Ecosystem change in a South African marine reserve (1960-2009): role of seagrass loss and anthropogenic disturbance

Seagrass ecosystems are disappearing throughout the world, raising several concerns for ecosystem functioning and biodiversity. In the present study we document changes in intertidal sandflat ecosystems over the last 5 decades in Langebaan Lagoon, a marine reserve on the west coast of South Africa, following large-scale losses of seagrass Zostera capensis. Aerial photography and GIS revealed a 38% loss of seagrass at sites in Langebaan Lagoon between 1960 and 2007. Longterm monitoring of one particular site, Klein Oesterwal, indicated a major shift in invertebrate communities between 1983, when Z. capensis was abundant, and 2009, when Z. capensis had virtually disappeared. Abundance and species richness of invertebrates declined significantly over this period. Seagrass-associated species such as the limpets Siphonaria compressa and Fissurella mutabilis, and the starfish Parvulastra exigua, declined almost to extinction locally. Generalists such as the gastropod Assiminea globulus also declined in abundance. However, sandflat species, particularly burrowers, increased in abundance and vertical range. Wading birds also appeared to be affected by changes in seagrass cover. The terek sandpiper, which depends on Z. capensis for feeding, showed 3 local population crashes, each corresponding to periods of seagrass collapse. Abundance of wading birds that were not dependant on this seagrass was more stable. The loss of seagrasses in the system, together with anthropogenic disturbance such as bait harvesting and trampling, may have had severe impacts on invertebrates, the most significant being the virtual extinction locally of the critically endangered limpet S. compressa. Cascading effects on wading birds, possibly through changes in availability of invertebrate prey, were also evident.