Results & Discussion.
What was found? What does it mean?
Ilybius ater, a species counted 14 times at the Zoological Museum between 1992-2009 (image link)
Oryctes nasicornis, a species counted just 1 time at the Zoological Museum between 1992-2009 (image link)
Yearly Abundance.
Yearly counts show that many Coleopteran Families have boom/bust patterns (Figure 11.). The more abundant Families (such as Carabidae and Staphylinidae) show this trend much clearer than less abundant families (such as Scarabaeidae and Heteroceridae). There is also no obvious trend towards a decline in abundance over time for any Families. Carabidae and Staphylinidae are two families of scavenging beetles that are typically the most abundant Families in various landscapes (Quodri et al., 2016). Because of their generalist life strategies, the strong boom/bust trends seen in Figure 11. are expected as their abundances are most likely driven by resource availability.
Figure 11. The yearly abundance of 10 Coleopteran Families trapped from 1992-2009 in a Robinson Light Trap atop the Zoological Museum of Copenhagen, Denmark. Insect counts are only from April-November and only families with at least one monthly count above 50 individuals are shown. Insect data sourced from Thomsen et al., 2016. Climate data was sourced from ClimateEU (Marchi et al., 2020).
Effects of Temperature and Precipitation:
When monthly Family counts were plotted against monthly temperatures (Figure 12A.), there appears to be some Families that are more sensitive to temperature than others. These more sensitive Families (such as Carabidae and Staphylinidae) are also the same Families who had very strong yearly boom/bust patterns as seen in Figure 11. Similar trends were found in Carabid and Staphylinid samples from other global locations (Saska et al., 2013; Tsafack et al., 2019; Hammond et al., 2021). The summer months, from May-August, appear to have the largest variation in insect abundance as temperature varies. Copenhagen is a mild climate city, and it is likely that the climate doesn't reach extreme enough temperatures to see stronger temperature effects within the insects. The apparent lack of patterns may also be due to scale.
When monthly Family counts were plotted against monthly precipitation (Figure 12B.), there does not appear to be any strong relationships between the two variables in most months. Only the months of July and August have a large variation in insect abundance as precipitation varies. This is not surprising since precipitation is quite similar between months and years within the city of Copenhagen (Figure 6.) masking any major precipitation dependencies.
The lack of visible trends among the climate variables is surprising because more extreme climate variables should have impacts on abundance. However, it is important to note that the months of December to March (winter months) were not considered in this analysis. Perhaps variations in winter temperature and precipitation are larger drivers for Coleopteran abundance than summer variations. This would be consistent with results from other works looking at overwintering temperature and precipitation effects on insects abundance (Mutshinda et al., 2011; Bowler et al., 2017a; Bowler et al., 2017b).
Figure 12. The abundance of 10 Coleopteran Families trapped from 1992-2009 in a Robinson Light Trap atop the Zoological Museum of Copenhagen, Denmark. Insect counts are only from April-November and only families with at least one monthly count above 50 individuals are shown. Insect data sourced from Thomsen et al., 2016. Climate data was sourced from ClimateEU (Marchi et al., 2020). A. Monthly Family counts against the temperature deviations from the monthly average (in degrees Celsius), each month for each Family is plotted as a separate point. B. Monthly Family counts against the precipitation deviations from the monthly average (in % mm), each month for each Family is plotted as a separate point.
Bootstrapped 95% Confidence Intervals.
The results indicate that the Coleopteran families like warmer temperatures in the months of May-Aug; where there is a positive relationship between temperature and insect abundance with a 95% probability (Figure 13). This would be expected since during this time frame there is the highest insect activity, and species could proliferate faster if conditions were optimal during their summer breeding seasons. Temperatures in the month of November also have a notable positive correlation with insect abundance as well. This was expected, and warmer winters typically result in less insect mortality (Ayres and Lombardero, 2000).
There is some indication that precipitation is correlated with overall Coleopteran abundance. However, these trends vary in relationship direction and have small correlation coefficients. General associations indicate that Coleopteran families enjoy less precipitation, especially in the months of July and November; where there is a negative relationship between precipitation and insect abundance with a 95% probability. It is also shown that a wet June followed by a dry July would have the highest impact on insect abundance.
Order Coleoptera.
Figure 13. The correlation analysis between climate variables and insect count of 10 Coleopteran Families trapped from 1992-2009 in a Robinson Light Trap atop the Zoological Museum of Copenhagen, Denmark. Larger points indicate significant correlation effects before bootstrapping using holmes' method of adjustment (P < 0.05). Bars show 95% confidence intervals for the Kendall correlation coefficients calculated using 1000 bootstraps, where dotted lines indicate insignificant bootstrap effects. Insect counts are only from April-November and only families with at least one monthly count above 50 individuals were used. The temperature was originally collected as average monthly temperature (in degrees Celsius) and the precipitation was originally collected as average monthly precipitation (in millimeters). Insect data sourced from Thomsen et al., 2016. Climate data was sourced from ClimateEU (Marchi et al., 2020).
Order Lepidoptera.
Agriphila straminella, a species counted 122 times at the Zoological Museum between 1992-2009 (image link)
Ennomos alniaria, a species counted 6 times at the Zoological Museum between 1992-2009 (image link)
Yearly Abundance.
Yearly counts show that Lepidopteran Families lack any obvious patterns over time (Figure 14.). In addition, there is no apparent trend towards a decline in abundance over time for any Families. It is interesting that there are no large declines in abundance over time since Lepidopteran abundance has been declining consistently across Europe (Wagner et al., 2021). Perhaps there would be different results if a lower taxonomic classification was analyzed (such as species). The city of Copenhagen could also simply be a very suitable habitat for Lepidopterans and since Lepidopteran declines are spatially heterogenous (Wagner et al., 2021), this is a plausible explanation.
Figure 14. The yearly abundance of 13 Lepidopteran Families trapped from 1992-2009 in a Robinson Light Trap atop the Zoological Museum of Copenhagen, Denmark. Insect counts are only from April-November and only families with at least one monthly count above 50 individuals are shown. Insect data sourced from Thomsen et al., 2016. Climate data was sourced from ClimateEU (Marchi et al., 2020).
Effects of Temperature and Precipitation:
When monthly Family counts were plotted against monthly temperature (Figure 15A.), there again appears to be some Families that are more sensitive to temperature than others. These more sensitive Families (such as Yponomeutidae and Noctuidae) are also the same Families that are more abundant. Yponomeutidae and Noctuidae contain some common introduced species (UBC, 2021) which may not be as adapted to the local climate as native species. Like the Coleopterans, Lepidopteran insect abundance seems to be more variable in the summer months. Explanations for this are similar to those provided for the Coleopterans. In addition, there does not appear to be more sensitivity among the Lepidopteran groups, which could be explained by their greater dispersal abilities (i.e. they can move farther and faster towards locations with optimal climates). Again, the apparent lack of patterns may be due to scale.
When monthly Family counts were plotted against monthly precipitation (Figure 15B.), there does not appear to be any strong variation in insect abundance as precipitation varies in most months (other than July and August). This is again not surprising since precipitation is temporally quite similar; masking any precipitation dependence.
Figure 15. The abundance of 13 Lepidopteran Families trapped from 1992-2009 in a Robinson Light Trap atop the Zoological Museum of Copenhagen, Denmark. Insect counts are only from April-November and only families with at least one monthly count above 50 individuals are shown. Insect data sourced from Thomsen et al., 2016. Climate data was sourced from ClimateEU (Marchi et al., 2020). A. Monthly Family counts against the temperature deviations from the monthly average (in degrees Celsius), each month for each Family is plotted as a separate point. B. Monthly Family counts against the precipitation deviations from the monthly average (in % mm), each month for each Family is plotted as a separate point.
Bootstrapped 95% Confidence Intervals.
The results indicate that the Lepidopteran families like warmer temperatures in the months of April, May, July, and August; where there is a positive relationship between temperature and insect abundance with a 95% probability (Figure 16). Again, this was expected since during this time frame there is the highest insect activity and reproductive capacity would be higher in warmer climates. Temperatures in the months of October and November also have a notable positive correlation with insect abundance. This was also expected, and warmer winters typically result in less insect mortality (Ayres and Lombardero, 2000).
There is an indication that the Lepidopteran families enjoy less precipitation in the months of July and August; where there is a negative relationship between precipitation and insect abundance with a 95% probability. The overall associations also indicate similar trends in the other months, however, these monthly trends are not significant.
Figure 16. The correlation analysis between climate variables and insect count of 13 Lepidopteran Families trapped from 1992-2009 in a Robinson Light Trap atop the Zoological Museum of Copenhagen, Denmark. Larger points indicate significant correlation effects before bootstrapping using holmes' method of adjustment (P < 0.05). Bars show 95% confidence intervals for the Kendall correlation coefficients calculated using 1000 bootstraps, where dotted lines indicate insignificant bootstrap effects. Insect counts are only from April-November and only families with at least one monthly count above 50 individuals were used. The temperature was originally collected as average monthly temperature (in degrees Celsius) and the precipitation was originally collected as average monthly precipitation (in millimeters). Insect data sourced from Thomsen et al., 2016. Climate data was sourced from ClimateEU (Marchi et al., 2020).