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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

1
Department of Environmental Sciences, Section of Conservation Biology, University of Basel, CH–4056 Basel, Switzerland
2
Department of Biology, Terrestrial Ecology Unit, Ghent University, 9000 Ghent, Belgium
*
Author to whom correspondence should be addressed.
Insects 2020, 11(4), 248; https://doi.org/10.3390/insects11040248
Received: 6 March 2020 / Revised: 9 April 2020 / Accepted: 10 April 2020 / Published: 15 April 2020
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. View Full-Text
Keywords: bottom-up trophic cascade; low abundance; orb-weaving spiders; prey scarcity; western European landscape bottom-up trophic cascade; low abundance; orb-weaving spiders; prey scarcity; western European landscape
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Nyffeler, M.; Bonte, D. 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. Insects 2020, 11, 248.

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