Coral Reefs in the Anthropocene

Plos OneIn a commentary Shifting Baselines, Local Impacts, and Global Change on Coral Reefs in PLoS Biology coral reef ecologists Nancy Knowlton and Jeremy Jackson write:

Imagine trying to understand the ecology of tropical rainforests by studying environmental changes and interactions among the surviving plants and animals on a vast cattle ranch in the center of a deforested Amazon, without any basic data on how the forest worked before it was cleared and burned. The soil would be baked dry or eroded away and the amount of rainfall would be greatly decreased. Most of the fantastic biodiversity would be gone. The trees would be replaced by grasses or soybeans, the major grazers would be leaf-cutter ants and cattle, and the major predators would be insects, rodents, and hawks. Ecologists could do experiments on the importance of cattle for the maintenance of plant species diversity, but the results would be meaningless for understanding the rainforest that used to be or how to restore it in the future.

This lack of a baseline for pristine marine ecosystems is particularly acute for coral reefs, the so-called rainforests of the sea, which are the most diverse marine ecosystems and among the most threatened [4–8]. Most of the world’s tropical coastal oceans are so heavily degraded locally that “pristine” reefs are essentially gone, even if one ignores changes associated with already rising temperatures and acidity [3]. Most modern (post-SCUBA) ecological studies have focused on reef ecosystems that are moderately to severely degraded, and we have a much better understanding of transitions between human-dominated and collapsed reefs than between human-dominated and quasi-pristine reefs.

Knowlton and Jackson’s essay is a comment on an article in PLoS One Baselines and Degradation of Coral Reefs in the Northern Line Islands by Stuart Sandin and others that describes a large scale marine community assessment across a gradient of human dominated to relatively little impacted reefs in the Pacific. The study found that large predatory fish and reef-building organisms dominated the reefs around unpopulated islands, but around populated islands the reefs were dominated by small planktivorous fishes and fleshy algae. The reefs around populated islands exhibited more coral disease and less coral recruitment, suggesting that protection from overfishing and pollution may increase the resilience of coral reefs. The authors write:

Thus, local protection from overfishing and pollution may enhance ecosystem resilience to warm episodes and coral bleaching that result from global warming. To test this we need to determine how do coral recruitment, growth, and survivorship respond to changes in local community structure due to fishing, and how do these responses interact with episodes of warming measured by DHW. We also need to determine how fish productivity, i.e., the key currency of fisheries management, varies with changes in food web structure such as those observed between Kingman and Kiritimati. The only way to answer these questions is by investigation of reefs like the northern Line Islands that have remained remarkably intact in comparison to the global norm. They are among the only baselines that remain.

Knowlton and Jackson propose a figure to summarize their understanding of the relationships between the intensity of local anthropogenic disturbance and biodiversity and ecosystem function. They state:

Most surprisingly, given the substantial attention of conservationists to “hot spots” of biodiversity [42], ecosystem function appears to decline long before any substantial decline in biodiversity. This is especially apparent for the diversity of fish species on the Northern Line Islands reefs that is negatively correlated with that of reef corals [39]. Thus, corals may be more sensitive to extinction due to human impacts than their associated species that can move to other habitats, an inference that is consistent with the observation that reef fishes recover rapidly following protection whereas corals may require several decades or more [25,31–33].

Maintaining reefs requires large protected areas. Simon Donner, one of the authors of the PlosOne paper, writes on his blog Maribo that this is happening:

As was reported last week, Kiribati is working with international conservation groups to create the Phoenix Islands Protected Area, the largest marine protected area in the world. The project has been in the works for a couple years.

It’s worth looking at a map of Kiribati to make sense of the announcement. The NY Times called Kiribati a ‘tiny island nation’. Yes, the islands themselves are tiny; the 32 atolls plus Banaba Island have a total area of ~729 sq. km (depending on the tide). The nation is not. It covers 3.5 million sq. km, which is about the area of India, and includes three distinct island chains.

The 410 000 sq km Phoenix Islands Protected Area will encompass the central island chain in Kiribati, to the east of the International Date Line. Around 30-40 people live on Kanton Atoll and serve as informal caretakers for the island chain. Otherwise, the Phoenix Islands are uninhabited. In fact, you could argue they are uninhabitable.

The majority of the 100,000 i-Kiribati live in western “Gilbert” Islands chain, particularly the capital of Tarawa Atoll (*). Previous attempts by the British to (forcefully) resettle i-Kiribati from the more crowded Gilbert island chain to the Phoenix Islands failed because of prolonged droughts and the lack of groundwater resources.

Most of the Kiribati government revenue comes from foreign fishing licenses. The protected area status will limit large-scale commercial fishing around the Phoenix Islands and any marine resource development efforts. So the real key to this plan is the creation of an international endowment raise funds to offset the lost revenues for the Kiribati government. If successful, PIPA may serve as a model for the creation of marine protected areas in the developed world.

Based on the scale and pressing nature of the problems facing coral reefs Knowlton and Jackson propose a set of key questions to address to enable the maintenacne of coral reefs in a human dominated world.

1. To what extent do overfishing and eutrophication increase the vulnerability of reef corals to bleaching, disease, and acidification caused by global climate change; and, conversely, does protection from these local stressors decrease the vulnerability of reef corals to the effects of climate change?

2. If local protection decreases the vulnerability of corals to climate change, what are the physiological or ecological mechanisms involved, including changes in associated microbial populations and their interactions with their coral hosts?

3. Does protection from overfishing and eutrophication increase rates of coral recruitment, growth, and reproduction that are essential to the reestablishment of coral communities following mass mortality due to the effects of climate change or natural disturbance?

4. Can we identify critical breakpoints and thresholds in the abundance and trophic composition of marine consumers below which coral populations will inevitably decline or fail to recover?

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