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

Abstract

Aerial web-spinning spiders (including large orb-weavers) depend, as a group of insectivores, completely on flying insects as a food source. The recent widespread loss of flying insects across large parts of western Europe, both in terms of diversity and biomass, can therefore be anticipated to have a drastic negative impact on survival and abundance of this type of spiders. To test the putative importance of such a to date neglected trophic cascade, a survey of population densities of the European garden spider Araneus diadematus - a large orb-weaving spider - was conducted in late summer 2019 on twenty sites of the Swiss midland. The data from this survey were compared with published population densities for this species from the previous century. The study verifies above-mentioned hypothesis that this spiders present-day overall mean population density has declined alarmly to densities much lower than can be expected from normal population fluctuations (0.7% of the historical densities). Review of other available records suggests this pattern is widespread and not restricted to this region. In conclusion, the here documented abundance 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.