Is Sugar a Preservative?

In a high enough concentration it can be. Cells (bacterial or otherwise) can only prevent osmosis across their membranes to a limited extent. When a solution is concentrated enough (i.e. hypertonic), it can dessicate the cells, therefore retarding microbial growth and rendering potential pathogens inert .

Some examples of sugar-based preservatives include various syrups (i.e. maple), honey, and fruit preserves (jams). In particular it is useful to note that jam does not require raising temperatures above 104C to kill botulinum spores; although most operations will pasteurize the stock to kill fungal spores which may grow on the top layer of the jam due to recondensed water).