Should a sourdough starter be left open/ajar after feeding? I keep my sourdough starter (spelt) in the fridge and leave it out after feeding it. Should the jar that the starter be kept in be left open/ajar in order to encourage more wild yeast to colonize it - or be left shut so that it doesn't go off/die?

I keep the starter in a glass kilner jar with a clip lid that keeps any air out.
It's actually a bit of an old wives' tale that the yeasts colonizing your starter are from the air. Almost all of the yeasts in a starter are from the grains used to make it. This is why starters are frequently started with either whole grain kernels or grains like rye that are left mostly intact when milled. All of the microorganisms on the outside of the grain kernel are then included in your starter.

As to leaving your jar open, I certainly wouldn't seal it up, especially if the jar is glass. The carbon dioxide produced by the starter could cause the jar to explode if it can't escape.