Abstract
Herbivory is a significant driver of algal community dynamics on coral reefs. However, abiotic factors such as the complexity and orientation of the benthos often mediate the impact of herbivores on benthic communities. We experimentally evaluated the independent and interactive effects of substrate orientation and herbivorous fishes on algal community dynamics on a coral reef in the Florida Keys, USA. We created horizontal and vertical substrates, mimicking the trend in the reduction of vertical surfaces of coral reefs, to assess how algal communities developed either with herbivory (open areas) or without herbivory (herbivore exclosures). We found that substrate orientation was the dominant influence on macroalgal community composition. Herbivores had little impact on community development of vertical substrates as crustose algae dominated these substrates regardless of being in exclosures or open areas. In contrast, herbivores strongly impacted communities on horizontal substrates, with upright macroalgae (e.g., Dictyota spp., articulated coralline algae) dominating herbivore exclosures, while filamentous turf algae and sediment dominated open areas. Outside of exclosures, differences between vertical and horizontal substrates exposed to herbivores persisted despite similar intensity of herbivory. Our results suggest that the orientation of the reef benthos has an important impact on benthic communities. On vertical surfaces, abiotic factors may be more important for structuring algal communities while herbivory may be more important for controlling algal dynamics in flatter areas. Thus, the decline in structural complexity of Caribbean coral reefs and the flattening of reef substrates may fundamentally alter the impact that herbivores have on benthic community dynamics.Electronic supplementary materialThe online version of this article (10.1007/s00227-018-3411-2) contains supplementary material, which is available to authorized users.
Highlights
Herbivory is a fundamental process on coral reefs that regulates algal species composition, algal abundance (Ogden and Lobel 1978; Lewis and Wainwright 1985; Carpenter 1986) and the interactions between corals and algae (Mapstone et al 2007; Trapon et al 2013a, b; Zaneveld et al 2016)
Grazing rates by surgeonfishes were similar on both substrate orientations with an average of 7.4 ± 2.0 bites h−1 400 cm−2 on horizontal substrates and 4.5 ± 1.5 bites h−1 400 cm−2 on vertical substrates (Fig. 2b)
We show that substrate orientation is a key driver of algal community dynamics
Summary
Herbivory is a fundamental process on coral reefs that regulates algal species composition, algal abundance (Ogden and Lobel 1978; Lewis and Wainwright 1985; Carpenter 1986) and the interactions between corals and algae (Mapstone et al 2007; Trapon et al 2013a, b; Zaneveld et al 2016). A die-off of sea urchins in the 1980s left fishes as the main herbivores on Caribbean reefs (Lessios 1988), they are currently overfished in many areas (Jackson et al 2014). Areas with higher structural complexity often have more abundant sea urchins (Fabricius et al 2014) and herbivorous fishes (Luckhurst and Luckhurst 1978; Graham 2014; Rogers et al 2014), which may increase topdown control on algal communities (Verges et al 2011). In the Caribbean, the structural complexity of coral reefs has declined by more than 50% since the 1960s, creating flatter, more horizontal reef surfaces (Alvares-Filip et al 2009, 2011). There is a critical need to understand how the loss of structural complexity and the flattening of coral reefs influence herbivory and algal community dynamics
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