Overview & Lesson Sequence

Focus on Standards & Assessment

Materials Needed, Preparation & Planning, Management Strategies

Background Info

Intro Activity
Why Study Populations?

Meet the Aphids

Research Project
Aphid Population Growth

Data Analysis I

Data Analysis II

Optional Activity
The Power of Exponential Growth

References

Glossary

Suppliers

Resource Sheets

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Background Information

Population Ecology
Ecology is the study of the interrelationships of organisms in their environment. The study of the processes that affect the distribution and abundance of those organisms is called population ecology. A population's distribution refers to its geographical range and how individuals are dispersed within that range,

whereas abundance refers to the population's size within that range. Population increase is caused by births and immigration,

and population decrease is caused by deaths and emigration. Environmental conditions such as the availability of food or major climatic changes also affect a population's size. The number of individuals an environment can support is called its "carrying capacity.

Populations can grow or decline rapidly. Unchecked population growth results in what is called an exponential growth rate, indicated by a "J" curve (see diagram). The population starts small then continues to grow faster and faster. As births and reproductive capacity increase, the growth rate of the population also increases. If organisms continue such unlimited growth, the result could be a population explosion. However, a population can grow only so long as it can be supported by its environment. When a population grows to the environment's carrying capacity, the population growth levels off, as illustrated by the "S" curve. (See diagram.) Given no environmental changes, the population will continue at a steady rate with births and deaths being more or less equal in number. If the population rapidly outstrips environmental resources beyond their ability to regenerate, the population may decrease dramatically, then level off at a lower number of individuals. In essence, population equilibrium results from a balance between the growth and death rates of the population and the available resources within the environment.

Aphid Biology
Aphids (or plant lice) are small (1-3mm), soft bodied insects in the order Homoptera, family Aphididae. They are normally found in large clusters feeding on the stems and leaves of plants during the warm spring and summer months. They are pear or oval in shape, have long antennae, needle-like mouthparts, a pair of siphunculi protruding from their posterior end, which can produce defensive secretions, and a pair of wings in some developmental stages. (See Aphid Anatomy sheets.)

Aphids can live up to six weeks in ideal conditions and molt four times to reach adulthood. Most temperate species have complex life cycles, but in general, they have a sexual reproductive stage and a parthenogenic (asexual) stage. Most of these species feed

and reproduce on one species of plant spending the winter months in the egg stage and then hatching into nymphs in the spring when the plant sprouts its new growth. These nymphs develop into winged females which stay on the plant or migrate to another of the same plant species and produce (by parthenogenesis) non-winged females. One unusual feature is that the young are born live. This allows them to quickly colonize the plant. In the autumn, after several parthenogenic generations, winged males and egg-laying females are produced. These sexual forms mate and eggs are laid that overwinter and the cycle begins again the next year. Sexual reproduction ensures genetic diversity and the ability to adapt to changing environments.

Some species of aphids live on more than one type of host plant. These species often spend one part of the year on one plant species and the rest of the year on another species. This is known as host alternation. A more detailed description of aphid life cycles can be found in the book, Biology of Aphids, and on Gordon Ramel's "Insect World" website (see references at the end of the lesson).

Aphids feed on the phloem sap of plants which carries nutrients to different plant parts through specialized plant cells called sieve tubes. Phloem sap is a liquid that is high in sugars but low in amino acids. Thus, aphids must drink large amounts of phloem sap to get enough amino acids; the excess sugar is excreted as liquid feces called honeydew. Aphids have a strong muscle associated with their mouthparts, called the cibarial pump, which helps them suck their food.

Because of their affinity for plants and their ability to reproduce so quickly, aphids are a crop pest and can affect a plant's growth in numerous ways. For example, their sticky, sugar-filled honeydew can coat the leaves of a plant and promote mold growth which can interfere with a plant's photosynthesis. In the case of the Cotton Aphid, Aphis gossypii, these sticky secretions can coat the open cotton bolls and render an entire cotton crop useless for industry. Sticky cotton clogs the cotton gins. In addition, many species are vectors of plant viruses and other plant pathogens.

Mathematical Models
Because nature is complex, scientists use mathematical models to help explain natural phenomena. Models are abstract representations that help simplify nature for a clearer understanding of ecological concepts. Population ecologists can develop mathematical expressions that account for birth, growth, movement (immigration and emigration), reproduction and death of individuals in a population. Products of these equations help scientists estimate species abundance and make predictions about future population trends. For example, numbers of individuals in a population at some future time (N_t) can be predicted by N_t = Nert where e = the base of the natural logarithm (e = 2.717), N = the number in the initial population, r = the rate of population increase [individuals/(individuals x time)] = the rate of population increase, and t = the time period.

Note
Some aphids have a special association with ants. Ants are attracted to the sugary honeydew that the aphids produce and this often becomes an important part of the ants' diet. The ants protect the aphids from predators and parasites such as ladybird beetles and parasitic wasps. In addition, when a heavily infested stem becomes stressed from aphid feeding, ants will move the aphids to a fresh stem on the host plant. Some people liken this relationship to a rancher tending cows and moving them to "greener pastures" when necessary.