Carbon becomes part of the carbon cycle in several ways, both by physical and chemical means. carbon enters a food web via through the process of

The carbon is among the most abundant element found in the living organisms. The carbon enters the food web through the producers via the process by which they make their food, which is known as photosynthesis. The plants during the process of photosynthesis, take in carbondioxide, water and capture the sunlight to make ATP. The carbon from all these elements enters the producers and then is passed on from one trophic level to another.

Hence, the blanks can be filled with 'producers, and photosynthesis'.

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.