Home spiny home: Ecological facilitation between a web-building spider and a rosette-shaped plant in subtropical grasslands

Arthropod--plant interactions span structural, mutualistic, and herbivorous relationships that may benefit both groups through nutrient cycling and habitat modification. In this study, we examined the co-occurrence patterns between the orb-weaver spider Alpaida quadrilorata and the rosette-shaped plant Eryngium horridum under experimentally-varying grazing intensities in the Pampa biome, southern Brazil. We hypothesize that spider occupancy depends on plant presence and that grazing pressure indirectly influences this interaction by affecting plant occupancy. Our study reveals a clear ecological association between A. quadrilorata and E. horridum, where occupancy and detection probabilities of the spider were significantly higher in the presence of the plant. This interaction reflects a facilitative, potentially commensal relationship, where the spider benefits from the plant's architecture without evidence of direct reciprocity. At the same time, E. horridum shows structural resilience to grazing, providing suitable substrate for the spider and other arthropods, with occupancy probabilities exhibiting a non-linear response to grazing intensity and peaking at intermediate levels. Considering the broader context of grassland management, our findings suggest that maintaining moderate grazing pressure can sustain E. horridum populations, which in turn, guarantee the persistence of A. quadrilorata and other arthropod populations, balancing conservation goals with productive land use.