COMPETITION & RESOURCE PARTITIONING
GENERAL ECOLOGY LABORATORY

Location: Garvin Brook

Objective: Examine potential competition and resource partitioning between two genera of caddisfly larvae attached to rocks in a small stream

Hypothesis: Glossosoma and Brachycentrus larvae will exhibit different patterns of distribution on stream rocks, and their densities will be affected by water depth and current velocity.

Agenda:

1) Select 20 rocks in different sections of the stream varying by depth and current velocity.

2) Measure and record water depth and current velocity on the stream bottom immediately in front of each rock.

3) Count and record the numbers of Glossosoma and Brachycentrus larvae attached to the various surfaces of each rock (top, bottom, sides, front, back).

4) Measure the total surface area of each rock (square cm).

5) Replace each rock to its original position within the stream.

Analysis:

1) Determine densities for Glossosoma and Brachycentrus larvae on each rock.

2) Use two goodness of fit tests (p. 14) to examine whether each genus of larvae distributes itself evenly on all rock surfaces.  Use VassarStats.net to perform the Chi-square tests (look under Frequency Data, Chi-square goodness of fit test).  Expected values should be equal numbers on each rock surface--see the data spreadsheet.

3) Use a contingency table (p. 16) to compare whether Glossosoma and Brachycentrus larvae distribute themselves similarly on rock surfaces.  Use the calculator on this website: (http://www.physics.csbsju.edu/stats/contingency_NROW_NCOLUMN_form.html).  Set up 2 rows (one for each species) and 6 columns (one for each rock surface). Record the Chi-square value and the probability (P) value.

3) Graph Glossosoma density versus water depth (one figure) and current velocity (another figure), and use two separate regression analyses (p. 17) to determine whether density is related to depth or velocity.  Use the Data Analysis function in Excel or simple linear regression in VassarStats.net to determine whether or not each regression is significant (that is, is the P value for the regression slope < 0.05?).

3) Graph Brachycentrus density versus water depth (one figure) and current velocity (another figure), and use two separate regression analyses (p. 17) to determine whether density is related to depth or velocity.  Use the Data Analysis function in Excel or simple linear regression in VassarStats.net to determine whether or not each regression is significant (that is, is the P value for the regression slope < 0.05?).

Equipment:

Current velocity meter

Meter tape

Meter stick

Data sheets

Waders