We spent our childhood hearing stories about various horrors and catastrophes, including tigers pulling away little children from play-fields or an entire village getting wiped out by cholera. They were not from story-books. People of our grandfather’s generation could recollect such events from personal experiences. In contrast, most parts of the world appear immensely safe now, even when diseases causes by insects and other parasites are taken into account.
What happens to a species, when it has no significant natural competitor or predator. One could hypothesize that the number of its members will reach some kind of steady state and stay there, unless there is a major change in environment. That hypothesis is not backed by an observation outside human world.
The Soay sheep is a primitive breed of domestic sheep (Ovis aries) descended from a population of feral sheep on the 250-acre (101 ha) island of Soay in the St. Kilda Archipelago, about 65 kilometres (40 mi) from the Western Isles of Scotland. It is one of the Northern European short-tailed sheep breeds.
The Hirta population is unmanaged and has been the subject of scientific study since the 1950s. The population make an ideal model subject for scientists researching evolution, population dynamics and demography because the population is unmanaged, is closed (no emigration or immigration) and has no significant competitors or predators.
The sheep exhibit a phenomenon known as overcompensatory density dependence, in which their population never reaches equilibrium. The population growth is so great as to exceed the carrying capacity of the island, which eventually causes a dramatic population crash, and then the cycle repeats. For example, in 1989, the population fell by two thirds within 12 weeks.
The age and sex structure of the population are important in determining when a crash happens; for instance, adult males enter winter in a poor condition after the autumn rut, whereas females have been grazing all summer and so enter winter in a good condition. Survival rates of males (and lambs) are influenced by weather throughout winter (dependent on the strength of the North Atlantic Oscillation), whereas survival rates of females (and adolescents) are influenced most strongly by rainfall at the end of winter, when they will normally be heavily pregnant (the rain soaks the wool, increasing energy expenditure).
Let us end the week on that cheerful note :)