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 m2) 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.
Highlights
Ecologists have long been fascinated by the diverse ways in which species interactions can shape the habitats in which they live
Halo widths in Heron lagoon were determined in two ways: first, by averaging the halo widths over remotely sensed images at the patch reefs where the surveys were conducted (Figure 1A), and second, by averaging benthic algal cover in 2013 from in-situ quadrats at the same reefs (Figure 1B; details in section)
Our remotely sensed outlines provide a binary metric of halo presence and indicated that halos at these reefs were generally no larger than ∼15 m wide (Figure 1A)
Summary
Ecologists have long been fascinated by the diverse ways in which species interactions can shape the habitats in which they live. From early studies in California grasslands (Bartholomew, 1970) to more recent studies of self-organized landscape patterns arising from different ecological processes across terrestrial (Getzin et al, 2016; Tarnita et al, 2017) and marine (Ruiz-Reynés et al, 2017) ecosystems, studies of how interactions among species shape their physical habitat abound. In many of these cases, vegetative patterns in particular result from the interplay of organisms’ abundance and/or behavior with existing physical habitat structure. The exact mechanisms controlling this common tropical and sub-tropical vegetation pattern have never been conclusively established, despite a common assumption dating back nearly 50 years that they arise from risk-averse herbivory (Randall, 1965; Ogden et al, 1973; Armitage and Fourqurean, 2006; Madin et al, 2011)
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