COS 58-1 - Ecological process in pattern generation in tropical coral-seagrass reefscapes: Is grazing sufficient to form grazing halos?

Wednesday, August 14, 2019: 1:30 PM
M109/110, Kentucky International Convention Center
Stephanie M. Bilodeau1, Austin W. H. Schwartz1, Gregory P. Asner2 and Miles R. Silman1, (1)Biology, Wake Forest University, Winston-Salem, NC, (2)Center for Global Discovery and Conservation Science, Arizona State University, Tempe, AZ
Background/Question/Methods

Landscape ecology links pattern to process, and the importance of pattern-producing organisms to ecosystem health and functioning has already been established in terrestrial systems. Near-shore tropical marine reefscapes also feature many diverse and easily-observable patterns at the intersection of distinct benthic cover types, including a series of bare sand “halos” around individual coral patches, which separate the corals from the darker seagrass or algal beds around them. Halos and similar spatial patterns present an opportunity to examine the interactions of reef-dwelling organisms at multiple spatial scales, from benthic primary producers to macrograzers and the predators that prey on them. Explanations of the phenomenon are both physical and biological, with halos most often attributed to grazing activity by reef herbivores, potentially controlled through a landscape of fear. We used a combination of manipulative grazing experiments, underwater cameras, and diver observations to address the role of grazing, herbivore behavior, and overall fish abundance in the maintenance of this spatial pattern over the extensive reefscape of Lighthouse Reef Atoll, Belize.

Results/Conclusions

Grazing and exclosure experiments revealed that seagrass blades are consumed far faster within the halo than meters away in the surrounding seagrass, that algal cover regenerates in the halo in the absence of grazing, and that this clear pattern begins to break down in areas subject to higher fishing pressure. Underwater images support the landscape of fear hypothesis: Herbivorous fish seen in the halo rarely, if ever, venture into the seagrass, refuting the idea that increased reef-adjacent grazing is due merely to the geometry of central-place foragers. Fish presence/absence data from over 75,000 underwater images reveals a measurable difference in fish detection inside and outside of the local marine protected area, which was indiscernible with diver observations alone. Pattern breakdown outside of protected area may be related to recent increases in seagrass cover across the atoll and dramatic decreases in the populations of seagrass- and sand-processing conch and holothurians. Taken together, this research supports fear-constrained grazing as a mechanism for halo maintenance and highlights the role of robust fish and invertebrate populations in the persistence of ecologically-significant patterns at Lighthouse Reef.