Handout 3A: Black tiger snake population simulation
Simulation
An island Black tiger snake population has three types of available prey. On average the prey are
available in the proportion of three mutton birds, two rats and one skink. There are three phenotypic
forms of the Black tiger snakes; large, medium and small. There are no predators of the Black tiger
snake on the island.
Assumptions for this simulation:
o The Black tiger snake population at the start of this study is made up of 30 individuals; 10
large, 10 medium and 10 small.
o Large snakes eat mutton birds, medium-sized snakes eat rats, small snakes eat skinks.
o In this simulation the sex of parents and offspring is not important.
o Each year the Black tiger snake population mate at random, and each pair produces one
offspring.
o After mating the following offspring are likely:
Phenotype of parents
large x large
medium x medium
small x small
large x medium
large x small
medium x small
o
Phenotype of offspring
large
medium
small
½ large, ½ medium,
½ large, ½ small,
½ medium, ½ large,
Each year the Black tiger snakes mate and produce offspring during April-June. During the
following summer one third of the population dies due to starvation.
Suggested approach
1. Begin with the initial population of 30; 10 large, 10 medium and 10 small snakes. An extra
pile of cards will be needed for the 15 offspring.
2. After shuffling 30 cards deal them out in pairs. These will represent the random mating of
each parent combination.
3. For each pair, produce one offspring by following the parent/offspring table.
L x L cross = L, M x M cross = M, S x S cross = S.
For L x M cross, roll dice - 1, 2 or 3 showing, the offspring will be large, 4, 5 or 6 offspring
will be medium.
For L x S cross, roll dice - 1, 2 or 3 showing, the offspring will be large, 4, 5 or 6 offspring
will be small.
For M x S cross, roll dice - 1, 2 or 3 showing, the offspring will be medium, 4, 5 or 6 offspring
will be small.
Starvation
4. The population now consists of 45 individuals, 15 of which are going to starve due to
unavailability of suitable prey. To determine which 15 will starve, roll the prey dice. If a
mutton bird is rolled select a large snake from the pile. If a rat is rolled select a mediumsized snake or if a skink is rolled select a small snake. Repeat this 29 times. The remaining
15 starved and are removed from the population.
5. Count up the numbers of each card type and record in a table to show population numbers
over several years of the study.
Year
1
Large
10
Medium
10
Complete the table for at least five years.
On Borrowed Time © CSIRO, 2008
Small
10
Total
30
30
30
30
30