Heterogeneity: spatial noise in parameter values can generate a series of bands of periodic travelling waves.[20] This is important in applications to oscillatory chemical reactions, where impurities can cause target patterns or spiral waves, which are two-dimensional generalisations of periodic travelling waves. This process provided the motivation for much of the work on periodic travelling waves in the 1970s and early 1980s. Landscape heterogeneity has also been proposed as a cause of the periodic travelling waves seen in ecology.[21] Invasions, which can leave a periodic travelling wave in their wake.[11][12][22] This is important in the Taylor-Couette system in the presence of through flow,[23] in chemical systems such as the Belousov-Zhabotinsky reaction[24][25] and in predator-prey systems in ecology.[26][27] Domain boundaries with Dirichlet or Robin boundary conditions.[28][29][30] This is potentially important in ecology, where Robin or Dirichlet conditions correspond to a boundary between habitat and a surrounding hostile environment.