Written by Matthew Tietbohl
Animals are well-known to be important ecosystem engineers, impacting their habitats in a number of different ways. Some predators may create landscapes of fear where herbivores avoid, resulting in mosaics of different plant communities across a habitat. Other terrestrial herbivores, like elephants can impact their environments by destroying trees and promote the growth of other vegetation in forests. The same is true of fish on coral reefs. Their feeding activity can impact animal species or algae they eat (top-down effects) and their waste products can fuel the growth of microbes or algae (bottom-up effects). However, these two different types of impacts are seldom studied in tandem.
A new study by Katrina Munsterman et al. (2021) looked at both top-down and bottom-up effects in herbivorous fish communities on backreefs in Moorea in the tropical Pacific Ocean. By looking at the feeding rate (bites per minute) and targeted dietary item, they were able to see how distinct the diet of different fishes were. An interesting example of some variation they found was that juvenile parrotfishes fed more on epibionts (plants or animals that grow on something else) compared to adults targeting different types of algae, like turf or bushy macroalgae. They also captured fishes and held them in sea-water filled plastic bags while measuring the amount of organic waste they produced. By normalizing these values to fish size (organic waste per gram of fish), they were able to show larger fishes had higher excretion.
Not only did they find interesting species-specific differences in feeding surfaces and waste production, but they also used this information to assess how these functions change over time and in different habitats. They used a dataset of fish surveys on backreefs from 2006-2018; some of these reefs had shifted to be dominated more by algae than by corals, while others remained dominated by corals. Using the counts of different species from the surveys, and factoring in the information they collected about species top-down (feeding) and bottom-up (waste production) effects, they were able to estimate how these processes have changed over time due to changes in the herbivorous fish communities found between different habitats.
Reefs that remained dominated by corals had a higher proportion of large excavating parrotfishes. These parrotfishes fed much more over turfing algal substrate, potentially helping to clear space for new coral recruits to settle. Reefs that had shifted to algal dominance had smaller parrotfishes and larger detritivorous (fish that feed on detritus) surgeonfishes. These species may contribute to the continuity of these algae-dominated reefs by feeding on epibionts growing on algae – by removing these, they can help large algae stay healthy. Larger detritivorous surgeonfishes also released more phosphorous in their waste, which could help macroalgae grow quickly at the cost of coral competitors.
This study was the first to definitively show how changes to the herbivorous fish community can impact top-down and bottom-up effects on a coral reef ecosystem. These important changes can lead to supporting factors that make it harder for algae dominated ecosystems to transform back to coral-dominated ones. More work is still needed, especially with browsing fishes, which were seldom ever recorded in fish surveys. However, this is still a very important study that allows us to better understand how to manage ecosystems to promote coral health and productive reefs.
Munsterman, K.S., Allgeier, J.E., Peters, J.R. and Burkepile, D.E., 2021. A View From Both Ends: Shifts in Herbivore Assemblages Impact Top-Down and Bottom-Up Processes on Coral Reefs. Ecosystems, pp.1-14.
Cover Photo: ©P. Kanstinger