Herbivory, Nutrients, Stochastic Events, and Relative Dominances of Benthic Indicator Groups on Coral Reefs: A Review and Recommendations

Abstract

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.

The effects of nitrogen and phosphorus enrichment on algal community development: Artificial mini-reefs on the Belize Barrier Reef sedimentary lagoon

“…However, some reef scientists recently have countered that such changes in benthic community structure are dependant on natural stochastic events (Precht and Miller, 2007;Littler et al, 2009), overfishing of herbivorous fish stocks (Hughes, 1994;Pandolfi et al, 2003) and/or loss of keystone grazers such as the long-spined sea urchin Diadema antillarum (Jackson et al, 2001). These last conclusions are supported by the numerous grazer-reduction experiments in nutrient-replete systems but not in strictly oligotrophic environments that, for the most part, have reported an expansion of low-growing fleshy algal forms, algal turfs and/or filamentous forms (<3 cm high) rather than macroalgal blooms (as predicted in the RDM; see Lapointe, 1999).…”

Evidence of Large-Scale Chronic Eutrophication in the Great Barrier Reef: Quantification of Chlorophyll a Thresholds for Sustaining Coral Reef Communities

“…The addition of nutrients to a pristine coral reef region will stimulate the growth of phytoplankton, benthic algae (including the symbiotic zooxanthellae), and heterotrophs (e.g., bacteria, viruses, and zooplankton including protozoa such as ciliates); this stimulated growth can cause significant changes in the coral reef community structure (e.g., see Smith et al 1981;Sander 1985, 1987;Lapointe 1989Lapointe , 1997Bell 1992;Littler et al 2009). For example, the phytoplankton compete with the symbiotic zooxanthellae for light and thus can interfere directly with coral growth.…”

“…For example, the relative-dominance model (RDM) (Littler et al 2009) suggests that for eutrophic systems coralline algae will eventually dominate the hard substrate if highly grazed and that frondose macroalgae will eventually dominate if lowly grazed. Observations in the GBR lagoon at Low Isles and on the reefs in the Cairns region show that soft corals can also out-compete with hermatypic corals in a highly grazed eutrophic system (Endean and Stablum 1973;Bell and Elmetri 1995).…”

“…Under such a situation it is less likely that the hermatypic corals will re-establish to dominate the benthos as before. Hence in eutrophic conditions the hermatypic corals should be considered as being fragile and the system overall would exhibit a low resilience Littler et al 2009). …”

Fringing Reefs of Reunion Island and Eutrophication Effects - Part 1: Long-Term Monitoring of Two Shallow Coral Reef Communities

“…Inconspicuousness or dominance of algae at the T and P sites were obviously linked to nutrient concentrations and/or the quantity of nutrient regeneration through organic matter depletion (Cuet et al, 1988;Mioche, 1998). However, regular echinoid grazing is also a major factor determining the dominance of primary producers (for instance, see Hughes et al, 1987;Littler et al, 2009). So, at both sites, the abundance (or rarity) of sea urchins provides a strong (or very weak) feedback on algal biomass (See part 2, Naim et al, 2013a).…”