Scientific Method and Hypothesis Testing
an exercise in applying the scientific method

Observations
The macroinvertebrate community structure in a stream can vary due to a number of physical, chemical, and biological factors. These factors can come from both terrestrial and aquatic sources. Changes in the terrestrial landscape may have direct or indirect effects on the number and types of macroinvertebrates present in a stream. In the eastern United States, hemlock forests are dying due to an exotic pest, the hemlock wooly adelgid (HWA). Because hemlocks can be an important component of the riparian terrestrial structure, the loss of these trees represents a potential changing factor for the stream ecosystem.

Hypothesis
In streams of similar size, type and gradient, streams in hemlock forests of different level of infestation with hemlock wooly adelgid will show different macroinvertebrate community structure.

Experiments
Nine stream sites in three different levels of HWA infestation in hemlock-dominanted forests were identified. The three levels of infestation were non-infested forest (healthy hemlock), 50% infestation, and 100% infestation (all hemlocks in area affected). Three sites were located in each infestation type. All sites were similar in watershed size, stream order, gradient, benthic substrate, and forest composition.

Stream macroinvertebrate samples were collected at all sites following the EPA's Rapid Bioassessment Protocol. The collected organisms were identified and placed into one of four functional feeding groups: scraper, shredder, predator, or collector. Relative percentage of total organisms in each functional feeding group was determined for each level of HWA infestation by averaging the percentages for each site within the level.

Data

Conclusion
Though there was some variability, functional feeding group analysis shows that there is a general trend toward a change in the macroinvertebrate community as hemlock wooly adelgid affects the health of the hemlock trees around streams. The most affected functional feeding groups appear to be the scrapers and the shredders, two groups that rely on outside inputs. Scrapers mainly feed on attached algae; algal growth would increase with increasing amounts of sunlight reaching the stream as the riparian tree cover is lost. Shredders rely mainly on organic input from the terrestrial environment; as hemlock trees are affected by the HWA infestation, the trees drop their needles, increasing organic input to the stream.

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