Caribbean reefs in the Anthropocene

Written by Matthew Tietbohl

It is no joke that Caribbean coral reefs have been hit hard by a variety of negative impacts and stressors which have drastically altered over the last century, nearly always for the ‘worse.’ Disease, excess nutrients, bleaching, and overfishing have transformed reefs from those once dominated by hard coral to many with turf and macroalgae abundant instead. The loss of live coral throughout the Caribbean has been very well documented, with the massive decline of key habitat forming species beginning in the 1950s (Cramer et al. 2020). Since this time, many scientists throughout the region have documented the loss of these corals and its impacts on the communities that rely on them (Jackson et al. 2014). With these losses, it is often agreed upon that these reefs are less able to supply what are known as ecosystem services, which are the direct and indirect contributions to human well-being. These include services such as protection against shoreline erosion, production of fishery species, as well as tourism and cultural value. The prevailing paradigm is that reefs with low coral cover are less able to supply these services, and in their degraded state, are less valuable overall. Perhaps surprisingly to some, this might not always be the case, and low coral cover reefs may still be able to provide a range of key services and processes.

When discussions begin about coral reef degradation, they often circle back to losses in coral cover and gains in turf or macroalgae cover. The loss of reef building corals can cause a shift in ecosystem states to those dominated by algae and prevent reefs from adequately allowing their normal processes to function and support ecosystem services. This is generally seen as a negative for coral reefs, as algae do not contribute to creating complex structure and do not maintain the structural integrity of the reef in the same way as corals. In many cases, especially throughout the Caribbean, a loss in coral cover is associated with an increase in macroalgae (Figure 1.) However, there is considerable variability within the data, meaning not all reefs with low coral cover have lots of macroalgae. This could also be true of other services on reefs too. Assuming all low coral cover reefs are equally inadequate to provide important services misses the potential to understand just how important even these low coral cover ecosystems can be. It is likely there will be many more reefs around the world like those of the current Caribbean in the future, so understanding how different processes occur on reefs with different amounts of coral is of great interest to scientists and people’ whose livelihood rely on these ecosystems. 

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Figure 1. Mean percent cover of macroalgae versus mean percent cover of live coral, across all AGRRA (Atlantic and Gulf Rapid Reef Assessment)sites (n=328). Grey shading represents 95% confidence interval. Note the considerable variability in the relationship between macroalgae and coral cover, especially at low coral cover. Reproduced from supplementary figure 5 in Lester et al. (2020).

A recent study published in the journal Global Change Biology tried to tackle this question. By using standardized surveys conducted throughout the Caribbean, Lester et al. (2020) compiled ten key measures from these surveys that directly relate to key processes, functions, and ecosystem services on coral reefs, like habitat building and herbivory. Because these surveys were conducted throughout the Caribbean in the same way at each site, the authors were able to compare their measures between sites to try and understand how these measures might be impacted by changes in coral cover. They broke up the locations into those with low (<10%), medium (10-20%), and high (>20%) coral cover, though it is worth noting as the authors did that the values used for high coral cover are not as high as they used to be. They then compared the range and variability of these values between reefs with different amounts of coral. This kind of analysis allowed them to identify if high cover coral reefs were better functioning than low coral reefs. Importantly too, it would show if these key processes could be sustained on reefs with lower coral cover, allowing them to identify bright spots where reef functions were doing better than expected.

What the authors found, was surprising, and not entirely in a bad way! As one might expect, their measures that related to coral functions on reefs, including calcification and settlement of baby corals, was much lower on reefs with low coral cover (Figure 1, left panels). This is not surprising, because if you have less coral you would expect to have less baby corals growing on those reefs, less calcification, and less species of coral. What was interesting though, was that their measures that related to fish instead of corals did not show a strong relationship with coral cover and metrics like fishery value, large fish density, and herbivore biomass showed considerable variability between sites with largely different coral cover. This reveals that some coral reef processes and functions can be supported equally well on reefs with high and low coral cover, meaning some of these processes are decoupled from coral cover. Other factors not included in their measures, such as wave exposure, water quality, and habitat connectivity, are likely playing a more important role in allowing these reefs to function similarly. This means there are some low coral cover reefs in the Caribbean that are bright spots and able to complete some key functions just as well as reefs with higher coral cover!

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Figure 2. Variability in the 10 metrics for three different mean live coral cover ranges on reefs in the Caribbean. High coral cover is >20%, medium is 10-20%, and low is <10% live coral cover. Black vertical lines represent individual data points, and p-values from linear mixed effects model results are shown in parenthesis after panel titles. Significant differences between the metrics are shown with different letters next to the boxes). Figure reproduced from figure 3 in Lester et al. (2020).

The variability the authors found highlights how difficult it can be to call a reef “degraded” or “ruined.” Though it is certainly true less hard coral is bad for multiple reef functions, not all processes are entirely dependent on hard corals. Bright spots and productive coral reefs can exist without a high abundance of hard coral. This is an important finding because understanding the drivers of variance in the ecosystem measures they used may allow scientists and managers to better target conservation measures to help achieve more bright spots where low coral reefs are still able to provide key ecosystem services. There is certainly more work to be done to better tease apart the specific mechanisms that drive these differences in functions between high and low cover coral reefs, but this research is a big step in the right direction towards better understanding how to think about coral reefs in a changing world. By understanding how “degraded” reefs function, we can learn more about how to ensure these reefs are still functioning in the future!

Citations:

Cramer KL, Jackson JBC, Donovan MK, Greenstein BJ, Korpanty CA, Cook GM, Pandolfi JM 2020. Widespread loss of Caribbean acroporid corals was underway before coral bleaching and disease outbreaks. Science Advances, 6(17): eaax9395 

Jackson JBC, Donovan MK, Cramer KL, Lam W 2014. Status and trends of Caribbean coral reefs: 1970–2012. Gland, Switzerland: Global Coral Reef Monitoring Network, IUCN.

Lester SE, Rassweiler A, McCoy SJ, Dubel AK, Donovan MK, Miller MW, Miller SD, Ruttenberg RI, Samhouri JF, Hay, ME (2020). Carribean reefs of the Anthropocene: variance in ecosystem metrics indicates bright spots on coral depauperate reefs. Global Change Biology, https://doi.org/10.1111/gcb.15253

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