Tropicalization strengthens consumer pressure on habitat-forming seaweeds

Salvador Zarco-Perello,Thomas Wernberg,Mathew A Vanderklift,Tim J Langlois

doi:10.1038/s41598-017-00991-2

Salvador Zarco-Perello, Thomas Wernberg + Show 2 more

Open Access

https://doi.org/10.1038/s41598-017-00991-2

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Abstract

Ocean warming is driving species poleward, causing a ‘tropicalization’ of temperate ecosystems around the world. Increasing abundances of tropical herbivores on temperate reefs could accelerate declines in habitat-forming seaweeds with devastating consequences for these important marine ecosystems. Here we document an expansion of rabbitfish (Siganus fuscescens), a tropical herbivore, on temperate reefs in Western Australia following a marine heatwave and demonstrate their impact on local kelp forests (Ecklonia radiata). Before the heatwave there were no rabbitfish and low rates of kelp herbivory but after the heatwave rabbitfish were common at most reefs and consumption of kelp was high. Herbivory increased 30-fold and kelp abundance decreased by 70% at reefs where rabbitfish had established. In contrast, where rabbitfish were absent, kelp abundance and herbivory did not change. Video-analysis confirmed that rabbitfish were the main consumers of kelp, followed by silver drummers (Kyphosus sydneyanus), a temperate herbivore. These results represent a likely indirect effect of the heatwave beyond its acute impacts, and they provide evidence that range-shifting tropical herbivores can contribute to declines in habitat-forming seaweeds within a few years of their establishment.

Highlights

A general poleward shift in distribution of marine species due to ocean warming was forecast decades ago[1,2,3,4]
Three species were identified as documented consumers of kelp: the temperate Kyphosus sydneyanus and Olisthops cyanomelas and the tropical Siganus fuscescens
Our results document the establishment of rabbitfish (Siganus fuscescens) on temperate reefs off Perth following the 2011 marine heatwave, and show a concurrent steep increase in consumption rates of kelp

Summary

Introduction

A general poleward shift in distribution of marine species due to ocean warming was forecast decades ago[1,2,3,4]. Increasing abundances of herbivorous fish on temperate reefs can translate into higher rates of herbivory and a reduced abundance of seaweed through direct consumption[35] or indirectly by affecting seaweed reproduction[36] and recruitment[14] This can have cascading effects that encompass multiple trophic levels, and reduce the resistance and resilience of kelp ecosystems[11, 14, 37]. In Western Australia, an exceptional marine heatwave during the summer of 2010–201149–51 allowed the poleward migration and population expansion of the tropical rabbitfish Siganus fuscescens in the region[14, 52, 53] This species is a characteristic herbivore of tropical reefs[54,55,56], but now is an important consumer of kelp in south-eastern Australia[35]. We present a temporal analysis of changes in abundance of herbivorous fish, kelp and rates of herbivory from years prior to the marine heatwave (2004 and 2007) to the present (2016)

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