DOI: 10.5061/dryad.8w9ghx3hx
title: Range-extending tropical herbivores increase diversity, intensity and extent of herbivory functions in temperate marine ecosystems

description: 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.
description: 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 (<i>Girella
 zebra</i>, <i>Olisthops cyanomelas</i>, <i>Pelates
 octolineatus</i>, <i>Kyphosus cornelii</i> and
 <i>Kyphosus sydneyanus</i>) and tropical herbivorous fish
 (<i>Scarus</i> spp. and <i>Siganus fuscescens</i>)
 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 m<sup>2</sup> 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 <i>et al.</i> 2015). Fish
 abundance was standardized to density (individuals 125
 m<sup>-2</sup>); 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>i.e</i>. school size; Beare 2002) and abundance estimates
 from static video methods (<i>i.e. </i>MaxN; Cappo <i>et
 al.</i> 2003).