Brine shrimp decline, phytoplankton increase. Australia goes through pronounced wet and dry climate cycles. Some perennial plants get established only in wet periods. Red kangaroos have most of their offspring during wet periods. And lots of trace stuff.
Concentration changes with altitude.
Concentrations have also changed over time. The early atmosphere contained little or no oxygen and probably more carbon dioxide. The addition of oxygen via photosynthesis probably resulted in large-scale extinctions of anaerobes. Above the troposphere to about 50 km. Above the mesosphere.