Cricket Populations

ESTIMATING CRICKET POPULATIONS


BACKGROUND

A population is the number of organisms of the same species in a given area. We study populations because they are the smallest subset of systems that we need to understand, in order to better understand the earth. However, before we can analyze data from our population studies, we must first determine the number of individuals in a population. Sampling a population may provide an accurate estimate of the number of individuals without the difficult and unrealistic task of manually counting each organism. Sampling is especially useful when estimating a population of moving organisms where some organisms may be counted twice or not at all. Sampling a large area is definitely preferable to doing a total count on a small area, especially if the area counted is too small to be representative of the population of interest.

Animals can be captured marked and released in the same area they were first captured. At a later date you can take another sample of your population, some organisms should be marked from the first sample and some organisms will not be marked as this is the first time they have been captured. It is then reasonable to assume that the proportion of new, unmarked animals caught in the lasted sample, is equivalent to the proportions in the entire population. Knowing the number of animals marked, you can draw a ratio and estimate the total number of animals in the population. Say that we have marked 50 animals and that we have just caught a sample of 20, 10 of which were marked in the first capture; the total population is then (20/10)50=100.

OBJECTIVES

  • To identify and describe the components that make a population
  • To estimate the members of a population using a marked sample

MATERIALS

  • crickets
  • Insect net
  • nail polish (or other permanent marker)
  • plastic bag/other container
PROCEDURES
1. A pre-determined number of crickets are inhabiting a given location. As a group of ecologists, we will capture a random sample of these crickets from their habitat. You are to remember that the utmost care and respect is to be shown to these animals while they are in your possession.

Capture as many crickets as you can during the amount of time that your teacher will give you. Be careful not to harm the crickets or break their legs as you catch them. Place them in the holding container.

2. Before releasing the animals back into their environment, we will count the number of individuals that we have observed and mark them with a harmless, but permanent substance, that will be recognized in future captures. This will be the total # marked.

When marking the crickets with nail polish, be sure to place the polish on the back of their abdomens. Be careful not to completely cover the crickets with polish or to polish their heads.

3. After an adequate amount of time has passed, we will return to the cricket habitat and capture another sample. After collecting the crickets, count the total number in the second capture, and the number of crickets marked in the second capture.

Using the following formula, we will estimate the number of crickets in the cricket population we sampled:

total # marked/total population= #marked in 2nd capture/total # in 2nd capture

total # marked = _______

# marked in 2nd capture= _______

total # in 2nd capture= _______

total population= _______

QUESTIONS
1.How did you find the crickets?



Might you use similar methods to located other animals? Why or why not?




2.How were the crickets captured and marked?


What might be the advantages/disadvantages to the methods you used?




3.Were the crickets any easier/harder to capture the second time? Why/why not?



Would this also apply to other animals?



4.After your teacher tells you how many crickets were in your area, how accurate was your estimated total number of animals?




Extensions

* Students can do population studies of other arthropods, and compare predator and prey populations such as crickets to spiders. Students should be warned of stinging/biting arthropods such as bees, wasps and centipedes.

* Students can use field guides to identify various other insects found in the same habitat as crickets.

* Students can research methods of population sampling used for other animals or plants.

Teacher's Notes

Cricket activity is proportional to temperature. On chilly damp days they will stay in one place and on warm sunny days they will move constantly. A good temperature to do this lab is around 75oF.

Students are usually hesitant at first to catch the crickets, but once they are assured that they do not bite, there is usually heavy competition, who can catch the most crickets.

House crickets can be purchased from biological supply houses or pet stores for about $.05/piece. However, if there is a courtyard or grassy playground area with its own population of house crickets, adding extra insects is not necessary.

The amount of time between samplings can vary. A few hours is best, although I have performed this lab in a 53 minute period. More time will give a more accurate estimation.

Question #2:

Answers will vary. Animals that are harmed during sampling, or have markers that make them less fit in their environment, might not survive long enough to be recounted. If this happens, then the estimation of the total population will be higher than actual. If the animals survive, but are prevented from escaping due to broken legs, then they will be easily recaptured and this will lead to an estimation lower than the actual population.

Question #3:

Answers will vary. Animals might be easier to capture the second time because students are more experienced, or animals were stressed after the first capture and less able to escape. Larger animals might be harder to recapture because they are more wary of capture methods. Both scenarios might affect the population estimation.