Fighting Mother Nature with Biotechnology - The Oxford Topic of Food, Politics, and Society - page 431

But these come at a price; Mother Nature demands strict adherence to some tough
rules. According to one such rule, from the realm of biology, every individual of every
species gets to reproduce, on average, just one progeny surviving long enough to also
generate progeny. Nature's 1:1 replacement ratio ensures relatively stable populations
of each species. Obviously, any population generating less than 1 reproductively suc-
cessful progeny per individual will eventually succumb to extinction. Indeed, spe-
cies extinction has occurred many times over the period of life on Earth. Conversely,
any species successfully reproducing to generate a replacement ratio higher than
1 will eventually inundate the planet. Unlike extinction, such inundation has not
happened—yet.
A popular high school thought experiment to illustrate this inundation through
exponential reproductive growth shows how a pair of fertile cockroaches (some-
times the hypothetical species will be flies, other insects, or even bacteria), given
unlimited resources and assuming a stated gestation period, number of fertile eggs
per litter, and breeding lifespan, will quickly smother the Earth (for a fascinat-
ing look at insects and how they've influenced human society over the years, see
Berenbaum 1996). Fortunately, Mother Nature ensures cockroaches—and all other
species—do not enjoy unlimited resources, and she keeps the cockroach popula-
tion in check with numerous constraints, including food and water scarcity, dis-
eases, insectivorous predators, and even cockroachphobic humans with thick soled
shoes. Taken as a whole, Mother Nature's replacement rule holds that in spite of the
potential to take over the Earth, each cockroach will, on average and calculated over
the course of many generations, leave behind exactly one reproductively successful
progeny. All other progeny will succumb to starvation, disease, predators, or, in at
least some cases, shoes.
Mother Nature metes out harsh punishment for violating her replacement rule.
The punishment for those populations falling below the 1:1 replacement—for other
than short periods—is extinction. All extinct species, from dinosaurs in ancient times
to dodos more recently, had a replacement ratio of less than 1:1 during their downfall,
and they paid the ultimate price for their lack of fecundity. If the species dies out due to
lack of food, or water, and those resources later replenish, well, by then it's too late. The
resources might return, but extinct species do not.
Certainly, many species diverge from the replacement ratio trajectory for short peri-
ods, and some do it with almost predictable frequency. Consider the population dance
of predators and their prey, such as hares and lynx or lions and gazelles, fluctuating
together almost in lockstep (the relationship is close but not perfect as other factors can
also influence either or both species). But averaged over several generations, the ulti-
mate ratio remains the same: 1:1 replacement for each species.
Species also exist that enjoy temporary reproductive capacity above 1:1, enjoying a
population boom giving rise to a sustained, long-term total population increase. We
see this in what population biologists call a sweepstakes route, such as when mammals
first discovered Australia. Plenty of food, no predators, and few limits to growth pre-
vailed, so the population expanded above the 1:1 replacement ratio until food and other