Simultaneous synergist, antagonistic and additive interactions between multiple local stressors all degrade algal turf communities on coral reefs

Abstract

Ecological communities are subjected to multiple anthropogenic stressors at both global and local scales that are increasing in number and magnitude. Stressors can interact in complex ways and are classified as additive, synergistic or antagonistic; the nature of the interaction is key to predicting changes and understanding community resilience. Coral reefs are among the most impacted communities and have shifted from coral‐ to algal‐dominated states, and overfishing, nutrient enrichment and sedimentation are local stressors that often co‐occur and may support degraded algal states. Short algal turfs are abundant benthic space holders on healthy reefs that may be pushed by local stressors to long algal turfs, a more degraded state that may prevent recovery to coral dominance. We conducted a fully crossed three‐factor field experiment on short algal turf communities manipulating herbivory pressure (+/−cages), nutrients (+/−fertilizer) and sediments (natural accumulation/removal). We applied stressors for 16 days, removed them and monitored turf height during and after manipulations. We found that significant pair‐wise interactions between all stressors pushed the community towards a degraded state with longer algal turfs. All three types of interactions (additive, synergistic and antagonistic) were common and occurred in equal frequency, suggesting more investigations into all types are needed to accurately predict community responses to multiple stressors. For example, when herbivores were present, nutrients and sediments interacted additively, while in the absence of herbivores, nutrients and sediments interacted synergistically. All interactions broke down following termination of experimental manipulations and all effects were undetectable after 49 days, indicating that this reef may be resilient, at least when stressors are applied on a short time‐scale. Synthesis. Because management of local stressors is often more tractable than global stressors, local management has been proposed as a means to offset global stressors. However, ecological communities often experience multiple local stressors simultaneously, and interactions between stressors, including synergisms and antagonisms, may be the source of nonlinear shifts in communities or “ecological surprises.” The majority of interactions in our study were both strong and nonlinear, and we suggest that, if pervasive across systems, nonlinear interactions may drive the recent global increase in “ecological surprises.”

Complex interactions among stressors evolve over time to drive shifts from short turfs to macroalgae on tropical reefs

“…To simulate the effects of overfishing, we limited access to herbivores by fully caging half the plots to exclude herbivorous fishes. Cages were constructed from metal mesh with 1 × 1 cm openings (as in Fong et al 2018). Cages were 15 × 15 cm square exclosures that were 15 cm tall and enclosed our natural algal turf communities.…”

“…To create ambient, press, and pulse nutrient treatments, we left ambient treatments in unmanipulated seawater and used Osmocote slow release fertilizer to provide added nutrients for both press and pulse nutrient treatment plots (Fong and Fong 2014, Fong et al 2018). This slow release fertilizer is comprised of 19:6 nitrogen:phosphorus.…”

“…Short algal turfs dominate in early successional recovery toward coral dominance as juvenile corals can recruit to short, sediment‐free algal turfs (Connell 1997, Birrell et al 2005, Adam et al 2011). Strong herbivory, low nutrient availability, and low sedimentation all maintain short‐cropped turfs, at least over the short term (Fong et al 2018). However, if these controlling forces are altered, turfs may transition to poor quality longer turfs that accumulate deep layers of sediments and have reduced productivity (Fong et al 2018, Tebbett and Bellwood 2020).…”

“…Strong herbivory, low nutrient availability, and low sedimentation all maintain short‐cropped turfs, at least over the short term (Fong et al 2018). However, if these controlling forces are altered, turfs may transition to poor quality longer turfs that accumulate deep layers of sediments and have reduced productivity (Fong et al 2018, Tebbett and Bellwood 2020). These long turfs may inhibit recovery because they experience reduced grazing and inhibit coral settlement, and some research suggests long, sediment laden turf may be an early indicator of community degradation (Birrell et al 2005, Goatley et al 2016).…”

“…Further, long turfs are less productive and are thus provide less food chain support, a crucial service in this system (Tebbett and Bellwood 2020). Over the longer term, long turfs may transition to mature, fleshy macroalgal communities via growth of cropped thalli or successful recruitment and growth of macroalgal propagules (Lewis 1986, Bellwood et al 2004, Birrell et al 2005, Smith et al 2010, Goatley et al 2016, Fong et al 2018). Short‐term experimental work has demonstrated that press stressors of decreased herbivory, increased nutrients, and increased sediment interact simultaneously to shift short algal turfs to longer turfs (Fong et al 2018).…”