Final answer:
Secondary plant compounds primarily serve as a defense mechanism against herbivores, deterring them from consuming the plants by being toxic.
Step-by-step explanation:
A secondary plant compound might be used for defense against herbivores. Many plant species produce secondary plant compounds that serve no other function for the plant than being toxic to animals, thus discouraging them from consumption. An example of this is the foxglove, which produces several compounds such as digitalis, that are extremely toxic when eaten. These toxic substances provide a form of chemical defense, which is a key adaptive strategy plants have developed over time to protect themselves from being consumed by herbivores, which includes a variety of animal species.