According to the law of conservation of matter, matter is never created or destroyed. yet in a food pyramid the amount of
biomass decreases as we move up through the pyramid, from producers to top level consumers. why?

In a food pyramid the amount of  biomass decreases as we move up through the pyramid, from producers to top level consumers because,  Matter changes to other forms that might not continue to be a part of the same food chain.

Explanation:

It is essential for any living organism to maintain a good health for survival. The good health refer in attaining the necessary nutrients. This can be achieved using the Food pyramid.  This helps in getting the nutrients at accurate amount that is helpful in maintaining a good health. There are similar types of nutrients that exists in a food type. These are classified under one group and they form the food pyramid.

Producers are those that have ability to make food for them on their own. For example plants. Plants can get carbon dioxide from atmosphere prepares food for them through photosynthesis process and releases oxygen that helps humans to survive. Consumers are those that depends on something for their food. In a food pyramid, when we move upwards, as the matter changes its forms, which will not sometime become a part of similar food chain, there will be a decrease in the biomass amount.

on island x the species needed to adapt because they were being subject to predators which didn't inhabit island y.