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 &amp; 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 <em>Quercus</em> for hundreds of years, partly former heathlands planted with <em>Pinus </em>and <em>Pseudotsuga </em>in the period 1900-1950. Locally, other tree species (<em>Fagus, Larix</em>) dominate; in the shrub layer <em>Sorbus, Amelanchier,</em> and <em>Rhamnus</em> are important. In the ground layer <em>Vaccinium myrtillus </em>dominates at many locations, next to some <em>Rubus </em>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 <em>Galium saxatile </em>and <em>Deschampsia flexuosa </em>decreased and <em>Rubus </em>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为探究荷兰森林中食蚜蝇（双翅目：食蚜蝇科Syrphidae）的丰度变化趋势，我们开展了长达40余年的食蚜蝇监测工作。在“Boeschoten”森林内，我们固定采用全长约3公里的样线，于每日上午后半段（10:00-13:00）开展时长约2小时的食蚜蝇普查。为完整记录该生态系统的物种组成，我们将样线周边30米范围内的森林区域均纳入调查范围，每次调查均力求覆盖所有出现的食蚜蝇物种。所有监测仅在晴天开展，不受当日气温影响。观测到的所有个体均通过捕虫网采集计数，部分类群的物种及性别可在野外直接鉴定，其余标本则被保存以供后续实验室鉴定（参考Barendregt 1978、van der Goot 1981、Bot & van de Meutter 2019、Speight et al. 2020等文献）。 本监测项目始于1974年，但直至1979年才开始完整记录物种名录，1982年起才正式统计各物种的观测个体数量。 Boeschoten是位于荷兰中部Garderen村西侧（52°13'24"N，5°40'31"E）的农业飞地，周边环绕大面积混交落叶-针叶林，海拔40-50米。该区域土壤为沙质，极度干燥，仅存在少量小型积水池。本次调查选取的森林片区面积约20公顷，其中部分区域数百年来以栎属（*Quercus*）植物为绝对优势种；其余区域则为1900-1950年间改造的原石楠荒地，后期种植了松属（*Pinus*）和黄杉属（*Pseudotsuga*）植物。局部区域的优势树种为山毛榉属（*Fagus*）、落叶松属（*Larix*）；灌木层以花楸属（*Sorbus*）、唐棣属（*Amelanchier*）和鼠李属（*Rhamnus*）为主；地表植被层则以越橘（*Vaccinium myrtillus*）为多数点位的优势种，林间路径旁则伴生有悬钩子属（*Rubus*）植物。 该森林的土地利用与经营管理模式至少60年未发生变更，仅林木持续自然成熟，未开展新的造林活动。1985年左右的强酸雨事件后，地表植被层发生了部分物种更替：蓬子菜（*Galium saxatile*）和曲芒发草（*Deschampsia flexuosa*）的种群丰度显著下降，而悬钩子属（*Rubus*）植物的数量则有所增加。Boeschoten周边分布有少量小型耕地，向南及向西5公里范围内则为集约化畜牧养殖区。 文件`counts.csv`记录了各监测日各物种的个体数量，其中包含以下变量： 1. `date`：监测日期（对应上午后半段的监测时段） 2. `Baccha.elongata`及后续104个变量：代表对应监测日某一物种的观测个体数量。1974-1978年的物种名录并不完整，因此该时期未被记录到的物种以`NA`（缺失值）表示。1982年起才启动系统性个体计数，1982年之前的物种出现情况以`1`标注，代表“至少观测到1个个体”。 3. `observer`：当日监测负责人的姓名首字母缩写：AB代表Aat Barendregt，TZ代表Theo Zeegers，WS代表Wouter van Steenis。 4. `temp`：监测当日10:00-13:00时段的平均气温（单位：摄氏度），数据来源于荷兰皇家气象研究所（Koninklijk Nederlands Meteorologisch Instituut，简称KNMI）代尔伦气象站的逐小时观测记录。 5. `sun`：监测当日10:00-13:00时段的直接日照时长占比均值，数据来源于KNMI代尔伦气象站的逐小时观测记录。 6. `precip`：监测当日10:00-13:00时段的总降水量（单位：毫米），数据来源于KNMI代尔伦气象站的逐小时观测记录。 7. `relhumid`：监测当日10:00-13:00时段的平均相对湿度（百分比），数据来源于KNMI代尔伦气象站的逐小时观测记录。 8. `T30d`：监测日前30天的平均气温（单位：摄氏度），数据来源于KNMI代尔伦气象站的逐日观测记录。 9. `S30d`：监测日前30天的直接日照时长占比均值，数据来源于KNMI代尔伦气象站的逐日观测记录。 10. `P30d`：监测日前30天的日均降水量（单位：毫米），数据来源于KNMI代尔伦气象站的逐日观测记录。 11. `Tapr`：监测当年4月的平均气温（单位：摄氏度），数据来源于KNMI代尔伦气象站的逐日观测记录。 12. `Sapr`：监测当年4月的直接日照时长占比均值，数据来源于KNMI代尔伦气象站的逐日观测记录。 13. `Papr`：监测当年4月的日均降水量（单位：毫米），数据来源于KNMI代尔伦气象站的逐日观测记录。 14. `seasonScore`：基于广义加性模型（Generalized Additive Model，简称GAM）预测的食蚜蝇总丰度的日序得分，经标准化后取值范围为0至1