Tropicalization strengthens consumer pressure on habitat-forming seaweeds

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.

Distribution models predict large contractions of habitat‐forming seaweeds in response to ocean warming

“…Unfortunately, the consequences of such events are difficult to predict by SDMs because extremes and local variability are poorly resolved in projected scenarios of ocean warming. Furthermore, indirect effects associated with changing consumer pressure, for example, due to current‐driven range‐shifting tropical herbivores (Nakamura, Feary, Kanda, & Yamaoka, ; Wernberg, Bennett et al., ), are unaccounted for in climate projections but could accelerate the retreat of temperate seaweeds (Vergés et al., , ; Zarco‐Perello, Wernberg, Langlois, & Vanderklift, ). Thus, thermal anomalies driven by variation in mesoscale warm currents, episodic heatwaves and cascading biological responses add a layer of concern above the predicted declines based on forecasted increases in baseline temperatures (this study, but also see Molinos et al., ).…”

Simultaneous exposure to a pulsed and a prolonged anthropogenic stressor can alter consumer multifunctionality

“…Ongoing global environmental change is dramatically altering the Earth's ecosystems (Sala et al 2000, Boyd et al 2014. Anthropogenic perturbations, such as global change and chemical pollution, can, for example, disrupt trophic links (Winder andSchindler 2004, Visser andBoth 2005) and food-web structure, thereby modifying energy transfer rates and efficiencies , Svensson et al 2017, Zarco-Perello et al 2017, Nordström and Bonsdorff 2017. Predicting the occurrence and magnitude of such ecosystemlevel affects is, however, difficult.…”

New Evidence of Marine Fauna Tropicalization off the Southwestern Iberian Peninsula (Southwest Europe)

“…The global trend is that this tropicalization phenomenon is occurring in many marine ecosystems [3]. Along the west and east coasts of Australia, dense kelp forests are disappearing, with records of 100 km range contractions on the west coast which are then followed by increases in the populations of subtropical and tropical herbivorous fish that suppress kelp recovery [4][5][6]. Other important habitat-forming species, such as coral reefs, have been expanding their poleward distribution along the Japanese coast at a rate of up to 14 km/year [7].…”