From colony to coastline: Using large-scale mapping to track benthic change on coral reefs

Background/Question/Methods

Advances in remote imagery were of fundamental importance to the development of novel and innovative perspectives of landscape ecology on land. Such landscape imagery allows the ecologist to investigate the spatial patterning of individuals, allowing inference about the dynamics that generated these patterns. Due to the challenges of light distortion, complementary remote imagery of underwater benthic habitats is not possible at similarly high resolution (the best being ground-truthed bathymetric reconstructions). However, with rapid developments in digital imagery it is now possible to gather images in situ of larger-scale benthic seascapes (100s - 1000s of square meters). While still orders of magnitude shy of terrestrial landscape images, these larger-scale photomosaics provide novel perspectives regarding the distributions of benthic organisms and, when replicated through time, context-specific patterns of change. We have been collecting photomosaics across a spectrum of coral reef habitats in the remote central Pacific to explore the linkage between spatial patterns and physical conditions, as well as considering how human activities shift these biophysical patterns.

Results/Conclusions

Coral reef communities are composed of a variety of organisms competing for limited space on the benthos. On little-impacted coral reef systems, such as those on uninhabited islands of the central Pacific, we find evidence that the competitive dynamics are structured principally by physical conditions. Specifically, we find that the competitive ability of stony corals relative to fleshy algae is linked tightly to nutrient availability (positive association) and wave energy (negative association). Further, among the stony corals we find similar physical association with wave energy across locations, playing off of a tradeoff between growth rate and ability to avoid breakage (in areas of high wave energy, there are more massive, slow-growing coral species). Interestingly, these linkages between benthic life history strategies and physical conditions tend to break down in areas with increasing human disturbance, a process that we refer to as biophysical decoupling. In particular, we find that in heavily disturbed reef habitats that the benthic dynamics are shifted to favor rapid colonization rather than long-term competition for space. As such, the relationship between benthic composition and physical conditions is weakened and the tightest relationship is simply between the cover of fleshy algae and the local density of herbivores (defined by local fishing pressure). By tracking benthic composition through time at larger spatial scales, we have a new wealth of information to inform ecological studies and marine management challenges.