Multi-Trophic Species Interactions Shape Seascape-Scale Coral Reef Vegetation Patterns

Abstract

How species interactions shape habitat structure is a longstanding question in ecology. A curious phenomenon reflecting ecological self-organization around reef habitat structures exists on coral reefs: large-scale (hundreds to hundreds of thousands of m 2) halo-like patterns surrounding patch reefs, i.e., individual coral reefs that are often separated by seagrass or macroalgal meadows. These "halos," long known to occur in various locations worldwide, reflect a distinct band of unvegetated sediments surrounding coral patch reefs. However, the full suite of mechanisms controlling them have never been rigorously explored, perhaps due to the common assumption dating back nearly 50 years that they arise solely from reef-based herbivory patterns shaped by anti-predator behavior. Here we provide empirical evidence from a set of halos within Australia's Great Barrier Reef that risk-averse foraging and a previously unrecognized functional group contribute to halo formation, demonstrating that these halos cannot be explained by any one mechanism in isolation. Our results show that halos are a more complex ecological phenomenon than previously assumed by the majority of studies of halos. Specifically, risk-averse grazing by herbivores is likely a key mechanism behind the formation of halos, as generally assumed, but bioturbators also play a central role. This knowledge furthers our understanding of how small-scale species interactions can structure habitat at landscape scales. These large-scale habitat features are important because they affect at least one important ecosystem function, carbon storage, and potentially others (e.g., biological nutrient transfer). These results also raise the question of whether other self-organized ecological patterns may be more nuanced than is currently assumed. This study capitalizes on recent advances in high resolution satellite imagery accessibility that allow ecologists to measure landscape-scale habitat features nearly everywhere on land and in shallow seas. Our results suggest that halos may hold potential as the basis for a tool for remotely observing ecological interactions and measuring large-scale ecosystem change on coral reefs.

Marine reserves shape seascapes on scales visible from space

“…The maximum distance that by halo-feeding herbivores ventured from the reef was unrelated to their relative abundance ( z = 1.66, p = 0.096; fig. 2A in Madin et al [8]). We found that piscivores likewise concentrated their time on and immediately around the reef, but, unlike herbivores, were also found throughout the halo zone and beyond its boundary into the algal meadows (figure 1 b ; red bars).…”

“…These mechanisms are not mutually exclusive with one another, nor with herbivory. For example, in our in situ study system of Heron Island lagoon, there is some evidence that bioturbation may contributing to halo formation along with the recognized key direct mechanism of herbivory [8]. Similarly, surface sediment particle size does vary significantly with distance from patch reefs [14] within Heron Island lagoon but was not found to have an effect on algal canopy height over the halo gradient [6].…”

Towards a macroscope: Leveraging technology to transform the breadth, scale and resolution of macroecological data

“…Camera traps remotely record still or video imagery of animals in their natural habitat. Given that they are small and stationary, camera traps are well suited to the generation of data on the presence, diversity, distribution and behaviour of mobile species that is largely free of human influence (e.g., Madin et al, ). Like drones, camera traps can be deployed at depth and have been instrumental in improving our understanding of both the pelagic zone and the deep sea (e.g., Jamieson et al, ).…”

“…Night‐time studies are particularly problematic: infrared radiation, which is invisible to fishes and needed for unobtrusive illumination, does not penetrate water. Extremely low‐light‐sensing cameras paired with far‐red illumination that is invisible to most fishes might help (Madin et al, ). Lastly, marine environments typically have low contrast and constant movement of particles, rendering automated image processing difficult, but not impossible (Villon et al, ).…”

Emerging Technologies and Coral Reef Conservation: Opportunities, Challenges, and Moving Forward

“…The field component of this project took place at Heron Island on Australia's southern Great Barrier Reef. The aim of the project was to determine the ecological mechanisms responsible for the formation of landscape-scale vegetation patterns surrounding coral reefs known as reef "halos" (Madin et al, 2019). In order to achieve this aim, it was necessary to obtain and process satellite imagery with specific characteristics.…”