  We study the response of the gaseous component of a galactic disc to the time
dependent potential generated by N-body simulations of a spiral galaxy. The
results show significant variation of the spiral structure of the gas which
might be expected to result in significant fluctuations in the Star Formation
Rate (SFR). Pronounced local variations of the SFR are anticipated in all
cases. Bursty histories for the global SFR, however, require that the mean
surface density is much less (around an order of magnitude less) than the
putative threshold for star formation. We thus suggest that bursty star
formation histories, normally attributed to mergers and/or tidal interactions,
may be a normal pattern for gas poor isolated spiral galaxies.
