Where Have All the Spiders Gone? Observations of a Dramatic Population Density Decline in the Once Very Abundant Garden Spider, Araneus diadematus (Araneae: Araneidae), in the Swiss Midland.

Martin Nyffeler,Dries Bonte

doi:10.3390/insects11040248

Abstract

Aerial web-spinning spiders (including large orb-weavers), as a group, depend almost entirely on flying insects as a food source. The recent widespread loss of flying insects across large parts of western Europe, in terms of both diversity and biomass, can therefore be anticipated to have a drastic negative impact on the survival and abundance of this type of spider. To test the putative importance of such a hitherto neglected trophic cascade, a survey of population densities of the European garden spider Araneus diadematus—a large orb-weaving species—was conducted in the late summer of 2019 at twenty sites in the Swiss midland. The data from this survey were compared with published population densities for this species from the previous century. The study verified the above-mentioned hypothesis that this spider’s present-day overall mean population density has declined alarmingly to densities much lower than can be expected from normal population fluctuations (0.7% of the historical values). Review of other available records suggested that this pattern is widespread and not restricted to this region. In conclusion, the decline of this once so abundant spider in the Swiss midland is evidently revealing a bottom-up trophic cascade in response to the widespread loss of flying insect prey in recent decades.

Highlights

When the Krefeld Entomological Society, together with colleagues, published their famous long-term study in 2017, it became known that the biomass of flying insects had declined by approx. 75% over the past three decades in over 60 nature protection areas of Germany [1].Another long-term study confirmed the Krefeld results, providing evidence that strong abundance declines of insect populations had occurred in farmland and forests across vast areas of Germany and Switzerland [2]
By comparing historical abundance data (20th century) with present-day data (2019), we examined whether the abundance of Araneus diadematus has changed over the past few decades
This study revealed that the large orb-weaving spider Araneus diadematus occurred in extraordinarily low population densities in the Swiss midland in 2019 (Table 1)

Summary

Introduction

75% over the past three decades in over 60 nature protection areas of Germany [1] Another long-term study (the “Munich study“) confirmed the Krefeld results, providing evidence that strong abundance declines of insect populations had occurred in farmland and forests across vast areas of Germany and Switzerland [2]. It is generally accepted that we live in an era of global insect meltdown [9,10,11,12,13] This has dramatic ecological implications: the fact that insects comprise the basis of many food chains and food webs [14,15] means that in a world without insects, countless insectivorous species will become extinct due to starvation [16].

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