A neighboring plant species creates associational refuge for consumer and host

Examples of plant-animal and plant-plant associational defenses are common across a variety of systems, yet the potential for them to occur in concert has not been explored. In salt marshes in the Gulf of Mexico, the marsh periwinkle (Littoraria irrorata) is an abundant and conspicuous member of the community, climbing up the stems of marsh plants to remain out of the water at high tide. Though Littoraria are thought to primarily utilize stems of marsh cordgrass Spartina alterniflora as a source of food and refuge, Littoraria were more abundant in mixed assemblages of Spartina and Juncus roemerianus than in Spartina-only areas at the same tidal height. Mesocosm experiments confirmed that Juncus provided a refuge for Littoraria, with predation by Callinectes sapidus (but not Melongena corona) reduced when Juncus was present. However, Littoraria's utilization of Juncus as well as the effectiveness of Juncus as a refuge depended strongly on plant height: when Juncus was experimentally clipped to a shorter height than Spartina, snail abundance on Spartina and snail predation by crabs increased. Interestingly, this plant-animal refuge led to a corresponding refuge for Spartina from Littoraria: Spartina plants lost less biomass to snail grazing when growing with Juncus in mesocosm and field experiments, and Spartina plants in natural assemblages were taller when growing with Juncus than when growing alone, even in the presence of abundant snails. This example highlights the potential importance of plant-plant and plant-animal associational refuges in competitive plant assemblages.