Artificial light at night (ALAN) is recognized as a widespread and increasingly important anthropogenic environmental pressure on wild species and their interactions. Understanding of how these impacts translate into changes in population dynamics of communities with multiple trophic levels is, however, severely lacking. In an outdoor mesocosm experiment we tested the effect of ALAN on the population dynamics of a plant-aphid-parasitoid community with one plant species, three aphid species and their specialist parasitoids. The light treatment reduced the abundance of two aphid species by 20% over five generations, most likely as a consequence of bottom-up effects, with reductions in bean plant biomass being observed. For the aphid Megoura viciae this effect was reversed under autumn conditions with the light treatment promoting continuous reproduction through asexuals. All three parasitoid species were negatively affected by the light treatment, through reduced host numbers and we discuss induced possible behavioural changes. These results suggest that, in addition to direct impacts on species behaviour, the impacts of ALAN can cascade through food webs with potentially far reaching effects on the wider ecosystem.
Coevolution shapes diversity within and among populations but is difficult to study directly.Time shift experiments, where individuals from one point in time are experimentally challenged against individuals from past, contemporary, and/or future time points, are a powerful tool to measure coevolution. This approach has proven useful in both directly measuring coevolutionary change and in distinguishing among coevolutionary models. However, these data are only as informative as the time window over which they were collected, and inference from shorter coevolutionary windows might conflict those from longer time periods. Previous time-shift experiments from natural microbial communities of horse chestnut tree leaves uncovered an apparent asymmetry, whereby bacterial hosts were more resistant to bacteriophages from all earlier points in the growing season while phages were most infective to hosts from only the recent past. Here we extend the time window over which these infectivity and resistance ranges are observed to across years and confirm that the previously observed asymmetry holds over longer timescales. These data suggest existing coevolutionary theory should be revised to include the possibility of differing models for hosts and their parasites, and examined for how such asymmetries might reshape the predicted outcomes of coevolution.
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