Using the Library catalogue to do an Advanced Search will allow you to search by subject - just choose 'in subject' from the drop-down menu on the left-hand-side.
The search shown in the screenshot below will look for resources with 'ecotourism' listed in the subject:
Other subject searches you could try are:
Subheadings or extensions, eg countries, relevant century, type of publication (catalogs, exhibitions, bibliographies etc) can be applied to subject headings to narrow the search. For example: