Project: Evaluating whether the sequence of human impacts to tropical reefs erodes resilience of short algal turf and drives the emergence of alternate algal communities

NSF Award Abstract:
Understanding ecological resilience (resistance to and recovery from disturbance) is critical as human impacts on marine communities accelerate. This is especially true on tropical reefs, where for half a century, communities have rapidly, and often unexpectedly, transitioned away from the desirable coral state. On healthy reefs, short turf algae replace corals after disturbance, but this is usually a temporary state before corals recover. Short turf is composed of fine hair-like filaments of highly nutritious algae that is targeted by many herbivorous fishes and allows for settlement of coral larvae during the recovery process. However, overfishing, nutrient pollution, and increased sedimentation may support the emergency of new seaweed communities that resist recovery, even if human impacts are reduced. This proposal addresses whether the order of human impacts that alter key ecological processes determines resilience of algal turf communities, a process that has never been evaluated. Field experiments are being performed in Moorea, French Polynesia, on a reef where novel seaweed communities have replaced corals over the last two decades. Both resistance and recovery of these emergent communities are being measured, as well as any processes or seaweed traits supporting these properties. The project provides research training for undergraduate students and supports a gap-year program for recent graduates from underrepresented groups.

On healthy tropical reefs, loss of coral often results in dominance by closely-cropped algal turf that can ultimately recover to coral. However, fishing of herbivorous fishes and increased nutrient and sediment supplies from developing watersheds can produce 'ecological surprises' - the emergence of novel algal states that resist a return to short turf, even after human impacts cease. Understanding resilience, defined as both resistance to and recovery from disturbance, of short turf communities is intrinsically linked to understanding the resilience of the novel emergent communities, which are often dominated by long turf or macroalgae. However, transitions among states, both forward and backward, are currently unpredictable, influenced not only by current environmental conditions, but also potentially by the order of human impacts that have altered key ecological processes, a theory that has never been evaluated. This project is interrogating the importance of the sequence of human impacts to community transitions and their resilience on tropical coral reefs. The team is 1) conducting a manipulative field experiment to examine the resistance of short turf communities by sequentially applying increased fishing pressure, enhanced nutrient supplies, and increased sedimentation rates in all possible combinations of order, with no-change and simultaneous change treatments as controls; 2) measuring recovery following termination of experimental manipulations to assess how the order of impacts affects resilience; 3) examining the mechanistic underpinnings of resilience of both the original short turf and the emergent macroalgal states by measuring stabilizing feedbacks in all experimental plots; and 4) for states that transition to macroalgae, relating measured algal traits to stabilizing feedbacks to generalize the results to other tropical reefs.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.