1. Any existing life there is, at this point, highly improbable
2. If there is any, how could terran life be competitive with it if the existing life has evolved to match the local environment over billions of years?
3. If there is existing life, how could a biologist not be able to easily distinguish it from terran life?
4. If the life there is ancient and now extinct, terran life isn't going to interfere with that
To answer part 2 with just one example: The native life of Mars, if it still exists, would exist in a state of homeostasis with its environment. It would have to in order to still remain existent. If terrestrial organisms were capable of replicating under martian conditions, they could easily eat everything up and then die off. Never quite getting the time necessary to adapt to the ecological limits of their new habitat. And by this process driving the native life to extinction as well.
To answer part 3: We're still discovering new kingdoms of life on Earth (though it's unlikely we'll discover new domains). If localized panspermia exists within our solar system (from meteor impacts or the like) it's possible martian life and terrestrial life are related enough for the martian life to fit within the already existing family tree of terrestrial life.
https://astronomy.com/news/2021/05/did-life-on-earth-come-fr...
2. They'd never eat all of it. Also, it the distribution of either form will never be even across the planet. There will be "islands" of one or the other.
3. Biologists are easily able to determine if they are new kingdoms are not. They're also able to estimate how long ago divergence from a common root happened.
There are many, many examples of parallel evolution in terran biology, but none of them are confused with each other. It's absurdly unlikely that a terran modern amoeba will be confused with a Martian amoeba.
2) Localized sure, I wasn't arguing about the entire planet. But introduced life could drive the native life to a local extinction. And if it did so fast enough we would never know the local life had been there.
3) Yes, I know. While this isn't my specialty I work at an organization that does have people that specialize in this. The difficulty would be in definitively concluding whether this is a native divergence that we've just never seen before, or the result of Martian evolution.
2. We've found fossilized remnants of bacteria in rocks, haven't we? There's also ice on Mars. If life existed, we'd find it frozen in the ice.
3. A billion years of evolutionary divergence, with local alien adaptations, is going to be very hard to confuse with anything brought over by a probe.
2. If there is any, how could terran life be competitive with it if the existing life has evolved to match the local environment over billions of years?
3. If there is existing life, how could a biologist not be able to easily distinguish it from terran life?
4. If the life there is ancient and now extinct, terran life isn't going to interfere with that