Intraspecific Competition between Insects and a Study of Predation on Haole Koa Seeds
In this lab we will ask how Anthribid (Araecerus levinennis) beetles select the laying sites for their eggs.
Haole Koa, Ekoa, or Leucaena leucocephala (the scientific name) is an abundant, fecund, fast growing tree typical of dry areas in the Hawaiian Islands. This plant is not native, but was introduced to the islands. It produces elongate seedpods containing numerous large seeds. The seeds occur in separate chambers within the pod and you can see the seeds as bumps on the pod surface. Haole Koa seeds suffer a high mortality caused by the larvae of an Anthribid beetle. The beetles chew a small hole in the pod, right above or below a seed, and deposit one egg in the pod, next to the seed. The egg hatches and the seed serves as a food source for the Anthribid larva which then pupate and metamorphose into an adult. The adult emerging adult chews a larger hole in the pod wall through which it escapes. Thus pods can be read like a book, the numbers and sizes of Anthribid holes telling the fate of the seeds inside.
If seed pods of appropriate age are available, we will attempt to determine if anthribid mothers lay eggs at random on the seeds or whether they space them out, e.g., only one egg to a seed, (what would be the selective advantage of spacing the eggs 1 per seed?) or cluster them.
If events such as "attacks" on seeds are rare, or relatively rare, and occur at random, the frequency distribution of these events approximates a Poisson distribution, named after a mathematician, Mr. Poisson.
100
50
01234567
Events per unit time (or space)
For instance if AM's (anthribid mothers) deposit eggs at random and if eggs are rare relative to the # of seeds, most seeds will have no eggs (scars), fewer will have 1, fewer still 2, etc.
10
8
6
4
2
0
0123456
Number of eggs per seed
If eggs are clustered (“clumped”) we would expect to see a distribution more like this:
8
6
4
2
0
0 1 2 345678
Number of eggs per seed
The principle point of this exercise is to show you that you can learn something about the behavior of these beetles by studying how they have dispersed their eggs.
Once we know the average number of eggs/seed we can use the Poisson distribution to calculate the number of seeds we expect to have 0, 1, 2, 3, etc. eggs if the MB's are laying eggs at random.
The formula for calculating the expected frequencies for each of the classes, 0 eggs/seed, 1 egg/seed, etc., is:
X r (# of seeds) = Expected frequency in the rth class
(r/r!ex ) (total number of seeds)
where: = average eggs/seed
r = respectively 0 eggs/seed;1 eggs/seed; 2 eggs/seed etc.
e = base of natural logs
r!= r factorial e.g. 3! = 3*2*1, 4! = 4*3*2*1
The observed frequencies of # of eggs/seed can be compared to the expected frequencies using a test you've learned about.
Reading
Krebs pgs. 41-54, 88-94, Chap 9
Andrewartha, M. G. 1961, Introduction to the study of animal populations. Univ. Chicago Press.
Southwood, T.R.E. 1966. Ecological Methods. Methuen.
Whittaker, R.H. 1975. Communities & Ecosystems. MacMillan.