What are the risks and consequences of colonizing an inhabited Earth-like planet?

Question

The planet I have in mind has an abundance of plants and animals, but no sapient species or civilizations (think Earth before humans came along). Wouldn't Earth micro-organisms wreak havoc on alien ecology? I have heard that it would not be a problem since Earth life would be too different to pose a threat to the native flora and fauna, and vice versa. But then NASA destroyed the Cassini spacecraft in order to not contaminate worlds. Apparently, "NASA maintains an Office of Planetary Protection in order to preserve our ability to study other worlds as they exist in their natural states; to avoid contamination that would obscure our ability to find life elsewhere — if it exists; and to ensure that we take prudent precautions to protect Earth’s biosphere in case it does.”

I am thinking of getting my colonists to take their chances and minimize the risk of wiping out entire alien species by establishing a colony in an isolated island cut off from large continents. They would be farming Earth plants and even try utilizing the native plant species. Is this plausible?

EDIT : This planet orbits an orange dwarf and is somewhat larger than Earth which means it has higher gravity. Plants would not grow as tall as that of Earth and they would be darker with hues of reds and oranges in order to make the most of the light from the star. Life is carbon based and use water as a solvent and have many other similarities to that of Earth life. EDIT: You're right, reds and oranges are the colors they would not reflect.

Answer 1

Sure - sorta:

There are a lot of variables in what you are suggesting, and it depends on how voluntary and complete your colonization efforts are. If your colonists are trying to recreate an exclusive bite of Earth on an island, they will inevitably need to wipe out an island of native life, likely a few species at least. Then, they need to recapitulate a complete Terran ecology to establish a viable environment for Terran Life, down to microorganisms. Somewhere, this Earth ecology will be bumping up against an alien one, and the degree of compatibility will be critical to understanding what happens.

• If life is reasonably compatible, there will be direct interaction and competition. Settlers may fish native organisms, and harvest native (fill in the blank kelp, etc) Terran fish will enter the oceans or native fish will enter your colony. New interactions will establish themselves and it will be difficult to prevent. Seeds will get scattered in both directions. Bird forms may move back and forth. Grass could be the equivalent to Kudzu and devastate whole ecologies.
• A very different biology means a different set of problems. There may be toxic native compounds that are harmless to native life, and vice versa. Alien bacteria will eat human plants and nothing will stop them, or terrestrial cyanobacteria will sweep native life from the oceans. Both things may be true - Earth bacteria spread like cancer across the land as native amino acids cause violent allergies.

There really isn't a way to be sure what will happen, as ecology is fickle and driven by ruthless survival. But it's unlikely that there can be a pure difference, unless the colonists are making a terran ecology in sealed greenhouses to prevent contamination. Even soil behaves a bit like sourdough - you need the stuff in it to let terran plants live and thrive. A worker coming into a greenhouse will track in organisms, and track them out.

There is an unavoidable risk of contamination, but it can be as real or perceived as you want. What is good for your story? Make it so, but acknowledge the concerns and dangers.