Are isolated central Pacific reefs really “healthier”?

In a new paper – that got a lot of media coverage – Smith et al 2016 quantified benthic reef composition “across 56 islands spanning five archipelagos in the central Pacific”.

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I think it’s an admirable project and an interesting data set, and there is a lot to like about the paper. However, some of the main interpretations, particularly in the press coverage, are off the mark. Although average coral cover was greater on reefs adjacent to uninhabited islands (24 vs 15%), this difference was non-significant; a key fact ignored or downplayed in the press. More importantly, the average coral cover of the human-dominated reefs Smith et al. surveyed isn’t representative of reefs in the region. Numerous synthesis of large survey and monitoring programs indicate the coral cover average used in Smith et al. for comparison to the uninhabited reef atolls is strangely low. This broader work also indicates the observed coral cover on uninhabited reefs isn’t at all exceptional. For example, mean coral cover across the Indian Ocean was 31% (2001-2005, Ateweberhan et al 2011), and in most subregions, cover was greater than the isolated reef average of 24% reported in Smith et al, e.g., Western Australia: 34%, Mozambique & South Africa: 28%, South Western Indian Ocean Islands: 36%.

The 15% value reported by Smith et al. is even lower than the average across the Caribbean – a highly disturbed and degraded region that lacks plating acroporiid corals (thus the central Pacific baseline is almost certainly 10-20% higher). And the value of 24% for uninhabited central Pacific reefs is a common, nearly universal subregional average these days, e.g., as seen across the Pacific (via Bruno and Selig 2007):

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And even across most the Caribbean (Schutte et al 2010):

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Some of the reefs Smith et al. surveyed clearly have very high coral cover:

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But the same is true of the broader Pacific, Indian ocean, and Caribbean, e.g., see this figure.

And I agree, their data indicates there is plenty worth preserving and fighting for and in that sense constitutes good news. But that finding and message is true of every regional synthesis of coral loss I’m familiar with – it isn’t particular to highly isolated reefs and regions. All regions have reefs and areas with especially high coral cover and far more fishes. It isn’t objective to attribute the coral cover values on the reefs in Smith et al. 2016 to isolation and the absence of people given similar observations are made around the word, often adjacent to developed coastlines.

In fact, to me, the lesson of Smith et al. is that even our most remote reefs are highly impacted and sensitive to (and not resilient to) ocean warming and subsequent bleaching, disease and coral loss. Just look what’s happening on the highly isolated northern Great Barrier Reef this week. Although isolated reefs could plausibly recover from bleaching more quickly than locally impacted reefs: 1) Given the growing frequency of mass bleaching I’m starting to question whether this even matters. 2) This doesn’t appear to be the case: if they did recover more quickly, coral cover should on average be greater (assuming the disturbance regime was equivalent). But it isn’t.

2 Responses to “Are isolated central Pacific reefs really “healthier”?”

  1. Douglas Fenner says:

    John,
    Thanks for this added perspective. I think it’s regrettable when the popular press misinterprets science papers, but we have little control over it. They won’t show us what they’ve written before they publish it because their publishing deadlines are so close, and because they can’t allow censorship. We just want to inform them to make sure they get their story accurate, not censor them. But we have different roles. Good to point out when the media get it wrong, though.
    I thought the central points of the paper were not that coral cover was significantly higher in unpopulated areas than populated (it wasn’t, as you say). They made the point that coral cover is unreliable, and macroalgae are unreliable. They show that the differences are large and significant for calcifiers = coralline algae PLUS coral cover, and for “fleshy algae” = macroalgae + turf. They found that coral cover and macroalgae were not inversely correlated, but that calcifiers and fleshy algae were. To me that supports the view that calcifiers and fleshy algae are the important functional groups, NOT corals and macroalage. Further, their main point was that reefs without people are healthier than those with reefs. They survey complete archipelagoes, so that helps to control for regional differences. Within archipelagoes they get significant differences in the calcifiers and for “fleshy algae.”
    I think their main point is valid, but I don’t find it surprising that coral cover is higher elsewhere. The Hawaiian islands are high latitude reefs, with the uninhabited portion being very high latitude for reefs. Most of the Marianas are young volcanoes, and the one I’ve been on had a lot of very bare volcanic rock or sand, I’ve never seen such bare surfaces in the tropics. The points were made within archipelagoes for good reason.
    All that said, I think you are exactly right that we need to have perspective on the coral covers reported, they are mostly low (American Samoa does pretty well!). But then I agree with them that coral cover isn’t everything, even adding macroalgae isn’t the whole story. Coral cover not only isn’t the whole story, it’s not the best metric, they’re showing that calcifiers are a better metric for some purposes, and I think they’re right.
    I think the important take home is that people not only reduce fish greatly on coral reefs near human populations, they also decrease coral cover. The former is not surprising, since humans directly remove fish in large quantities. The latter is something we’ve all thought, but hard evidence of it is good to add.

    • John Bruno says:

      Thanks Doug for reading, replying and for your thoughts, which I especially appreciate given where you live! (I’m pretty envious)

      I largely agree, except that I don’t see the evidence for local human effects on coral cover in their study. I also don’t agree that CCA cover is a good measure of reef “health” or even resilience. I agree coral cover isn’t the only measure of reef “health” (a term I’m try to avoid using), but it is better than anything else.

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