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Rapid collapse of extensive kelp forests and a regime shift to tropicalized temperate reefs followed extreme heatwaves and decades of gradual warming. Abstract:Ecosystem reconfigurations arising from climate driven changes in species distributions are expected to have profound ecological, social and economic implications. Here, we reveal a rapid climate driven regime shift of Australian temperate reef communities, which lost their defining kelp forests and became dominated by persistent seaweed turfs. Following decades of ocean warming, extreme marine heatwaves forced a 100 km range contraction of extensive kelp forests, and saw temperate species replaced by seaweeds, invertebrates, corals and fishes characteristic of subtropical and tropical waters. This community wide tropicalization fundamentally altered key ecological processes, suppressing the recovery of kelp forests. Main Text:Broad scale losses of species which provide the foundations for habitats cause dramatic shifts in ecosystem structure because they support core ecological processes (1-3). Such habitat loss can lead to a regime shift where reinforcing feedback mechanisms intensify to provide resilience to an alternate community configuration, often with profound ecological, social and economic consequences (4-6). Benthic marine regime shifts have been associated with the erosion of ecological resilience through overfishing or eutrophication, altering the balance between consumers and resources, rendering ecosystems vulnerable to major disturbances (1, 2,6,7). Now, climate change is also contributing to the erosion of resilience (8,9), where increasing temperatures are modifying key physiological, demographic and community scale processes (8, 10), driving species redistribution at a global scale and rapidly breaking down long-standing biogeographic boundaries (11,12). These processes culminate in novel ecosystems where tropical and temperate species interact with unknown implications (13). Here we document how a marine heatwave caused the loss of kelp forests across ~2,300 km 2 of Australia's Great Southern Reef, forcing a regime shift to seaweed turfs. We demonstrate a rapid 100 km rangecontraction of kelp forests and a community-wide shift toward tropical species with ecological processes suppressing kelp forest recovery.To document ecosystem changes we surveyed kelp forests, seaweeds, fish, mobile invertebrates and corals at 65 reefs across a ~2,000 km tropical to temperate transition zone in western Australia (14). Surveys were conducted between 2001 to 2015, covering the years before and after an extreme marine heatwave impacted the region.The Indian Ocean adjacent to western Australia is a 'hotspot' where the rate of ocean warming is in the top 10% globally (15), and isotherms are shifting poleward at a rate of 20 -50 km per decade (16). Until recently, kelp forests were dominant along >800 km of the west coast (8), covering 2,266 km 2 of rocky reefs between 0 -30 m depth south of 27.7°S (Fig. 1). Kelp forests along the midwest section of this ...

The distribution of biomass among trophic levels provides a theoretical basis for understanding energy flow and the hierarchical structure of animal communities. In the absence of energy subsidies [1], bottom-heavy trophic pyramids are expected to predominate, based on energy transfer efficiency [2] and empirical evidence from multiple ecosystems [3]. However, the predicted pyramid of biomass distribution among trophic levels may be disrupted through trophic replacement by alternative organisms in the ecosystem, trophic cascades, and humans preferentially impacting specific trophic levels [4-6]. Using empirical data spanning >250 coral reefs, we show how trophic pyramid shape varies given human-mediated gradients along two orders of magnitude in reef fish biomass. Mean trophic level of the assemblage increased modestly with decreasing biomass, contrary to predictions of fishing down the food web [7]. The mean trophic level pattern is explained by trophic replacement of herbivorous fish by sea urchins at low biomass and the accumulation of slow-growing, large-bodied, herbivorous fish at high biomass. Further, at high biomass, particularly where fishers are not selectively removing higher trophic level individuals, a concave trophic distribution emerges. The concave trophic distribution implies a more direct link between lower and upper trophic levels, which may confer greater energy efficiency. This trophic distribution emerges when community biomass exceeds ∼650 kg/ha, suggesting that fisheries for upper trophic level species will only be supported under lightly fished scenarios.