Data from: Forest hoverfly community collapse: abundance and species richness drop over four decades

To study abundance trends in hoverflies (Diptera: Syrphidae) in a Dutch forest, we monitored hoverflies over the course of 4 decades. Within the ‘Boeschoten’ forest the same permanent route of approximately 3 kilometres was inspected for the presence of hoverflies in the second half of the morning (10:00-13:00), for a duration of approximately 2 hours. The forest was included up to 30 meters from the route to obtain a complete inventory of the ecosystem; a complete list of all present hoverflies was aimed for each time. Monitoring was done only on sunny days, independent from temperature. All observed specimens have been counted and collected with an insect net, species and sex identified in the field, or preserved for identification later on (with e.g. Barendregt 1978; van der Goot 1981; Bot & van de Meutter 2019; Speight et al. 2020). While monitoring started in 1974, it was only from 1979 onwards that complete species lists were kept, and from 1982 onwards that the number of observed individuals per species was recorded. Boeschoten is an (extensively used) agricultural enclave within large mixed deciduous-coniferous forests in the center of The Netherlands, 40-50 m a.m.s., west of the village of Garderen (52º13'24"N, 5º40'31"E). The sandy soil is very dry and open water is only available in some small pools. The selected forest sections (ca. 20 ha) are partly dominated by Quercus for hundreds of years, partly former heathlands planted with Pinus and Pseudotsuga in the period 1900-1950. Locally, other tree species (Fagus, Larix) dominate; in the shrub layer Sorbus, Amelanchier, and Rhamnus are important. In the ground layer Vaccinium myrtillus dominates at many locations, next to some Rubus along paths. The forest did not change for at least 60 years in land use or management: only the trees matured further (no new planting) and in the ground layer the vegetation changed in some species after the period of intense acid rain around 1985, when Galium saxatile and Deschampsia flexuosa decreased and Rubus increased in abundance. In the surroundings of Boeschoten there are some smaller arable fields; 5 km to the south and west there is intensive livestock farming. 

The file ‘counts.csv’ contains the counts per species per monitoring day. The file contains the following variables:

date:                      date of monitoring (in the second half of the morning)

Baccha.elongata and next 104 variables:

                              number of observed individuals of a particular species per day of monitoring. Species lists were not complete in 1974-1978, hence the NAs when a species was not recorded in that period. Systematic counting started in 1982. Presence before 1982 is indicated with a ‘1’, meaning ‘at least 1’.

observer:               initials of the person responsible for a day of monitoring. AB = Aat Barendregt, TZ = Theo Zeegers, WS = Wouter van Steenis.

temp:                     mean temperature in degrees Celsius between 10:00 and 13:00 (i.e. during the general time of monitoring) on the day of monitoring, based on hourly data from KNMI weather station Deelen.

sun:                       mean proportion of time with direct sunlight between 10:00 and 13:00 (i.e. during the general time of monitoring) at the day of monitoring, based on hourly data from KNMI weather station Deelen.

precip:                   total amount of precipitation in millimeters between 10:00 and 13:00 (i.e. during the general time of monitoring) at the day of monitoring, based on hourly data from KNMI weather station Deelen.

relhumid:               mean relative humidity (percentage) between 10:00 and 13:00 (i.e. during the general time of monitoring) at the day of monitoring, based on hourly data from KNMI weather station Deelen.

T30d:                     mean temperature in degrees Celsius in the 30 days preceding the day of monitoring, based on daily data from KNMI weather station Deelen.

S30d:                    mean proportion of time with direct sunlight in the 30 days preceding the day of monitoring, based on daily data from KNMI weather station Deelen.

P30d:                    mean amount of precipitation (in millimeters) per day in the 30 days preceding the day of monitoring, based on daily data from KNMI weather station Deelen.

Tapr:                     mean temperature in April of the year of monitoring, based on daily data from KNMI weather station Deelen.

Sapr:                     mean proportion of time with direct sunlight in April of the year of monitoring, based on daily data from KNMI weather station Deelen.

Papr:                     mean amount of precipitation (in millimeters) per day in April of the year of monitoring, based on daily data from KNMI weather station Deelen.

seasonScore:        day-of-the-year-specific GAM prediction of the total abundance of hoverflies, rescaled between 0 and 1