to soil-dwelling animals that are periodically active on the soil surface. A prime example is the earthworm Lumbricus terrestris L. (the dew-worm), an ecosystem engineer that emerges nocturnally on the soil surface. In the summer the northernmost populations of L. terrestris are exposed to a time interval with no daily dark period. Over a two-week period preceding the summer solstice, we studied the constraints that boreal night illumination imposes on L. terrestris activity by comparing their behaviour under ambient light with artificially induced darkness. Looking for evidence of geographical divergence in light response, we compared the behaviour of native L. terrestris (Jokioinen, S-W Finland; 60º48’N) with two markedly more southern populations, from Preston (Lancashire, UK; 53º47’N) and Coshocton (Ohio, USA; 40º22’N) where the nights have a period of darkness throughout the year (total latitudinal range ca. 2 300 km). Under ambient light conditions, L. terrestris emergence on the soil surface was diminished by half compared with the darkened treatment and it peaked at the darkest period of the night. Also mating rate decreased considerably under ambient light. The native dew-worms were generally the most active under ambient light. They emerged earlier in the evening and ceased their activity later in the morning than dew-worms from the two more southerly populations. The differences in behaviour were, however, significant mainly between native and UK dew-worms. In the darkened treatment, the behaviour of the three earthworm origins did not differ. Light condition was the dominant environmental factor controlling surface activity, but elevated night-time air temperature and humidity also encouraged dew-worm emergence without discernible differences among geographical origins. Our results show, that in boreal summer, the high level of night illumination strongly limits soil-surface activity of dew-worms. Considering the important regulatory role of L. terrestris in many ecosystem processes, this can have significant corollaries in dew-worm impacts on the environment. Although evidence for geographical differentiation in behaviour was obtained, the results point to phenotypic flexibility in L. terrestris light response.