Key Resilience Indicators
Scientists have prioritized the following indicators and anthropogenic stressors as likely to be the most important to support the resilience of coral reef ecosystems; (ranked from highest to lower perceived importance ref). The first six (*) are considered essential and will be important to include in nearly all resilience assessments. See Table 1 for descriptions and potential methods for assessing these indicators and stressors.
Key ecological resilience indicators:
- *Resistant coral species
- *Coral diversity
- *Herbivore biomass
- *Coral disease
- Temperature variability
- Herbivore diversity
- Habitat/structural complexity
- Mature coral colonies
- Light (stress)
- Coral class-size distribution
- Substrate suitability
Key anthropogenic stressors:
Table 1. A description of the recommended resilience indicators, along with common units, and a list of potential field methods (from Maynard et al. 2017).
|Resilience indicators||Description||Possible methods||Common units|
|Resistant coral species||Proportion of the reef-building coral community made up by species that have demonstrated or are thought to be relatively resistant to thermal coral bleaching (Marshall and Baird 2000; McClanahan et al. 2004).||Timed swims, quadrats, belt transects, point-intercept transects||% of community|
|Coral diversity||A quantitative measure that reflects how many different coral species there are in a dataset, while simultaneously taking into account how evenly the species are distributed. Common indices express the probability that two species selected at random from a community will be different.||Indices: Shannon or Simpson's Index||Unitless|
|Herbivore biomass||Weight per unit area of herbivorous fish|
and invertebrates. can be inclusive of all major herbivore functional groups (scrapers, grazers, excavators, browsers) or can separate these.
|Timed swims, belt transects, stationary point counts||kg/100m2, g/m²|
|Coral disease||Proportion of the coral community that is affected by diseases. You may choose to use a ‘total prevalence’, which combines all diseases and all corals, or a subset of diseases or corals to assess effects from a particular disease or on a particular coral.||Belt transects||% (of colonies affected; a 'total prevalence'; i.e., all or a subset of diseases combined)|
|Recruitment||Abundance and density of recently settled corals that are less than 2 years old.||Quadrats||#/m²|
|Temperature variability||Variability of temperatures during the warm season. Higher variability has been associated with bleaching resistance.||Remotely sensed, available for all coral reefs at 4-km resolution from NOAA remote sensing archives||Unitless|
|Herbivore diversity||See 'coral diversity' description; same for herbivorous fish and invertebrates. Can also be assessed as the number of key herbivore functional groups present at a minimum abundance (e.g., scrapers, grazers, browsers and excavators).|
Timed swims, belt transects, stationary point counts
|Unitless (diversity indices), or number present at a minimum abundance|
|Habitat/structural complexity||Three-dimensionality of the substrate and crack and crevice depth and diversity. Ratio of reef surface contour distance to linear distance.||Chain over substrate||m|
|Mature colonies||Proportion of the benthic community made up by long-lived corals (i.e., >10 years old).||Timed swims, belt transects, stationary point counts||% of community|
|Light (stress)||Amount of light per square meter reaching the substrate during typical oceanographic conditions during the warm season.||Requires instrumentation||watts/cm²|
|Coral size-class distribution||Evenness of corals within a range of size classes that includes recruits and mature colonies.||Timed swims, quadrats, belt transects, point-intercept transects||Unitless|
|Substrate suitability||Ratio expressing available substrate for coral recruits as being suitable and unsuitable for coral settlement.||Timed swims, quadrats, belt transects, point-intercept transects||Unitless|
A recent paper ref assessed recovery patterns of 21 reef sites in the Seychelles over a 17 year period, spanning a major climate-induced bleaching event. The authors identified a range of factors affecting recovery patterns, but found that quantifying structural complexity and water depth before the bleaching event accurately predicted ecosystem response following bleaching.
“Several factors can affect reef ecosystem trajectories following bleaching but, where necessary, just depth and structural complexity may be useful predictors of ecosystem fate.” ref
Graham et al. (2015) found that recovery was favored when reefs were:
- structurally complex (when values before disturbance were >3.1) and in deeper water (>6.6 m)
- density of juvenile corals and herbivorous fishes was relatively high (i.e., >6.2 per m2 of juvenile corals; herbivorous fish biomass of 177 kg ha-1)
- nutrient loads were low (carbon: nitrogen ratios in macroalgae >38)
Although this study focused on reefs in the Seychelles, the authors note that the predictors are relevant for reefs globally; specifically, depth and structural complexity were consistent predictors of recovery patterns across 6 other countries from East Africa to the South Pacific.
Structural complexity captures the structure provided by corals and the underlying reef matrix, and influences a range of ecological processes, substantially contributing to overall diversity and productivity of many reef associated organisms. Deeper sites may recover better due to the relationship between light penetration and algal growth (shallower areas receive greater light penetration which stimulates algal growth) or greater vulnerability of shallower reefs to disturbances such as recurrent coral bleaching or storm damage. Researchers have debated whether levels of nutrients or herbivory are more important for coral reef regime shifts or recovery, but these results suggest that although both are important, they are less certain predictors than structural complexity, depth, and the density of juvenile corals.
How to Select Indicators
While the list of indicators above may be used to help managers prioritize which ones to include in a resilience assessment or monitoring program, it may also be useful for managers to review key publications to identify additional indicators to use in their local context. ref
Indicators should be selected that are thought to have strong links to resistance or recovery based on local knowledge, and that can be reliably assessed using the same methodology for all sites. In addition, the assessment of all indicators must be within the expertise and resources available.
It is useful to select indicators through a collaborative process that includes representatives from all agencies and groups likely to use the results and outputs of the assessment or monitoring program.
How Many Resilience Indicators Should be Included in an Assessment?
Six priority ecological resilience indicators should be included in any resilience assessment, including: resistant coral species, coral diversity, herbivore biomass, coral disease, macroalgae cover, and coral recruitment. Additionally, it is important for managers to consider that the total number of indicators included in an assessment will affect the power that each indicator has to determine resilience potential; because the importance of each individual indicator is diluted with each indicator that is included.
Managers may use existing data or may need to collect new data for all or some of the indicators. There are many suitable methods for assessing resilience indicators (see the Resources sections on the Designing a Monitoring Plan and Assessing and Monitoring Reef Resilience; see Analyzing Relative Resilience for detailed information on how to analyze the data once collected and compiled.