Question

During the 1940s, military ships from the South Pacific accidentally introduced brown tree snakes from Australia to the island of Guam. These snakes eat birds, lizards, and small mammals in their native range of Australia. Since no species on Guam eats the snakes, their population has grown rapidly. Researchers estimate that two million tree snakes now inhabit the island. So far, 10 species of birds and 5 species of lizards have disappeared from Guam; small mammals have also decreased in abundance.

Use this information and your knowledge about biology to answer questions 1 – 5

1. Since the brown tree snake is a keystone species in Guam, it must also be a keystone species in Australia. True or false?

2. Native predators of birds on Guam likely decreased in abundance after brown tree snakes arrived. True or false?

3. If birds on Guam usually disperse seeds, brown tree snakes likely had a negative indirect effect on plants. True or false?  

4. Since more prey and fewer predators occur on Guam than in Australia, the density of brown tree snakes in Guam likely exceeds the density in Australia. True or false?

5. If some species of the now-extinct lizards used to eat bird eggs, brown tree snakes had a positive direct effect and a negative indirect effect on birds. True or false?

This is also the information provided

Answer 1

Question 1 - The statement is FALSE

Explanation: In Australia Brown tree snake may not be the key stone predator, because there are other top predators already present and the brown tree snake eaters are also there. So it is not keystone species in Australia.

Question 2 - The statement is TRUE

Explanation: As brown tree snakes began to eat birds, their number of birds reduced and as a result, the native predators who used to eat on birds, get less food. This may cause decline n their number.

Question 3- The statement is TRUE

Explanation: As brown tree snakes began to eat birds, their number of birds reduced and as a result, the number of birds which disperse the seeds of plants also reduced. This obviously affect the seed dispersal of plants and thus negatively affect plant population.

Question 4- The statement is TRUE

Explanation: There are huge prey and no such predators for brown tree snake in Australia. On the other hand, there are predators of brown tree snake in Australia and also there are other members of the same trophic level of brown tree snake which consume same kind of food resource. So there are both intra and inter specific competition in Australia. This is absent in new island and for this reason there population density of brown tree snake exceeds than the population density in Australia.

Question 5 - The statement is FALSE

Explanation: The brown tree snake eats the birds, so there is direct negative effect on birds. On the other end brown tree snakes consumes lizards which eats the eggs of birds and thereby help in survival of eggs. This is indirect positive effect on birds by brown tree snake.