Invasive rats need to be eradicated from certain tropical islands to help conserve already-threatened coral reefs, a researcher part of a team that studied the matter said.
Researcher Aaron MacNeil is an associate professor of biology at Dalhousie University and a Canada Research Chair in fisheries ecology.
The research, titled “Seabirds enhance coral reef productivity and functioning in the absence of invasive rats,” was published on July 11.
The field work was conducted in 2016, MacNeil said. Part of the funding for the project came from the Natural Sciences and Engineering Research Council of Canada, and by co-author Nick Graham via the Royal Society, he said.
MacNeil was interviewed about the project in Halifax on Friday.
What is the research about?
The research is about the role that invasive rats have played in coral reef islands. About 90 per cent of the tropical islands in the world have invasive rats on them, and we were sort of interested in what the effects of those rats have been on the surrounding coral reefs. People have known for a long time that invasive rats really interrupt the functioning of terrestrial ecosystems — the islands themselves — but we got to thinking about, ‘Well, it has all these big effects on the islands, what are the effects on the reefs surrounding those islands?’
Which islands in particular?
These islands that we investigated are quite unique. They’re in the middle of the Indian Ocean, called the Chagos Archipelago, and because of the presence of an American airbase there, it has been a de facto marine-protected area for more than 40 years. It’s one of the most remote and pristine island chains in the world, and it has a series atolls that are just absolutely stunning, full of sharks and other fish. It’s a really amazing place.
What was your role in this project?
My role was to help design the study. We were looking at the effects of rats interrupting the flow of energy from birds that feed out in open ocean and then how that energy goes out onto the reef. I helped design the study and to do the analysis and write the paper around it.
What are the takeaways?
The biggest takeaway is that the effects of these rats are really severe and significant. This study was a bit of a high-risk study. We didn’t really know or expect to see any real result, and the results were absolutely clear: The rats are having a devastating effect on this flow of nutrients that comes from birds that feed out in the open ocean that come back onto the island and release their poo, their guano onto the island, which acts like a fertilizer. That fertilizer gets washed out onto the reef through rain and wave action, and it actually fertilizes the reefs, so the reefs where rats are present have much lower productivity, and the ecosystem itself is not functioning properly. That interruption of that trophic flow, we call it, that energy flow is a really big and significant effect. So much so, it’s one of the most important energy transfers for a reef itself. Reefs occur in very nutrient-poor water, and so we realize now that these subsidies that come from land are actually really important.
What about the rats’ impacts on the bird population?
The rats basically eat the young, so when a bird’s nesting there, the rats climbs the trees and they’ll eat the eggs or the young chicks. Basically, the birds figure out that they can’t roost on those islands, and so there are a few birds that sort of try their luck but, for the most part, they go onto the islands without rats. It’s a very noticeable thing. If you go to these islands and you stand on an island that have birds, that kind of natural ecosystem, it’s noisy, there are birds all over the place, it smells terrible, and then you go to the islands that have rats on them, and it’s beautiful, it’s quiet, you can hear the waves slapping on the shore. It’s just night and day in terms of the difference between them.
So what needs to be done?
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This study really shows that eradication is the way forward for rats on islands. Coral reefs are threatened quite heavily by climate change. They’re right on the boundary of the temperatures in the world, and as the temperatures rise, corals are really struggling to deal with those elevated temperatures, but how to help coral reefs through this change as we start to try to deal with climate change is very uncertain. They’re very complicated ecosystems, there are a lot of interacting species, and so trying to understand what to do is not often clear, but here is a clear-cut case where there are so many islands with invasive rats on them that this is a low-hanging fruit, it’s a really obvious and clear thing we can do to help reef ecosystem function and to help them function better and just by eradicating rats. In Chagos, there are about 35 islands that have rats on them, and we’ve calculated it would be between $2 and $3 million to eradicate all those rats which, given the benefits for the ecosystem, is actually a very small amount of money and a very big return on investment.
Where there any things learned that might be relevant to Canada?
I think so. I mean, the surprising part of this study was the positive effects of these nutrients. So excessive nutrients from things like golf courses or river inputs have negative effects on coral reefs that cause the reefs to recover less quickly from disturbances, for example. This is another case of increased nutrients, but the nutrients are beneficial and we think that it has to do with the actual composition of the nutrients themselves where run-off from fertilizer tends to be just very high in nitrogen, whereas these nutrients tend to have right blend of nitrogen and phosphorous and other things. So that idea that there are good nutrients and bad nutrients for marine ecosystems that come from land could play out here. We have nutrient issues surrounding things like eelgrasses in Prince Edward Island, and so, because of the run-off from fertilizer in P.E.I., a lot of those eelgrass beds are in trouble. Trying to understand that maybe if we change the mixture of nutrients that end up going out into the marine ecosystem, maybe there are benefits to be seen there.
The other thing that a colleague brought up to me recently was what about cormorants? We have a lot of cormorants that are in lakes and marine ecosystems around here. Are they having similar effects in the areas where they roost? Fisherman killed off a lot of cormorants because they compete with them for fish. As cormorants recover, are we going to see those nutrient subsidies going into ocean and actually improving what our sort of reefs are like, which are actually algae. A lot of the fish communities in coastal Nova Scotia benefit from really big kelps and algae, and they’ve been decimated over the past few decades, and so maybe this is another type of subsidy that could help locally, but we really just don’t know.
Does this highlight how interconnect life is on earth, and its ripple effects?
Absolutely. I mean, one of the unique things about this study for anybody who’s interested in ecology is that here you have one animal, the black rat, which has interrupted the flow of energy from the open ocean onto land, the terrestrial ecosystem, and then out onto a coral reef. So these are three connected ecosystems, and very connected. The productivity of the open ocean is actually fuelling the productivity of a coral reef, and the fact that one species is able to interrupt this gives you an idea of how much this may have happened in other ecosystems that we haven’t even looked at yet.
So, in this context, bird poop good, rat poop bad?
Yeah, the rat poop is just recycled. It doesn’t come somewhere else. So it’s a big like soil; if you don’t add nutrients into soil, you can just deplete the soil, and it’s just cycling through over time. The rats are not adding anything to the island. All of the nutrients that the rats get come from the island itself, essentially, whereas the birds are going out in the very productive open ocean, bringing it back, releasing it out onto the island and feeding this energy cycle.
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