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Nat Commun
2017 Jan 23;8:14160. doi: 10.1038/ncomms14160.
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Prehistorical and historical declines in Caribbean coral reef accretion rates driven by loss of parrotfish.
Cramer KL
,
O'Dea A
,
Clark TR
,
Zhao JX
,
Norris RD
.
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Caribbean coral reefs have transformed into algal-dominated habitats over recent decades, but the mechanisms of change are unresolved due to a lack of quantitative ecological data before large-scale human impacts. To understand the role of reduced herbivory in recent coral declines, we produce a high-resolution 3,000 year record of reef accretion rate and herbivore (parrotfish and urchin) abundance from the analysis of sediments and fish, coral and urchin subfossils within cores from Caribbean Panama. At each site, declines in accretion rates and parrotfish abundance were initiated in the prehistorical or historical period. Statistical tests of direct cause and effect relationships using convergent cross mapping reveal that accretion rates are driven by parrotfish abundance (but not vice versa) but are not affected by total urchin abundance. These results confirm the critical role of parrotfish in maintaining coral-dominated reef habitat and the urgent need for restoration of parrotfish populations to enable reef persistence.
Figure 1. Reef
accretion rates over last 3,000 years in Bocas del Toro, Panama.(a) Reef matrix cores (n=4) analysed for subfossil and sediment composition and U–Th dates (n=23) obtained (in year AD) along length of cores. AP=Airport Point, CA=Cayo Adriana, PD1 and PD2=replicate cores from Punta Donato. (b) Map of coring locations. Stars indicate population centers, including Smithsonian Tropical Research Institute (STRI)'s research station. Turquoise=AP, blue=CA, pink and green=PD1 and PD2, respectively. (c) Age-depth plot showing reef accretion trends Age reversals excluded from linear interpolations of age estimates; rates for PD2 were assumed to be equivalent to those for same core position in PD1.
Figure 2. Major subfossil groups preserved in reef matrix cores.(a) Fish teeth functional groups and variety of parrotfish tooth morphotypes. Teeth range from 500–63 μm in size but majority are 250–104 μm; images not to scale. (b) Common coral taxa identified from skeletal fragments. (c) Common urchin taxa identified from spines.
Figure 3. Millennial-scale trends in abundance of major reef subfossil groups and accretion rates.(a) Total fish tooth abundance measured as number of individual teeth divided by dry weight of all sediment size fractions combined. (b) Total coral abundance measured as weight of coral fragments from>2 mm sediment fraction divided by dry weight of>2 mm fraction. (c) Total urchin abundance measured as weight of urchin spines divided by dry weight of 0.5–2 mm sediment fraction. (d) Proportion of all teeth belonging to parrotfish. (e) Reef accretion rate represented as mm of reef sediment accumulated per year. (f) Abundance of Echinometra urchins measured as total spine weight of genus divided by dry weight of 0.5–2 mm sediment fraction. Number samples=154. Turquoise=Airport Point, blue=Cayo Adriana, pink and green=Punta Donato 1 and 2, respectively. Coloured lines are loess smoothed estimated trends for individual cores and semitransparent black lines are loess smoothed estimates for all cores combined; smoothing parameter=0.9.
Figure 4. Analysis of causality between reef accretion and abundance of herbivores or dominant coral.(a) Parrotfish relative abundance and reef accretion rate. (b) Urchin abundance and reef accretion rate. (c) Parrotfish relative abundance and urchin abundance. (d) Branching Porites coral relative abundance and reef accretion rate. Lines and shaded regions show mean±s.d. from 100 bootstrapped iterations. Significant causal forcing (*) determined from bootstrap test with 100 iterations, and indicated when the Pearson correlation coefficient is significantly greater than zero for larger sample sizes (number of core samples, including all spatial replicates in the composite time series) and when correlation coefficient increases significantly with increasing number of core samples.
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