Rats to Reefs: How furry critters are devastating coastal and marine ecosystems in the Chagos Archipelago

Written by Danielle Moloney

Article: Graham, N. A., Wilson, S. K., Carr, P., Hoey, A. S., Jennings, S., & MacNeil, M. A. (2018). Seabirds enhance coral reef productivity and functioning in the absence of invasive rats. Nature559, (7713), 250.

Introduction

When you think about coral reefs, the last thing that comes to your mind is probably rats. A new study has indicated a surprising connection between these furry critters and the colorful, diverse underwater ecosystems that provide a home for a wide array of marine life (Graham et. al 2018). Research on nutrient flow in the Chagos Archipelago offers a window into one of the many threats that humans have posed on coral reefs: rats.

Coral Reefs and Nutrient Flow

Corals reefs, fragile biodiversity hotspots that support 25% of marine life while occupying just 0.1% of the ocean surface, have come under threat in recent years due to climate change and human impact (United Nations Environment Programme 2000). Their sensitive nature puts a spotlight on their health as they battle rising sea temperatures and the threats that come with rapidly rising global environmental crises. There are many factors to consider in assessing the health of a reef, one of which is nutrient flow in their abiotic (non-living) environment. Studying the movement of organic matter between ecosystems in their natural state is a difficult task, especially given the current extent of human impact that must be worked around. Coral reefs are no exception in this instance, as the connection between them and terrestrial ecosystems is poorly understood.

Understanding the logic behind nutrient flow in an ecosystem helps to clarify how two seemingly very different animals, like rats and corals, can still affect one another. The chain of events that Graham et. al point to as the link between rats and corals is, in short, as follows: humans (inadvertently) bring rats to an island near coral reefs, and rats dramatically decrease the number of nesting seabirds on the island by preying on eggs. Less nitrogen is deposited into the surrounding water and plant life (because the seabirds produce nitrogen rich excrement, which is transferred into their environment, including oceans), therefore fish become smaller and more scarce due to lack of nitrogen rich food (Graham et. al 2018, Figure 1). This decrease in fish population leads to a decrease in the rate of algae grazed, allowing it to overrun corals and for the health of the reef to suffer. It may seem contradictory that nitrogen rich excrement is a key link in this chain because coral reefs are known to thrive in nutrient poor waters. However, if there is an abundant level of fish living on the reef, they will consume the nitrogen as it is absorbed into plants, thereby keeping nutrient levels in the water itself low.

Picture2
Figure 1. This nesting seabird chick lives on a rat free island in the Chagos Archipelago. (Photo by Nick Graham)

Methods

In order to untangle these complex networks, Graham et. al capitalized on the unique setup in the Chagos Archipelago, settled in the heart of the Indian Ocean. Due to patterns of human movement, the northern islands in the archipelago are rat free, while southern islands are rat infested. This allowed for a wide scale comparison in which abundance of seabirds, defecation rate, and nitrogen content of excrement were all analyzed. To quantify the disparity in fish size between rat free and rat infested islands, the researchers measured the size of growth rings in the otoliths (ear bones) of damselfish. Furthermore, the net biomass (total mass of all organisms in the area) gain, number of times grazed per year, and erosion rate were all compared between island groups to determine the effect of rats on the adjacent marine habitat (Figure 2). Nitrogen isotopes played a key role in findings, as they were used to understand where nitrogen sources derived from. This is important because rats are merely recycling nitrogen that is already present on the islands, while by contrast, seabirds feed in the open ocean and bring nitrogen back to the island via their excrement.

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Figure 2. Comparative analysis of biomass gain, times grazed, and erosion rate between rat free and rat infested islands. (a) Net biomass gain in six different fish types indicated that higher biomass existed around rat free islands than rat infested islands, as shown by the positive values in the effect size plot. (b) The number of times the reef was grazed per year was over three times higher around rat free islands than rat infested islands. (c) The erosion rate was lower around rat free islands than rat infested islands.

What does all of this mean for conservation?

Based on Graham et. al’s striking findings, rat removal from coral reef islands should become a priority for conservation efforts. Not only would removing rats lead the ecosystem back towards its natural state before human impact, but it would also serve to benefit both the marine and terrestrial environments in the region. While it may seem discouraging to add yet another task to the long and overdue burden of supporting coral reefs, rat eradication has proven effective in the past (Keitt et. al 2015). With reefs providing food, jobs, and resources to people around the globe in a multibillion dollar industry, it is imperative to take concrete steps towards a future with more successful and resilient coral reefs.

So next time you’re sitting in your basement or walking through a crowded city street and a rat races past your feet, think about a future with healthy coral reefs and how we can get there.

Bibliography

Béné, C., Barange, M., Subasinghe, R., Pinstrup-Andersen, P., Merino, G., Hemre, G. I., &

Williams, M. (2015). Feeding 9 billion by 2050–Putting fish back on the menu. Food

Security, 7(2), 261-274.

Graham, N. A., Wilson, S. K., Carr, P., Hoey, A. S., Jennings, S., & MacNeil, M. A. (2018).

Seabirds enhance coral reef productivity and functioning in the absence of invasive rats.

Nature, 559(7713), 250.   https://doi.org/10.1038/s41586-018-0202-3

Keitt, B., Griffiths, R., Boudjelas, S., Broome, K., Cranwell, S., Millett, J., &

Samaniego-Herrera, A. (2015). Best practice guidelines for rat eradication on tropical

islands. Biological Conservation, 185, 17-26.

United Nations Environment Programme, Coral Reef Unit (2000). Coral Reefs: Valuable but

Vulnerable. UNEP.

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