Do thousands of runs, and look at the distribution of the outcomes:
What proportion of the simulations went extinct?
What proportion fell below 10 individuals? "quasi-extinction" (where it will just spiral into extinction)
What was the average time to extinction?
How do these results change if we alter survival? Reproduction?
What factors have the largest proportional impact on the outcomes?
Key stochastic (random) factors for small populations:
1. Genetic stochasticity
2. Demographic stochasticity
3. Environmental stochasticity
Random changes in the population's genetic composition. Due to founder effects, inbreeding, genetic drift, etc.
Easy to model using genetic theory (applied probability), but in reality this has little impact compared to the other factors.
Random events in the sex and age structure of the population.
Of final 13 health hens, 11 were males
Restricted to island
Only about three hundred more
Once they thought they were protected, that's where the problem occurred
The heath hen was brought to the brink of extinction by deterministic factors: overhunting and habitat destruction
Once the population became very small, it was vulnerable to stochastic factors, which eventually caused its extinction
Bottleneck: population becomes very very small in comparison to what it used to be.
So they brought a lot back and they thought they were successful. And then they had a wildfire in that area and killed a bunch of the birds. And then they had a bad winter. And then they had an influx of predators. So the population had an 80% decline. And then a chicken brought a disease to the island which the heath hens weren't resistant to.
Of final 6 dusky seaside sparrows (they only had males left in captivity and basically had to watch them die), all were males
This becomes very important at very small population sizes
Random year-to-year changes in the environment, such as food supply, populations of predators or parasites, diseases
Ex: Normal cycles of fires, weather, drought, and climate (El Nino)
Larger populations can better withstand these effects.
Low frequency, high intensity disturbance event.
Higher magnitude and lower frequency that environmental stochasticity
i. Cane Toad (also known as Marine Toad or Giant Toad)
1) Native to northern south america and central america
2) People introduced them to australia as biological control to a non-native pest to a non-native plant (cane sugar)
3) They didn't eat the pest they
4) The pest was brought down by pesticides
5) The Toad is still a huge problem (50,000 offspring per year), generalist
6) The toads are toxic
7) Spreading across northern Australia
ii. Kudzu (Invasive vine)
b. Domestic animals (feral) and ornamental plants
c. Acclimatization Societies
i. (people like things that remind them of their homeland or some improvement to the wild)
ii. European Rabbits at Australian water hole in 1880s (note lack of vegetation)
1) Thousands of rabbits that had no predator
2) So introduced mammalian carnivores: weasels, stoats, mongoose, red fox, etc.
3) Generalist: which are terrible biological control species because once they bring the population down then it'll eat anything else
4) So the weasels became more of a problem than the rabbits
5) Myxamatosis: fatal disease that they introduced to kill off the rabbits and control that population
iii. European Starling
1) Successfully introduced in central park in 1987
2) Spread all over into the surrounding states and into Canada and now all over America
3) Monopolize resources (cavity nesters) from native species
d. Game: Sport hunting, fishing, commercial fishing
i. Feral pigs: generalist, can eat just about anything, disturb top 2-3 inches of soil, damaging
ii. CA wild horses
iii. Feral cats, and free-ranging housecats
1) Cats are listed among the world's most problematic invasive species (IUCN)
iv. Nile Perch, introduced to Africa's Lake Victoria to provide commercial/sport fishing
1) Greatest water body in Africa
2) Hundreds of endemic cyclic fish that had no commercial value
3) Introduced Nile Perch as either a sport fish or to provide a resource for the local people to sell in the market but it ate all the cyclic fish