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Prey selection by generalist predators can be highly dynamic depending on the prey community structure. However, the dynamics of prey selection at the stage of prey entrapping are rarely investigated in trap-building predators, probably because their traps have been previously considered to intercept mobile prey proportionally to its availability in environment. Here we investigated the dynamics of prey selection by the orb-weaving spider Gasteracantha hasselti (Araneidae) depending on the composition of the available prey in tropical lowland forests located in north-eastern Thailand. We found that Gasteracantha captured a wide variety of prey but selected, on average, mostly Coleoptera and Diptera. The selectivity of Gasteracantha’s webs for Coleoptera was constant across the changes in overall prey availability and prey composition. The web selectivity for Hemiptera decreased rapidly with increasing relative densities of Hemiptera in the environment. The selectivity for Diptera and Hymenoptera increased and decreased, respectively, with their absolute densities in the environment. The relative selectivity of Gasteracantha’s traps for a particular prey type was driven by the presence and density of the highly selected prey rather than overall prey density. The results show that the selectivity of Gasteracantha’s traps for prey had both fixed and dynamic components and the dynamic component was determined by the relative as well as absolute densities of the particular prey types in the environment.

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