During the 14th century, Europe was ravaged by the
bacterium yersinia pestis, also known as the black plague.
It killed as many as 30 to 80%
of the people, depending on the geographic location.
And took more than 150 years, for most of the human population to recover in number.
However, they did eventually all recover and prosper.
Populations of other species, have not been so lucky.
During the last 150 years, several species
of Hawaiian honeyeater birds, have died out
altogether from avian malaria, following the introduction
of the parasite and its culex mosquito vector.
Viruses, like canine distemper virus, were
associated with the recent near extinction of
the Black-footed Ferret, and are a significant
conservation threat to many large African carnivores.
Ebola is now the nail in the coffin, for
some of the few remaining wild populations of gorillas.
In the face of this, it's natural to ask, what type of
pathogens might drive its host to the brink of extinction and beyond?
Surprisingly, it may not be the grizzly ones, of we most quickly think of.
When we think of pathogens with a major impact of human population, we
might think of acute, highly contagious
pathogens, that cause sudden and major epidemics.
Such as small pox or
pandemic flu.
With high case fatality rates, such agents may reduce population size.
However, they rarely drive populations or species extinct.
The reason is the inherently self limiting nature of such pathogens.
As susceptible hosts are being depleted through the epidemic, transmission slows.
And the chain of transmission is almost always broken,
while some fraction of susceptible individuals still remains uninfected.
The simple susceptible infected recovered model,
predicts an exponentially decaying relationship between
the pathogen basic reproductive number, and
the fraction or hosts escaping infection.
There's a flu like pathogen with an R 0 of around 1.5, that's expected
to leave around 20% of the population
unharmed, even if it killed all infected individuals.
A lethal small pox like pathogen with an
R 0 as high as 5, would be absolutely devastating.
But the epidemic, would still be expected to
burn out with a percentage of the population remaining.
Even in the absence of any control
measures, or behavioral changes in the host.
Thus acute directly transmitted pathogens, may bring a
population near the brink of extinction, but unlikely beyond.
The inherent self limitation of these pathogens,
comes about because the force of infection
acting on the susceptible population, drops quickly
during the second half of the epidemic.
Thus from the point of view of extinction, we
are concerned with characteristics that prevent this from happening.
Two key characteristics that reduce pathogen
self limitation, and increase the chance of
host extinction, are alternative hosts that can serve as a reservoir, and vector
born transmission.
Having an alternate and resistant host, means that the pathogen
can still circulate robustly, as the susceptible population is depleted.
This mechanism is an important driver in many Chytrid
fungus driven extinction of frogs throughout the Americas.
Some species appear resistant, while in other
species, the disease is almost always fatal.
Squirrel pox which is 100% lethal in some species,
is responsible for the disappearance of England's red squirrels.
The North American Grey Squirrel, which is resistant to this
disease, was introduced to England and brought the virus with it.
The red squirrels has been wiped out, while they grey squirrels now
has a thriving population that serves as a robust reservoir for the virus.
Having a
blood feeding vector, can also secure
robust circulation as the hosts become rarer.
The vector needs its blood meal, and will increase its focus on the remaining hosts.
This is how flea born plague, recurrently kills
entire prairie dog villages in the Midwestern US.
Once a flea is infected, the bacterium often even blocks
it's digestive tract to increase it's biting rate.
Thus the force of infection even increases, as the epidemic progresses.
Jumping from host to host, plague quickly spreads through prairie
dog families and villages, leaving nothing but death in its wake.
Native Hawaiian birds evolved in the absence of avian
malaria, and many species are extremely susceptible to the parasite.
These bird, birds experienced a double whammy of getting infected
by highly virulent vector born parasite, and being surrounded by
numerous introduced bird species that could evolve with the parasite
in their native range, and serve as a resistant reservoirs.
Avian malaria, has been implicated as a significant contributor
to the extinction of least a dozen Hawaiian bird species.