Some species have experienced population size reductions (bottlenecks),
but have since recovered. The Mauritius kestrel was reduced to a
single pair but has now recovered to 400–500 birds (Box 8.1). Northern
elephant seals dropped from many thousands to 20–30 but now number
over 100000 (Box 8.2), while the Lord Howe Island woodhen declined to
20–30 individuals but has since built up to about 200 birds. These populations
pay a genetic cost for their bottlenecks; they have reduced
genetic diversity, higher levels of inbreeding, lower reproductive fitness
and compromised ability to evolve