This study investigated population dynamics in
*Colpidium* protozoa,
single-celled organisms that live in freshwater environments and
eat bacteria. Holdridge and colleagues (2016) grew groups of
protozoa at four different initial densities. After three days,
they measured the final densities to calculate the realized per
capita population growth rate (${r}_{\text{realized}}$) for each group.

The scientists also wanted to know how the population growth rate at a given density is affected by resource availability. So, they studied the four densities of protozoa under two different conditions: high and low resource availability. Protozoa under the “high resource availability” condition got about four times as much food (bacteria that grew on dried bloodworms) as protozoa under the “low resource availability” condition did.

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The effect of initial density and resource (food)
availability on the realized per capita population growth
rate (${r}_{\text{realized}}$) of *Colpidium* protozoa

${r}_{\text{realized}}$ was calculated by subtracting the initial cell density from the final cell density, then dividing that difference by the initial cell density.

**Use what you've learned above to answer the following
questions:**