Elucidation needed on textbook's description of the effect of increased Interest rates on the money supply curve? A textbook, when explaining the effects of higher interest rates, claims that:


  The initial equilibrium is with real output at $Y_0$,the price level
  at $P_0$ and the rate of interest $r_0$. An increase in the rate of
  interest to $r_1$ will need to be balanced by a decrease in money
  supply to maintain money-market equilibrium.


The illustration below accompanies the claim.



How/why does the money supply shift leftwards to accommodate a return to "money-market equilibrium", and what is such an equilibrium to begin with? 

Additionally, is this shift automatic or is it one where the government must intervene?
The graph is a little misleading. 

Imagine the interest rates increase without $MS$ shifting to $MS1$. Here the demand for money is less than the supply, so there is a surplus. If the MS does not shift, eventually the interest rate will fall again until the shortage is no longer (we are back in equilibrium). If the Fed wants to maintain the higher interest rate, $r_1$, it also must shift $MS$ to $MS1$.