Herbivory, nutrients, stochastic events, and relative dominances of benthic indicator groups on coral reefs: a review and recommendations

Abstract

Threshold levels (i.e., tipping points where the probability of community phase shifts is increased and the potential for recoverability is reduced) for critical bottomup interactions of productivity (e.g., nutrients) and those for top-down disturbances (e.g., herbivory) must be known to manage the competitive interactions determining the health of coral-dominated reefs. We further posit that latent trajectories (reduced resiliencies/ recoverability from phase shifts) are often activated or accelerated by large-scale stochastic disturbances such as tropical storms, cold fronts, warming events, diseases, and predator outbreaks. In highly diverse and productive reef ecosystems, much of the overall diversity at the benthic primary producer level is afforded by the interaction of opposing nutrient-limiting/nutrient-enhancing and herbivory controls with the local physical and spatial variability, such that a mosaic of environmental conditions typically occur in close proximity. Although the relative dominance model (RDM) appears straightforwardly simple, because of the nature of direct/indirect and stimulating/limiting factors and their interactions it is extremely complex. For example, insuffi cient nutrients may act directly to limit fl eshy algal domination (via physiological stress); conversely, abundant nutrients enhance fl eshy algal growth, with the opposite effect on reef-building corals (via toxic inhibition or increased diseases). Furthermore, the effects of controls can be indirect, by infl uencing competition. Even this seemingly indirect control can have further levels of complexity because competition between algae and corals can be direct (e.g., overgrowth) or indirect (e.g., preemption of substrate). High herbivory (via physical removal) also acts indirectly on fl eshy algae through reduced competitive ability, whereas lowered herbivory and elevated nutrients also indirectly inhibit or control corals and coralline algae by enhancing fl eshy algal competition. Other ecologically important bottom-up factors, such as reduced light, abrasion, allelopathy, disease vectoring, and sediment smothering, also result from indirect side effects of fl eshy algal competition. These factors tend to selectively eliminate the long-lived organisms in favor of weedy fast-growing species, thereby reducing desirable complexity and biodiversity.