Functional restructuring of kelp forest macroinvertebrate communities under tropicalisation

Climate change is driving the poleward expansion of warm-affinity species into cooler latitudes, a process known as tropicalisation, which is fundamentally restructuring temperate reef communities worldwide. Mobile benthic invertebrates are key mediators of energy flow and trophic function in these ecosystems, yet how tropicalisation reshapes these roles has received little attention. To understand how temperate reef tropicalisation may reshape the energetic and trophic functions of associated invertebrate communities, we examined long-term trends (2006-2025) in mobile benthic macroinvertebrate communities along the Western Australia (WA) coastline. Species were assigned into functional groups based on max body size and feeding guild to assess changes in community functional composition. The study spans a period encompassing tropicalisation at the warm range edge, where the 2011 marine heatwave triggered a permanent shift from kelp to subtropical Sargassum spp., enabling comparisons between 'tropicalised' and 'un-tropicalised' reefs across two range positions. We found macroinvertebrate communities at both locations have experienced substantial compositional changes over time, which translated into a shift in functional composition in the tropicalised reefs only. In tropicalised reefs, macroinvertebrate communities showed an increase in warm-affinity species, accompanied by a decline in medium-sized functional groups and an increase in small-sized herbivorous and predatory functional groups through time. In contrast, un-tropicalised forest macroinvertebrate communities showed compositional change over time but retained similar functional attributes. Our findings demonstrate that tropicalisation leads not only to species turnover but also to the restructuring of trophic functions within mobile macroinvertebrate communities. These functional shifts may have consequences for energy flow and ecosystem resilience in temperate reefs undergoing tropicalisation.