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Grasshopper Population Density versus Size of Patch

Jeff Weld

Overview

What is the impact of patch size on an insects' population? The model organism for this study is the common grasshopper, Brachystola magna, andthe intent is to determine its population as a function of the size of grassland patch. But the larger issue is one of habitat fragmentation in general. Certainly grasshopper populations have been as subjected to this phenomenon as any organism as a result of roads, housing, public parks, etc. infringing on, and carving up, grassy habitats. Thus you might consider this study a model for examining more generalizable issues by investigating the following questions: What are the implications of habitat fragmentation or elimination in terms of population genetics, species diversity, and ecosystem stability in these altered environments?

Ideally, you'll need to make two visits to each patch: first visit establishes a count of grasshoppers within each quadrat, and to mark every captured grasshopper. Multiple quadrats at both the large and small patch are recommended. The second visit is to again collect organisms at each quadrat of both sites, this time to determine the marked versus unmarked grasshoppers. The species of interest will be active in the adult form from August to October. This accords you plenty of time to contrast several large patches to several small patches in order to bolster your findings for statistical analysis. A paired t-test can be used to determine the statistical significance of the difference between population sizes atlarge versus small patches.

Prepare to consult the literature referenced below and elsewhere in order to complete a picture of habitat fragmentation, patch size, population densities, and the implications from work that others have done.

Suggested Methods

  1. Locate large and small undisturbed and unmowed grassy fields for this study. Ideal small patches are roadside ditches- bordered on both sides by roads and farm fields. Ideally you'll be able to make comparisons between a number of pairs of large sites versus small sites during different weeks throughout the fall.

  2. Stake out quadrats for conducting your sampling. Squares of perhaps 2 meters x 2 meters should be adequate. Mark boundaries using corner stakes. The number of quadrats at each sampling site should exceed three. Postpone captures until your next visit.

  3. Use a bed sheet to drape over the marked quadrat in order to capture grasshoppers. Members of your group should encircle the draped sheet and crawl toward center, herding grasshoppers inward to be captured in the ballooning sheet.

  4. Remove the grasshoppers from this initial capture one-by-one, marking each prior to release. Mark the grasshoppers by placing a dab of neon orange modeling paint upon the upper prothorax (chest cavity) of the grasshopper in plain view.

  5. Repeat this procedure for each quadrat in both your small wild patch and large wild patch. RECORD the number of captured and marked grasshoppers at each quadrat of both sites, and total the number of marked grasshoppers at each site.

  6. Return to the sites after one to two weeks, and conduct the bed-sheet capturing technique described above. Count and record the total number of grasshoppers captured and the number of marked recaptured.

  7. Prepare to represent your data in table form depicting captures and recaptures at each quad of each site as well as totals for each patch.

  8. In order to draw conclusions regarding the population density of each patch, use the Lincoln Index (N=nM/x), where
    • N = estimated population,
    • n=total captured,
    • M= total marked, and
    • x= the number of marked recaptured.


  9. To test the accuracy of your Lincoln Index population estimation, apply a standard error calculation of

    • N= N[(N-M)(N-n)/nM(N-1)].


    You can have a 95 percent certainty that the true number of grasshoppers lies between two standard errors above and two standard errors below N. Provide theseconfidence intervals for the population estimates at each site.

  10. Each site is to be compared using a paired t-test to identify significant differences between populations of sites. Statistics books are referenced on the following page for your consultation.

  11. Based upon estimated populations derived from the Lincoln Index and the consultation of sources listed below and elsewhere, what generalizations can be made regarding:
    1. population density of grasshoppers in large versus small patches;
    2. reproductive ramifications of small patch populations versus large patch populations.

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