Coral reefs in a human-dominated environment : implications of altered disturbance regimes and reduced resilience

Sammanfattning: Coral reefs are facing an "Anthropocene " era where humans have advanced from a minor species with limited influence on coral reefs to a major source of disturbance. Disturbance arriving from human influences may not only interact with each other, but also with natural disturbance regimes, leading to synergistic effects. Several studies have addressed the influence of natural disturbance regimes on coral reefs. However, experimental studies on disturbance complexity and physiological responses of corals to sublethal stresses, anthropogenic in particular, are surprisingly few. This thesis investigates the effects on corals exposed to single and multiple stress and highlights human influences on the interaction between altered disturbance regimes and ecological resilience. The first three studies have an ecophysiological approach showing that a stressor that do not affect corals when arriving as a single parameter can have a substantial effect when occurring in combination (simultaneously or sequentially) with other stressors. The studies also show that, in the short-term, antagonistic effects may occur.Interacting disturbance regimes can result in ecological surprises, such as phase shifts. A common manifestation of such phase shifts in coral reefs is the shift from coral to algal dominance where overfishing, mass mortality of herbivorous organisms, and eutrophication seem to be some of the major driving forces. This thesis also explores the dual and interactive role of marine protected areas and physical reduction of macroalgae as potential management strategies in reefs that have undergone a phase shift. However, results show that herbivorous fishes are not able to keep algae growth in check after the algae once are removed which might indicate that under prevailing conditions the phase shift might be irreversible.Instead of focusing on recovery of certain species and populations of species within disturbed sites of individual reefs, this thesis also address spatial resilience i.e. the dynamic capacity of a reef matrix to reorganize and maintain ecosystem function following disturbance. The final two papers begin an identification of spatial sources of resilience in dynamic seascapes where the interplay between altered disturbance regimes and ecological resilience is highlighted. Managing for resilience in dynamic seascapes enhances the likelihood of conserving coral reefs and provides insurance to society by sustaining essential ecosystem services.