Grasshopper Population Density versus Size of Patch: Instructor Supplement

Background Information:

Depending upon the suitability of the habitats chosen for study, students may encounter less lubber grasshoppers (Brachystola magna) and more field crickets (Gryllis pennsylvanicus). Typically, grasshoppers will prevail in more sparsely vegetated fields, crickets in more dense and rich vegetation. Both occur as adults from August to October, but begin to die off upon first frost. It will make no difference which organism is the object of this study, so long as students are encouraged to carefully identify the experimental organism and to remain consistent in its study.

Habitat fragmentation and boundary constriction, as discussed in a number of the articles listed above, are perhaps a primary cause for both the loss of diversity, and for the subsequent local extinction, of species. Many effects are directly witnessed by patch size decrease: resource depletion due to over-foraging (as is the case for grasshoppers) and subsequent habitat destruction, deleterious mutation accumulation in isolated and inbred genetic lines, and loss of species diversity. Students who explore the broader dimensions of this topic armed with background from their own sampling and application of the Lincoln Index will arrive to appreciate the importance of expansive patches for the long-term maintenance of single species populations as well as for inter-species diversity.

Logistically, the idealized study will involve at least three different comparisons between large and small patches. Each comparison should take two weeks: one for establishing baseline counts and marking, and one for a follow-up count of marked and unmarked. Therefore the total number of weeks devoted to field work should be a minimum of six, with actual hours in the field totaling 30 hours. Enough sites with a robust number of quadrats per site need to be sampled to provide sufficient power of a t-test. The technique for capturing and marking might be improved if a fine insect net or a bridal veil were substituted for a bed sheet.

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