An annual breeding success as low as 27% is unprecedented among Diomedea
albatrosses (typically 60–75%; e.g. Croxall et al. 1990
; Weimerskirch 1992
; Elliott & Walker 2005
). Cuthbert et al. (2004)
reported a 28% decrease in the Tristan albatross population over 46 years and their population model identified low chick survival as a significant driver. Similarly, a population model for the Atlantic petrel predicts a decrease, given the observed low chick production (Cuthbert 2004
). Mice appear to be largely responsible for high rates of chick mortality in both the species on Gough, and negative population trends are probable unless predation is reduced.
This is the first unequivocal proof that house mice are significant predators of healthy seabird chicks. Three published records of possible house mouse predation (Fugler et al. 1987
; Ainley et al. 1990
; Campos & Granadeiro 1999
) were all on small chicks (less than 50
g) and could have been the result of mice scavenging/killing moribund chicks or taking abandoned eggs. Gough mice are relatively large compared with natural populations elsewhere (Berry et al. 1979
). Nevertheless, they are much smaller than rats and it is astonishing that they can successfully attack Tristan albatross chicks that are more than 300 times heavier. Tristan albatross chicks thwart avian predators such as sub-Antarctic skuas, Catharacta antarctica
, and southern giant petrels Macronectes giganteus
. Why then are healthy, well-developed albatross chicks, weighing more than 8
kg, incapable of defending themselves against such diminutive predators? Island birds are particularly vulnerable to attacks from novel predators because they lack appropriate behavioural responses (Kepler 1967
; Atkinson 1985
). Also, not all attacks were fatal. Albatross chicks typically took several days to die, during which time the mice fed repeatedly on them, often opening multiple wounds. In some ways, this is closer to parasitism than to predation, analogous to finches in Galapagos pecking at tail feathers of seabird chicks and drinking their blood (Grant 1986
), although, in that case, the chick seldom dies.
This is the first record of widespread and devastating predation by house mice on seabird chicks, despite mice having been introduced to many seabird islands. Has it been overlooked? Or is there something peculiar about Gough Island? The climate and native biota are not unusual, and it seems unlikely that some other condition, as yet undefined, could have given rise to predatory behaviour. Perhaps of more importance is that among 385 islands with bird species known to be sensitive to invasive species, only six, including Gough, have house mice as the only invasive mammal (Brooke & Hilton 2002
). We suspect that where house mice are part of a complex of invasive mammals, the effects of dominance, competition and predation by larger species render mice less of a threat to native vertebrates (Courchamp et al. 1999
). Recent events on Marion Island (46°
E) support this hypothesis. House mice became the sole introduced mammal following the eradication of cats, Felis catus
, in the 1990s (Bester et al. 2000
). Since 2004, several wandering albatross Diomedea exulans
chicks have succumbed to wounds consistent with mouse attacks. This is the first time in over 20 years of intensive study that wounds of this nature have been recorded (Peter G. Ryan, unpublished data).
These findings are significant for both Gough and Marion islands and, more generally, for island conservation and the emerging discipline of invasion biology. On Gough, unmitigated predation by mice could contribute to the local extinction of Tristan albatrosses and Atlantic petrels. The winter timing of attacks makes grey petrels, Procellaria cinerea
, and great-winged petrels, Pterodroma macroptera
, both winter-breeding, burrowing species, likely to experience mouse predation. The Gough bunting population is likely to have decreased as a result of nest predation by mice (Cuthbert & Hilton 2004
). If attacks on albatrosses are confirmed on Marion Island, several other species (e.g. winter-breeding, burrowing petrels) are likely to be subject to mouse predation there.
In a broader context, the conservation status of seabirds breeding on islands where mice are the sole introduced mammals needs to be studied. We predict that this phenomenon is more widespread than has been documented. In addition, costly mammal eradications are premised on projected conservation benefits (Krajick 2005
). Our findings support the mesopredator release and competitor release hypotheses (Courchamp et al. 1999
; Caut et al. in press
) that the value of eradicating competitors and predators of mice (e.g. rats and cats) would be greatly enhanced also by eradicating mice. Conversely, some long-term benefits could be compromised if they are not. In light of these results, the prioritization given to mouse eradications in island restoration projects should be reviewed.
Invasion biologists seek to predict the risks and consequences of alien species introductions. Islands are often studied to understand the general principles of invasive patterns and processes. It is surprising that major discoveries can still be made about the behaviour of a well-studied species with a history of invasion stretching back several centuries. Our findings reveal how the characteristics of invasive species can be context specific and thus difficult to predict. In a more positive vein, they also generate testable predictions and suggest avenues for research into ecological interactions, which will benefit invasion biology theory and conservation practice.