Massive spider web aggregations in South American grasslands after flooding

• Published in: Ecological entomology Vol. 46; no. 6; pp. 1333 - 1341
• Main Authors: Piacentini, Luis N., Grismado, Cristian J., Aisenberg, Anita, Toscano‐Gadea, Carlos A., Laborda, Alvaro, Simó, Miguel, Postiglioni, Rodrigo, Peralta, Luciano, Proud, Dan N., Ramírez, Martín J.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.12.2021; Wiley Subscription Services, Inc
• Subjects: Agricultural land; Araneae; ballooning; Density; dispersal behavior; Flooding; Floods; Grasses; Grasslands; Literature reviews; riparian habitats; silk; Spiders; Webs
• ISSN: 0307-6946; 1365-2311
• DOI: 10.1111/een.13080

1. Major floods of riparian and grassland habitats may lead to a shift in the availability of resources (e.g., food, space) that produces cascading effects on the organisms that rely on it. After flooding occurs, particularly in natural grasslands or agricultural fields, massive aggregative spider webs are occasionally observed. However, given it is an infrequent and unpredictable event, it is seldom reported in detail. 2. We characterise two instances of massive web aggregations that followed major floods in grasslands and agricultural fields in Argentina and Uruguay in 2015. In the Argentinean event, we compare the composition of spider assemblages on the webs, in grasses below the webs, and in grasses from nearby habitats, with samples taken in the same location but when the fields were not flooded. 3. The assemblage of spiders in the massive web aggregations largely consisted of immature Diapontia uruguayensis (Lycosidae). In the grass below the webs, we recorded an impressive density of 1007 specimens m−1 belonging to several spider species, 6–15 times the density of similar samples without aggregated webs, co‐occurring and showing low levels of aggression among them. 4. Our observations and review of the literature support the idea massive webs produced by lycosids are a byproduct of an unusually high density of spiders simultaneously attempting to disperse through the air via ballooning and escaping from floods. This is the first detailed report of aggregative spider webs in South America with an extensive survey of the spiders on the webs and in the surrounding vegetation. After two major floods in grasslands and agricultural fields in Argentina and Uruguay, we found multispecific aggregations 6–15 times the density of similar samples without aggregated webs. Results and literature review suggest this phenomenon as a byproduct of an unusually high density of spiders simultaneously attempting to disperse through the air via ballooning and escaping from floods.