Climate‐driven shift in coral morphological structure predicts decline of juvenile reef fishes

Abstract

Rapid intensification of environmental disturbances has sparked widespread decline and compositional shifts in foundation species in ecosystems worldwide. Now, an emergent challenge is to understand the consequences of shifts and losses in such habitat‐forming species for associated communities and ecosystem processes. Recently, consecutive coral bleaching events shifted the morphological makeup of habitat‐forming coral assemblages on the Great Barrier Reef (GBR). Considering the disparity of coral morphological growth forms in shelter provision for reef fishes, we investigated how shifts in the morphological structure of coral assemblages affect the abundance of juvenile and adult reef fishes. We used a temporal dataset from shallow reefs in the northern GBR to estimate coral convexity (a fine‐scale quantitative morphological trait) and two widely used coral habitat descriptors (coral cover and reef rugosity) for disentangling the effects of coral morphology on reef fish assemblages. Changes in coral convexity, rather than live coral cover or reef rugosity, disproportionately affected juvenile reef fishes when compared to adults, and explained more than 20% of juvenile decline. The magnitude of this effect varied by fish body size with juveniles of small‐bodied species showing higher vulnerability to changes in coral morphology. Our findings suggest that continued large‐scale shifts in the relative abundance of morphological groups within coral assemblages are likely to affect population replenishment and dynamics of future reef fish communities. The different responses of juvenile and adult fishes according to habitat descriptors indicate that focusing on coarse‐scale metrics alone may mask fine‐scale ecological responses that are key to understand ecosystem functioning and resilience. Nonetheless, quantifying coral morphological traits may contribute to forecasting the structure of reef fish communities on novel reef ecosystems shaped by climate change.

Extreme environmental conditions reduce coral reef fish biodiversity and productivity

“…In contrast, even the optimistic estimate of 0.38 g ha −1 d −1 for the Arabian Gulf compared poorly with the same degraded GBR-reef. Notably, the study site on the GBR had undergone a sequence of severe disturbances 98 , yet it retained a diverse assemblage of cryptobenthic fish species that were likely able to satisfy their energetic demands due to benign temperature profiles 26 . At the time of our survey, reefs in the Arabian Gulf had undergone extensive bleaching in previous years 99 – 102 , which may have negatively affected the diversity and abundance of cryptobenthic fishes compared to the less disturbed reefs in the Gulf of Oman 24 , 103 , 104 .…”

“…Scleractinian corals, the foundation species of tropical reefs, show high sensitivity to thermal extremes, which has led to the rapid global decline of coral reef ecosystems 23 . In the wake of losing coral habitat, communities of the most prominent reef consumers, teleost fishes, also decline or shift in composition [24][25][26][27] , which directly affects the provision of resources to people dependent on reef fisheries 28 . Although recent evidence suggests that some fish species will be able to cope with (or even benefit from) live coral loss, at least in the short term [28][29][30][31] , tropical reef fishes are typically adapted to a relatively narrow suite of environmental conditions.…”

Seascape Configuration and Fine-Scale Habitat Complexity Shape Parrotfish Distribution and Function across a Coral Reef Lagoon

“…This highlights the critical role that fine-scale soft complexity can play for functionally important fish. However, loss of coral cover (in particular structurally complex branching corals) can induce changes in the fish community, especially causing decline in the abundance of juveniles of certain species [23], and also inhibit settlement of others [95]. Considering the changes in coral community and cover in the Mo'orea lagoon during the past decades, we highlight that the current seascape with the presence of macroalgae and areas of dead coral substrate may favor some species of parrotfish associated with degraded stages of coral reefs (i.e., C. spilurus) [96], but may not be attractive for all species.…”

“…For example, habitat heterogeneity may influence ecological processes, such as predation and herbivory [20,21], as the benthic structural complexity provides refuge from abiotic stressors, reduces predation risk, and enhances resource availability [15]. In tropical seascapes, loss of structural complexity could therefore negatively impact reef fish assemblages, and ultimately, small-scale fisheries [18,22,23].…”

Disentangling the nonlinear effects of habitat complexity on functional responses

“…Changes in trophic interactions (Dean & Connell, 1987; Kovalenko et al., 2012), along with less diversified niches (Jeffries, 1993) and smaller amounts of total food and habitat area (Fontoura et al., 2019; Parker et al., 2001), drive community responses to habitat simplification. Trophic interactions determine the fluxes of energy and nutrients within food webs and their alterations can thus have cascading effects on the structure and stability of the underlying communities (Berlow et al., 2004).…”