Recall that in week $6$ we had a prac looking at Emma Wilkie's data on native vs invasive oysters. She measured the number of different types of organisms on native, invasive and control tiles at high and low shore. We used factorial ANOVA to test whether the total number of organisms differed between the different treatments. Now let's use PCA to visualise how the community composition differed between the different treatments. Run the code below to import the data into a dataframe called oysters.

library$($curl$)$

u $<-\text{'}$https:$//$docs$.$google.com$/$spreadsheets$/$d$/1$Auccj$72-$wEnH$0$QgPK$\_$KCt$0$HKn$6$H$1$D$3$vcBaV$4$ehjRCac$/$pub$?$gid$=0$&single$=$true&output$=$csv$\text{'}$

curl$\_$download$($u$,$'oysters.csv$\text{'})$

oysters $<-$ read.csv$(\text{'}$oysters$.$csv$\text{'})$

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Carry out a Principal Components Analysis on the taxonomic counts $($columns $4$ to $7)$ by submitting the command

prcomp$($oysters$[4$:$7],$scale$=$TRUE$)$

How many principal components are there?