Which of the following is a growing threat to animals that are believed to be critical to the success of natural remedies?
competition for resources
disease and illness
poaching by hunters
increase in predators

Option C, poaching by hunters

Explanation:

All the other threats except the one mentioned in option C “poaching by hunters” are natural processes and thus they occur to balance the ecosystem. While poaching is an induced activity of limitless killing of animals by humans  

Unrestricted poaching has not only disturbed the ecological balance (by interrupting food chains) but also led to the extinction of several species. Therefore poaching must be avoided to save animals that are critical to success of natural remedies

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.

if the type of the food available changes, then the frequency of beak types will change, because birds with beaks more suited to the available food will be more successful over time.