When playing a card that gives resources and has an "additional effect" on the bottom, you must always do everything on a card when it is played or you cannot play it. This means both the resource and the additional effect are mandatory, even if the additional effect is detrimental. The only exception is for cards with the trash symbol; these can be played for their resources or trashed for the listed additional effect, but not both.

This is supported by the following rule:

• "When playing a card which gives resources and has an 'additional effect' on the bottom, do you always get both effects or must you choose one or the other?
  You must always do everything on a card when it is played or you cannot play it. The only exception are cards with the [trash] symbol. These cards are played for their resources and may be trashed for the listed additional effect."
  (Page 28)

Summary:
Both effects are mandatory unless the card has the trash symbol, in which case you choose one or the other.
Source: page 28.

Source: Core Rulebook (pp. 13, 14, 18, 28, 29)