Multi-decadal changes in structural complexity following mass coral mortality on a Caribbean reef

In recent decades, extensive mortality of reef-building corals throughout the Caribbean region has led to the erosion of reef frameworks and declines in biodiversity. Using field observations, structural models, and high-precision U–Th dating methods, we quantify changes in structural complexity in the major framework-building coral Orbicella annularis over a 20-year period at Long Caye (Belize). Despite extensive mortality following the mass coral bleaching event of 1998, the structural complexity of frameworks remained largely unchanged between 1998 (rugosity index, R, of 2.35±0.1) and 2018 (R of 2.29±0.1). Colony-scale structural complexity was maintained, as the rapid growth of surviving ramets (0.69±0.1 cm yr−1) offset the slower bioerosion of dead ramets (-0.11±0.16 cm yr−1). Despite the apparent stability of the structural complexity at colony scales, bioerosion of individual dead ramets over 2 decades led to declines in microhabitat complexity, with an overall reduction in the depth of microhabitats within frameworks. Altered microhabitat complexity appears to have negative effects on cryptic fauna, with the grazing urchin Echinometra viridis declining from 1.5±0.4 individuals m−2 in 1998 to 0.02± individuals m−2 in 2018. Changes in microhabitat complexity have the potential to alter ecological interactions that can impact recovery dynamics on coral reefs in ways that are undetectable using traditional rugosity metrics of structural complexity.

Microhabitat complexity influences fear acquisition in fathead minnows

Habitat varies in structure, with animals often preferring a certain degree of microhabitat complexity that facilitates fitness-related activities such as predator avoidance. Environments with high predation risk can induce elevated baseline fear and neophobia in prey, but whether microhabitat complexity influences the acquisition of neophobia has yet to be reported. Here, we tested whether exposure to predation risk induces different levels of fear in microhabitats that differed in complexity. We exposed fathead minnows, Pimephales promelas, to predation risk repeatedly (12 times over 4 days) in the form of damage-released chemical alarm cues (compared to water control) in tanks with vertical plant structure distributed either throughout the tank (complex habitat) or clumped together (simple habitat). Then, we tested minnows before and after exposure to a novel odor in tanks with either the same microhabitat complexity (i.e., familiar habitats) or in tanks with novel habitat that had different substrate structure and no vertical structure. Minnows in the complex habitat showed less overall movement one day after the background risk period, whereas individuals in the simple habitat showed reduced movement regardless of prior risk exposure. We observed stronger effects in the novel habitat, where background risk in both simple and complex habitats caused neophobia. However, individuals from the simple background habitat showed higher baseline fear behaviors. Hence, for minnows, low microhabitat complexity appears to lead to elevated fear, which remains even after a habitat change.

Multi-decadal changes in structural complexity following mass coral mortality on a Caribbean reef

In recent decades, extensive mortality of reef-building corals throughout the Caribbean region has led to erosion of reef frameworks and declines in biodiversity. Using field observations, structural models and high-precision U-th dating methods, we quantify changes in structural complexity in the major framework building coral Orbicella annularis over a 20-year period at Long Cay (Belize). Despite extensive mortality following the mass coral bleaching event of 1998, structural complexity of frameworks remained largely unchanged between 1998 (rugosity index = 2.35 ± 0.1) and 2018 (2.29 ± 0.1). Reef-scale structural complexity was maintained through rapid growth of surviving ramets (0.69 ± 0.1 cm yr−1) offsetting slower bioerosion of dead ramets (−0.11 ± 0.16 cm yr−1). Despite apparent stability of structural complexity at reef-scales, bioerosion of individual dead ramets over two decades led to declines in microhabitat complexity, with an overall reduction of the depth of microhabitats within frameworks. Altered microhabitat complexity appears to have negative effects for cryptic fauna, with the grazing urchin Echinometra viridis declining from 1.5 ± 0.4 per m2 in 1998 to 0.02 ± 0.02 per m2 in 2018. Changes in microhabitat complexity have the potential to alter ecological interactions that can impact recovery dynamics on coral reefs in ways that are undetectable using reef-scale metrics of structural complexity.