Ecologists like myself often have a bit of a blindspot for diseases. We don’t see their impacts very often in wild populations – this may be a particular failing of ornithologists. But there are some good examples of diseases having big impacts on populations – usually, of course, introduced diseases that arrive in a new place thanks to humankind’s domestic animals or plants.
In my book about the extinction of the Passenger Pigeon, A Message from Martha, I discuss the possibility of disease playing a part in the demise of the most numerous bird species the world has ever seen (pp140-2) and even at this distance I’m fairly happy with what I wrote back in 2013.
Passenger Pigeons weren’t at all good at social distancing – they nested in colonies of millions of nests, they roosted in winter roosts of millions of birds, they travelled the land in flocks of millions and they fed in flocks, sometimes of millions. The life of a Passenger Pigeon was spent, every minute of it, and really like no other bird, in close contact with huge numbers of conspecifics.
But from where might a new disease have come? A candidate would be from some of the non-native bird species that we Europeans took to the New World. House Sparrows, were taken to North America by Europeans; 8 pairs were released in Brooklyn in 1854 and more birds were released in Cincinnati in the 1870s as well as other transfers of birds around the USA. Soon, House Sparrows were common across the range of the Passenger Pigeon, I wonder whether any of them brought diseases with them.
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The role of disease as a potential cause of population decline and extinction is perhaps more obvious in plants where there are well documented examples – e.g. Dutch Elm disease, Ash die-back and American Chestnut blight. Sadly, awareness of this doesn’t seem to have had much impact on our habit of importing tree saplings from abroad along with – potentially – their diseases and invertebrate disease vectors.
Pathogens have been transmitted from animals to humans since the beginning of time, but the intensity has increased over the last century as human populations have grown. With 7.5 billion people on this planet the pathway from animal microbe to human pathogen has a clear pathway.
Covid-19 is the consequence of our inhuman treatment of animals, bad husbandry, contempt and our ever increasing demands on these animals as food and destruction of their habitats. By radically changing nature (e.g. planting non-native) fundamentally propels the emergence and transmission of novel and as yet unknown human infectious diseases.
It seems this particular virus came to us via live wild animal markets in Wuhan, China, it is probably a bat or pangolin virus according to what I have read. Perhaps this will provide the impetus needed to control or get rid of such markets. Since we started to closely associate with animals historically viruses have crossed the species divide, TB and influenza are both originate in other species, bovines and early domesticated ducks.
Of course when we “discovered” the New World we inflicted uncountable casualties on the native inhabitants with the viral and bacterial infections we imported that they had no immunity to, indeed far more than died in violent conquest.
It is entirely possible that we gave other native species diseases we carried ourselves or with other species we imported and it is always possible that disease contributed to the demise of the Passenger Pigeon although do the facts fit? Over to you Mark.