Evolution and Ecology Lab (BIOL 3113)

Spring 2003


Biotic factors regulating population size


Last week you collected data that allowed you to examine the effects of nutrient levels on population growth rates in the Spotted False Duckweed Landoltia punctata. This week you will use the same experimental setup to look for evidence that the populations in these same canisters are approaching their carrying capacity. What might this evidence be? Well, if the populations are continuing to grow at an exponential rate, then the per capita growth rate the first week of the experiment should not be significantly different from the per capita growth rate for the second week. Thus, we need to do little more than record population sizes, calculate per capita growth rates for each week, and look for significant differences over time. Because there were more than likely significant differences in growth rates among the different nutrient levels, you will need to analyze the weekly growth rates of each nutrient level separately.

Procedures

Repeat last week's procedures. That is, record the number of Landoltia thalli in each canister, and enter the data in one of the laptops at your bench. To test your hypotheses, you will need to convert raw population sizes to per capita growth rates. This is simple to do in Excel. To calculate per capita growth rates during the first week, create a new column with an Excel formula that is equivalent to: (C1 / C0) - 1, where C0 = the starting population size (which should be 10!), and C1 = the population size at the first census. Likewise, to calculate per capita growth rates during the second week, create a new column with an Excel formula that is equivalent to: (C2 / C1) - 1, where C2 = the population size at the second census.

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