By Thomas E. Lovejoy
In many senses, the recent publication in PNAS by Ceballos et al. (1) on population losses and declines in vertebrates can be traced back to efforts early in the 20th century led by the American Committee for International Wildlife Protection to document the extinction phenomenon (2⇓–4). The focus at the time was very much on individual species, not on extinction rates, and certainly not on precursor population declines.
It is interesting to reflect on one of the most famous extinctions caused by humans, namely, the Passenger Pigeon. The original populations were staggeringly large and so impressive that in the 1870s, the Passenger Pigeon was chosen to adorn one of the gateposts of the British Museum of Natural History in South Kensington to represent the Nearctic region.
Its extinction would have seemed inconceivable, given the enormous populations and the unpredictability of nesting sites. With the advent of a predator aided by telegraph to advertise the otherwise unpredictable nesting sites and by rail to transport the harvest to markets, the populations declined and winked out one by one until all that remained was the single female Martha, which died at the Cincinnati Zoo in 1916. Basically, there was no easy way to keep track, whereas today, in contrast, not only is there an army of birdwatchers but also, in the digital age, the ability to monitor population trends by, in this case, an electronic database, e-bird.