Plastic is a man-made material, most often derived from petrochemicals. It is designed to last. Most of it is not biodegradable, which means it cannot be decomposed by bacteria or other living organisms. Instead, it persists in the environment and, under the influence of the elements, is torn into smaller parts without changing its chemical makeup.
These small pellets, known as microplastics, are often swallowed by birds and fish, resulting in death by starvation (animals mistake fullness for satiety) or gastrointestinal complications. This is especially true for larger pieces of plastic (aka macroplastic) with which birds often feed their young.
Furthermore, many harmful chemicals are associated with plastics. These chemicals, many of which are toxic, carcinogenic or mutagenic, are released from the plastic as it fragments in the environment.
It is now believed that there are 5.25 trillion pieces of plastic debris in the ocean. Of that mass, 269,000 tons float on the surface, while some four billion plastic microfibers per square kilometer litter the deep sea.