You might think that increasing the nutrient resources available would elevate the number of plant species that an area could support, but the evidence suggests otherwise. The data in the accompanying table are from the Park Grass Experiment at Rothamsted Experimental Station in the U.K., where grassland field plots have been fertilized annually for the past 150 years (collated by Harpole and Tilman 2007). The number of plant species recorded in 10 plots is given in response to the number of different nutrient types added in the fertilizer treatment (nutrient types include nitrogen, phosphorus, potassium, and so on).

a. Draw a scatter plot of these data. Which variable should be the explanatory variable (X) , and which should be the response variable (Y) ?

b. What is the rate of change in the number of plant species supported per nutrient type added? Provide a standard error for your estimate.

c. Add the least squares regression line to your scatter plot. What fraction of the variation in the number of plant species is “explained” by the number of nutrients added?

d. Test the null hypothesis of no treatment effect on the number of plant species.

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Plot 11 22 33 44 55 66 77 88 99 Number of nutrients added 00 00 00 1 1 22 33 1 1 33 44 Number of plant species 3636 3636 3232 3434 3333 3030 2020 2323 2121