Less, lesser, smaller, fewer are comparable terms that approach each other in meaning but are not synonyms and are rarely interchangeable.
Less means not as much, especially in degree, value, or amount, and its opposite is usually more. It applies chiefly to collective nouns or nouns denoting a mass or abstract whole.
Lesser means not as great, as important, or as significant as that with which the thing so qualified is compared and implies opposition to greater or major. In vernacular names lesser implies that the kind of plant or animal so designated is distinguished from a very similar one carrying the same name chiefly by its comparative smallness of size.
Smaller means not as large as that with which the thing so qualified is compared (as in size, dimensions, or quantity).
Fewer means not as many and implies a difference in number of individuals or units; the term therefore modifies a plural noun.