Overwintering tropical herbivores accelerate detritus production on temperate reefs

Abstract

The tropicalization of temperate marine ecosystems can lead to increased herbivory rates, reducing the standing stock of seaweeds and potentially causing increases in detritus production. However, long-term studies analysing these processes associated with the persistence of tropical herbivores in temperate reefs are lacking. We assessed the seasonal variation in abundances, macrophyte consumption, feeding modes and defecation rates of the range-extending tropical rabbitfish Siganus fuscescens and the temperate silver drummer Kyphosus sydneyanus and herring cale Olisthops cyanomelas on tropicalized reefs of Western Australia. Rabbitfish overwintered in temperate reefs, consumed more kelp and other macrophytes in all feeding modes, and defecated more during both summer and winter than the temperate herbivores. Herbivory and defecation increased with rabbitfish abundance, but this was dependent on temperature, with higher rates attained by big schools during summer and lower rates in winter. Still, rabbitfish surpassed temperate herbivores, leading to a fivefold acceleration in the transformation of macrophyte standing stock to detritus, a function usually attributed to sea urchins in kelp forests. Our results suggest that further warming and tropicalization will not only increase primary consumption and affect the habitat structure of temperate reefs but also increase detritus production, with the potential to modify energy pathways.

Range‐extending tropical herbivores increase diversity, intensity and extent of herbivory functions in temperate marine ecosystems

“…Thus, although tropical and temperate species inhabit the same location, their overlap decreases at finer spatial scales (Streit, Cumming, & Bellwood, 2019). Moreover, specific feeding behaviours can also increase niche dissimilarity among species; for instance, species that tend to feed on drifting macrophytes, such as S. fuscescens , reduce resource competition in the benthic space (Zarco‐Perello et al., 2019). Hence, there seem to be ample niche spaces available in temperate ecosystems for range‐shifting tropical herbivores to occupy, without directly competing against their temperate counterparts (Vergés et al., 2019).…”

“…The composition of fish assemblages in coastal habitats of western Australia has changed following a marine heatwave that struck the region in 2011 (Wernberg et al., 2016). Tropical rabbitfish Siganus fuscescens have since formed self‐recruiting populations in temperate ecosystems (Lenanton, Dowling, Smith, Fairclough, & Jackson, 2017; Zarco‐Perello et al., 2019) while parrotfish Scarus ghobban and S. schlegeli became more abundant (Parker et al., 2019). To identify the effects of these changes, 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.…”

“…Herbivory is a key ecological function which regulates the bottom‐up energy flow between trophic levels and the top‐down control on habitat structure by reducing seagrass and seaweed canopy cover and mediating competition for space between sessile taxa (Nowicki, Fourqurean, & Heithaus, 2018; Poore et al., 2012). Consequently, the strengthening of herbivory by range‐extending tropical herbivores could shift temperate habitats primarily regulated by production towards states regulated by consumption (Vergés et al., 2014), thereby accelerating the integration of biomass from habitat‐forming macrophytes into the food chain (Zarco‐Perello, Langlois, Holmes, Vanderklift, & Wernberg, 2019) and increasing the cover of turf seaweed and sessile invertebrates (Filbee‐Dexter & Wernberg, 2018; Ling, Barrett, & Edgar, 2018).…”

“…Herbivorous fishes, particularly species from the family Siganidae, have been some of the most successful at expanding their distributions poleward (Vergés et al., 2014). Current evidence suggests that where they have expanded, there has not been a displacement of temperate species but rather additions of new populations (Bennett, Wernberg, Harvey, Santana‐Garcon, & Saunders, 2015; Vergés et al., 2014; Zarco‐Perello et al., 2019; Zarco‐Perello, Wernberg, Langlois, & Vanderklift, 2017).…”

“…The spatial overlap of temperate and range‐expanding tropical herbivores will determine changes in functional diversity and redundancy of herbivory; however, since many species have functional plasticity (i.e. they can graze or browse on different macrophytes; Bennett et al., 2015; Ebrahim, Martin, Mumby, Olds, & Tibbetts, 2020; Zarco‐Perello et al., 2019), the execution of each function also depends on the spatial overlap between consumers and food resources. The concept of spatial overlap within ecological communities has been applied to estimate trophic interactions between species of predators and their prey (e.g.…”