Multiple predators often have effects on their common prey populations that cannot be predicted by summing the effects of each predator at a time. When predators forage on the same vegetation substrate, intraguild interactions might cause emergent outcomes for the plants on which the predators co-occur. We experimentally evaluated the effects of spiders and ants on herbivory and reproduction in the extrafloral nectary-bearing tree Qualea multiflora (Vochysiaceae). Plants were divided in four experimental groups, depending on the presence or absence of ants and spiders. We compared the effects of each treatment on richness and abundance of chewing and sucking herbivores and on herbivory (leaf area loss). We also evaluated the impact of predators on the production of buds, fruits and seeds, and weight of the fruits. The presence of ants reduced the abundance and richness of spiders, but spiders did not affect the abundance and richness of ants. Only the removal of ants resulted in a significant increase in the abundance of herbivores and herbivore richness. Herbivory, however, was also affected by spiders. In addition, we found a significant interaction effect of ants and spiders on herbivory, indicating an emergent multiple predator effect. Neither ants nor spiders had an impact on the number of buds produced, number of fruits per bud, and seeds per fruits or fruit weight. This study highlights the importance of evaluating the effect of the predator fauna as a whole and not only one specific group on herbivory.Abstract in Portuguese is available in the online version of this article.
Despite their essentially carnivorous habits, spiders can supplement their diets with other food sources, such as extrafloral nectar. The extrafloral nectar can constitute an excellent complementary energy source for spiders because it is rich in mono and disaccharides besides other compounds. We investigate the presence of fructose and its concentration in the bodies of wandering and web weaver spiders collected on extrafloral nectary-bearing plants from a neotropical savanna area by means of cold anthrone test. The tested spiders belonged to seven families: Anyphaenidae, Araneidae, Oxyopidae, Pisauridae, Salticidae, Theridiidae and Thomisidae. A total of 265 (88.04%) of all tested individuals were fructose intake positive. In general, there was no difference in the percentage of positive spiders between wandering and web weaver spider species or between adult and immature individuals. Results found in this study corroborate those from other studies that suggest that nectar feeding is a common activity among spiders and it includes a new family (Theridiidae) in the nectivory spiders list.
The central tendency in ecological studies to explain variations in the outcomes of biotic interactions is to suppose that the majority of meaningful functional diversity occurs at the species level. However, individuals are rarely identical and behavioral ecology shows that consistent individual differences alter the roles that individuals play within populations and possibly communities, but the intraspecific variation is commonly ignored in studies of species interactions. Here, throughout examples of field work studies, we discuss that the knowledge of individual aspects (including genetic variation) and natural history are basic tools and fundamental to a real and whole comprehension of species interaction networks in qualitative and quantitative terms.
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