Thousands of insects living on a single urban rooftop in Copenhagen for the past 18 years have demonstrated how local insect community turnover may be impacted by climate change, say researchers from the University of Copenhagen. Their study is shedding light on species most sensitive to change. 

"As temperature increases we see a corresponding change in the insect community, specifically for the resource specialists – the insects that feed on only one species of plant. Earlier studies have confirmed that specialist species also respond rapidly to destruction of their habitats, so we are dealing with a very sensitive group of animals," Philip Francis Thomsen, one of the lead authors of the study from the Center for Geogenetics, said in a news release. 

For their study, researchers monitored 1543 different species of moths and beetles. In total, more than 250.000 individuals have been registered on a single urban rooftop in Copenhagen over the course of an 18-year time period. These insect populations represent 42 percent of all the species of moths in Denmark and 12 percent of all beetles, the release noted.

So what did they find? Researchers observed significant changes in insect communities – specifically, the nut weevil (Curculio nucum) which is an example of a resource specialist that feeds solely on hazel. Compared to the closely related acorn weevil (Curculio glandium), nut weevils can be found living much more further north in Europe. The acorn weevil is another example of a resource specialist that only feeds on acorns. 

When observing these two insects researchers found that the nut weevil was only registered in the first half of the study, while the acorn weevil only appeared in the last part of the study. This indicates that the specialist species are moving northwards in Europe, the release noted. This explains the increased environmental pressures northern species are encountering as they are forced out of their natural habitat range.

"We are likely to lose some specialist species as they retreat north, but more new specialist species will arrive from the south. This trend is theoretically expected but extremely rare to confirm with observations across this many species. Insects are often over-looked and under prioritized for long term studies," Peter Søgaard Jørgensen, the other lead author of the study from the Center for Macroecology, Evolution and Climate, explained in the release. 

Over the course of 18 years, researchers monitored the species' populations every week from 1992 to 2009. Observations were recorded during spring, summer and fall using a light trap on the roof of the Natural History Museum of Denmark. These light traps were placed 17.5 meters height. 

"Long-term monitoring, even without a pre-defined purpose, can be of incredible value when trying to understand and predict biodiversity in a changing world. Species monitoring is under prioritized in Denmark and primarily driven by personal interest from committed enthusiasts. Without those individuals we would basically be in the dark about the majority of species in Denmark. The same is probably true for many other European countries. We hope this study can push nature monitoring back onto the political agenda," Thomsen added. 

During their study, seven species of moths and two species of beetles were registered for the first time in Denmark. This includes the multicolored Asian lady beetle (Harmonia axyridis), which has since been seen spreading throughout most of country, so much so that researchers consider it an invasive species. 

"Some insects are very mobile and only eat as larvae. It is therefore not unusual to find them further from their habitats as adults. However, it is an impressive diversity of species registered. Even though the study is limited to one site, there is no reason to believe that the trend we see here would be different at other sites," Jørgensen said in a statement. 

To better understand the role climate change plays in this insect diversification, researchers calculated an index for the temperature changes experienced throughout the entire habitat range in Europe during the 18-year study period. This allowed them to target specific species that may be particularly impacted by rising temperatures. 

"The results confirm that climate change is impacting biodiversity right now. It is not something that will happen far into the future or only if we reach a two degree temperature increase," Jørgensen added. 

Their study was recently published in the Journal of Animal Ecology. 

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