Presence of a second foundation species alters seagrass ecosystem structure and function
Foundation species form the structural basis for many ecosystems and are characterized by positive interactions with community members. Multiple foundation species often co-occur within the landscape, frequently increasing diversity beyond that expected from either species in isolation. Seagrasses are a classic example of foundation species, creating structured habitat both above and below ground and influencing important ecosystem processes. Sponges, which can also be foundation species, alter the physical environment in many ways, including provisioning of structured habitat. Although sponges are a common component of seagrass beds, there is a paucity of knowledge regarding how the two types of species interact to structure associated communities. In a long-term experiment (17 months), we determined the effect of a sponge, Ircinia felix, on the structure of a Thalassia testudinum dominated seagrass bed on Abaco Island, The Bahamas. In June 2013, fifteen 25m2 plots were established; five containing a single, live I. felix, five with a polypropylene model sponge, and five controls. Seagrass density, community structure and growth, as well as macroalgae, macroinvertebrate, and fish abundance and diversity were measured prior to live or model sponge placement. All variables have been measured twice a year in July and November since establishment.
By November 2014, plots containing live I. felix contained more species-rich fish and macro-invertebrate communities. Over the course of the experiment, T. testudinum growth increased in plots containing I. felix. The increased growth did not lead to a significant change in the shoot density of T. testudinum; however, in plots containing an I. felix the shoot density of two additional seagrass species, Syringodium filiforme and Halodule wrightii, increased significantly. Taken together, these results show that the presence of a sponge may increase diversity of seagrass communities.