Answer to Question 1
Ehrlich's theory states that all species are fairly equal in their importance to the ecosystem and the ecosystem will fail after the removal of species places a sufficient strain on the system. Walker's theory states that there are certain keynote species, which, if removed, severely damage the health of the ecosystem. Under this philosophy, it does not necessarily matter how many species you remove it is more important which species are removed from the ecosystem.
Answer to Question 2
As the sun ages, it becomes brighter. This results in a higher incoming solar flux at Earth orbit. The carbonate-silicate cycle dictates that this higher solar flux will lead to decreased CO2 concentrations, stabilizing the surface temperature. Eventually, the CO2 concentration will fall below the CO2 compensation point and photosynthesis is no longer energetically favorable. This could result in the end of life as we know it, as the primary producers currently on Earth will likely go extinct. Sometime after this, Earth's oceans should be lost as the planet develops a wet stratosphere, and water vapor is photolyzed, with the hydrogen escaping into space. This can all be greatly postponed, though, if we build a solar shield at the Earth-Sun L1 Lagrange point.
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