Diptera: Syrphidae) Using Swedish Citizen Science Data

Linköping University | Department of Physics, Chemistry and Biology Bachelor thesis, 16 hp | Biology Spring term 2021 | LITH-IFM-G-EX—21/3998--SE

Examining the Link between Temperature and Flight Phenology in Hoverflies (Diptera: Syrphidae) Using Swedish Citizen Science Data

Malin Magnusson Rundqvist

Examiner, Karl-Olof Bergman Tutor, Per Milberg

1. Abstract ...... 1

2. Introduction ...... 1

3. Methods ...... 3

3.1. Syrphidae dataset ...... 3 3.2. Temperature data ...... 3 3.3. Analyses ...... 4 4. Results ...... 5

4.1. Temperature data ...... 5 4.2. Flight phenology ...... 6 4.3. First flight and length of flight ...... 9 5. Discussion ...... 13

6. Societal and ethical considerations ...... 16

6.1. Societal considerations ...... 16 6.2. Ethical considerations ...... 16 7. Acknowledgements ...... 17

8. References ...... 18

1. Abstract

Global warming is causing a general trend of rising temperatures worldwide. Simultaneously there is also a decline in populations of pollinators all over the world. Therefore, it is important to examine the effect warming temperatures might have on different pollinator species. The focus of this study was to look at how flight phenology of hoverflies in southern Sweden is affected by rising summer temperatures using two regions differing in temperature, and 11 years of citizen science data on hoverfly observations. Summer temperature and observations of 13 species were used. Although four species had a significantly earlier first flight in years with warmer weather, there were overall no apparent trends toward earlier or longer flight periods due to temperature deviation. However, geographical location had a strong impact on flight behaviour of hoverflies in Sweden with hoverflies in Götaland having an earlier first flight compared to Svealand (located further north). This might be the result of an earlier onset of spring and summer in Götaland than in Svealand. The results of this study indicate that more factors than temperature affect flight phenology in hoverflies. Keywords: climate change, hoverfly, phenology, pollinator, Sweden, Syrphidae.

2. Introduction

The global climate is under change due to anthropogenic effects, and it can be seen on every continent of the world. Temperature is rising globally, and weather conditions are becoming more extreme (IPCC, 2014). In Europe, springtime is advancing on an average of 2.5 days per decade, and it is apparent that both animal and plant phenology adjust accordingly (Menzel et al., 2006). Phenology is a term used to describe the study of how recurring events in nature occurs in relation to seasonal weather changes i.e., time for bird migration, leafing of trees, flowering of plants as well as freezing of lakes (Lechowicz, 2001). An extensive study with data from over two centuries in England (Sparks & Carey, 1995), found that some species were leafing and blooming earlier in spring, while others came on later. Sparks and Carey (1995) predict that most phenological events would start earlier in the year due to climate change. Some birds have also been found to lay their eggs earlier in spring in the United Kingdom (Crick & Sparks, 1999).

Although there seems to be many factors contributing to the decline of pollinator biodiversity all over the world, three important factors that seem to stand out are habitat loss, climate change,

1 and the changing of plant phenology (mainly length of growth season) (Vasiliev & Greenwood, 2021). In Sweden pollinators are almost solely insects. Of the 299 anthophilan species (wild bees) in Sweden 33 % is on the national red list, 20 % of 2 645 lepidopteran species (butterflies and moths) and 11 % of the 400 Syrphidae species (hoverflies) also appears on the national red list (Bergström et al., 2018).

Almost all Swedish adult hoverflies are generalists, in contrast to bees and butterfly species, meaning they pollinate a wide variety of flowers. The relatively low percentage of species found on the national red list is likely due to the fact that most adult hoverfly species are generalists (Bergström et al., 2018). Since most hoverflies are generalists with good flying capacity, they can visit many flowers and are effective pollinators (Bergström et al., 2018). Therefore, hoverflies is one of the most important families of pollinators in Sweden (Bartsch, 2009a; Bartsch, 2009b) and hence the focus of the current study.

Studies regarding rising temperatures in Europe and the response of different hoverfly species, have led to the conclusion that hoverflies in general emerge earlier each year (Graham-Taylor et al., 2009; Hassal et al., 2016; Olsen et al., 2020). Despite their importance as a pollinator in Sweden, there is a paucity of knowledge about their phenology and response to the Swedish weather (Bergström et al., 2018). In 2009 the Encyclopaedia of Swedish flora and fauna (Bartsch, 2009a; Bartsch, 2009b) was published and made it significantly easier for amateurs to identify different hoverfly species. This resulted in an increase of reported sightings of hoverflies, both red-listed and non-threatened all over Sweden (Bergström et al., 2018). The aim of the current study is therefore to address whether rising summer temperatures affects flight phenology of hoverflies in Sweden. This is studied in two Swedish regions using 11 years of temperature data as well as observational citizen science data on hoverflies.

3. Methods

To examine if there is a connection between flight period of hoverflies and temperatures, observational reports from Swedish citizen science data as well as historical temperature data were used. These were divided into two geographical regions of southern Sweden: Götaland and Svealand. Götaland is located at approximate latitude 55.40 to 59.20 while Svealand is located north of Götaland at approximate latitude 58.70 to 62.20.

3.1. Syrphidae dataset

The observational data for different hoverfly species were obtained from the open database available on Artportalen.se (SLU Artdatabanken, 2021). Artportalen.se is a website hosted by the Swedish University of Agricultural Sciences and funded mainly by the Swedish Environmental Protection Agency. In total there are over 56 million reported observations of plants, animals, and fungi, starting at the late 1990s (SLU, 2016). The data uploaded to Artportalen.se is reported by both professionals and amateurs (SLU Artdatabanken, 2020). The number of observed hoverfly species per year varied, and only the years 2010-2020 were deemed to have sufficient number of observations for meaningful analyses.

In April 2021, I downloaded the data from Artportalen.se filtered by species name, years 2010- 2020, months March-October and Imago/adult. I chose period March-October to remove outliers outside of the general flight period for hoverflies in Sweden. The different species were chosen based upon three criteria: 1) how common they were in Sweden, 2) total number of observations reported during 2010-2020 and 3) species found in both Götaland and Svealand. The third Swedish province Norrland was initially considered, but there were too few observations to be useful in the current analysis.

The minimum number of observations 2010-2020 was set to 900, as lower numbers is unlikely to work well when broken down per year and region. I also considered the number of observations in each region and set 250 as the minimum. Among the many potential species, 13 were selected for this study.

3.2. Temperature data

Temperature data was compiled from the website of Swedish Meteorological and Hydrological Institute, SMHI, and open data for year- and monthly statistics (SMHI, 2021). I downloaded

3 monthly weather data (temperature and wind) from March-October, and 2010-2020, from all active weather stations, as well as the monthly values for the reference period 1961-1990. From this, I calculated how much each regions temperature had deviated from the reference period based on available weather stations and annual weather attributes, in this study referred to as mean deviance from reference period. This was calculated through the following three steps: 1. For each weather station the deviation between mean value of the month and the reference period was calculated. 2. For each month and region, the deviation from the reference period was calculated into one mean value for the whole month. 3. Lastly each mean value for March-October was summarised, per year, into one mean deviation value.

3.3. Analyses

Data was handled in RStudio (R Core Team, 2020). The following were included for each species observation dataset: scientific name, number of observations, date of observation, activity, and province. I filtered away any observations of dead hoverflies (noted as dead under activity).

The observations were split into Götaland and Svealand with the help of observed province. The observed date was converted into the Julian day number i.e., the number of days from 1 January. Horizontal violin plots were created per region, as a representation of flight pattern. Two features of the flight data were also extracted by year and region: first flight and length of flight period (number of days between first and last observation).

To statistically analyse whether flight phenology was affected by temperature deviation from reference period 1961-1990, a GLM (generalized linear model, normal distribution, identity- link) was conducted for each species. The geographical region was used as a categorical variable and temperature deviation (square root transformed) as continuous explanatory variable in the GLM.

4. Results

Total observations per species ranged between 903 and 3280 (sum over the eleven years).

4.1. Temperature data

The mean temperature deviance from the reference period (1961-1990) for March-October during 2010-2020, varied substantially, but was consistently above the reference period (Figure 1). The trend seen is that temperature deviation in general have been increasing for both regions (Figure 1).

Figure 1. Mean temperature deviation (°C) for March-October during 2010-2020, from reference period 1961-1990. A: Götaland. B: Svealand (located north of Götaland).

4.2. Flight phenology

Violin plots were produced and analysed for all 13 species, but only five are shown in this section to illustrate different findings (Figure 2-5). The violin plots showed distribution of reported observations layered with a boxplot to give more detail and readability. Violins were sorted by year from the lowest temperature deviation to highest deviation.

Volucella pellucens showed no apparent change in length of flight, but there was a trend of earlier first flight in Götaland (Figure 2A). Observations of first flight in Götaland (Figure 2A) tended to indicate earlier first flight in warmer years while no such trend is seen in Svealand (Figure 2B).

Figure 2. Flight period of Volucella pellucens 2010-2020. Colour coordinated by year and sorted from smallest (bottom) to largest (top) temperature deviation. A: Observations in Götaland. B: Observations in Svealand (located north of Götaland).

The flight period of Eristalis tenax was both longer and had an earlier first flight in Götaland (Figure 3A) compared to Svealand (Figure 3B).

Figure 3. Flight period of Eristalis tenax 2010-2020. Colour coordinated by year and sorted from smallest (bottom) to largest (top) temperature deviation. A: Observations in Götaland. B: Observations in Svealand (located north of Götaland).

For Sphaerophoria scripta, there was a trend of earlier flight for years with higher temperature deviance in both regions. Also, observations in Götaland (Figure 4A) were generally earlier than for Svealand (Figure 4B).

Figure 4. Flight period of Sphaerophoria scripta 2010-2020. Colour coordinated by year and sorted from smallest (bottom) to largest (top) temperature deviation. A: Observations in Götaland. B: Observations in Svealand (located north of Götaland).

For Helophilus pendulus, a trend of earlier first flight in years with higher temperature deviance were seen in both regions (Figure 5).

Figure 5. Flight period of Helophilus pendulus 2010-2020. Colour coordinated by year and sorted from smallest (bottom) to largest (top) temperature deviation. A: Observations in Götaland. B: Observations in Svealand (located north of Götaland).

4.3. First flight and length of flight

When considering all 13 species, first flight seemed stable around day 120-180 for most species in both regions, regardless of temperature deviation from reference period 1961-1990 (Figure 6).

Figure 6. Reported first observation (day of year from 1 January) for each year, over an 11- year period compared to temperature deviance from reference period 1961-1990. A single point represents the first observation, between March-October, for one year. A: Götaland. B: Svealand (located north of Götaland).

Species within the same tribe seemed to have similar day of first flight. For tribe Eristalini (Eristalis intricaria, Eristalis interrupta, Eristalis tenax, Helophilus pendulus and Sericomyia silentis) all species, except E. tenax, had very similar day of first flight in both geographical regions. Both Volucella bombylans and Volucella pellucens in tribe Volucellini had a similar day of first flight in both regions, and the same thing could be observed for Syritta pipiens and Xylota segnis in tribe Xylotini. Lastly the species in tribe Syrphini (Episyrphus baltaetus, Eupeodes corollae, Sphaerophoria scripta and Syrphus ribesii) all had similar day of first flight in both regions, except E. baltaetus (Figure 6).

An overview of all 13 species (Figure 7) with length of flight period in day of year were plotted against temperature deviation. There was no apparent trend between length of flight and temperature deviation from reference period 1961-1990, in neither Götaland nor Svealand. It was however evident that most species within the same tribe had similar length of flight (Figure 7).

Figure 7. Flight period (number of days between first observation and last observation), for each year, over an 11-year period compared to temperature deviance from reference period 1961-1990. A single point represents length of flight between March-October, for one year. A: Götaland. B: Svealand (located north of Götaland).

In tribe Eristalini (E. intricaria, E. interrupta, E. tenax, H. pendulus and S. silentis) all species had a similar length of flight, except E. tenax which had a longer flight period. Both V. bombylans and V. pellucens in tribe Volucellini had a similar pattern of flight period in either region. In tribe Xylotini it seemed that S. pipiens had longer flight period than X. segnis in both

11 regions. Looking at the tribe Syrphini (S. ribesii, E. baltaetus, E. corollae and S. scripta) all species had similar length of flight in both regions except E. baltaetus that generally had a longer flight period in both geographical regions (Figure 7).

Temperature deviation could explain first flight in only 4 out of 13 species (Table 1). Furthermore, temperature deviation seemed independent of length of flight period (Table 1). In contrast, differences between regions had a clear trend for both first flight and length of flight. In almost all species (11 out of 13) an earlier first flight was observed in Götaland compared to Svealand. Length of flight was also significantly longer in Götaland than in Svealand for 6 species.

Table 1. Result of GLM of first flight and of length of flight period for 13 hoverfly species in southern Sweden over an 11-year period as explained by temperature deviation and region. Test statistic is shown for temperature deviance from the reference period 1961-1990 and two geographic regions (Götaland and Svealand). First Flight: Z-value Flight period: Z-value Species Temp.deviance Region1 Temp.deviance Region1 Eristalis intricaria -0.947 -3.172** 0.391 3.435*** Eristalis interrupta 0.427 -1.536 -0.230 0.432 Eristalis tenax -1.326 -3.867**** 1.395 3.922**** Helophilus pendulus -2.931** 0.736 0.378 -0.095 Sericomyia silentis 1.196 -5.311**** -0.883 2.958** Volucella bombylans -2.473* -2.251* 1.692 0.143 Volucella pellucens -0.685 -2.125* 1.378 1.040 Syritta pipiens -0.760 -3.116** 0.703 3.031** Xylota segnis 1.901 -5.321**** 0.009 3.446*** Episyrphus balteatus -0.748 -3.098** 0.716 4.431**** Eupeodes corollae 0.042 -2.325* 0.510 1.440 Sphaerophoria scripta -3.142** -2.101* 1.175 1.315 Syrphus ribesii -2.208* -3.453*** 0.197 1.361 1Götaland compared to Svealand: earlier first flight if negative, longer flight period if positive. Significant at * P < 0.05, ** P < 0.01, *** P < 0.001, **** P < 0.0001.

5. Discussion

The mean temperature deviance from the reference period for March-October during 2010- 2020 was consistently above the reference period 1961-1990 (Figure 1) and goes well in line with IPCCs conclusions of increasing temperatures as a result of climate change (IPCC, 2014). Only ~31 % (4 out of 13 species) of species examined in this study had a significant change between first flight and temperature deviance. In addition, there was a consistent difference in flight phenology when comparing observations of the same species in the two geographical regions of southern Sweden.

It was further found that most species had a similar flight phenology regardless of tribe or genus (Figure 6-7). However, two species, E. tenax and E. baltateus, differed from this pattern. Both are adult migratory species but have on a few occasions been found to hibernate in southern Sweden (Bartsch, 2009a). Them being migratory species can explain why both had visibly different flight phenology compared to other species in the same tribe (Figure 6-7). The same pattern with E. tenax and E. baltateus has also been found in other studies (Graham-Taylor et al., 2009; Olsen et al., 2020). The species S. scripta is also a migrating species but there was no indication of that in Figure 6-7, as there was for E. tenax and E. baltateus. However, the test statistic (Table 1) clearly shows that these three migratory species were all seen to have an earlier first flight due to region. One reason behind this could be that the geographical region of Götaland is further south than Svealand and therefore where migratory species first land on their northward migration. That the geographical region of Götaland is further south than Svealand, would also results in earlier arrival of spring. That could explain why ~85 % (11 out of 13 species) had a significantly earlier first flight in Götaland than in Svealand, only with 3 of these species also significant to temperature deviance.

In this study all observations between day 60 and 304 were analysed, making the data sensitive to outliers. One observation around day 60 would change the flight phenology analyses significantly for a species with their mean flight period around 100 days later. This can clearly be seen when analysing the distribution of reported observations via violin plots layered with boxplots (Figure 2-5). The GLM analyses in this study only account for outliers (first flight is first reported observation, and length of flight is calculated from first and last observation) and does not take distribution into account. For that reason, the result from the statistical analyses could be skewed for some species. Further examining the violin plots for all species, as well as

13 their datasets, there are more outliers in early spring than in autumn. This all indicate that the results of this study may not be fully representative of hoverflies flight period. To account for this, a better method in the future would be to analyse e.g., observations between 10th and 90th percentile, as has been done in more extensive studies (Brooks et al., 2014; Olsen et al., 2020).

Even though my study only resulted in ~31 % (4 out of 13 species) of species having an earlier first flight due to temperature deviance, more extensive studies show a significant correlation between rising temperatures and earlier onset of flight for many hoverfly species (Graham- Taylor et al., 2009; Hassal et al., 2016; Olsen et al., 2020). A study that also used citizen science data, involving 37 species in Denmark, concluded that hoverflies generally seem to have an earlier first flight due to warmer temperatures (Olsen et al., 2020). Olsen et al. (2020) also found that migratory species in general have an earlier first flight. An interesting find in another study (Ellwood et al., 2012) was that many insects had a later first flight when plotted against temperature, but when analysed as function of temperature and precipitation, first flight came earlier.

It has also been noted by Graham-Taylor et al. (2009) that lower temperatures in autumn (the year before reported observation) with higher temperatures during winter and spring resulted in an earlier first flight, while summer and autumn temperatures during the flight year was not affecting their first flight. However, warmer autumn temperature led to a later first flight the next year. Precipitation did not affect this result (Graham-Taylor et al., 2009). There could be many reasons why autumn and winter temperatures are affecting flight phenology more than spring and summer temperatures, one being the effects temperature have on larval and pupal stages. It could be that hoverflies lay their eggs later when autumn temperatures are higher, resulting in slower development through their life cycle and a later first flight in spring (Graham-Taylor et al., 2009). This is also supported by another study (Iler et al., 2013) which showed that years with an earlier snow melt correlated with an earlier first flight. The same study also found that hoverflies and the plants they pollinate had similar patterns of phenology. Together this indicate that spring and summer temperatures has little to do with first observed flight and it would therefore be interesting to expand future studies to involve temperature deviation in several different time-intervals, like September-March.

In conclusion this study showed that flight phenology for 13 syrphid species in southern Sweden was more affected by geographical region than by summer temperatures. It was also concluded

14 that flight phenology for hoverflies was not unaffected by temperature deviation during March- October. This all indicates that more factors than temperature affects flight phenology in hoverflies and that future studies might contribute towards our understanding of how flight period of hoverflies is affected by interannual variation in weather, as well as climate change.

6. Societal and ethical considerations

6.1. Societal considerations

This study was made possible thanks to observations on Artportalen.se (SLU Artdatabanken, 2021), showing the importance and possible uses of citizen science when studying possible trends in nature. Easy and available knowledge is important for society to learn more about how to protect pollinators and what actions are needed. Society's view on pollinators can therefore be positively affected by this study. This study touches upon goal 15 about life on land and biodiversity from the 17 sustainable development goals (UNDESA, 2021). It also gives valuable information about how rising temperatures, as a consequence of global warming, may affect flight phenology of hoverflies in southern Sweden in the future.

6.2. Ethical considerations

The analysed hoverfly data in this study was collected from Artportalen.se (SLU Artdatabanken, 2021), where both professionals and amateurs at species identification report observations of animals, plants, and fungi. In this study, reports from 2010 to 2020 have been analysed for 13 different hoverfly species. It is not known how the different observations have been handled by the observer. Though in general, observations reported are carried out in a non- destructive manner. If species are hard to identify, most people use a hoop-net to capture and look closer at the individual species. After identification, the insect is most likely released again. However, some people do collect insects which would mean killing the insect after capture. Despite how the hoverflies have been reported, this study does not inflict any further possible harm, since the study is based upon already reported data.

7. Acknowledgements

I would like to thank my supervisor Per Milberg for being patient and answering my questions and helping me finish this study. Also thank you to Lars Westerberg for giving valuable tips in RStudio. Lastly, I would like to thank Anna Palm for always being there to discuss ideas and problems that occurred during the making of this study.

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