Segregação espacial horizontal em pequena escala entre duas espécies da família Theridiidae (Araneae)

Abstract

Niche theory predicts that coexistence between related species occurring simpatrically is unlikely. Due to the shared phylogenetically conserved ecological attributes, the stronger competitor may exclude competitively the related heterospecific. However, coexistence mechanisms may allow similar species to persist together. In this study, we investigated the coexistence between two related theridiid spider species: Helvivis longicauda and Chrysso intervales. Along riparian habitats of a tropical forest, H. longicauda occurs close to river margins, while C. intervales occurs in higher abundance in further regions, after the zone occupied by H. longicauda, but always in shaded, humid regions. In this sense, this study aimed to investigate this fine-scale horizontal segregation pattern, exploring the relative contribution of different factors in generating the arrangement. Our results confirmed spatial segregation between spider species in both summer and winter, with H. longicauda populations being restricted to river margins and C. intervales to contiguous regions. These species were positively related to the abundance of suitable sites used for web-building and negatively to the relative abundance of fern leaves, which seem to be unsuitable sites. Apparently, spiders also present niche differentiation in the height they build webs, with C. intervales occurring at lower average heights. Helvibis longicauda was the only one infested by fungi and the mortality rates were higher during winter. At this time, we found more C. intervales individuals occupying areas close to the river than we did in summer. Helvibis longicauda trophic niche breadth was wider than that of the heterospecific. However, we did not found differences in prey abundance across distinct distances from the river. Only the temperature was different, increasing away from the river. There were no differences in establishment rates between species in both environments. However, it is still possible that H. longicauda success far from the margins is lower during summer, once transplant experiments were performed during winter, in which temperature did 2 not differ between species zones. Helvibis longicauda was the most common species colonizing potential web-building sites at marginal regions, while C. intervales colonizers were found mainly in areas further away. Agonistic interactions demonstrated that resident spiders present advantage in both intra and interspecific contests, once they remained alone in the web in most contests. Resident spiders effectively defended the web even against larger intruders. However, much larger intruders presented an increase in the probability of remaining in the alien web, sometimes taking it over. Although C. intervales average body mass is higher than that of H. longicauda, this difference seems to represent no effect in C. intervales reaching marginal regions. We hypothesized that capture rates of H. longicauda at further distances is compromised due to high temperature acting on the smaller viscid droplets, reduc...