Conservation organisations struggle to directly assist all threatened species, so deciding where to spend limited resources is a common problem. In a new paper, Oppel and colleagues show that, for long-lived species, a population may decrease long before this trend becomes evident in the part of the population that can be counted.

Albatrosses are among the largest flying birds in the world, and they can get incredibly old, with one female named “Wisdom” who was marked >65 years ago still breeding today. Albatrosses achieve this long life by reproducing very slowly – they often need 5-15 years before they can start breeding. In the largest species, a breeding pair can only raise one chick every 2 years because it takes almost 12 months for the chick to grow large enough to fly, and parents need a long rest between raising chicks.

Despite being amongst the largest birds, albatrosses can be threatened by some of the smallest mammals – mice. On several islands such as Marion (South Africa) or Midway (USA), introduced non-native house mice (Mus musculus) have started to eat albatross chicks and sometimes even adults. As a consequence, the albatross species breeding on those islands have very low breeding success as many chicks are lost to hungry mice.

Although this problem has been known for two decades, the consequences of mouse predation have so far been difficult to evaluate due to the long lifespan of albatrosses. For example, the Critically Endangered Tristan Albatross (Diomedea dabbenena) has lost on average half of each season’s chicks to mouse predation since monitoring began in 2004. Yet, over the same period, the breeding population has remained remarkably stable at ~1500 pairs every year, leaving conservationists puzzled what the impact of mice might be, and whether albatrosses would benefit from an ambitious operation to eradicate mice from their main breeding island.

Our work provides a compelling answer. A consortium of researchers funded by the Agreement on the Conservation of Albatrosses and Petrels used a sophisticated population model that accounts for all the young albatrosses, and adults taking a break from breeding, that roam the southern oceans and can therefore not be counted by ornithologists. The scientists found that the total population of the Tristan Albatross has in fact decreased by >2000 birds since 2004 – despite the stable number of breeding pairs.

Extrapolating 30 years into the future, the researchers further concluded that eradicating mice from their main breeding island would most likely result in a Tristan Albatross population that was 2-8 times larger in 2050 than if the mice remained.

The population projections come with large uncertainty though – mostly because it is very difficult to know whether young albatrosses are still alive. After fledging, albatrosses can spend 2-20 years at sea where they cannot be accounted for. This uncertainty renders the estimates of population size somewhat imprecise, and when extrapolating the population 30 years into the future, the range of uncertainty spans several thousand birds. Nonetheless, the new estimates are the most robust yet and provide a lot of new information for guiding management decisions.

Overall, the conclusions from the study support the decision that investing in a mouse eradication on islands where mice kill albatrosses is likely to be a highly effective strategy to restore populations of these ocean wanderers.

Read the full paper Cryptic population decrease due to invasive species predation in a long-lived seabird supports need for eradication in Journal of Applied Ecology