Jim Occi, BugPics, Bugwood.org
What determines whether an insect population explodes or just moves along at a steady, relatively low level? This questions has intrigued people for centuries. As usual, there are a combination of factors that determine growth potential of a population in a place:
1. Resources available: Quantity and Quality of the food, water, light, air, space)
2. Climate/temperature: Average, high and low, variation in temperature
4. Predation/Parasitisms: Every organism is preyed upon by some others
5. Movement: Immigration and emigration from one spot to another
6. Biological characteristics: Each organism has their own reproductive characteristics under specific conditions
- Birth rate
- Death rate
- Sex ratio of offspring
- Age of first reproduction
- Life span
Consider how changing any one of these will change the potential for growth in the population.
A Special Case: Mighty Morphin' Aphids
Aphid pests are wizards of reproduction. They quickly adapt to changes in their environments by producing different "morphs", or physical and biological forms. Food source drying up? They produce winged forms called "alates" which can be carried many miles by the wind to new food sources. At a new, lush food source? The produce big fat fecund wingless females "gynoparae" that give live birth to only female offspring, which in turn give birth to only female offspring, and on and on. They do this without males, by parthenogenesis. Once the food source dries up in the fall, it's back to winged forms, this time male and female. These alates mate and females lay eggs that overwinter. Eggs may hatch in spring into parthenogenetic females, the beginning of the new line, called "stem mothers". Got all that?? This is why aphid reproductive rates are hard to get a grip on!! We will try a very simple experiment in this activity to see if we can observe any of these phenomena on captive aphids on cereal (rye and oats).
1. Learn about the factors affecting population growth
2. Learn about the different types of population growth (linear, exponential)
3. Learn about "carrying capacity" of a resource (i.e. food, space, air, water)
4. Learn about the particular biology of the experimental organism
Materials Needed: Any population of small insects and their food.
Charts on which to graph results.
Transfer 5-10 parthenogenetic female aphids from the lab colony to new, week-old seedlings. Cover.
Make sure plants are moist.
Check new colonies daily and count total number of aphids present.
Graph # added per day and cumulative number.
What is the rate of increase for these aphids?
What will happen when their daughters start to reproduce?
Can you estimate the population level in one month?
From a pest management point of view - when is the best time to try to control such a pest as aphids?
From an ecological perspective, is unlimited growth of any population possible?
What will happen eventually?