The most extreme, sometimes alarmist, claims about the growth of wildlife populations are easily repudiated because they are biologically impossible.
Biology has limits. Population growth is biologically capped. I am not speaking here of the limits imposed by resources, like water, food and shelter. Resource limits vary. I am speaking about the absolute limits imposed by many millennia of evolution. Evolution is contingent but also imposes constraints on the future. Evolution invents the biologically possible and impossible at the same time.
Species have different evolutionary histories and so have their own unique, absolute biological limits. These limits are known and can be used to estimate the fastest growth rates possible for species.
For example, feral horses are placental mammals. Offspring are conceived and develop in the mother for a period called gestation that ends in birth. The gestation takes almost an entire year – around 11 months . It is not possible for a mare to reduce gestation length so that she could breed more than once a year – Limit 1.
It is also not possible for mares to produce more than one foal per year – Limit 2. Twins in horses are extremely rare. They have only been known to survive in domestic mares with human assistance. Even then successful twin births occur in only 1 of 10,000 pregnancies. Thus, feral horses cannot produce more than one offspring each breeding attempt. More rapid population growth in horses cannot be achieved by increasing litter size such as can occur in feral rats, cats, dogs, goats, and sheep.
Maternal age also imposes limits. A female cannot conceive before sexual maturity. The earliest sexual maturity is the second breeding season (spring-summer) after birth. Thus, the earliest a mare can foal is towards the end of her second year – Limit 3.
Some extraordinary ages have been reported for domestic horses, especially geldings (desexed males). But domestic horses typical live to 25-30 years old and domestic mares only rarely breed into their late-20s.
Variation in the longevity of feral and wild horses is, unfortunately, not as well understood because so few individual horses of known age and living wild have been studied for their entire lives. Available studies of longevity are in small populations where mare breeding is controlled by contraception. In those populations mares, like domestic geldings, have lived much longer lives – sex shortens lives. Mares whose reproduction is controled have reached 30 years of age and bred as 26-year olds.
Unmanaged mares, however, will live shorter lives and often die with the demands of their last breeding attempt. It appears unlikely that unmanaged feral mares breed past 20 years of age [2, 3, 4] – Limit 4.
Taken together these limits mean that it is improbable for a feral mare to produce more than 18 foals in a lifetime – maximum.
One can use values like these to estimate the maximum possible growth rate of a closed feral horse population. One might also use the most extreme values of reproduction and survival reported to estimate the largest possible growth rates from real data for comparison.
Scientists have conducted and reported these types of study twice in the last 30 years. William Conley in 1979  and colleagues and I in 2001 . The results are revealing. They help us identify hyperbole in claims of feral horse population growth.
To be continued…
1. Parker, R. 2013. Equine Science, 4th edn. New York: Delmar Cengage Learning.
5. Conley, W. 1979. The potential for increase in horse and ass populations: a theoretical analysis. In: Symposium on the ecology and behavior of wild and feral equids (Ed. by R. H. Denniston), pp. 221-234. University of Wyoming, Laramie: University of Wyoming, Laramie.
6. Cameron, E., Linklater, W., Minot, E. & Stafford, K. 2001. Population dynamics 1994-98, and management of Kaimanawa wild horses. Wellington, New Zealand: Science & Research Unit, Science, Technology and Information Services, Department of Conservation.