1. Climate change is modifying species distributions around the world, forcing some species poleward, where they can alter trophic interactions. Many tropical herbivorous fishes have successfully expanded their ranges into temperate ecosystems, and while it is clear they drive increases in herbivory rates in specific localities, little is known about how they might affect the diversity of herbivory functions across large spatial scales, considering their interaction with assemblages of native herbivores in temperate habitats. 2. We assessed the spatial overlap and habitat associations of native temperate and range-expanding tropical herbivorous fishes in six sub-regions of south-western Australia to determine how incursions of tropical species may have affected the diversity, redundancy (index of uniqueness) and the ‘spatial extent’ (addition of functions in new areas) and ‘intensity’ (increasing density of functional groups) of specific herbivory functions in recipient ecosystems. 3. Tropical herbivores were more abundant in temperate ecosystems, forming schools from 40 (parrotfish) to 200 (rabbitfish) individuals strongly associated with seagrass meadows and reefs with high cover of turf algae. Overlap with temperate herbivores was highest in the northern sub-regions, forming unique assemblages, with no apparent species displacements. The addition of tropical species increased functional diversity and uniqueness (the complement of redundancy), introducing novel herbivory functions to many locations. Seagrass browsing increased in spatial extent (27%) and intensity (15 x), while seaweed browsing and grazing increased in intensity by up to 2.5 x in regions with high abundances of tropical herbivores. 4. Our results suggest that the diversity, intensity and spatial extent of different herbivory functions can change as tropical species with different habitat affinities, behaviors and diets shift their distributions poleward. Changes in functional redundancy are likely to be heterogeneous in space and might not increase initially because the diversity of herbivory functions is relatively low in some temperate marine ecosystems. However, there is the potential for greater redundancy as further tropical species arrive, their abundances increase and the spatial and functional overlap of communities rises.

Surveys of herbivorous fish were conducted over inshore habitats along the coastline of south-western Australia during summer (April-May) of 2018 and 2019, from the Perth metropolitan area to Cape Naturaliste in the south of the state. The abundances of temperate (Girella zebra, Olisthops cyanomelas, Pelates octolineatus, Kyphosus cornelii and Kyphosus sydneyanus) and tropical herbivorous fish (Scarus spp. and Siganus fuscescens) were measured via a GPS-tracked roving underwater visual census with 15 minutes per sampling location. One surveyor swam at a constant pace, registering all individuals of the species of interest in an approximate area of 2000 m² per transect. A second surveyor followed the same path recording the seascape with an underwater camera for subsequent assessment of habitat features. Videos were analyzed with the program TransectMeasure (SeaGIS), each video transect was paused at one minute intervals (n=15 per transect) to extract information on topographic complexity (Wilson, Graham & Polunin 2007), substrate type, species composition, species diversity, and abundance of morpho-functional groups of sessile biota based on the CATAMI classification scheme (Althaus et al. 2015). Fish abundance was standardized to density (individuals 125 m^-2); however, we also considered a new metric: MaxS (maximum number of fish per school in each survey), a similar approach to abundance assessments of pelagic schooling fish (i.e. school size; Beare 2002) and abundance estimates from static video methods (i.e. MaxN; Cappo et al. 2003).