01 October 2008

Predation of seabirds by mice: a case of mesopredator release?

Ross M. Wanless, Andrea Angel, Richard J. Cuthbert, Geoff M. Hilton, Peter G. Ryan (2007). Can predation by invasive mice drive seabird extinctions? Biology Letters, 3 (3), 241-244 DOI: 10.1098/rsbl.2007.0120

In an ecosystem, the top predators may predate on both the intermediate predators (mesopredators) and smaller prey species. Therefore, the removal of a top predator is likely to cause an increase in the numbers of mesopredators. If the increase in the numbers of mesopredators more than compensates for the numbers of top predators removed, a longterm result of the removal of a top predator may, somewhat paradoxically, be a drop in the numbers of smaller prey species. This is known as the mesopredator release hypothesis and it was the subject of this post.

A corollary of MRH is that two predator species, one top and one mesopredator, may be better for the long-term survival of a prey species preyed on by both predators if the top predator also controls the numbers of the mesopredator.

That introduction paves the way for the paper by Wanless et al. (pdf and video). They monitored the chicks of 3 bird species on Gough Island where the only alien mammal is the house mice (Mus musculus) that was introduced before 1888. The birds were the endangered Tristan albatross (Diomedea dabbenena), the vulnerable Atlantic petrel (Pterodroma incerta) and the great shearwater (Puffinus gravis).

They produced video evidence showing mice attacking and killing healthy
chicks of all the 3 bird species. A video clip from the journal page shows several mice attacking an albatros chick that was eventually killed and eaten by the mice. As you can see in the video, the peculiar thing about the behavior of the chick, which was much larger than its attackers, is that it seems to be incapable of defending itself. The authors note:

No chicks displayed appropriate behavioural responses to attacks, even though mice had eaten through the body wall of one filmed albatross chick and were consuming the contents of the chick’s abdominal cavity.
This immediately made me suspect that the chicks killed by the mice were perhaps unhealthy and were going to die anyway. But Wanless et al. discount that possibility:
Chicks were all apparently healthy...when attacked, whereas we found three dead/moribund chicks without wounds. Mice do not appear to target weak or sick individuals.
The authors note that the findings of their study support the mesopredator release and competitor release hypotheses. In this case, the mice have no predators of their own and also no competitors. The main factor that controls their population is probably the availability of food and, which they don’t have to share with another species.

The implications of these findings for the conservation of threatened seabird species that nest on islands are clear, but will be difficult to put to practical use.

There is something else that is bothering me, though. If the mice and the birds have coexisted since at least 1888, why did the mice apparently start to threaten the bird populations recently? This was something I discussed in this post.

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