Rehabilitation of intertidal biodiversity on breakwaters could be compromised by grazing pressure and environmental context

Abstract

Sustaining biodiversity on coastal built structures is a priority to maintain ecosystem functioning in the face of increasing urban sprawl. This goal has been generally pursued by adding physical features to built structures to generate multiple microhabitats, without considering key species interactions, such as grazing. Here, combining descriptive studies across a latitudinal gradient along the coast of Chile (18°S to 41°S) and a field experiment, we examined if topographic enhancement of breakwaters increases grazer abundances. Specifically, we explored whether grazer densities and trait diversity influence the effectiveness of topographic microhabitats (crevices, grooves, and pits) in enhancing biodiversity on wave-exposed and wave-sheltered breakwaters. High abundances of grazers and a positive relationship between scraping-grazer densities and bare rock area were recorded on mid-latitude breakwaters. In most cases, topographic microhabitats did not enhance Shannon’s diversity index or species number of the sessile community on both wave-exposed and wave-protected breakwaters. Only at high shore levels of a wave-exposed breakwater adding crevices resulted in increased diversity, species number and cover of filter feeders and calcareous algae. These changes also corresponded with lower grazer densities and bare space cover. Our results suggest that the effectiveness of eco-engineering solutions in sustaining intertidal biodiversity on coastal structures can be significantly reduced by grazing and influenced by wave-exposure, and tidal level. Innovative designs to rehabilitate biodiversity should attract native predators to control the abundances of grazers and their impacts. Promoting balanced consumer-resource interactions would enhance the ecological value of built structures and could aid in halting anthropogenically-driven coastal biodiversity loss.