Asynchrony in coral community structure contributes to reef-scale community stability

Abstract

Many aspects of global ecosystem degradation are well known, but the ecological implications of variation in these effects over scales of kilometers and years have not been widely considered. On tropical coral reefs, kilometer-scale variation in environmental conditions promotes a spatial mosaic of coral communities in which spatial insurance effects could enhance community stability. To evaluate whether these effects are important on coral reefs, we explored variation over 2006-2019 in coral community structure and environmental conditions in Moorea, French Polynesia. We studied coral community structure at a single site with fringing, back reef, and fore reef habitats, and used this system to explore associations among community asynchrony, asynchrony of environmental conditions, and community stability. Coral community structure varied asynchronously among habitats, and variation among habitats in the daily range in seawater temperature suggested it could be a factor contributing to the variation in coral community structure. Wave forced seawater flow connected the habitats and facilitated larval exchange among them, but this effect differed in strength among years, and accentuated periodic connectivity among habitats at 1-7 year intervals. At this site, connected habitats harboring taxonomically similar coral assemblages and exhibiting asynchronous population dynamics can provide insurance against extirpation, and may promote community stability. If these effects apply at larger spatial scale, then among-habitat community asynchrony is likely to play an important role in determining reef-wide coral community resilience.

Figure 1

Map showing the four habitats (fringing reef, backreef, 10 m, and 17 m on the fore reef) and thermistor locations in Moorea, French Polynesia. Photo credit: Google Earth.

Figure 2

Changes over time in coral cover and community structure across habitats at LTER 1: (A,E) fringing reef, (B,F) back reef, (C,G) 10-m depth on the fore reef, and (D,H) 17-m depth on the fore reef. (A–D) Mean cover (± SE) of the most abundant corals, and (E–H) non-metric multidimensional scaling (NMDS) of coral community structure over time, with circle size showing the summed percent cover for all corals. For fringe and fore reef habitats, N = ~ 40 year⁻¹, and for back reef, N = 5 year⁻¹.

Figure 3

Coral cover from 2006 to 2019 at LTER1, pooled among habitats: (A) mean (± SE, n = 4 habitats year⁻¹, SE bars smaller than symbol size where they are not shown) percent cover of the four most common corals that account for 43–95% of the coral cover in each habitat, and (B) synchrony (phi) and community variability (CV) over time.

Figure 4

The relationship between community variability (CV) and synchrony (phi) for the reef-wide coral community over 13 years from 2006 to 2019. Line is fitted by model II major axis regression of CV on phi.

Figure 5

Variation in (A) diurnal temperature range °C (DTR) during the inferred period of coral spawning and pelagic larval duration (September to January) across habitats, and (B) mean wave energy flux (kW m⁻¹) across the reef crest on the north shore during the same period. Missing points indicate periods for which DTR or wave energy flux were not calculated due to gaps in the dataset.