Reef fishes weaken dietary preferences after coral mortality, altering resource overlap

Abstract

The direct and indirect effects of climate change can affect, and are mediated by, changes in animal behaviour. However, we often lack sufficient empirical data to assess how large‐scale disturbances affect the behaviour of individuals, which scales up to influence communities. Here, we investigate these patterns by focusing on the foraging behaviour of butterflyfishes, prominent coral‐feeding fishes on coral reefs, before and after a mass coral bleaching event in Iriomote, Japan. In response to 65% coral mortality, coral‐feeding fishes broadened their diets, showing a significant weakening of dietary preferences across species. Multiple species reduced their consumption of bleaching‐sensitive Acropora corals, while expanding their diets to consume a variety of other coral genera. This resulted in decreased dietary overlap among butterflyfishes. Behavioural changes in response to bleaching may increase resilience of coral reef fishes in the short term. However, coral mortality has reduced populations of coral‐feeders world‐wide, indicating the changes in feeding behaviour we document here may not be sufficient to ensure long‐term resilience of butterflyfishes on coral reefs.

Rapid resource depletion on coral reefs disrupts competitor recognition processes among butterflyfish species

“…If this hypothesis were supported, we would have expected to see even closer encounters between conspecifics as resources deplete, yet this was not observed. Counterintuitively, dietary overlap between butterflyfishes decreased after bleaching and there was greater dietary variation within species [15]. Therefore, in our system, if aggregation at a shared resource was the mechanism underlying proximity, conspecifics would be expected to reduce proximity after bleaching yet we observed no change in conspecific proximity before and after coral mortality.…”

“…were some of the most susceptible corals to bleaching-induced mortality at our field sites [7]. In response to this severe resource depletion, these fish species broadened their diet, leading to decreased dietary overlap [15]. At the same time, dietary breadth increased because of variation between individuals (niche variation hypothesis), reinforcing the idea that the lines of battle need to be redrawn because established niche partitioning was disrupted.…”

“…Heterospecific encounters resulted in aggression across a wide range of proximities before coral mortality, yet after resources were depleted, only close encounters (less than or equal to 25 cm) initiated aggression. A mechanistic hypothesis for this change is that altered dietary overlaps [7,15] led to a mismatch between evolved neural templates to recognize competitors and the actual competitive threat that they posed. To counteract this mismatch, heterospecifics may have shifted, or added to, their competitor recognition cue to include direct feeding observations on a shared resource to confirm a competitor.…”

“…Guild-wide dietary shifts can occur after sudden disturbance events (e.g. [15]) with the potential to alter dietary overlap between species. It is unclear to what extent recognition processes can keep pace with this shift.…”

“…Here, we test the predictions of Peiman & Robinson [10] using 3770 observations of coral-feeding butterflyfish encounters between 32 species across 17 reefs, before and after a mass coral mortality event. Butterflyfishes reduced aggression and shifted diets in response to rapid reductions in resource availability following the 2016 global mass bleaching event where coral—the major food resource of butterflyfishes—decreased by 18–65% of initial cover [7,15]. This response was replicated at multiple locations, providing a natural experiment to explore the effect of environmental change on competitor recognition processes.…”