Fit the species-area line

You will write functions for the species in an area, for the slope Z from two plots, and for the share of species lost when a habitat shrinks.

The idea. The species are $S = C A^{Z}$. The slope from two plots is $\dfrac{\log S_2 - \log S_1}{\log A_2 - \log A_1}$. When the area falls to a fraction $f$, the species fall to $f^{Z}$.

Your tasks.

1. Finish species. It gives $S$ from $C$, $Z$ and $A$.
2. Finish slope. It gives $Z$ from two plots.
3. Finish percent_lost. It gives the per cent of the species lost when the area left is the fraction left.