Tropicalization and kelp loss shift trophic composition and lead to more winners than losers in fish communities

Abstract

Climate‐mediated species redistributions are causing novel interactions and leading to profound regime shifts globally. For species that expand their distribution in response to warming, survival depends not only on their physiological capacity, but also on the ability to coexist or be competitive within the established community. In temperate marine reefs from around the world, the range expansion of tropical species, known as ‘tropicalization’, has been linked to the disappearance of temperate habitat‐forming kelps and shifts to dominance by low‐biomass turfing algae. The consequences of these range expansions and habitat changes on resident fish communities are, however, unclear. Here, we use data derived from baited remote underwater video (BRUV) surveys to analyse changes in diversity and abundance of marine fishes over a 17‐year period in warming reefs that have experienced kelp loss (occurring c. 2009). Despite the loss of kelp, we found that species richness and overall abundance of fishes (measured as probability of occurrence and relative abundance), including both tropical and temperate species, increased through time. We also found dramatic shifts in the trophic composition of fish assemblages. Tropical herbivorous fish increased most markedly through time, and temperate‐associated planktivores were the only group that declined, a potential consequence of tropicalization not previously identified. At the species level, we identified 22 tropical and temperate species from four trophic guilds that significantly increased in occurrence, while only three species (all temperate associated) declined. Morphological trait space models suggest increases in fish diversity and overall occurrence are unlikely to be driven by uniqueness of traits among tropical range expanders. Our results show more winners than losers and suggest that pathways of energy flow will change in tropicalized systems, as planktonic inputs become less important and a higher proportion of algal productivity gets consumed locally by increasingly abundant herbivores.

Temperate Kelp Forest Collapse by Fish Herbivory: A Detailed Demographic Study

“…In marine environments, poleward range shifts have been documented for several species in response to rising seawater temperatures (Lima et al, 2007;Sorte et al, 2010;Poloczanska et al, 2013). These range shifts are especially noticeable in temperate-subtropical transition zones where species of both climatic regimes co-exist (Horta e Costa et al, 2014;Troast et al, 2020;Smith et al, 2021). On a community level, the ecological impact of these shifts may be more widespread when they involve foundation or habitat-forming species (Zarnetske et al, 2012).…”

“…On the one hand, water warming has increased the feeding rate on kelp reefs and forest of other brown seaweeds by some herbivorous fish native to temperate zones (Gianni et al, 2017). On the other hand, warm-water fishes have been driven by currents and overwinter into cold-water ecosystems in recent times, and their occurrence is predicted to increase with ocean warming and marine heatwaves, adding new herbivory pressure (Vergés et al, 2014a;Basford et al, 2015;Smith et al, 2016Smith et al, , 2021Beck et al, 2017). One of the best studied examples of warm-water fish range expansion into temperate regions (tropicalization) linked to the disappearance of kelp forests occurred in Western Australia (Wernberg et al, 2012).…”

Absolute Abundance Estimates for Red Snapper, Greater Amberjack, and Other Federally Managed Fish on Offshore Petroleum Platforms in the Gulf of Mexico

“…As these findings are tangential to our overall study, we relegate details on these methods and results to Appendix A for simplicity of presentation. Our core findings are based on the MaxN metric as in a number of other recent studies (e.g., Parsons et al 2016;Reynolds et al 2018;Smith et al 2021;Jones et al 2021).…”

Tropicalization unlocks novel trophic pathways and enhances secondary productivity in temperate reefs

“…As organic mass inside coralline algae is unavailable to most fish consumers in our study area (parrotfishes are scarce in the Solitary Islands; Smith et al, 2021), it was categorized in a separate 'unconsumable organic matter' component. To determine the relative or- (n = 8) and Amphiroa anceps (n = 10), exposed them to 5% hydrogen chloride until no bubbles were produced and then re-dried and re-weighed the remaining material.…”

“…secondary productivity) remain poorly resolved, limiting our understanding of how energy flow changes at the ecosystem level. This is particularly true for mobile taxa such as reef fishes, which are key conduits of energy, but often show mixed, species‐specific, responses to shifts in habitat structure (Pratchett et al., 2008; Smith et al., 2021).…”