National Geographic

Effects of invading island rats ripple across land and sea

Brownrat.jpgHumans have explored the entire face of the planet, but we haven’t done so alone. Animals and plants came along for the ride, some as passengers and other as stowaways. Today, these hitchhikers pose one of the greatest threats to the planet’s biodiversity, by ousting and outcompeting local species.

Islands are particularly vulnerable to invaders. Cut off from the mainland, island-dwellers often evolve in the absence of predators and competitors, and are prone to developing traits that make them easy pickings for invaders, like docile natures or flightlessness.

Two years ago, I wrote about the ability of invasive predators to change entire island landscapes. In the Aleutian archipelago that runs between Alaska and Russia, Arctic foxes have turned some islands from grassland to tundra by killing the seabirds whose droppings provided the islands’ only fertiliser.

Now, we return to the ill-fated Aleutians to discuss another study that shows how the actions of immigrant predators can even domino into the surrounding waters. This time, the stars are not foxes, but that other ubiquitous opportunist – the brown rat.

Islands the world over have suffered at the paws of rats. When they land, the local bird populations plummet, either because the eggs and chicks are eaten, or because the adults flee elsewhere to breed. The Aleutians are no exception and there, the brown rat (Rattus norvegicus) feasts on the chicks of ground-nesting birds like the Glaucous-winged gull and the black oystercatcher. But not all islands have been invaded.

To Carolyn Kurle from the University of California, the existence of rat-free islands meant that the Aleutians could act as a natural laboratory for studying the effects of rat invasions. Together with Donald Croll (who led the fox study) and Bernie Tershy, she compared the density of different local species on 32 islands throughout the archipelago. Fifteen of them were free of rats but others were plagued by them, to the extent that their noctural scurrying sent Kurle reaching for earplugs.
Glaucousbackedgull.jpg

As expected, the rats took their toll on gulls and oystercatchers, which were ten times more common on rat-free islands than infested ones. Kurle found that these direct effects led to more unexpected ones that extended to the very margins of the islands. Both gulls and oystercatchers prey upon invertebrates living in the tidal zone.

With fewer hungry beaks around, these animals thrived in the presence of the rats and Kurle found that the coasts of invaded islands were home to 50 times more sea stars, 30 times more mussels and 6 times more barnacles than rat-free ones. Snails and limpets also became more common and the presence of these grazers halved the amount of algae in the tidal zone. With less algae taking up space on the rocks, other invertebrates including sea anemones and sponges flourished, even though they aren’t directly eaten by birds.

On the Aleutians at least, rats have triggered a chain of effects usually reserved for top predators. Their assault on the seabirds has unwittingly shifted the balance of power in the waters surrounding the island, from algae to invertebrates. It’s likely that similar events, known to ecologists as “trophic cascades”, have happened all over the world, for rats are some of the most successful of island pests. Who knows how far their influence truly extends?

Ratcascade1.jpg

Further reading: Check out the excellent Invasive Species Weblog for regular updates on the threat of alien animals. Image of cascade from PNAS paper.

Reference: Kurle, C.M., Croll, D.A., Tershy, B.R. (2008). Introduced rats indirectly change marine rocky intertidal communities from algae- to invertebrate-dominated. Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.0800570105

Add Yours.

  1. Rat
    February 25, 2008

    Whaddya mean, we’re pests? My family has lived on this island for generations!

  2. John Mark
    February 25, 2008

    ^Yeah, but by whose generations?
    As a history major, bravo to your theme of explaining science without all the messy jargon.

  3. Jack Hynes
    February 25, 2008

    Sadly I can’t get your Atom/RSS 2.0 feeds to work! Hopefully they’ll be up-and-running soon, and it’s just teething problems.

  4. Ed Yong
    February 26, 2008

    “that other ubiquitous opportunist – the brown rat.”
    So ubiquitous that one has even invaded the blog. Now it’ll eat all the good links, reproduce madly, and before I know it, I’ll have a virtual ecological disaster.
    Welcome all.
    Oh and re: feeds, I’ve noticed this – hopefully the wonderful people at Seed will swoop down and make everything better.

  5. Brigit
    February 26, 2008

    But look at those pretty, beady eyes. That rat wouldn’t hurt a fly. I swear, she’s just here for some pies, then she’ll go on her merry way.

  6. Brian English
    February 26, 2008

    I understand the little rodents are causing lots of damage to wildlife on Maquarie Island in the southern ocean. Aided and abetted by their cousin rodents in the rabbit…..

  7. Brian English
    February 26, 2008

    That was Macquarie Island of course…..

  8. Rat
    February 26, 2008

    @Brigit: thank you for your kind comment. Actually I’m not here for pies … though if you have some handy … er, I’m here to find long-lost family members. There’s a story handed down from my many-greats-grandma that some of my distant relatives were recruited by scientists to assist them in experiments… though that particular grandma was said to be a little confused before the fox… uh, never mind. I’d love to meet some long-lost cousins.

  9. Ed Yong
    February 26, 2008

    The rat is disturbingly more eloquent than some of the commenters I’ve had in the past…
    Oh and for anyone interested in the effects of invading island rodents, have a look at this post from the classic site.

  10. autumn
    February 26, 2008

    For an island biome, where does one objectively place the “native flora and fauna” in the time line of the place?
    I can state very confidantly that Hawai’i’s ecology is not native, but only if I allow that the “nativeness” of a location is somehow plausible, and that it is to be compared to some standard of “nativeness”.
    Do we stop after a couple thousand years, or do we attempt to preserve every rafting species, as these are the most pure invaders.
    Not trying to be dickish, merely asking when the divide between native and invasive apply to a once sterile rock.

  11. Salim Fadhley
    February 26, 2008

    What might happen if somebody introduced a blight of kittens to chomp on the rats?
    If the island is so over-run by rats how long does it normally take for some kind of opportunistic predator to take up residence? And what were the rats eating once they ran out of eggs & chics?

  12. Noe
    February 26, 2008

    Autumn- in the context of small island ecology, we generally consider a native species to be one which arrived on its own, without human assistance. Obviously, in your Hawaii example, people have been living there for several hundred years and have brought a whole suite of non-native species with them (though there are still a few ecosystems there which have a high proportion of native species). Not all of these are invasive, though- for example, lychee trees are cultivated, but have not spread & propagated on their own. Think “invasive”=”weedy”. Rats are definitely an example of a species which has been very successful at spreading worldwide with human assistance.
    If a new species were to get to an island on its own, I think the usual approach would be to leave it there, UNLESS the species in question was known to be disruptive in other ecosystems and the island was functioning as a refuge for species which were there previously (in which case land managers would try to prevent it from establishing itself). Unfortunately, nowadays so many island species are threatened that these isolated islands are the last refuge for many, and they are managed so as to preserve what’s left.
    Salim- if you dropped a bunch of cats off on the island, they would likely kill some of the rats, but they would also eradicate the ground-nesting birds. A similar idea was tried in Hawaii with mongooses to kill rats, although there it turned out that rats were nocturnal and the mongooses were active in the daytime, so the mongooses turned to the birds as a food source instead. Oops! Cats are also extremely effective at eliminating bird populations because they kill for play- they don’t stop when they have enough to eat.

  13. Ed Yong
    February 26, 2008

    The feed is working now. Thanks to Ginny and Tim at ScienceBlogs for their help. Feel free to start subscribing!

  14. Dave Briggs
    February 27, 2008

    Whaddya mean, we’re pests? My family has lived on this island for generations!
    Posted by: Rat | February 25, 2008 10:58 PM
    Don’t take it personally Rat! LOL! One man’s pest is another man’s cute furry little critter!
    Dave Briggs :~)

Add Your Comments

All fields required.

Related Posts